Article pubs.acs.org/JPCA
Molecular Docking via Olefinic OH···π Interactions: A Bulky Alkene Model System and Its Cooperativity Robert Medel, Matthias Heger, and Martin A. Suhm* Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstrasse 6, D-37077 Göttingen, Germany S Supporting Information *
ABSTRACT: Complexes of t-butyl alcohol with norbornene and its monocyclic constituents cyclopentene and cyclohexene are studied via their OH stretching fundamental transitions in supersonic jet expansions. Compared to OH···OH hydrogen bonds, the spectral shifts due to OH···π bonding in the mixed dimers are reduced by a factor of 2. Mixed trimers show substantially different spectral signatures due to cooperative effects. Regioselective docking on the two sides of the double bond in norbornene is observed. Harmonic modeling of the spectra using dispersion-corrected hybrid functionals is quite successful, suggesting a high predictive power for this poorly explored class of complexes between alcohols and alkenes.
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INTRODUCTION
The molecular recognition of unsaturated hydrocarbons by functional groups in other organic molecules is of interest in several fields of science, such as the mechanisms of olfaction1,2 or chromatography.3 For example, it is remarkable that the olefinic terpenes limonene and α-pinene cause some of the most prominent enantioselective smell impressions in the human olfactory system.1 Quantum chemical modeling of the underlying subtle intermolecular interactions can be quite demanding, and it is desirable to have a few test cases at hand to assess the reliability of a chosen theoretical approach. Vibrational spectroscopy provides an attractive bridge between theory and experiment because it supplies sensitive frequency information, in particular, for hydride chromophores such as the OH group interacting with an alkene. In combination with the low temperatures of a supersonic jet, spectroscopy can also help to discriminate different docking motifs on the kJ/mol energy scale. Such docking motifs of OH toward alkenes are also of some relevance for low-temperature OH radical addition on alkenes in cold planetary atmospheres,4 which proceed via a hydrogen-bonded prereactive complex similar to that of alcohol−alkene dimers. One bottleneck in the theory− spectroscopy comparison is the role of anharmonicity, which is not easily captured by theoretical models for larger molecules or complexes. Here, we study a model system that should be relatively free from such anharmonic interferences, the interaction of t-butyl alcohol (T) with bulky alkenes (A), specifically norbornene (No) and its monocyclic constituents cyclopentene (Pe) and cyclohexene (Hx) (Figure 1). We denote the complexes by a sequence of the molecular acronyms, starting with the alcohol donor. TPe means that a t-butyl alcohol coordinates a cyclopentene unit. The docking of an alcohol to cyclopentene © XXXX American Chemical Society
Figure 1. Most stable conformations, abbreviations, and point groups of the monomeric compounds. The curved lines represent faces of cyclopentene and norbornene, which interact differently with hydrogen bond donors, giving rise to possible regioisomers.
can be syn or anti relative to the floppy out-of-plane CH2 group, which should be mobile even under supersonic jet conditions due to the low ring flip barrier.5 In contrast, the barrier for cyclohexene is about an order of magnitude larger,6 such that the planar chirality of cyclohexene may be frozen in the jet expansion, whereas it is variable at room temperature. Norbornene offers two docking sites, which expose the fiveSpecial Issue: 25th Austin Symposium on Molecular Structure and Dynamics Received: August 20, 2014 Revised: October 21, 2014
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FTIR scan at 2 cm−1 spectral resolution is recorded. The vacuum system recovers during a waiting time of 30−60 s, being permanently pumped at 500−2500 m3/h, before the next gas pulse is synchronized with another FTIR scan. For the spectra shown in this work, typically 80−100 pulses were coadded. The spectrometer (Bruker IFS 66 v/S) is equipped with CaF2 optics, a band-pass filter, and a photovoltaic InSb detector. For further details, see ref 30. Although the experimental method is not strictly sizeselective, relative cluster sizes are determined by changing the composition of the expansion in terms of the alcohol and the alkene. Usually, this allows for a discrimination between mixed dimers and alcohol- or alkene-rich trimers if they are spectrally separated, whereas tetramers and larger clusters tend to overlap and cannot be easily discriminated, in particular, if they are alkene-rich. Details on spectral variation with expansion conditions are provided in the Supporting Information (Figures S1−S7). Exploratory quantum chemical studies were performed at the B97D/TZVP/TZVPFit level using the Gaussian 09 program package.31 Local minima were reoptimized at the B3LYP/def2TZVP level, including empirical dispersion correction D332 (B3LYP-D3) and optional Becke−Johnson−Damping 33 (B3LYP-D3(BJ)) with Turbomole V6.5.34,35 Geometry optimizations were run with tight convergence criteria, 10−9 Eh for the energy convergence and 10−6 Eh/a0 for the gradient norm. Furthermore, some systems were reoptimized at the B2PLYPD3(BJ)/TZVP/TZVPFit level using Gaussian 09 with tight convergence criteria as well. BSSE corrections were not applied, and none of the presented structures feature any imaginary frequency. Cartesian coordinates of all displayed structures are given in the Supporting Information. Inclusion of electron correlation is mandatory for such OH···π complexes.36 For the system size relevant to this work, London dispersion corrections on top of density functional calculations37 represent a reasonable choice, whereas correlated wave-function-based methods are still out of reach for the larger complexes under investigation.
membered ring (No5) or the boat form of the cyclohexene ring (No6) to the alcohol. Norbornene is well-known for unusually high exoselectivity in addition reactions;7,8 therefore, it will be interesting to see if this strong preference for one side also translates into hydrogen bonding acceptor qualities. The slightly bent geometry of the RHCCHR unit is another indication for the electronic inequality of the two sides of the π system.7,8 The idea is to compare the complexes of T with No to those of T with Pe and Hx, which are obtained by removing the ethylenic or methylenic bridge in the No bicycle. Variation of the olefinic acceptor is expected to provide general insights into the bond strength via its qualitative correlation with hydrogen-bond-induced frequency shifts.9 Because the interaction is weak, one can hope that the OH oscillator largely retains its diagonal anharmonicity in the complex, thus canceling out in the analysis of hydrogen-bondinduced frequency shifts rather than absolute frequencies. The binding partners are large, shifting most intermolecular vibrations to low frequencies, where they cannot distort the interaction by pronounced zero-point-energy effects. Furthermore, conformational flexibility within the binding partners is largely absent. The only remaining large-amplitude motion of concern is the OH torsion, which turns into a libration upon binding to the π-system. However, the π acceptor is larger and more diffuse than an oxygen atom lone electron pair, justifying the expectation that its interaction is less distortive than that in alcohol dimers. To our knowledge, there is no experimental study of vacuum-isolated aliphatic alcohol−alkene complexes in the literature. There are a few studies of corresponding intramolecular interactions between alkene and alcohol groups,10 typically near room temperature, where thermal excitation and intramolecular strain combine to make these interactions relatively weak.11 Most of these12−14 and related intermolecular studies15−17 are carried out in solution, where solvent shifts have to be considered on top of the subtle complexation shifts. Furthermore, numerous complexes of alcohols with aromatic acceptors have been studied,18−24 and the infrared spectrum of the complex of phenol with ethene is also very well characterized.25,26 The acceptor qualities of cyclopentene, cyclohexene, and other alkenes have been compared before in CCl4 solution with phenol as the donor, showing that the doubly substituted alkenes studied in this work shift more than singly and less than triply substituted alkenes.16,17 Finally, the prototypical water−ethene complex has been studied in rare gas matrices27 and using rotational spectroscopy.28 Replacement of ethene by ethine increases the CH acidity and turns methanol into a hydrogen bond acceptor in Ar matrices.29 However, the balance is subtle, and a switch to N2 matrices re-establishes methanol as a donor. It will be interesting to see whether the CH···O interaction also plays a role in the alcohol−alkene complexes, which are the subject of this work.
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EXPERIMENTAL RESULTS The main spectroscopic cluster results are summarized in Figure 2. The coordination of T with Pe (upper trace) results in a strong and sharp band with a red shift (bathochromic shift) of 73 cm−1 relative to the t-butyl alcohol monomer (3642 cm−1, not shown), about half of the value found for coordination with T in the t-butyl alcohol dimer (TT, 145 cm−1) and quite comparable to the shift of HCl by ethene38 of 88 cm−1 or the red shift of phenol by ethene25 of 77 cm−1. Reports on the red shift of phenol by Pe in CCl4 fall in the range of 78−93 cm−1.16,17 Surprisingly, the shift of HCl by Pe in liquid argon is much higher (181 cm−1),39 exceeding that of HCl−ethene in the jet38 and cryosolution phases40 by a factor of 2. For TPe, additional Pe units lead to a range of weak absorptions redshifted from TPe by 5−40 cm−1, indicative of conformational diversity and a significant enhancement of the primary OH···π bond by CH···O interactions and possibly also by CH···π bonds. With increasing Pe concentration and cooling, these absorptions grow in line with their Pe-rich origin but shall not be analyzed further (additional spectra are provided in the Supporting Information (Figures S1 and S2)). Addition of a T unit to the TPe complex (TTPe) leads to two bands with increases of the red shifts by 44 cm−1 compared to TPe and by 45 cm−1 compared to TT, in line with the expected
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METHODS Cyclopentene (Aldrich, 96%), norbornene (Aldrich, 99%), cyclohexene (Fluka, >99.5%), and t-butyl alcohol (Roth, ≥ 99%) were used without further purification. Mixtures of an alkene with t-butyl alcohol and a large excess of helium carrier gas are generated in a 67 L reservoir at a pressure between 0.7 and 1.2 bar and expanded through six pulsed valves into a preexpansion zone, from where they escape through a 600 mm × 0.2 mm slit into a large vacuum chamber, while a complete B
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cm−1 from TNo5. A corresponding peak may also be discussed for Pe, but it is substantially more prominent in No expansions. Its concentration dependence points at a likely TNoNo origin, and we will come back to it in the comparison with calculations. Further shifted from TNo5 by 44 cm−1, a likely candidate for TTNo5 is detected as a weak band at the onset of an absorption pattern, whereas the strong band shifted from TT by 55 cm−1 is another likely absorption feature for this complex, again followed by a weaker and more red-shifted absorption. It is difficult to decide whether these additional absorptions correspond to isomers of TTNo5 or to further No aggregation on it. For all systems, the mixed TA dimer OH···π shifts are found to be 50 ± 6% smaller than that of the corresponding TT dimer. This is slightly less than the 61% found for phenol− ethene relative to phenol−phenol,25,42 probably due to the poor acceptor quality of phenol. We further note that the shifts induced by intramolecular OH···π hydrogen bonds in suitable conformations of syn-7norbornenol36 and 3-cyclopenten-1-ol,11 if compared to conformations without such hydrogen bond interactions, are substantially smaller than the values observed in this work. This is certainly related to the intramolecular restraints on the geometry but also to the elevated temperatures used in these studies and possibly also the solvent used in the syn-7norbornenol investigation.
Figure 2. OH stretching spectra of t-butyl alcohol (T) in coexpansions with an order of magnitude excess of (top to bottom) cyclopentene (Pe), norbornene (No), and cyclohexene (Hx) (99% He as carrier). Arrows indicate additional red shifts due to cooperativity in OH··· OH···π patterns. Doublets in the TTHx trimer are due to diastereoisomerism (see text) in the interaction of planar chiral Hx with transiently chiral TT. Docking to the five-ring and six-ring faces of No is indicated by superscripts.
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COMPARISON TO HARMONIC PREDICTIONS The spectral assignments from the preceding section can be used to judge quantum chemical methods in their ability to predict energy sequences and OH stretching spectra for this previously unexplored class of compounds. The comparison of all three related systems is more powerful than an isolated comparison for a given compound because it may also reveal systematic trends with the size of the alkene. If the experimental OH stretching wavenumber of t-butyl alcohol (3642 cm−1) is corrected by twice the anharmonicity constant of 87 cm−1,43 a harmonic OH stretching wavenumber close to 3816 cm−1 is predicted. This extrapolation is of limited accuracy if off-diagonal anharmonicity contributions do not cancel, but it may serve as an orientation for the quantum chemical predictions. The B2PLYP-D3(BJ) prediction of 3818 cm−1 is fully consistent with experiment, whereas the B3LYPD3(BJ) prediction of 3785 cm−1 (3783 cm−1 without BJ damping) falls somewhat short of the estimate. We note that B97D and ωB97XD undershoot and overshoot by about 100 cm−1, respectively. The TPe dimer is predicted to have the puckering CH2 group syn to the alcohol group, probably because of a stabilizing CH···O contact44 (Figure 3). We formally and somewhat arbitrarily define such a contact by a distance of less than 2.9 Å between H and O without accounting for the contact angle. Slightly different electronic acceptor qualities of the faces of the π system could also be responsible for the syn arrangement. The same side preference was predicted for HCl−Pe and BH3− Pe complexes before.39 In THx, a similar binding motif is found, and the organic moiety of the alcohol again prefers the proximity to the syn-CH2 group of the half-chair. In TNo5 and TNo6, the higher steric demand of the methylenic and ethylenic bridges is visible, forcing the bulky part of T further away and modifying the geometry of the OH···π hydrogen bond. In TNo6, only one of two possible close CH···O contacts is
cooperativity in the OH···OH···π sequence.41 No evidence for conformational diversity is found for TPe or TTPe. This indicates that the Pe unit realizes its optimum conformation via the low puckering barrier (or the shifts induced are too similar). In cryosolution, no second conformer was observed for HCl− Pe either.39 Hx (bottom trace) induces a slightly larger red shift of 80 cm−1 in T, in line with solution-phase comparisons of Pe and Hx with phenol as the donor.16,17 Additional alkene units again lead to increases of the shifts by 5−40 cm−1. Addition of a second T for TTHx complexes now results in two structures with two transitions each, further shifted than THx by 39 and 45 cm−1 and further shifted than TT by 46 and 51 cm−1. The very close analogy to TTPe supports this assignment, and the fairly robust chirality of the half-chair Hx conformation invites an explanation as diastereomeric complexes because the TT unit is transiently chiral as well. These spectra set the stage for complexes of No (center trace), which exhibits cyclopentene and cyclohexene faces. However, the latter ring is forced in No into a boat conformation, which is a transition structure for Hx itself. Indeed, the stronger TNo5 complex band is almost coincident with TPe, with a red shift of 71 cm−1, whereas the less abundant TNo6 complex has a reduced shift of 63 cm−1. This parallels the enhanced reactivity and electron density of the five-ring side of norbornene.7,8 Again, a complex band pattern further red-shifted by 20−40 cm−1 behaves like No-rich complexes with one T unit embedded. However, there is now a single narrow transition that is only red-shifted by 14 C
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of π-coordinated t-butyl alcohol in a satisfactory and predictive manner. Although not accessible in the present experiment, it is instructive to discuss alcohol−alkene dimer dissociation energies (Table 2). We tabulate and discuss the experimentally Table 2. Zero-Point-Corrected Dissociation Energies D0 (in kJ/mol) of Different Dimers of t-Butyl Alcohol (T) with Alkenes and with Itself
Table 1. Harmonic Red Shifts (in cm−1) Induced in the OH Stretching Fundamental of t-Butyl Alcohol (T) by Different Acceptors in Mixed Dimers No6
No5
Pe
Hx
T
B2PLYP-D3(BJ)/def-TZVP B3LYP-D3(BJ)/def2-TZVP B3LYP-D3/def2-TZVP experiment(anharmonic)
64 69 61 63
79 86 77 71
77 86 77 73
84 93 86 80
174 176 171 145
TNo6
TNo5
TPe
THx
TT
B2PLYP-D3(BJ)/def-TZVP B3LYP-D3(BJ)/def2-TZVP B3LYP-D3/def2-TZVP
17.3 18.3 19.7
18.5 19.2 20.0
19.1 20.1 21.0
20.5 21.8 22.8
29.1 28.9 30.6
observable D0 values, but the electronic dissociation energies De are only 18−20% larger. The dissociation energies fall in the 20 kJ/mol range and correlate qualitatively with the OH stretching shifts. The highest values are found for THx and the lowest for TNo6. The larger size of No compared to the monocyclic compounds does not contribute significantly to the cohesion. This indicates a competition between steric repulsion and dispersion attraction in No coordination. A comparison of TPe and TNo5 across all theoretical methods indeed reveals that the red shift is the same within 2 cm−1, whereas the binding energy of TNo5 is consistently smaller by 0.5−1 kJ/mol, the hydrogen bond more linear, and the infrared intensity 20% higher. This points at some compensation of subtle structural trends for the spectral shift. TT binding is about 50% stronger than binding to the alkenes, underscoring the relevance of the hydrogen bond in the cohesion of alcohols. Turning now to trimers with one alkene and two alcohol units, an attractive cooperative hydrogen bond pattern OH··· OH···π may be expected.27,41 The anticooperative OH···π···HO pattern is indeed confirmed to be much less favorable by about 20 kJ/mol. In TTPe, the syn preference of the docking side is less pronounced than that in TPe, so that anti isomers might also contribute to the experimentally observed sharp peak. In any case, the hydrogen bond pattern is folded to allow for an optimization of dispersion interactions of the second alcohol with the Pe ring (Figure 4). The additional OH stretching red shift compared to the dimers (44 and 45 cm−1 from experiment) is reproduced rather faithfully (47−54 cm−1) for OH···π (Table 3) and somewhat overestimated (57−61 cm−1) for OH···O (Table 4), in line with the dimer performance and strongly supporting the assignment. The OH···π band gains 50% in intensity through cooperativity, whereas the OH···O band intensity gain is on the order of 10%. TTHx shows chirality recognition because the planar chiral Hx interacts differently with the two mirror images of TT. An interconversion would require Hx inversion or a T acceptor lone pair switch, which may not be feasible under supersonic jet conditions. Because the energy difference between the diastereomers is predicted to be in the range of 0.5 kJ/mol or less but the OH stretching wavenumbers differ significantly, the experimentally observed doubled bands are easily explained. However, a detailed assignment of the doublet components to the two diastereomers is difficult without conformationally selective spectroscopy. The slightly more stable conformer TTHx1 (at the B3LYP-D3(BJ) level) has a slightly (2−3 cm−1) reduced OH···π red shift but a significantly (8−9 cm−1) enhanced OH···O red shift. If one uses this subtle qualitative
Figure 3. Structures of mixed t-butyl alcohol−alkene dimers at the B3LYP-D3(BJ)/def2-TZVP level. Marked are OH···π hydrogen bonds and intermolecular CH···O contacts of less than 2.9 Å.
method
method
established (2.72 Å), while the neighboring hydrogen keeps a larger distance (2.96 Å.) These structural trends are reflected in the sequence of OH red shifts, TNo6 < TNo5 ≈ TPe < THx (Table 1). The predicted absolute shifts are also reasonably close, with typically positive deviations of less than 10% from (anharmonic) experiment. The exception is B3LYP-D3 with Becke−Johnson damping,33 which overshoots by up to 20%, in contrast to the standard zero damping. Despite a first-order cancellation of diagonal anharmonicity effects in this shift analysis, residual diagonal and off-diagonal corrections could be of comparable size. Therefore, we cannot exclude that BJ damping provides a better description at the anharmonic level, but it appears less likely. We note that deviations between experimental shifts and predicted harmonic shifts are larger for the classical OH···O hydrogen bond in the TT dimer, on the order of 20% or 30 cm−1. This systematic discrepancy between the predictive power of harmonic hydrogen bond shifts to O and π acceptors has been noted before45,46 and could be due to anharmonic effects or deficiencies at the electronic structure level. Without dispersion correction, the harmonic B3LYP shift prediction for TT is coincidentally perfect,47 and that for TPe is too low. We further note that the spectral visibility of OH···π hydrogen bonds is predicted to be about two times inferior to that for OH···O hydrogen bonds for T. This should be taken into account when visually interpreting the measured spectra. In summary, the D3-corrected hybrid and double-hybrid functionals investigated in this work are able to describe the absolute values and the relative trends of OH stretching shifts D
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of the alkene also play a role is not clear. With about 60 kJ/mol, the cohesion in TTA trimers is comparable to that of the double hydrogen-bonded formic acid dimer.48 This shows nicely how dispersion interactions and cooperativity in two intrinsically rather weak alcoholic hydrogen bonds can compensate for two intrinsically strong but noncooperative acidic hydrogen bonds. With only one O−H donor in TAA trimers, the double OH···π coordination option realized for water−alkene−alkene complexes49 is not available. TAA trimers might therefore be structurally rather diverse with no strong structural preference of the second alkene unit other than overall cluster compactness. The increase in dissociation energy is thus only about half of that for an extra T. However, the prominent TNoNo band at 3557 cm−1 deserves a closer analysis as it has no equally intense counterpart in Pe or Hx. Figure 5 shows a plot of a range of B3LYP-D3(BJ) TNo5No trimer structures in terms of energy (relative to the most stable one) and harmonic OH stretching red shift (relative to TNo5). The color coding indicates whether the second No unit coordinates the oxygen lone pairs with one or more C(sp3)−H bonds (red) or with one C(sp2)−H bond (blue) or not at all (black). Clearly, C−H coordination and the associated dispersion energy gain contribute to the binding energy and induce an additional red shift in the OH···π hydrogen bond. No clear energetic or spectroscopic differentiation between C(sp3)−H and C(sp2)−H coordination is predicted. The most stable structure (confirmed at B97D, B2PLYD-D3(BJ), and B3LYPD3 levels) features a quadruple C(sp3)−H environment around one oxygen lone pair (Figure 6). However, energetic differences are too small to make a definite assignment. All possible alternatives within 2 kJ/mol also reproduce the experimental additional red shift reasonably well. No possible band for a TNo6No trimer could be identified. The calculations suggest a widened gap between the five- and six-regioisomers of TNoNo similar to the case of TTNo trimers, such that only the most stable isomer is formed for both trimer compositions. Exploratory calculations of TNo5NoNo tetramers indicate that additional C−H coordination of the second oxygen lone pair might be able to shift as far as the coordination of the first lone pair. This makes tetramers and bigger No-rich clusters a possible explanation for the broad spectral feature at around 3541 cm−1, which especially profits from high alkene concentrations and strong cooling, although this must remain speculative at this point. Figure 7 shows such a possible fully oxygen-solvating TNo5NoNo structure. Similar broad absorptions are found for Pe and Hx at higher pressures of the expanded gas mixture (Figures S1−S7 in the Supporting Information). The calculated additional red shift relative to TNo5 induced by only two solvent norbornene molecules is already on the order (20 cm−1) of estimated shifts between the
Figure 4. Structures of mixed trimers consisting of one alkene and two alcohol units at the B3LYP-D3(BJ)/def2-TZVP level. Marked are OH···π hydrogen bonds and intermolecular CH···O contacts of less than 2.9 Å.
correlation for a peak assignment, one indeed observes that the relative TTHx1 abundance is somewhat higher in the more dilute and thus colder expansion, but this may also be coincidental. A cross assignment would lead to more uniform differences between OH···π and OH···O cooperativity enhancements for the two isomers. Therefore, we refrain from a definitive assignment of the two structures, but the existence of two structures is fully compatible with the calculations. In TTNo, the preference for the five-ring coordination site is enhanced by the second alcohol molecule beyond 1 kJ/mol. This explains the absence of a spectrally significant second isomer for the mixed trimers when no major kinetic hindrance is present. The intensity ratio between the two dominant TTNo5 bands is distorted by an underlying No monomer transition in the more red-shifted band, which may thus also be somewhat uncertain in its band position. Upon adding a second T to TPe, THx, and TNo5, the hydrogen atom that formed the weak CH···O contact in the dimers now realizes a closer contact to the second oxygen atom. The exception to this is TTHx2, where a different hydrogen takes this part. The availability of this additional binding site in Hx might be one of the requirements for the coexistence of two energetically nearly equal diastereomers of TTHx. An inspection of TTA trimer dissociation energies shows no monotonous increase with alkene size. The predicted binding energy gain compared to the sum of TT and TA dimers amounts to 9−10 kJ/mol, independent of the alkene size. This confirms that the dispersion energy gain in norbornene is not sufficient to compensate for the steric demand of the bulky bicycle. Whether ring strain arguments on the acceptor ability
Table 3. Additional Harmonic Red Shifts (in cm−1) Induced in the OH···π Stretching Fundamental of Mixed t-Butyl Alcohol Alkene Dimers (TA) upon Formation of an OH···OH···π Pattern by Further T Additiona method B2PLYP-D3(BJ)/def-TZVP B3LYP-D3(BJ)/def2-TZVP B3LYP-D3/def2-TZVP experiment(anharmonic) a
TTNo6 45 42
TTNo5
TTPe
TTHx1
TTHx2
47 49 44
47 51 54 44
48 48 39 (45)
51 50 45 (39)
Alternative assignments are in parentheses. E
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Table 4. Additional Harmonic Red Shifts (in cm−1) Induced in the OH···O Stretching Fundamental of the t-Butyl Alcohol Dimer (TT) upon Formation of an OH···OH···π Pattern by Alkene Additiona method B2PLYP-D3(BJ)/def-TZVP B3LYP-D3(BJ)/def2-TZVP B3LYP-D3/def2-TZVP experiment(anharmonic) a
TTNo6
TTNo5
TTPe
TTHx1
TTHx2
60 60 55
57 59 61 45
60 61 51 (46)
51 53 46 (51)
42 45
Alternative assignments are in parentheses.
Figure 7. Speculative structure of a TNo5NoNo tetramer with an additional red shift of 21 cm−1 relative to TNo5 induced by two solvent norbornene molecules at the B3LYP-D3(BJ)/def2-TZVP level. Full view (left) and immediate surroundings of the hydrogen bond (right) featuring three C(sp3)H···O contacts for the left and two for the right oxygen lone pair.
expansions based on their OH stretching signature. The spectral fingerprint of the (OH···)OH···π bonds is clearly distinct from that of alcohol dimers and trimers, with intermediate shifts that allow for unambiguous assignments without rigorously size-selective tools. A second alkene attached to an alcohol−alkene dimer shows substantial spectral effects on the OH stretching frequency, although its only options besides unspecific London dispersion forces are CH···π and CH···O coordination. The latter appears to dominate in the case of norbornene. Even a double norbornene solvation of both oxygen lone pairs in reinforcement of the primary OH···π interaction from the alcohol to norbornene may be spectroscopically evident, but the resulting complex supramolecular structures call for conformationally selective tools. Such specific spectral signatures for a simple alcohol make it easier to rationalize olfactory discrimination of similar alkenes, although olfaction happens at environmental temperature. The latter is probably compensated for by a larger number of docking sites and the three-dimensional binding pocket of odor receptors, features that our simple model system obviously lacks. Nevertheless, our spectra provide a first rigorous opportunity to judge the performance of quantum chemical methods in such olefinic OH···π interactions and their cooperativity. The performance of B3LYP and B2PLYP calculations with polarized triple-ζ basis sets is remarkably good, once they are augmented by a posteriori dispersion correction. Absolute OH···π wavenumber shifts are surprisingly sensitive to the type of dispersion damping. Remaining systematic discrepancies can easily be blamed on the harmonic approximation, which is closer to reality for OH···π than for OH···O hydrogen bonds. The regioselectivity of the OH stretching shift is pronounced. It clearly distinguishes between five- and six-ring sites and between oxygen lone pair diastereomers, both in experiment and in the quantum calculations. These harmonic calculations therefore have predictive quality for other OH···alkene systems, which are up to now rather poorly characterized by spectroscopy, if one discounts solution studies, aromatic systems, and intramolecular hydrogen bonds.
Figure 5. Diagram of 34 TNo5No trimer structures at the B3LYPD3(BJ)/def2-TZVP level. Shown are relative energies (most stable set to zero) and additional red shifts upon adding a second norbornene unit to TNo5. The experimental data indicate a single structure shifted by 14 cm−1 (green cross). The colors represent whether the second No unit coordinates the oxygen lone pairs with one or more C(sp3)− H contacts (red) or with one C(sp2)−H contact (blue) or not at all (black).
Figure 6. Most stable structure found for TNo5No in the plot in Figure 5 with an additional red shift of 10 cm−1 relative to TNo5. The distances of the marked CH···O contacts are (from top to bottom) 2.89, 2.71, 2.59, and 2.79 Å. The second oxygen lone pair remains free for possible further coordination.
vapor phase and hydrocarbon solutions for OH···π oscillators.15 With the OH···π hydrogen bond now shielded, less pronounced effects can be expected from further aggregation of No, which corresponds to the concept of secondary solvation shells. However, thermal effects are not accounted for in the jet expansion.
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CONCLUSIONS Mixed dimers and trimers of t-butyl alcohol with (bi)cyclic C5− C7 alkenes are observed and assigned in supersonic jet F
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(14) Bakke, J. M.; Bjerkeseth, L. H. The Conformational Composition of 3-Buten-1-ol, the Importance of Intramolecular Hydrogen Bonding. J. Mol. Struct. 1998, 470, 247−263. (15) O̅ sawa, E.; Yoshida, Z. The Effects of Solvents on the Hydrogen Bonded OH Stretching Frequency. Spectrochim. Acta, Part A 1967, 23, 2029−2036. (16) Kuhn, L.; Bowman, R. The Hydrogen Bond−VII. The Intermolecular Hydrogen Bond between Phenol and Olefins. Spectrochim. Acta, Part A 1967, 23, 189−193. (17) Yoshida, Z.; Ishibe, N. Intermolecular Hydrogen Bond Involving a π Base as the Proton Acceptor. IX. Hydrogen Bonding of Phenol to Monoolefins. Bull. Chem. Soc. Jpn. 1969, 42, 3263−3266. (18) Zwier, T. S. The Spectroscopy of Solvation in HydrogenBonded Aromatic Clusters. Annu. Rev. Phys. Chem. 1996, 47, 205−241. (19) Wickleder, C.; Droz, T.; Bürgi, T.; Leutwyler, S. Accurate Intermolecular Binding Energies of 1-Naphthol to Benzene and Cyclohexane. Chem. Phys. Lett. 1997, 264, 257−264. (20) Mons, M.; Robertson, E. G.; Simons, J. P. Intra- and Intermolecular π-Type Hydrogen Bonding in Aryl Alcohols: UV and IR−UV Ion Dip Spectroscopy. J. Phys. Chem. A 2000, 104, 1430− 1437. (21) Buchhold, K.; Reimann, B.; Djafari, S.; Barth, H.-D.; Brutschy, B.; Tarakeshwar, P.; Kim, K. S. Fluorobenzene and p-Difluorobenzene Microsolvated by Methanol: An Infrared Spectroscopic and Ab Initio Theoretical Investigation. J. Chem. Phys. 2000, 112, 1844−1858. (22) Kim, K. S.; Tarakeshwar, P.; Lee, J. Y. Molecular Clusters of πSystems: Theoretical Studies of Structures, Spectra, and Origin of Interaction Energies. Chem. Rev. 2000, 100, 4145−4185. (23) Seurre, N.; Sepioł, J.; Lahmani, F.; Zehnacker-Rentien, A.; Le Barbu-Debus, K. Vibrational Study of the S0 and S1 States of 2Naphthyl-1-ethanol/(Water)2 and 2-Naphthyl-1-ethanol/(Methanol)2 Complexes by IR/UV Double Resonance Spectroscopy. Phys. Chem. Chem. Phys. 2004, 6, 4658−4664. (24) Suhm, M. A. Hydrogen Bond Dynamics in Alcohol Clusters. Adv. Chem. Phys. 2009, 142, 1−57. (25) Fujii, A.; Ebata, T.; Mikami, N. An Infrared Study of πHydrogen Bonds in Micro-solvated Phenol: OH Stretching Vibrations of Phenol−X (X = C6H6, C2H4, and C2H2) Clusters in the Neutral and Cationic Ground States. J. Phys. Chem. A 2002, 106, 8554−8560. (26) Yamada, Y.; Kayano, M.; Mikami, N.; Ebata, T. Picosecond IR− UV Pump−Probe Study on the Vibrational Relaxation of Phenol− Ethylene Hydrogen-Bonded Cluster: Difference of Relaxation Route/ Rate between the Donor and the Acceptor Site Excitations. J. Phys. Chem. A 2006, 110, 6250−6255. (27) Engdahl, A.; Nelander, B. A Matrix-Isolation Study of the Ethylene−Water Interaction. Chem. Phys. Lett. 1985, 113, 49−55. (28) Andrews, A. M.; Kuczkowski, R. L. Microwave Spectra of C2H4· H2O and Isotopomers. J. Chem. Phys. 1993, 98, 791−795. (29) Jovan Jose, K. V.; Gadre, S. R.; Sundararajan, K.; Viswanathan, K. S. Effect of Matrix on IR Frequencies of Acetylene and Acetylene− Methanol Complex: Infrared Matrix Isolation and Ab Initio Study. J. Chem. Phys. 2007, 127, 104501. (30) Suhm, M. A.; Kollipost, F. Femtisecond Single-Mole Infrared Spectroscopy of Molecular Clusters. Phys. Chem. Chem. Phys. 2013, 15, 10702−10721. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision D.01; Gaussian Inc.: Wallingford, CT, 2009. (32) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H−Pu. J. Chem. Phys. 2010, 132, 154104. (33) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456−1465. (34) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Electronic Structure Calculations on Workstation Computers: The
An accurate evaluation of anharmonicity in the OH···π bond, the detailed experimental characterization of the methanol− ethene prototype, and a systematic investigation of ring strain effects and spectral effects on normal modes localized at the alkene site remain on the supersonic jet agenda for this class of weak hydrogen bonds.
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ASSOCIATED CONTENT
S Supporting Information *
A summary of all experimental bands positions, additional spectra, and Cartesian coordinates for all displayed structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support by the German Research Foundation (DFG) via Grant Su 121/4-1 is gratefully acknowledged, as is support for R.M. by M. Buback.
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REFERENCES
(1) Laska, M.; Teubner, P. Olfactory Discrimination Ability of Human Subjects for Ten Pairs of Enantiomers. Chem. Senses 1999, 24, 161−170. (2) Anselmi, C.; Centini, M.; Fedeli, P.; Paoli, M. L.; Sega, A.; Scesa, C.; Pelosi, P. Unsaturated Hydrocarbons with Fruity and Floral Odors. J. Agric. Food Chem. 2000, 48, 1285−1289. (3) Li, S.; Purdy, W. C. Cyclodextrins and Their Applications in Analytical Chemistry. Chem. Rev. 1992, 92, 1457−1470. (4) Daranlot, J.; Bergeat, A.; Caralp, F.; Caubet, P.; Costes, M.; Forst, W.; Loison, J.-C.; Hickson, K. M. Gas-Phase Kinetics of Hydroxyl Radical Reactions with Alkenes: Experiment and Theory. ChemPhysChem 2010, 11, 4002−4010. (5) Leong, M. K.; Mastryukov, V. S.; Boggs, J. E. Structure and Conformation of Cyclopentene, Cycloheptene and trans-Cyclooctene. J. Mol. Struct. 1998, 445, 149−160. (6) Ocola, E. J.; Brito, T.; McCann, K.; Laane, J. Conformational Energetics and Low-Frequency Vibrations of Cyclohexene and Its Oxygen Analogs. J. Mol. Struct. 2010, 978, 74−78. (7) Spanget-Larsen, J.; Gleiter, R. Structure and Reactivity of Norbornene and syn-Sesquinorbornene. Tetrahedron 1983, 39, 3345− 3350. (8) Steinmann, S. N.; Vogel, P.; Mo, Y.; Corminboeuf, C. The Norbornene Mystery Revealed. Chem. Commun. 2011, 47, 227−229. (9) Engdahl, A.; Nelander, B. Water−Olefin Complexes. A Matrix Isolation Study. J. Phys. Chem. 1986, 90, 4982−4987. (10) Miller, B. J.; Lane, J. R.; Kjaergaard, H. G. Intramolecular OH···π Interactions in Alkenols and Alkynols. Phys. Chem. Chem. Phys. 2011, 13, 14183−14193. (11) Ocola, E. J.; Al-Saadi, A. A.; Mlynek, C.; Hopf, H.; Laane, J. Intramolecular π-Type Hydrogen Bonding and Conformations of 3Cyclopenten-1-ol. 2. Infrared and Raman Spectral Studies at High Temperatures. J. Phys. Chem. A 2010, 114, 7457−7461. (12) Baker, A. W.; Shulgin, A. T. Intramolecular Hydrogen Bonds to π-Electrons and Other Weakly Basic Groups. J. Am. Chem. Soc. 1958, 80, 5358−5363. (13) Schleyer, P. v. R.; Trifan, D. S.; Bacskai, R. Intramolecular Hydrogen Bonding Involving Double Bonds, Triple Bonds and Cyclopropane Rings as Proton Acceptors. J. Am. Chem. Soc. 1958, 80, 6691−6692. G
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Program System TURBOMOLE. Chem. Phys. Lett. 1989, 162, 165− 169. (35) TURBOMOLE V6.5 2013, 1989−2007. TURBOMOLE GmbH; University of Karlsruhe and Forschungszentrum Karlsruhe GmbH; http://www.turbomole.com (2007). (36) Bjerkeseth, L. H.; Bakke, J. M.; Uggerud, E. Inter- and Intramolecular O−H···π Hydrogen Bonding in the Methanol−Ethene Complex and syn-7-Norborneol, Probed by IR, 1H NMR and Quantum Chemistry. J. Mol. Struct. 2001, 567−568, 319−338. (37) Grimme, S. Density Functional Theory with London Dispersion Corrections. WIREs Comput. Mol. Sci. 2011, 1, 211−228. (38) Ç arçabal, P.; Seurre, N.; Chevalier, M.; Broquier, M.; Brenner, V. Infrared Spectra of C2H2− HCl Complex. J. Chem. Phys. 2002, 117, 1522−1528. (39) Herrebout, W.; Gatin, A.; Everaert, G.; Fishman, A.; van der Veken, B. A Cryosolution Infrared and Ab Initio Study of the van der Waals Complexes of Cyclopentene with Hydrogen Chloride and Boron Trifluoride. Spectrochim. Acta, Part A 2005, 61, 1431−1444. (40) Herrebout, W. A.; Everaert, G. P.; van der Veken, B. J.; Bulanin, M. O. On the Ethene/HCl van der Waals Complexes Observed in Liquefied Argon and Liquefied Nitrogen. J. Chem. Phys. 1997, 107, 8886−8898. (41) DuPré, D. B.; Yappert, M. C. Cooperative Hydrogen- and π HBonded Interactions Involving Water and the Ethylenic Double Bond. J. Phys. Chem. A 2002, 106, 567−574. (42) Kayano, M.; Ebata, T.; Yamada, Y.; Mikami, N. Picosecond IR− UV Pump−Probe Spectroscopic Study of the Dynamics of the Vibrational Relaxation of Jet-Cooled Phenol. II. Intracluster Vibrational Energy Redistribution of the OH Stretching Vibration of Hydrogen-Bonded Clusters. J. Chem. Phys. 2004, 120, 7410−7417. (43) Kollipost, F.; Papendorf, K.; Lee, Y.-F.; Lee, Y.-P.; Suhm, M. A. Alcohol Dimers How Much Diagonal OH Anharmonicity? Phys. Chem. Chem. Phys. 2014, 16, 15948−15956. (44) Takahashi, O.; Kohno, Y.; Nishio, M. Relevance of Weak Hydrogen Bonds in the Conformation of Organic Compounds and Bioconjugates: Evidence from Recent Experimental Data and HighLevel Ab Initio MO Calculations. Chem. Rev. 2010, 110, 6049−6076. (45) Altnöder, J.; Bouchet, A.; Lee, J. J.; Otto, K. E.; Suhm, M. A.; Zehnacker-Rentien, A. Chirality-Dependent Balance between Hydrogen Bonding and London Dispersion in Isolated (±)-1-Indanol Clusters. Phys. Chem. Chem. Phys. 2013, 15, 10167−10180. (46) Altnöder, J.; Oswald, S.; Suhm, M. A. Phenyl- vs CyclohexylSubstitution in Methanol: Implications for the OH Conformation and for Dispersion-Affected Aggregation from Vibrational Spectra in Supersonic Jets. J. Phys. Chem. A 2014, 118, 3266−3279. (47) Zimmermann, D.; Häber, T.; Schaal, H.; Suhm, M. A. Hydrogen Bonded Rings, Chains and Lassos: the Case of t-Butyl Alcohol Clusters. Mol. Phys. 2001, 99, 413−425. (48) Kollipost, F.; Larsen, R. W.; Domanskaya, A. V.; Nörenberg, M.; Suhm, M. A. Communication: The Highest Frequency Hydrogen Bond Vibration and an Experimental Value for the Dissociation Energy of Formic Acid Dimer. J. Chem. Phys. 2012, 136, 151101. (49) Thompson, M. G. K.; Lewars, E. G.; Parnis, J. M. Observation of the π···H Hydrogen-Bonded Ternary Complex, (C2H4)2H2O, Using Matrix Isolation Infrared Spectroscopy. J. Phys. Chem. A 2005, 109, 9499−9506.
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Molecular Docking via Olefinic OH···π Interactions: A Bulky Alkene Model System and Its Cooperativity Robert Medel, Matthias Heger, and Martin A. Suhm* Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstrasse 6, D-37077 Göttingen, Germany S Supporting Information *
ABSTRACT: Complexes of t-butyl alcohol with norbornene and its monocyclic constituents cyclopentene and cyclohexene are studied via their OH stretching fundamental transitions in supersonic jet expansions. Compared to OH···OH hydrogen bonds, the spectral shifts due to OH···π bonding in the mixed dimers are reduced by a factor of 2. Mixed trimers show substantially different spectral signatures due to cooperative effects. Regioselective docking on the two sides of the double bond in norbornene is observed. Harmonic modeling of the spectra using dispersion-corrected hybrid functionals is quite successful, suggesting a high predictive power for this poorly explored class of complexes between alcohols and alkenes.
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INTRODUCTION
The molecular recognition of unsaturated hydrocarbons by functional groups in other organic molecules is of interest in several fields of science, such as the mechanisms of olfaction1,2 or chromatography.3 For example, it is remarkable that the olefinic terpenes limonene and α-pinene cause some of the most prominent enantioselective smell impressions in the human olfactory system.1 Quantum chemical modeling of the underlying subtle intermolecular interactions can be quite demanding, and it is desirable to have a few test cases at hand to assess the reliability of a chosen theoretical approach. Vibrational spectroscopy provides an attractive bridge between theory and experiment because it supplies sensitive frequency information, in particular, for hydride chromophores such as the OH group interacting with an alkene. In combination with the low temperatures of a supersonic jet, spectroscopy can also help to discriminate different docking motifs on the kJ/mol energy scale. Such docking motifs of OH toward alkenes are also of some relevance for low-temperature OH radical addition on alkenes in cold planetary atmospheres,4 which proceed via a hydrogen-bonded prereactive complex similar to that of alcohol−alkene dimers. One bottleneck in the theory− spectroscopy comparison is the role of anharmonicity, which is not easily captured by theoretical models for larger molecules or complexes. Here, we study a model system that should be relatively free from such anharmonic interferences, the interaction of t-butyl alcohol (T) with bulky alkenes (A), specifically norbornene (No) and its monocyclic constituents cyclopentene (Pe) and cyclohexene (Hx) (Figure 1). We denote the complexes by a sequence of the molecular acronyms, starting with the alcohol donor. TPe means that a t-butyl alcohol coordinates a cyclopentene unit. The docking of an alcohol to cyclopentene © XXXX American Chemical Society
Figure 1. Most stable conformations, abbreviations, and point groups of the monomeric compounds. The curved lines represent faces of cyclopentene and norbornene, which interact differently with hydrogen bond donors, giving rise to possible regioisomers.
can be syn or anti relative to the floppy out-of-plane CH2 group, which should be mobile even under supersonic jet conditions due to the low ring flip barrier.5 In contrast, the barrier for cyclohexene is about an order of magnitude larger,6 such that the planar chirality of cyclohexene may be frozen in the jet expansion, whereas it is variable at room temperature. Norbornene offers two docking sites, which expose the fiveSpecial Issue: 25th Austin Symposium on Molecular Structure and Dynamics Received: August 20, 2014 Revised: October 21, 2014
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FTIR scan at 2 cm−1 spectral resolution is recorded. The vacuum system recovers during a waiting time of 30−60 s, being permanently pumped at 500−2500 m3/h, before the next gas pulse is synchronized with another FTIR scan. For the spectra shown in this work, typically 80−100 pulses were coadded. The spectrometer (Bruker IFS 66 v/S) is equipped with CaF2 optics, a band-pass filter, and a photovoltaic InSb detector. For further details, see ref 30. Although the experimental method is not strictly sizeselective, relative cluster sizes are determined by changing the composition of the expansion in terms of the alcohol and the alkene. Usually, this allows for a discrimination between mixed dimers and alcohol- or alkene-rich trimers if they are spectrally separated, whereas tetramers and larger clusters tend to overlap and cannot be easily discriminated, in particular, if they are alkene-rich. Details on spectral variation with expansion conditions are provided in the Supporting Information (Figures S1−S7). Exploratory quantum chemical studies were performed at the B97D/TZVP/TZVPFit level using the Gaussian 09 program package.31 Local minima were reoptimized at the B3LYP/def2TZVP level, including empirical dispersion correction D332 (B3LYP-D3) and optional Becke−Johnson−Damping 33 (B3LYP-D3(BJ)) with Turbomole V6.5.34,35 Geometry optimizations were run with tight convergence criteria, 10−9 Eh for the energy convergence and 10−6 Eh/a0 for the gradient norm. Furthermore, some systems were reoptimized at the B2PLYPD3(BJ)/TZVP/TZVPFit level using Gaussian 09 with tight convergence criteria as well. BSSE corrections were not applied, and none of the presented structures feature any imaginary frequency. Cartesian coordinates of all displayed structures are given in the Supporting Information. Inclusion of electron correlation is mandatory for such OH···π complexes.36 For the system size relevant to this work, London dispersion corrections on top of density functional calculations37 represent a reasonable choice, whereas correlated wave-function-based methods are still out of reach for the larger complexes under investigation.
membered ring (No5) or the boat form of the cyclohexene ring (No6) to the alcohol. Norbornene is well-known for unusually high exoselectivity in addition reactions;7,8 therefore, it will be interesting to see if this strong preference for one side also translates into hydrogen bonding acceptor qualities. The slightly bent geometry of the RHCCHR unit is another indication for the electronic inequality of the two sides of the π system.7,8 The idea is to compare the complexes of T with No to those of T with Pe and Hx, which are obtained by removing the ethylenic or methylenic bridge in the No bicycle. Variation of the olefinic acceptor is expected to provide general insights into the bond strength via its qualitative correlation with hydrogen-bond-induced frequency shifts.9 Because the interaction is weak, one can hope that the OH oscillator largely retains its diagonal anharmonicity in the complex, thus canceling out in the analysis of hydrogen-bondinduced frequency shifts rather than absolute frequencies. The binding partners are large, shifting most intermolecular vibrations to low frequencies, where they cannot distort the interaction by pronounced zero-point-energy effects. Furthermore, conformational flexibility within the binding partners is largely absent. The only remaining large-amplitude motion of concern is the OH torsion, which turns into a libration upon binding to the π-system. However, the π acceptor is larger and more diffuse than an oxygen atom lone electron pair, justifying the expectation that its interaction is less distortive than that in alcohol dimers. To our knowledge, there is no experimental study of vacuum-isolated aliphatic alcohol−alkene complexes in the literature. There are a few studies of corresponding intramolecular interactions between alkene and alcohol groups,10 typically near room temperature, where thermal excitation and intramolecular strain combine to make these interactions relatively weak.11 Most of these12−14 and related intermolecular studies15−17 are carried out in solution, where solvent shifts have to be considered on top of the subtle complexation shifts. Furthermore, numerous complexes of alcohols with aromatic acceptors have been studied,18−24 and the infrared spectrum of the complex of phenol with ethene is also very well characterized.25,26 The acceptor qualities of cyclopentene, cyclohexene, and other alkenes have been compared before in CCl4 solution with phenol as the donor, showing that the doubly substituted alkenes studied in this work shift more than singly and less than triply substituted alkenes.16,17 Finally, the prototypical water−ethene complex has been studied in rare gas matrices27 and using rotational spectroscopy.28 Replacement of ethene by ethine increases the CH acidity and turns methanol into a hydrogen bond acceptor in Ar matrices.29 However, the balance is subtle, and a switch to N2 matrices re-establishes methanol as a donor. It will be interesting to see whether the CH···O interaction also plays a role in the alcohol−alkene complexes, which are the subject of this work.
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EXPERIMENTAL RESULTS The main spectroscopic cluster results are summarized in Figure 2. The coordination of T with Pe (upper trace) results in a strong and sharp band with a red shift (bathochromic shift) of 73 cm−1 relative to the t-butyl alcohol monomer (3642 cm−1, not shown), about half of the value found for coordination with T in the t-butyl alcohol dimer (TT, 145 cm−1) and quite comparable to the shift of HCl by ethene38 of 88 cm−1 or the red shift of phenol by ethene25 of 77 cm−1. Reports on the red shift of phenol by Pe in CCl4 fall in the range of 78−93 cm−1.16,17 Surprisingly, the shift of HCl by Pe in liquid argon is much higher (181 cm−1),39 exceeding that of HCl−ethene in the jet38 and cryosolution phases40 by a factor of 2. For TPe, additional Pe units lead to a range of weak absorptions redshifted from TPe by 5−40 cm−1, indicative of conformational diversity and a significant enhancement of the primary OH···π bond by CH···O interactions and possibly also by CH···π bonds. With increasing Pe concentration and cooling, these absorptions grow in line with their Pe-rich origin but shall not be analyzed further (additional spectra are provided in the Supporting Information (Figures S1 and S2)). Addition of a T unit to the TPe complex (TTPe) leads to two bands with increases of the red shifts by 44 cm−1 compared to TPe and by 45 cm−1 compared to TT, in line with the expected
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METHODS Cyclopentene (Aldrich, 96%), norbornene (Aldrich, 99%), cyclohexene (Fluka, >99.5%), and t-butyl alcohol (Roth, ≥ 99%) were used without further purification. Mixtures of an alkene with t-butyl alcohol and a large excess of helium carrier gas are generated in a 67 L reservoir at a pressure between 0.7 and 1.2 bar and expanded through six pulsed valves into a preexpansion zone, from where they escape through a 600 mm × 0.2 mm slit into a large vacuum chamber, while a complete B
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cm−1 from TNo5. A corresponding peak may also be discussed for Pe, but it is substantially more prominent in No expansions. Its concentration dependence points at a likely TNoNo origin, and we will come back to it in the comparison with calculations. Further shifted from TNo5 by 44 cm−1, a likely candidate for TTNo5 is detected as a weak band at the onset of an absorption pattern, whereas the strong band shifted from TT by 55 cm−1 is another likely absorption feature for this complex, again followed by a weaker and more red-shifted absorption. It is difficult to decide whether these additional absorptions correspond to isomers of TTNo5 or to further No aggregation on it. For all systems, the mixed TA dimer OH···π shifts are found to be 50 ± 6% smaller than that of the corresponding TT dimer. This is slightly less than the 61% found for phenol− ethene relative to phenol−phenol,25,42 probably due to the poor acceptor quality of phenol. We further note that the shifts induced by intramolecular OH···π hydrogen bonds in suitable conformations of syn-7norbornenol36 and 3-cyclopenten-1-ol,11 if compared to conformations without such hydrogen bond interactions, are substantially smaller than the values observed in this work. This is certainly related to the intramolecular restraints on the geometry but also to the elevated temperatures used in these studies and possibly also the solvent used in the syn-7norbornenol investigation.
Figure 2. OH stretching spectra of t-butyl alcohol (T) in coexpansions with an order of magnitude excess of (top to bottom) cyclopentene (Pe), norbornene (No), and cyclohexene (Hx) (99% He as carrier). Arrows indicate additional red shifts due to cooperativity in OH··· OH···π patterns. Doublets in the TTHx trimer are due to diastereoisomerism (see text) in the interaction of planar chiral Hx with transiently chiral TT. Docking to the five-ring and six-ring faces of No is indicated by superscripts.
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COMPARISON TO HARMONIC PREDICTIONS The spectral assignments from the preceding section can be used to judge quantum chemical methods in their ability to predict energy sequences and OH stretching spectra for this previously unexplored class of compounds. The comparison of all three related systems is more powerful than an isolated comparison for a given compound because it may also reveal systematic trends with the size of the alkene. If the experimental OH stretching wavenumber of t-butyl alcohol (3642 cm−1) is corrected by twice the anharmonicity constant of 87 cm−1,43 a harmonic OH stretching wavenumber close to 3816 cm−1 is predicted. This extrapolation is of limited accuracy if off-diagonal anharmonicity contributions do not cancel, but it may serve as an orientation for the quantum chemical predictions. The B2PLYP-D3(BJ) prediction of 3818 cm−1 is fully consistent with experiment, whereas the B3LYPD3(BJ) prediction of 3785 cm−1 (3783 cm−1 without BJ damping) falls somewhat short of the estimate. We note that B97D and ωB97XD undershoot and overshoot by about 100 cm−1, respectively. The TPe dimer is predicted to have the puckering CH2 group syn to the alcohol group, probably because of a stabilizing CH···O contact44 (Figure 3). We formally and somewhat arbitrarily define such a contact by a distance of less than 2.9 Å between H and O without accounting for the contact angle. Slightly different electronic acceptor qualities of the faces of the π system could also be responsible for the syn arrangement. The same side preference was predicted for HCl−Pe and BH3− Pe complexes before.39 In THx, a similar binding motif is found, and the organic moiety of the alcohol again prefers the proximity to the syn-CH2 group of the half-chair. In TNo5 and TNo6, the higher steric demand of the methylenic and ethylenic bridges is visible, forcing the bulky part of T further away and modifying the geometry of the OH···π hydrogen bond. In TNo6, only one of two possible close CH···O contacts is
cooperativity in the OH···OH···π sequence.41 No evidence for conformational diversity is found for TPe or TTPe. This indicates that the Pe unit realizes its optimum conformation via the low puckering barrier (or the shifts induced are too similar). In cryosolution, no second conformer was observed for HCl− Pe either.39 Hx (bottom trace) induces a slightly larger red shift of 80 cm−1 in T, in line with solution-phase comparisons of Pe and Hx with phenol as the donor.16,17 Additional alkene units again lead to increases of the shifts by 5−40 cm−1. Addition of a second T for TTHx complexes now results in two structures with two transitions each, further shifted than THx by 39 and 45 cm−1 and further shifted than TT by 46 and 51 cm−1. The very close analogy to TTPe supports this assignment, and the fairly robust chirality of the half-chair Hx conformation invites an explanation as diastereomeric complexes because the TT unit is transiently chiral as well. These spectra set the stage for complexes of No (center trace), which exhibits cyclopentene and cyclohexene faces. However, the latter ring is forced in No into a boat conformation, which is a transition structure for Hx itself. Indeed, the stronger TNo5 complex band is almost coincident with TPe, with a red shift of 71 cm−1, whereas the less abundant TNo6 complex has a reduced shift of 63 cm−1. This parallels the enhanced reactivity and electron density of the five-ring side of norbornene.7,8 Again, a complex band pattern further red-shifted by 20−40 cm−1 behaves like No-rich complexes with one T unit embedded. However, there is now a single narrow transition that is only red-shifted by 14 C
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of π-coordinated t-butyl alcohol in a satisfactory and predictive manner. Although not accessible in the present experiment, it is instructive to discuss alcohol−alkene dimer dissociation energies (Table 2). We tabulate and discuss the experimentally Table 2. Zero-Point-Corrected Dissociation Energies D0 (in kJ/mol) of Different Dimers of t-Butyl Alcohol (T) with Alkenes and with Itself
Table 1. Harmonic Red Shifts (in cm−1) Induced in the OH Stretching Fundamental of t-Butyl Alcohol (T) by Different Acceptors in Mixed Dimers No6
No5
Pe
Hx
T
B2PLYP-D3(BJ)/def-TZVP B3LYP-D3(BJ)/def2-TZVP B3LYP-D3/def2-TZVP experiment(anharmonic)
64 69 61 63
79 86 77 71
77 86 77 73
84 93 86 80
174 176 171 145
TNo6
TNo5
TPe
THx
TT
B2PLYP-D3(BJ)/def-TZVP B3LYP-D3(BJ)/def2-TZVP B3LYP-D3/def2-TZVP
17.3 18.3 19.7
18.5 19.2 20.0
19.1 20.1 21.0
20.5 21.8 22.8
29.1 28.9 30.6
observable D0 values, but the electronic dissociation energies De are only 18−20% larger. The dissociation energies fall in the 20 kJ/mol range and correlate qualitatively with the OH stretching shifts. The highest values are found for THx and the lowest for TNo6. The larger size of No compared to the monocyclic compounds does not contribute significantly to the cohesion. This indicates a competition between steric repulsion and dispersion attraction in No coordination. A comparison of TPe and TNo5 across all theoretical methods indeed reveals that the red shift is the same within 2 cm−1, whereas the binding energy of TNo5 is consistently smaller by 0.5−1 kJ/mol, the hydrogen bond more linear, and the infrared intensity 20% higher. This points at some compensation of subtle structural trends for the spectral shift. TT binding is about 50% stronger than binding to the alkenes, underscoring the relevance of the hydrogen bond in the cohesion of alcohols. Turning now to trimers with one alkene and two alcohol units, an attractive cooperative hydrogen bond pattern OH··· OH···π may be expected.27,41 The anticooperative OH···π···HO pattern is indeed confirmed to be much less favorable by about 20 kJ/mol. In TTPe, the syn preference of the docking side is less pronounced than that in TPe, so that anti isomers might also contribute to the experimentally observed sharp peak. In any case, the hydrogen bond pattern is folded to allow for an optimization of dispersion interactions of the second alcohol with the Pe ring (Figure 4). The additional OH stretching red shift compared to the dimers (44 and 45 cm−1 from experiment) is reproduced rather faithfully (47−54 cm−1) for OH···π (Table 3) and somewhat overestimated (57−61 cm−1) for OH···O (Table 4), in line with the dimer performance and strongly supporting the assignment. The OH···π band gains 50% in intensity through cooperativity, whereas the OH···O band intensity gain is on the order of 10%. TTHx shows chirality recognition because the planar chiral Hx interacts differently with the two mirror images of TT. An interconversion would require Hx inversion or a T acceptor lone pair switch, which may not be feasible under supersonic jet conditions. Because the energy difference between the diastereomers is predicted to be in the range of 0.5 kJ/mol or less but the OH stretching wavenumbers differ significantly, the experimentally observed doubled bands are easily explained. However, a detailed assignment of the doublet components to the two diastereomers is difficult without conformationally selective spectroscopy. The slightly more stable conformer TTHx1 (at the B3LYP-D3(BJ) level) has a slightly (2−3 cm−1) reduced OH···π red shift but a significantly (8−9 cm−1) enhanced OH···O red shift. If one uses this subtle qualitative
Figure 3. Structures of mixed t-butyl alcohol−alkene dimers at the B3LYP-D3(BJ)/def2-TZVP level. Marked are OH···π hydrogen bonds and intermolecular CH···O contacts of less than 2.9 Å.
method
method
established (2.72 Å), while the neighboring hydrogen keeps a larger distance (2.96 Å.) These structural trends are reflected in the sequence of OH red shifts, TNo6 < TNo5 ≈ TPe < THx (Table 1). The predicted absolute shifts are also reasonably close, with typically positive deviations of less than 10% from (anharmonic) experiment. The exception is B3LYP-D3 with Becke−Johnson damping,33 which overshoots by up to 20%, in contrast to the standard zero damping. Despite a first-order cancellation of diagonal anharmonicity effects in this shift analysis, residual diagonal and off-diagonal corrections could be of comparable size. Therefore, we cannot exclude that BJ damping provides a better description at the anharmonic level, but it appears less likely. We note that deviations between experimental shifts and predicted harmonic shifts are larger for the classical OH···O hydrogen bond in the TT dimer, on the order of 20% or 30 cm−1. This systematic discrepancy between the predictive power of harmonic hydrogen bond shifts to O and π acceptors has been noted before45,46 and could be due to anharmonic effects or deficiencies at the electronic structure level. Without dispersion correction, the harmonic B3LYP shift prediction for TT is coincidentally perfect,47 and that for TPe is too low. We further note that the spectral visibility of OH···π hydrogen bonds is predicted to be about two times inferior to that for OH···O hydrogen bonds for T. This should be taken into account when visually interpreting the measured spectra. In summary, the D3-corrected hybrid and double-hybrid functionals investigated in this work are able to describe the absolute values and the relative trends of OH stretching shifts D
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of the alkene also play a role is not clear. With about 60 kJ/mol, the cohesion in TTA trimers is comparable to that of the double hydrogen-bonded formic acid dimer.48 This shows nicely how dispersion interactions and cooperativity in two intrinsically rather weak alcoholic hydrogen bonds can compensate for two intrinsically strong but noncooperative acidic hydrogen bonds. With only one O−H donor in TAA trimers, the double OH···π coordination option realized for water−alkene−alkene complexes49 is not available. TAA trimers might therefore be structurally rather diverse with no strong structural preference of the second alkene unit other than overall cluster compactness. The increase in dissociation energy is thus only about half of that for an extra T. However, the prominent TNoNo band at 3557 cm−1 deserves a closer analysis as it has no equally intense counterpart in Pe or Hx. Figure 5 shows a plot of a range of B3LYP-D3(BJ) TNo5No trimer structures in terms of energy (relative to the most stable one) and harmonic OH stretching red shift (relative to TNo5). The color coding indicates whether the second No unit coordinates the oxygen lone pairs with one or more C(sp3)−H bonds (red) or with one C(sp2)−H bond (blue) or not at all (black). Clearly, C−H coordination and the associated dispersion energy gain contribute to the binding energy and induce an additional red shift in the OH···π hydrogen bond. No clear energetic or spectroscopic differentiation between C(sp3)−H and C(sp2)−H coordination is predicted. The most stable structure (confirmed at B97D, B2PLYD-D3(BJ), and B3LYPD3 levels) features a quadruple C(sp3)−H environment around one oxygen lone pair (Figure 6). However, energetic differences are too small to make a definite assignment. All possible alternatives within 2 kJ/mol also reproduce the experimental additional red shift reasonably well. No possible band for a TNo6No trimer could be identified. The calculations suggest a widened gap between the five- and six-regioisomers of TNoNo similar to the case of TTNo trimers, such that only the most stable isomer is formed for both trimer compositions. Exploratory calculations of TNo5NoNo tetramers indicate that additional C−H coordination of the second oxygen lone pair might be able to shift as far as the coordination of the first lone pair. This makes tetramers and bigger No-rich clusters a possible explanation for the broad spectral feature at around 3541 cm−1, which especially profits from high alkene concentrations and strong cooling, although this must remain speculative at this point. Figure 7 shows such a possible fully oxygen-solvating TNo5NoNo structure. Similar broad absorptions are found for Pe and Hx at higher pressures of the expanded gas mixture (Figures S1−S7 in the Supporting Information). The calculated additional red shift relative to TNo5 induced by only two solvent norbornene molecules is already on the order (20 cm−1) of estimated shifts between the
Figure 4. Structures of mixed trimers consisting of one alkene and two alcohol units at the B3LYP-D3(BJ)/def2-TZVP level. Marked are OH···π hydrogen bonds and intermolecular CH···O contacts of less than 2.9 Å.
correlation for a peak assignment, one indeed observes that the relative TTHx1 abundance is somewhat higher in the more dilute and thus colder expansion, but this may also be coincidental. A cross assignment would lead to more uniform differences between OH···π and OH···O cooperativity enhancements for the two isomers. Therefore, we refrain from a definitive assignment of the two structures, but the existence of two structures is fully compatible with the calculations. In TTNo, the preference for the five-ring coordination site is enhanced by the second alcohol molecule beyond 1 kJ/mol. This explains the absence of a spectrally significant second isomer for the mixed trimers when no major kinetic hindrance is present. The intensity ratio between the two dominant TTNo5 bands is distorted by an underlying No monomer transition in the more red-shifted band, which may thus also be somewhat uncertain in its band position. Upon adding a second T to TPe, THx, and TNo5, the hydrogen atom that formed the weak CH···O contact in the dimers now realizes a closer contact to the second oxygen atom. The exception to this is TTHx2, where a different hydrogen takes this part. The availability of this additional binding site in Hx might be one of the requirements for the coexistence of two energetically nearly equal diastereomers of TTHx. An inspection of TTA trimer dissociation energies shows no monotonous increase with alkene size. The predicted binding energy gain compared to the sum of TT and TA dimers amounts to 9−10 kJ/mol, independent of the alkene size. This confirms that the dispersion energy gain in norbornene is not sufficient to compensate for the steric demand of the bulky bicycle. Whether ring strain arguments on the acceptor ability
Table 3. Additional Harmonic Red Shifts (in cm−1) Induced in the OH···π Stretching Fundamental of Mixed t-Butyl Alcohol Alkene Dimers (TA) upon Formation of an OH···OH···π Pattern by Further T Additiona method B2PLYP-D3(BJ)/def-TZVP B3LYP-D3(BJ)/def2-TZVP B3LYP-D3/def2-TZVP experiment(anharmonic) a
TTNo6 45 42
TTNo5
TTPe
TTHx1
TTHx2
47 49 44
47 51 54 44
48 48 39 (45)
51 50 45 (39)
Alternative assignments are in parentheses. E
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Table 4. Additional Harmonic Red Shifts (in cm−1) Induced in the OH···O Stretching Fundamental of the t-Butyl Alcohol Dimer (TT) upon Formation of an OH···OH···π Pattern by Alkene Additiona method B2PLYP-D3(BJ)/def-TZVP B3LYP-D3(BJ)/def2-TZVP B3LYP-D3/def2-TZVP experiment(anharmonic) a
TTNo6
TTNo5
TTPe
TTHx1
TTHx2
60 60 55
57 59 61 45
60 61 51 (46)
51 53 46 (51)
42 45
Alternative assignments are in parentheses.
Figure 7. Speculative structure of a TNo5NoNo tetramer with an additional red shift of 21 cm−1 relative to TNo5 induced by two solvent norbornene molecules at the B3LYP-D3(BJ)/def2-TZVP level. Full view (left) and immediate surroundings of the hydrogen bond (right) featuring three C(sp3)H···O contacts for the left and two for the right oxygen lone pair.
expansions based on their OH stretching signature. The spectral fingerprint of the (OH···)OH···π bonds is clearly distinct from that of alcohol dimers and trimers, with intermediate shifts that allow for unambiguous assignments without rigorously size-selective tools. A second alkene attached to an alcohol−alkene dimer shows substantial spectral effects on the OH stretching frequency, although its only options besides unspecific London dispersion forces are CH···π and CH···O coordination. The latter appears to dominate in the case of norbornene. Even a double norbornene solvation of both oxygen lone pairs in reinforcement of the primary OH···π interaction from the alcohol to norbornene may be spectroscopically evident, but the resulting complex supramolecular structures call for conformationally selective tools. Such specific spectral signatures for a simple alcohol make it easier to rationalize olfactory discrimination of similar alkenes, although olfaction happens at environmental temperature. The latter is probably compensated for by a larger number of docking sites and the three-dimensional binding pocket of odor receptors, features that our simple model system obviously lacks. Nevertheless, our spectra provide a first rigorous opportunity to judge the performance of quantum chemical methods in such olefinic OH···π interactions and their cooperativity. The performance of B3LYP and B2PLYP calculations with polarized triple-ζ basis sets is remarkably good, once they are augmented by a posteriori dispersion correction. Absolute OH···π wavenumber shifts are surprisingly sensitive to the type of dispersion damping. Remaining systematic discrepancies can easily be blamed on the harmonic approximation, which is closer to reality for OH···π than for OH···O hydrogen bonds. The regioselectivity of the OH stretching shift is pronounced. It clearly distinguishes between five- and six-ring sites and between oxygen lone pair diastereomers, both in experiment and in the quantum calculations. These harmonic calculations therefore have predictive quality for other OH···alkene systems, which are up to now rather poorly characterized by spectroscopy, if one discounts solution studies, aromatic systems, and intramolecular hydrogen bonds.
Figure 5. Diagram of 34 TNo5No trimer structures at the B3LYPD3(BJ)/def2-TZVP level. Shown are relative energies (most stable set to zero) and additional red shifts upon adding a second norbornene unit to TNo5. The experimental data indicate a single structure shifted by 14 cm−1 (green cross). The colors represent whether the second No unit coordinates the oxygen lone pairs with one or more C(sp3)− H contacts (red) or with one C(sp2)−H contact (blue) or not at all (black).
Figure 6. Most stable structure found for TNo5No in the plot in Figure 5 with an additional red shift of 10 cm−1 relative to TNo5. The distances of the marked CH···O contacts are (from top to bottom) 2.89, 2.71, 2.59, and 2.79 Å. The second oxygen lone pair remains free for possible further coordination.
vapor phase and hydrocarbon solutions for OH···π oscillators.15 With the OH···π hydrogen bond now shielded, less pronounced effects can be expected from further aggregation of No, which corresponds to the concept of secondary solvation shells. However, thermal effects are not accounted for in the jet expansion.
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CONCLUSIONS Mixed dimers and trimers of t-butyl alcohol with (bi)cyclic C5− C7 alkenes are observed and assigned in supersonic jet F
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An accurate evaluation of anharmonicity in the OH···π bond, the detailed experimental characterization of the methanol− ethene prototype, and a systematic investigation of ring strain effects and spectral effects on normal modes localized at the alkene site remain on the supersonic jet agenda for this class of weak hydrogen bonds.
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ASSOCIATED CONTENT
S Supporting Information *
A summary of all experimental bands positions, additional spectra, and Cartesian coordinates for all displayed structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support by the German Research Foundation (DFG) via Grant Su 121/4-1 is gratefully acknowledged, as is support for R.M. by M. Buback.
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REFERENCES
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H
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