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O-H···S Hydrogen Bonds Conform to the Acid-Base Formalism Surjendu Bhattacharyya, Aditi Bhattacherjee, Pranav R Shirhatti, and Sanjay J. Wategaonkar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp405414h • Publication Date (Web): 30 Jul 2013 Downloaded from http://pubs.acs.org on August 5, 2013

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O−H···S Hydrogen Bonds Conform to the Acid-Base Formalism

Authors: Surjendu Bhattacharyya, Aditi Bhattacherjee, Pranav R. Shirhatti,† and Sanjay Wategaonkar* Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, Mumbai, India.

Abstract: Hydrogen bonding interaction between the ROH hydrogen bond donor and sulfur atom as an acceptor has not been as well characterized as the O−H···O interaction. The strength of O−H···O interactions for a given donor has been well documented to scale linearly with the proton affinity (PA) of the H-bond acceptor. In this regard, O−H···O interaction conforms the acid-base formalism. The importance of such correlation is to be able to estimate molecular property of the complex from the known thermodynamic data of its constituents. In this work, we investigate the properties of O−H···S interaction in the complexes of the H-bond donor and sulfur containing acceptors of varying proton affinity.

The hydrogen bonded complexes of p-Fluorophenol (FP) with four different sulfur

containing acceptors and their oxygen analogues, namely H2O/H2S, MeOH/MeSH, Me2O/Me2S and tetrahydrofuran (THF)/tetrahydrothiophene (THT) were characterized in regard to its S1-S0 excitation spectra and the IR spectra.

Two-color resonantly enhanced multiphoton ionization (2c-R2PI),

resonant ion-dip infrared (RIDIR) spectroscopy, and IR-UV hole burning spectroscopic techniques were used to probe the hydrogen bonds in the aforementioned complexes. The spectroscopic data along with the ab initio calculations were used to deduce the strength of the O−H···S hydrogen bonding interactions in these system relative to that in the O−H···O interactions. It was found that despite being dominated by the dispersion interaction the O−H···S interactions conforms the acid-base formalism as in the case of more conventional O−H···O interactions. The dissociation energies and the red shifts in the O−H stretching frequencies correlated very well with the proton affinity of the acceptors. However, the O−H···S interaction did not follow the same correlation as that in the O−H···O H-bond. The energy decomposition analysis showed that the dissociation energies and the red shifts in the O−H stretching frequencies follow a unified correlation if these two parameters were correlated with the sum of the charge transfer and the exchange component of the total binding energy.

Keywords: p-Fluorophenol, O−H···S hydrogen bond, acid-base formalism, proton affinity. * Corresponding author: E-mail: [email protected]; Phone: +91-22-2278-2259 1 ACS Paragon Plus Environment

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Introduction Hydrogen bonding interaction (HB) is the most important non-covalent interaction which plays a key role in chemistry and biology.1,2

In general it can be represented as R–D–H···A–R′,

where the D (proton donor) and A (proton acceptor) are two electronegative atoms. Formation of hydrogen bond is facilitated by the partially polar nature of the D–H bond and the electron rich acceptor center A. However, theoretical and recent experimental studies have revealed that even less polar bonds like C–H and S–H can also behave as Hydrogen-bond donor.

Likewise, less

electronegative atoms like Sulfur (S) or diffuse charge like π electron density can act as acceptors of HB.3-7 Though these are not usually considered as classical HBs, their significance in structures of the biopolymers and in living processes like enzyme catalysis or molecular recognition is unarguable.8 In fact, recently it has been shown that N–H···π interaction can overwhelm the classical N–H···O interaction in terms of strength and stability in Indole-Furan hetero dimer.9 There also exist a few experimental evidences of blue shifted hydrogen bonds3,10-16 which have been explained by the increment of s-character of D–H bond upon complex formation.17 Sulfur, despite being much less electronegative compared to other second row elements such as O, N and F, has been found to be strongly involved in hydrogen bonding interactions. The abundance of structural information on the D–H⋅⋅⋅S interaction in crystallographic database of proteins and many organic molecules such as therapeutic agents, agrochemicals motivated the spectroscopic investigation of this interaction.18,19 In terms of strength, these interactions fall within the moderate regime. Both intermolecular and intramolecular hydrogen bonding involving S and various proton donors such as H2O, HF, HCl, HNO2, HNO3, and CF3CCH in isolated matrices has been studied by several groups.20-29 Arunan and co-workers investigated the rotational spectra of weakly bound benzene–H2S complex under jet cooled condition.30 Biswal et al. showed the existence of O–H⋅⋅⋅S hydrogen bonding interaction in gas phase using supersonic jet expansion method for the first time.5 In addition, it has been shown that S can act as an acceptor in O–H⋅⋅⋅S, N–H⋅⋅⋅S, C–H⋅⋅⋅S as well as donor in S–H⋅⋅⋅π interaction.5-7,31-35

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Apart from the “three-centered-four electron shared-proton interaction”, HB can be thought of as “a single proton sharing two lone pairs from two adjacent electronegative atoms or groups: R–D:– H+⋅⋅⋅:A–R′.”36 This depicts that both D and A have to be more electronegative than the central hydrogen atom and greater the electronegativity stronger the HB will be. It is evident that HB strength can be modulated by varying the proton affinities (PA) of both donor and the acceptor atoms. Hence, it is expected higher the proton affinity of the HB acceptor, stronger will be the H-bond. This is a sort of acid-base formalism because the proton affinity of an acceptor is a measure of its gas phase basicity. It has been shown that there exists a good correlation for red shift in O–H frequency of phenol with the proton affinity of acceptor bases.37,38 N. Biswas et al. showed that there exists a linear correlation between the red shifts in band origin (BO) of the complexes and the difference in the pKa values of the S1 and S0 states for a few phenol–H2O complexes.39 All these correlations are well established for conventional O−H⋅⋅⋅O hydrogen bonding where electrostatic interaction is dominant. To the best of our knowledge no such correlations have been reported for S centered HB. Usefulness of these correlation plots is to estimate the basic characteristics of HB from the known thermodynamic data. The aim of the present work was to investigate the strength of the H-bond between p-Fluorophenol (FP) as hydrogen bond donor and several oxygen and sulfur containing solvents of varying proton affinity and to test whether O–H⋅⋅⋅S interaction obeys the similar acid base formalism in the gas phase. For the present study FP was chosen because fluorine (F) weakly deactivates the aromatic ring through its back donation owing to similar orbital size with carbon. Basicity or the proton affinity of the acceptor solvents was modulated by alkyl substitution on the central O or S atom. We have investigated the spectroscopic characteristics of the complexes of FP with seven different solvents, namely, H2O/H2S, Me2O/Me2S, MeOH, and THF/ THT. Due to unavailability of the sulfur analog of MeOH, i.e., the MeSH solvent, only the computational results are presented for its complex with FP in order to obtain a more complete view.

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Methods Experimental The complexes were prepared under jet cooled condition. A detailed description of the experimental apparatus has been discussed in our earlier works.40 In brief, a cold molecular beam was produced by supersonic jet expansion technique which also allows preparing size selected clusters. The electronic excitation spectra of the monomer and its complexes with various solvents were obtained by the two-color resonantly enhanced two-photon ionization (2c-R2PI) method coupled with time of flight mass spectrometry (TOFMS). The vibrational stretching frequencies were determined by the resonant ion-dip infrared (RIDIR) technique. IR-UV double resonance spectroscopy, also commonly known as IR-UV hole burning technique was performed to identify the number of conformers present for each complex. In this method, vibrational excitation is carried out by IR laser and the depopulation of the ground state is probed by the electronic transition. The UV laser is delayed with respect to the IR laser by 50 ns and electronic excitation spectra are recorded. The electronic transitions corresponding to the conformer excited by the IR laser are identified as those which are reduced in intensity /eliminated from the 2c-R2PI excitation spectrum. The 2c-R2PI experiments were carried out using dye lasers pumped by 10 Hz, Nd+3: YAG lasers. A Sirah dye laser (Cobra Stretch-LG-18) pumped by a Q-Switched Nd+3:YAG laser (SpectraPhysics Quanta-Ray, PRO-Series, 10Hz, ~10 ns FWHM) was used as an excitation source and a Quantel dye laser (TDL90) pumped by a Brilliant-B Q-Switched Nd+3:YAG laser (10 Hz, ~6 ns FWHM) was used as the ionization laser. The UV beams were generated by doubling the dye laser outputs by KDP crystals. Both the lasers were synchronized by an electronic delay generator (SRS model no: DG645) and spatially overlapped in a co-propagating geometry by a biconvex quartz lens of focal length of 50 cm. The ion current was detected using a single channel electron multiplier (dia. 25 mm, Dr. Sjuts Optotechnik GmbH; KBL25RS). Signal was amplified by ORTEC VT120 FastTiming Preamplifier and digitized by LeCroy LT354M Digital Oscilloscope. The data acquisition was performed through PC using our own LabView program. Absolute wavelength calibration for the

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dye lasers were performed using a Fe–Ne hollow cathode lamp by optogalvanic method. The uncertainty in the UV wavelengths was less than 2 cm-1. For RIDIR measurements, a LiNbO3 optical parametric oscillator (OPO) with a line narrowing etalon (LaserSpec, 2.6-4.0 µm, bandwidth ~ 0.5 cm-1) pumped by Brilliant-B Q-Switched Nd+3:YAG laser was used. The frequency calibration of the IR-OPO in the O−H stretch region (3500 to 3900 cm-1) was performed by recording the photo-acoustic spectrum of water and comparing it with the standard spectra available in the HITRAN database.41 The uncertainty in the IR frequency was of the order of 0.5 cm-1. The IR and the UV beams were spatially overlapped in a counter propagating manner. IR was focused by a 50 cm focal length CaF2 biconvex lens. With the help of an electronic delay generator (SRS model no: DG645), the IR and UV lasers were synchronized such that the IR pump pulse preceded the UV (probe) pulse by approximately 50 ns. p-Fluorophenol (99 %, GC), dimethyl ether (≥ 99 %), dimethyl sulfide(≥ 99 %), tetrahydrofuran (≥ 99.9 %), and tetrahydrothiophene (99 %) were from Sigma Aldrich. Methanol (AR) was purchased from SD Fine Chemicals Limited. Zero Grade Hydrogen Sulfide (99.5 %) was locally purchased from Ultra-Pure Gases (I) Pvt. Ltd. Millipore water purified by Elix 5 was used for preparation of water premix. p-Fluorophenol was heated to 65° C to produce enough vapor pressure for optimum signal to noise ratio whereas nozzle temperature was maintained at 85°C. The 1:1 complexes were prepared by co-expanding p-Fluorophenol vapor with 0.2-2 % premix in Helium of the corresponding solvent under stagnation pressure of 1−3 kg-cm-2. The band origin transitions for the complexes were recorded by keeping the ionization laser at wavelengths such that total energy of excitation and ionization laser was ~400–500 cm-1 above the ionization energy of the corresponding complex. The premix concentrations, stagnation pressures, laser powers and wavelengths of the ionization laser were critically optimized to maximize the signal to noise ratio in each case. Computational methods All possible conformers were optimized due to the presence of various donor and acceptor sites for some of the protic solvents. The geometry optimization and frequency calculations were performed using Gaussian-09 suite of programs42 without any constraints using several initial 5 ACS Paragon Plus Environment

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structures for all the solvents at the B3LYP and MP2 level using aug-cc-pVDZ basis set. The dissociation energies (D0) for all the conformers for each complex were calculated after the ZPE and BSSE corrections. In almost all cases the conformer in which phenolic O−H group acts as the H-bond donor and the O/S atom of the solvent as an acceptor was the most stable conformer. These conformers were further refined using a wide range of methodologies such as MP2 and DFT methods using various dispersion corrected hybrid density functionals to estimate the binding energies and calculate vibrational frequencies using the aug-cc-pVDZ basis. Three different functionals, namely ωB97X-D, LC-ωPBE, M06-2X were used that have explicit dispersion corrections.43-45 All the computations were done using counterpoise corrected gradient.

Natural bonding orbital (NBO)

analysis was performed to identify the overlapping orbitals between donor and acceptor as well as to calculate the change in natural bond order (∆ΝBO) upon complex formation. NBO 5.9 was linked with Gaussian-09 suite of programs for this calculation.46 Output files were analyzed by NBO View 1.1.46 Topology of electron density was analyzed by AIM2000 set of programs.47 Electron density (ρH···Y) and Laplacian of the electron density (∇2ρH···Y) at the bond critical point (BCP) upon H-bond formation was calculated from the wave functions computed at the MP2 level. Natural Energy decomposition analysis (NEDA)46 was performed by incorporating NBO 5.9 in GAMESS, USA.48

Results Experimental results Two-colour R2PI Excitation spectra The individual 2c-R2PI spectra of monomer and its complexes with various solvents with detailed analysis have been provided in the supporting information (Figures S1–S8). The figures given here show only the assignments of the prominent intra and intermolecular modes. Various vibronic modes observed in these spectra are identified and listed in Table 1. The assignments of the observed normal modes of the monomer and the complexes were made using the Varsanyi’s nomenclature.49

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Table 1: Summary of the observed fundamental normal modes in the 2c-R2PI and RIDIR experiments. All numbers are in cm-1. *O–H frequencies computed at the cp-MP2/aug-cc-pVDZ level scaled by a factor of 0.9613.

Molecule/Complexes Modes

FP

FP–H2O

FP–MeOH

FP–Me2O

FP–THF

FP–H2S

FP–MeSH

35119

34712 -407

34633 -486

34589 -530

34554 -565

34848 -271

– –

FP– Me2S 34725 -394

A

25

27

18

35



20

14

B

31



53

23

C

47



85

105

93



156

162

340



362

360

BO ∆BO

1

σ

16a 6a 1

2

1

29

34676 -443



160

172

143

338

343

353

356

422

426

426

427

428

424



419

428

821

822

824

821

818



811

821

1

186

FP–THT

νΟ−Η

3663

3523

3457

3405

3330

3548



3393

3359



3663

3513

3457

3409

3329

3549

3510

3431

3407

νΟ−Η

FP, FP−H2O, and FP−H2S Figure 1 shows the 2c-R2PI spectra of the FP monomer and its 1:1 complexes with H2O and H2S. The red most transition for the FP monomer, i.e., the band origin (BO) transition was observed at 35119 cm-1 in good agreement with the earlier reports.50

The other transitions, observed at

35457 cm-1 (+338) and 35541 cm-1 (+422) were assigned as 16a2 and 6a1, respectively.

The

former assignment is based on the spectroscopic data for p-aminophenol51 and it differs from that given in Reference 50.

For most para di-

substituted benzene derivatives the S1 vibronic Figure 1: 2c-R2PI excitation spectra of (a) FP, (b) FP−H2O, and (c) FP–H2S. Transitions within the black rectangles are artifacts due to the baseline distortion caused by strong transitions of the monomer at those frequencies and not real features of the complexes.

states have been well characterized and the assignments given here are based on the

established literature data. In the Figure S1 the numbers in parentheses refer to the energy relative to the band origin and have been given next to each transition. For the FP–H2O complex, the BO transition appeared at 34712 cm-1 which is red shifted by 407 cm-1 with respect to that of the 7 ACS Paragon Plus Environment

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monomer. A moderately strong intermolecular vibrational mode was found at 34872 cm-1 (+160) along with two other lesser Franck-Condon (FC) active modes at 34866 cm-1 (+154) and 34844 cm-1 (+132) (Figure S2).

The 160 cm-1 transition was assigned as the intermolecular stretch, σ1,

transition.32,52 The features observed at 35055 cm-1 (+343), 35138 cm-1 (+426), and 35533 cm-1 (+821) were assigned as 16a2, 6a1, and 11, respectively. Two relatively weak transitions were observed in the association with Mode 11 at 35565 cm-1 (+853) and 35599 cm-1 (+887). Combination bands of all the three low frequency transitions with the 6a1 transition were observed at 35270 cm-1 (+558; 6a1+132), 35292 cm-1 (+580; 6a1+154), and 35298 cm-1 (+586; 6a1+160). The peaks at 35360 cm-1 and 35460 cm-1, marked by a rectangle were not the transitions of the FP–H2O complex, but these were artifacts due to strong transitions of the monomer at these positions which distorted the base line in the FP– H2O mass channel. The band origin of FP–H2S complex was observed at 34848 cm-1 which is red shifted by 271 cm-1 relative to that of the monomer. A strong transition was observed at 34941 cm-1 (+93) and a relatively weaker transition was observed near BO at 34883 cm-1(+35) (Figure S3). The former one was assigned as the intermolecular stretch, σ1, transition.32

Other FC active intramolecular

fundamental modes such as 16a2, 6a1 and 11 were found at 35188 cm-1 (+340), 35272 cm-1 (+424) and 35666 cm-1 (+818) only. The intermolecular stretch also appeared in combination with the 6a1 transition at 35366 cm-1 (+518; 6a1+93). As in the case of the FP−H2O complex, the transitions marked by rectangles correspond to the monomer/FP–H2O complex and not the FP–H2S complex. These artifacts were either due to the monomer and/or the FP−H2O complex. FP−MeOH, FP−Me2O, and FP−Me2S The 2c-R2PI spectra of the 1:1 complexes of FP with MeOH, Me2O, and Me2S are shown in Figure 2. The band origin of the FP–MeOH complex appeared at 34633 cm-1 which is 486 cm-1 red shifted with respect to the monomer. Several strong as well as weak features were observed within 200 cm-1 region of the BO at 34658 cm-1 (+25; A), 34664 cm-1 (+31; B), 34680 cm-1 (+47; C), 34689 cm-1 (+56; A+B), 34705 cm-1 (+72; A+C), 34730 cm-1 (+97; 2A+C) (Figure S4). It appears that all the observed transitions can be attributed to three low frequency fundamental intermolecular modes, 8 ACS Paragon Plus Environment

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A, B, and C and their progressions and combinations. The other possibility is that the some of these transitions may be due to different conformers of the complex, vide infra. Two moderately strong intermolecular transitions were found at 34805 cm-1 (+172) and 34814 cm-1 (+181) along with a weaker one at 34836 cm-1(+203; 172+B). The transition at 34805 cm-1 was assigned to the intermolecular stretching mode and the weaker feature at 34836 cm-1 is very likely due to the combination

of

the

B

mode

with

the

intermolecular stretching mode. The 181 cm-1 transition was assigned to the intramolecular in-plane bending mode, 151, that is coupled with the intermolecular stretch. The transition Figure 2: 2c-R2PI excitation spectra of (a) FP–MeOH, (b) FP–Me2O, and (c) FP–Me2S complexes.

at 34986 cm-1 (+353) was assigned as 16a2

while the other two stronger transitions at 35059 cm-1 (+426) and 35455 cm-1 (+822) were assigned as the 6a1 and 11 transitions, respectively. The same pattern of transitions in the vicinity of the band origin was found to appear in combination with the latter two most FC active intramolecular modes. The band origin transition of the FP–Me2O complex was found at 34589 cm-1 which was redshifted by 530 cm-1 with respect to that of the monomer. Two equally spaced transitions were observed at 34616 cm-1 (+27; A) and 34643 cm-1 (+54; 2A) near the BO which are most likely due to the progression in a low frequency (27 cm-1) intermolecular mode of the complex. The intermolecular stretching (σ1) mode was found at 34732 cm-1 (+143) followed by its very weak combination band with the +27 cm-1 low frequency vibrational mode at 34757 cm-1 (+168). The transitions observed at 34945 cm-1 (+356), 35016 cm-1 (+427), and 35413 cm-1 (+824) were assigned as the intramolecular 16a2, 6a1, and 11 normal modes. As in the case of FP−MeOH complex, all the intermolecular features also appeared in combination with the 6a1 and 11 modes. In contrast to its oxygen analog, the FP−Me2S complex gave a very rich 2c-R2PI spectrum. The band origin transition was identified at 34725 cm-1 which is red-shifted by 394 cm-1 with respect 9 ACS Paragon Plus Environment

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to the monomer. Unlike all the other complexes, the BO transition of FP–Me2S was not the most intense transition. The most intense transition was observed at 34745 cm-1 (+20; A) followed by several closely lying features at 34765 cm-1 (+40; 2A), 34778 cm-1 (+53; B), 34786 cm-1 (+62; 3A), 34797 cm-1 (+73; A+B), 34810 cm-1 (+85; C), 34830 cm-1 (+105; A+C), and 34850 cm-1 (+125; 2A+C). The 20 cm-1 mode, A, was the most Franck-Condon active mode. The progression in this mode also appeared in combination with the other two modes, namely the B (53 cm-1) and C (85 cm-1) mode. The intensity distribution pattern in the A progression indicates that there is a significant structural change in the equilibrium geometry in the excited state of the complex along the A normal coordinate. The transitions at 34870 cm-1 (+145), 34881cm-1 (+156), 34903 cm-1 (+178) have been identified as intermolecular vibrations.

The transition at 34881 cm-1 was assigned as the

intermolecular stretch and the one at 34903 cm-1 as its combination with the most FC active mode A. The FC activity similar to one near the BO was also observed in combination with both 16a2, 6a1, and 11 transitions at 35536 cm-1 (+362), 35144 cm-1 (+419) and 35536 cm-1 (+811), respectively. All the transitions along with their energy spacing from the BO have been annotated in Figure S6. FP−THF and FP−THT The 2c-R2PI spectra of the 1:1 complexes of THF and THT with FP are shown in Figure 3. The band origin of FP–THF complex was observed at 34554 cm-1 which was 565 cm-1 red shifted with respect to the monomer. This was the highest red shift in the BO transition among all the complexes. Relatively weaker transitions were observed at 34572 cm-1 (+18; A), 34583 cm-1 (+29; B), 34599 cm-1 (+45; A+B), 34610 cm-1 (+56; 2B), and 34638 cm-1 (+84; 3B) near the band origin transition. The Figure 3: 2c-R2PI excitation spectra of (a) FP–THF, and (b) FP–THT complexes. Transitions within the black rectangles are artifacts due to the baseline distortion caused by strong transitions of the monomer at those frequencies and not real features of the complexes.

intermolecular stretch was identified at 34740 cm-1 (+186). Other FC active intramolecular modes such

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as 6a1 and 11 were found at 34982 cm-1 (+428) and 35375 cm-1 (+821), respectively. Weak transitions due to the intermolecular modes with the FC activity similar that observed near the BO transition were also found in combination with both these intramolecular modes. The band origin of the FP–THT complex was observed at 34676 cm-1 which was 443 cm-1 red shifted with respect to that of the monomer. Many low frequency transitions were observed at 34690 cm-1 (+14; A), 34699 cm-1 (+23; B), 34714 cm-1 (+38; A+B), 34725 cm-1 (+49; 2B), 34747 cm-1 (+71; 3B), 34781 cm-1 (+105; C), and 34805 cm-1 (+129; B+C) which may either be vibrational progressions or due to multiple conformers, vide infra. The transition at 34838 cm-1 (+162) was assigned as the intermolecular stretching mode (σ1). Other FC active intramolecular fundamental modes such as 6a1 and 11 were observed at 35104 cm-1 (+428) and 35497 cm-1 (+821). The pattern of transitions similar to that near the BO was observed in combination with the σ1, 6a1, and 11 transitions. In summary, the 2c-R2PI spectra of the complexes indicate that the stabilization of the complexes in the excited state increased with the increased substitution of alkyl groups on the acceptor atoms, be it oxygen or sulfur. The red shifts of the BO transitions were 407, 486, 530, and 565 cm-1 for H2O, MeOH, Me2O, and THF, respectively. The same for the sulfur centered solvents were 271, 394, and 443 cm-1 for H2S, Me2S, and THT, respectively. In all the cases, the red shifts for the sulfur bound complexes were smaller than those of their oxygen bound analogues. In each of the 2c-R2PI spectra, there were clear appearances of intermolecular stretch in the excited state (σ1) arising upon HB formation.

RIDIR spectra and IR-UV hole burning spectra The RIDIR technique was applied to measure the vibrational frequencies of the monomer as well as of the complexes. This is a potentially powerful technique for identifying the sites of interactions in the complexes, usually by detecting the shift in the stretching frequencies of the H-bond donors, X−H, with respect to those in the monomers upon complex formation.

In

combination with the IR-UV double resonance spectroscopy it is also very useful in identifying whether multiple conformers of a specific complex are present in molecular beam. In the context of the work presented here, in principle, multiple conformations are possible for the FP–H2O, FP–H2S, 11 ACS Paragon Plus Environment

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and FP–MeOH complexes as these protic solvents can act both as H-bond donors as well as acceptors. However, since in the case of FP the phenolic O−H is fairly acidic (pKa=9.95)53 most likely it will behave as a proton donor and therefore the phenolic O–H stretching mode was probed by RIDIR spectroscopy. The RIDIR spectra of FP monomer and all its complexes recorded by probing their respective BO transitions (for Me2S complex, however, the IR spectrum was

Figure 4: RIDIR spectra of (a) FP, (b) FP–H2O, (c) FP– MeOH, (d) FP–Me2O, (e) FP–THF, (f) FP–H2S, (g) FP–Me2S, and (h) FP–THT recorded by probing them at their respective band origins except for the FP–Me2S complex which was probed at 34745 cm-1, i.e., at the first strongest transition adjacent to the band origin at 34725 cm-1. Numbers in parenthesis indicate the shifts with respect to the IR transition observed for the monomer.

recorded by exciting the peak at 34745 cm-1 (A), next to BO for better S/N ratio) are shown in Figure 4. Oxygen containing solvents

For the FP monomer (trace a), O–H frequency was found at 3663 cm-1 in good agreement with the literature.54 The phenolic O–H frequency for the FP−H2O complex (trace b) was observed at 3523 cm-1 which was 140 cm-1 red shifted with respect to that in the monomer. The antisymmetric stretch of the bound water molecule (not shown in the figure) was observed at 3747 cm-1, which is consistent with that observed in the most phenol-water complexes.39,55 In the case of FP−MeOH complex (trace c), two O–H stretching frequencies were observed at 3457 cm-1 and 3680 cm-1. The O−H stretch in free methanol occurs at 3686 cm-1 and therefore the 3680 cm-1 feature was assigned to the free methanolic O−H group.56 The 3457 cm-1 was assigned as H-bond donating phenolic O−H group. The amount of red shift (206 cm-1) in phenolic O–H frequency was 56 cm-1 higher compared to that in FP−H2O. The 2c-R2PI excitation spectrum of FP−MeOH complex showed several low frequency vibronic features, vide supra. As mentioned earlier these transitions could either be due to different conformers or due to the low frequency intermolecular vibronic transitions and their progressions. To affirmatively determine the true picture, the IR-UV hole burning spectra (Figure S11) was acquired by keeping the IR laser in resonance with the measured phenolic O–H frequency. 12 ACS Paragon Plus Environment

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Uniform near-90% depletion of all the peaks in the excitation spectrum confirmed that only one conformer was present in the beam and observed three sets of peaks (A, B and C) were due to the low frequency modes. All the transitions observed in the 2c-R2PI spectrum (Figure S4) could be assigned to these three modes and their combinations/progressions. The O–H frequency in the FP−Me2O complex was observed at 3405 cm-1 (trace d). The red shift for the Me2O complex was 258 cm-1. The IR-UV hole burning spectrum recorded by probing the IR transition at 3405 cm-1 (Figure S-12) confirmed that only one conformer was present in the beam. In the case of Me2O complex, only one low frequency mode of 27 cm-1 was observed along with its overtone in the 2c-R2PI spectrum. The IR spectrum in trace (e) shows the RIDIR spectrum of FP-THF complex. The 2c-R2PI excitation spectrum of the FP−THF complex showed a few low frequency vibronic features near the BO transition. The IR spectra were recorded by probing a couple of them and it was found that they were all alike. Figure S-13 shows two representative IR spectra recorded using the transitions at 34572 and 34583 cm-1 as probe. The O−H stretching frequency in this complex appeared at 3330 cm-1, which was red shifted from that in the monomer by 333 cm-1. The IR-UV hole burning spectrum (Figure S-14) carried out by keeping IR frequency at 3330 cm-1 showed 100% depletion of all the peaks in the excitation spectra. This confirmed that there was only one conformer present. The transitions at 34572 cm-1 (A) and 34583 cm-1 (B) were assigned to two intermolecular modes and the transition at 34599 cm-1 was as their combination band. A two member progression was observed in mode B. Sulfur containing solvents In the FP−H2S complex (trace f), the O–H frequency was found at 3548 cm-1 which was relatively less red shifted (115 cm-1) compared to that for the FP−H2O complex. The IR-UV hole burning spectra recorded for both H2O and H2S complexes (Figure S9 and S10, respectively) indicated that there was only one conformer present for both the complexes and those were phenolic O–H bound. Trace g shows the RIDIR spectrum for the FP−Me2S complex. The excitation spectrum of the FP−Me2S complex showed many low energy vibronic features with a strong progression in a 20 cm-1 vibrational mode. The IR spectra were recorded by probing a few transitions observed in the 2c- R2PI spectrum and in all the spectra (Figure S-15) the O–H stretching frequency was observed at 13 ACS Paragon Plus Environment

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3393 cm-1 corresponding to the red shift of 270 cm-1. The IR-UV hole burning spectrum carried out by keeping IR frequency at 3393 cm-1 showed depletion of all the peaks in the excitation spectra (Figure S-16). This confirmed that there was only one conformer present. Incidentally, the red shift in the O−H stretch in this case was more than its oxygen analog, namely, the FP−Me2O complex; 258 vs. 270 cm-1.

In addition, as evident from the transitions near the BO, the low frequency

intermolecular mode A was the most FC active mode in the case of Me2S complex. As in the previous case the 2c-R2PI spectrum of the FP−THT complex was also quite rich in transitions near the BO transition. The exercises similar to the ones in the preceding cases were carried and it turned out in this case too only one conformer was observed. The trace (h) shows the IR spectrum of this complex. The IR transition was observed at 3359 cm-1. The red shift in the OH stretching frequency was 304 cm-1.

All the transitions near the BO were assigned to three

intermolecular modes A, B, and C and their combinations as given in the 2c-R2PI section.

Computational results: In principle, the complexes of FP with the protic solvents, namely, H2O, H2S, MeOH and MeSH could have multiple conformers such as 1) FP(O–H)···A (A=O or S atom of the solvent) or 2) A–H···(O/F/π)FP, i.e., O, F atom, or the π electron density for FP. Although the conventional wisdom dictates that the former possibility is predominant, for the sake of completeness the dissociation energies of all the conformers were calculated at the B3LYP and MP2 levels using the aug-cc-pVDZ basis set incorporating the BSSE and ZPE corrections. These values for all the four solvents are given in Table 2. Table 2: Dissociation energy (D0) including zero point energy (∆ZPE) contribution and basis set superposition error (BSSE) correction for various possible conformations computed using aug-cc-pVDZ basis.

D0 (kcal/mol) Conformations A=(O/S)

MP2

B3LYP

FP–H2O FP–H2S FP–MeOH FP–MeSH FP–H2O FP–H2S FP–MeOH FP–MeSH O−H···A A−H···OFP A−H···FFP A−H···πFP

4.73 2.56 1.96 1.62

2.72 1.54 1.12 2.10

6.15 2.71 2.65 2.72

4.39 2.84 − 3.15

4.35 1.78 1.10 −

1.87 0.37 0.14 −

5.23 2.11 1.61 −

3.10 0.41 0.58 − 14

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It was found that in all the cases at both the levels of calculations, the O−H···A type conformers were most stable. For the other solvents such ambiguity does not arise. Therefore only the O−H···A type structures of the complexes of all the solvents were subjected to further computational studies. The structural optimizations and frequency calculations were carried out at the MP2 level with and without counterpoise corrected surface. Computations were also carried out using various dispersion corrected DFT functional methods (using the counterpoise corrected surface) in order to test their relative performances. The absence of imaginary frequencies in all the cases ensured that the reported structures were the true minima for the specific conformation of the complexes. The resulting red shifts in the H-bond donating O−H stretching frequencies are given in Table 3 along with the computed dissociation energies. A few selected geometrical parameters of the Table 3: Red shift in phenolic O−H frequency (∆νO−H) and dissociation energy (D0) including zero point energy contribution computed by various methods at aug-cc-pVDZ level on counterpoise corrected surface. Scaling factors for frequencies are 0.9613, .9377, 0.9388, 0.9423, and 0.9586 for MP2, ωB97X-D, LC-ωPBE, M06-2X, and B3LYP respectively. For the (∆νO−H), the numbers in the parenthesis represent deviation from the observed frequency shifts and for the dissociation energies (D0) the numbers in the parentheses represent the % deviation from the D0 value at the cp-MP2 level.

∆νΟ−Η (cm-1) method EXPT cp-MP2 MP2 cp-ωB97X-D cp-LC-ωPBE cp-M06-2X cp-B3LYP cp-MP2 cp-ωB97X-D cp-LC-ωPBE cp-M06-2X cp-B3LYP

FP– H2O

FP– MeOH

FP–THF

FP– H2S

FP– MeSH

FP–Me2O

140 150 (10) 177 (37) 196 (56) 195 (55) 159 (19) 194 (54)

206

258

333

115

206 (0)

254 (-4)

334 (1)

114 (-1)

239 (33)

307 (49)

394 (61)

234 (28)

250 (-8)

318 (-15)

246 (40)

272 (14)

329 (-4)

216 (10)

231 (-27)

309 (-24)

113 (-2)

245 (39)

269 (11)

326 (-7)

144 (29)

N.A. 153 (N.A.) 151 (N.A.) 187 (N.A.) 189 (N.A.) 182 (N.A.) 208 (N.A.)

4.99 5.36 (7.4) 4.45 (-10.8) 5.61 (12.4)

6.59 6.89 (4.6) 5.43 (-17.6) 6.98 (5.9)

7.20 7.11 (-1.3) 5.57 (-22.6) 7.01 (-2.6)

8.54 11.5 (34.7) 9.62 (12.6) 11.63 (36.2)

4.41 (-11.6)

5.28 (-19.9)

5.36 (-25.6)

6.33 (-25.9)

136 (21) 144 (29) 127 (12)

D0(kcal/mol) 3.01 5.23 3.37 5.09 (12.0) (-2.7) 2.13 3.45 (-29.2) (-34.0) 2.80 4.14 (-7.0) (-20.8) 1.94 (-35.5)

3.3 (-36.9)

Mean Absolute Error

FP–Me2S

FP–THT

270

304

232 (-38)

256 (-48)

14.6

260 (-10)

191(-113)

46.3

242 (-28)

295 (-9)

24.7

252 (-18)

278 (-26)

24.1

196 (-74)

268 (-36)

27.4

269 (-1)

295 (-9)

21.4

6.44 6.57 (2.0) 4.44 (-31.1) 6.76 (5.0)

7.53 12.99 (72.5) 11.17 (48.3)

4.17 (-35.2)

17.1 25.8

13.93 (85.0) 4.61 (-38.8)

21.9 28.7

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most stable conformations of all the complexes obtained at the MP2 level on counterpoise corrected surface using aug-cc-pVDZ basis (cp-MP2/aug-cc-pVDZ) level are given in Table 4. Table 4: Summary of ab-initio computations, QTAIM and NBO analysis. Optimization and frequency calculations were performed at the cp-MP2/aug-cc-pVDZ level. Frequencies are obtained by scaling the computed frequency with factor 0.9613. Dissociation energy (D0) was calculated after adding zero point energy. QTAIM and NBO analysis was performed upon that optimized geometry at aug-cc-pVDZ basis set.

Computed

Complexes

dH···A (Å)

FP–H2O 1.9205

FP– MeOH 1.8724

FP– Me2O 1.8325

FP–THF 1.7824

FP–H2S 2.5184

FP– MeSH 2.4773

FP–Me2S 2.3574

FP–THT 2.3464

RO···A (Å)

2.8952

2.8316

2.8052

2.7551

3.4811

3.3430

3.2521

3.2524

∆rO−H (Å) θ (deg) ψ ( deg) Φ (deg) Φ' (deg) BSSE (kcal/mol)

0.0079 176.8 134.9 0.0 0.1

0.0106 166 133 7.0 16.3

0.0128 170.9 134.7 6.3 12.8

0.0167 168.9 119.6 7.0 14.1

0.0056 170 92.4 0.0 0.0

0.008 147.7 90.5 17.7 40.1

0.0116 151.5 87.2 13.6 34.8

0.0129 153.3 88.6 18.5 38.4

–1.41

–2.03

–2.38

–3.01

–1.48

–2.75

–3.10

–3.84

parameters

∆ZPE (kcal/mol) D0 (kcal/mol) ∆νΟ−Η (cm-1) ρH···A (au)

–1.65

–1.32

–1.15

–1.08

–1.03

–0.89

–0.83

–0.78

4.99 150 0.0251

6.59 206 0.0292

7.20 254 0.0324

8.54 334 0.0345

3.01 114 0.0136

5.23 153 0.0160

6.44 232 0.0217

7.53 256 0.0223

∇2ρH···A(au)

0.0918

0.1038

0.1135

0.1415

0.0364

0.0418

0.0484

0.0491

RH···BCP

0.6594

0.6393

0.6181

0.5818

0.7938

0.8000

0.7539

0.7470

RBCP···A

1.2614

1.2339

1.215

1.2015

1.7247

1.679

1.6044

1.6008

0.13 13.27 0.0072

2.31 13.22 0.0095

5.32 11.68 0.0121

3.52 17.67 0.0143

0.06 9.16 0.0076

0.4 9.66 0.0095

0.96 14.07 0.0143

1.08 15.74 0.0159

(2)

E (i-j) (kcal/mol) ∆ΝBO

The shifts in the O−H stretching frequencies given in Table 3 have been scaled by a factor obtained by dividing the experimentally determined O–H frequency for the monomer by that computed at the respective level. It can be seen that the shifts calculated at the cp-MP2 level are in excellent agreement with the observed frequency shifts, except for the Me2S and THT complexes. For these two latter complexes the cp-MP2 level computation predicts lower red shifts than those observed experimentally. It is also highlighted that the MP2 frequency shifts without the counterpoise

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corrected surface are uniformly higher and are as good or as bad as those predicted using other methods. At present, it is difficult to comment on calculated frequency shift for the FP−MeSH complex because of the lack of experimental data. Table 3 also lists the binding energies for all the complexes. The binding energies for the FP−H2O

and

FP−MeOH

complexes

were

computed to be 4.99 and 6.59 kcal/mol at the cpFigure 5: The optimized structure of FP−Me2S at cpMP2/aug-cc-pVDZ level. The atom number 23 is the dummy (D) one such that the line A14D23 bisects the ∠C19A14C15 (A=O/S); θ=∠O7H13A14; ψ= ∠H13A14D23; Φ = Dihedral angle formed by C4C1O7H13; Φ' = Dihedral angle formed by C4C1O7A14.

MP2 level. These are in good agreement with the experimentally

determined

values

for

the

Figure 6: Side view of minimum energy conformers of (a) FP−H2O, (b) FP−MeOH, (c) FP−Me2O, (d) FP−THF, (i) FP−H2S, (j) FP−MeSH, (k) FP−Me2S, and (l) FP−THT complex; Top view of minimum energy conformers of (e) FP−H2O, (f) FP−MeOH, (g) FP−Me2O, (h) FP−THF. 17

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phenol−H2O (5.60 kcal/mol), phenol−MeOH (6.11 kcal/mol) complexes.57 Therefore, we report the values computed at this level with confidence. For the group of solvents with oxygen atom as the Hbond acceptor the binding energies were 4.99, 6.59, 7.20, and 8.54 kcal/mol for H2O, MeOH, Me2O, and THF, respectively. For the sulfur centered analogs of these solvents these were 3.01, 5.23, 6.44, and 7.53 kcal/mol for H2S, MeSH, Me2O, and THT, respectively. The dissociation energy of the H2O complex was ~1.66 times greater than that of the H2S complex. The dissociation energy significantly increases upon successive substitution of hydrogen atoms with the methyl group to the HB acceptor. The THF/THT complexes were found to be most stable (8.54/7.53 kcal/mol, respectively) in the series. Although the binding energies of the O-bound the cp- complexes were always greater than those of their S-bound counterparts, th e relative stability of the S-bound complexes increased in the analogous pairs going from H2O/H2S to THF/THT. The binding energies computed at the other levels will be discussed later. A few selected geometrical parameters such as the H-bond distance, dH⋅⋅⋅A (A = O or S), the distance between OFP and the acceptor atom, RO⋅⋅⋅Α, elongation of the phenolic O–H bond (∆rO–H), the H-bond angle, i.e., ∠OHA (θ), the angle made by the bisector of ∠R'AR'' (R', R'' = Alkyl group) with the H–A bond (the H-bond) (Ψ), the dihedral angles C4C1O7H13 (Φ), and C4C1O7A14 (Φ'), (see Figure 5 for the atom numbering) have been listed in Table 4. The dihedral angles Φ and Φ' indicate the deviation of the O−H bond and the HB acceptor atom (A) from the aromatic plane of the donor, respectively. All the optimized structures at MP2/aug-cc-pVDZ level are provided in Figure 6. The range of the hydrogen bond distances for the O−H···O type complexes was 1.7824 -1.9205 Å and that for the OH···S type complexes was 2.3464 to 2.5184 Å. The magnitude of Ψ was ~135º for the oxygen bound complexes except for the THF complex for which it was ~120º. However, for the sulfur bound complexes the Ψ was around ~90º for all the complexes, vide infra. The dihedral angles Φ and Φ' were ~0º for the H2O and H2S complexes. The other O-bound complexes showed slight non-planarity; the values of Φ and Φ' were ~7 and ~15º, respectively. However, the Φ was much higher (~13 to 18º) for the S bound complexes other than the H2S complex and further, the Φ' was

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even larger (~35 to 40º). For these complexes the alkyl groups appeared to be folded back onto the aromatic ring due to the larger contribution of the dispersion interaction. Quantum theory of atoms-in-molecules (QTAIM) analysis was carried out to identify the (3,-1) bond critical points which indicate bond formation between the two nuclei.58,59 The electron density maps generated using the wave function obtained at the MP2/aug-cc-pVDZ level are shown in Figures S-19–S-26. The electron densities (ρ) and Laplacian of electron densities (∇2ρ) at the BCPs for various complexes were found to be in range from 0.0136–0.0345 a.u. and 0.0364–0.1415 a.u., respectively, as listed in Table 4. These Figure 7: The correlation plots of electron density at the BCP (ρH···A) vs. the dissociation energy (D0) kcal/mol for the O–H⋅⋅⋅O and O–H⋅⋅⋅S bound complexes.

values are within the range specified for existence of HB.60 Figure 7 shows the plot of

the ρ vs. the dissociation energy for all the complexes. It can be seen that the data falls on two distinct straight lines, of course assuming the linear dependence of ρ on the binding energy. From the regression coefficients it does not seem to be a bad assumption. A clear trend shows that the same correlation does not hold for the oxygen vs. sulfur bound complexes. Natural bonding orbital analysis was performed to obtain natural bond order (NBO) and the second-order perturbative interaction energies (E(2)(i-j)) of the interacting orbitals between donor-acceptor. These are listed in Table 4.

The natural orbitals and their three

dimensional pictures for the individual complexes Figure 8: Percentage contribution of NEDA interaction energy (EintNEDA) and dispersion interaction (EintDISP) to the total interaction energy (EintMP2).

are depicted in Figures S-27 to S-38.

It was

found that for FP−MeOH, FP−Me2O and FP−THF, 19

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there are two significant interactions involving both the lone pairs of acceptor O with the antibonding orbital, σ* of phenolic O−H group. However, for the complexes of FP with H2O and all S containing acceptors, significant interaction was only with one of the lone pairs from the HB acceptor atom. Total second order perturbative interaction energy was always greater for the O centered complexes compared to that for the analogous S-centered complex. The change in the intramolecular O−HFP bond order was calculated from the difference in the electronic occupancies of the O−H bonding and antibonding orbitals. Based on this analysis, for the FP−H2O complex the reduction on the bond order was obtained as 0.0072 whereas that for the FP−THF complex it was the 0.0143. Incidentally this scales very well with the relative magnitudes of O−H frequency red shifts for these two complexes. Similar trend was observed for the sulfur containing solvents; for the H2S complex the reduction in the bond order was 0.0076 while that in the case of the THT complex was 0.0159. Natural energy decomposition analysis (NEDA) was performed to extract various components of the total interaction energy such as NEDA interaction energy (EintNEDA), dispersion interaction (EintDISP), attractive contributions of charge transfer (Ect), Electrostatic (Ees), exchange (Eex), polarization (Epol), and repulsive core (Ecore) energy. These are listed in Table S-28 of the supporting information. The pictorial representation of percentage contribution of (EintNEDA) and dispersion interaction energy (EintDISP) to the total interaction energy (EintMP2) is provided in Figure 8. It was found that the dispersion stabilization of the sulfur bound complexes is relatively more than their oxygen counterparts. For the FP−H2S complex the dispersion stabilization component was ~53%. As the H atoms on sulfur atoms are replaced by the alkyl groups the dispersion interaction further dominated over electrostatic interaction and it was as high as 76% in FP−THT complex. On the other hand the oxygen bound complexes are stabilized largely by the electrostatic interaction. The common trend between the oxygen and sulfur bound complexes was that the dispersion contribution increases upon substitution of H-atoms by methyl groups on oxygen/sulfur.

The dispersion

contribution in the oxygen based solvents was in the order H2O (~18%) < MeOH (~33%) < Me2O (~35%) < THF (~38%).

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Discussion Table 3 summarizes the red shifts in the O−H stretching frequency in all the complexes obtained by various computational methods. It can be seen that the red shifts computed at the cp-MP2 level matched the experimentally determined shifts remarkably well. The mean absolute error was only 14.6 cm-1 which was the lowest among all the methods that were tested. The cp-MP2 method has been reported to do a very good job of frequency computations for the hydrogen bonded complexes that are largely stabilized by the dispersion interaction.10,61-63 The results reported in this work are consistent with this expectation except for two specific complexes, namely, the FP−Me2S and FP−THT complexes. In these two cases the computed red shifts were actually 38 cm-1 and 48 cm-1 lower than the experimentally determined red shifts, respectively. This indicates that the cp-MP2 method is still not correcting enough in these cases. At all other levels of computations the mean absolute deviations were greater than 21 cm-1. The dissociation energies for all the complexes computed using various methods are also listed in Table 3. We report the energies at the cp-MP2 level with some degree of confidence for all the complexes. Among the other methods that were tested, those estimated at the cp-ωB97XD level came close to the cp-MP2 numbers. The mean absolute error was of the order of 17%. Actually, the statistical agreement looks bad only because of the two complexes, namely the THF and the THT complexes, and especially due to the latter one where the deviation from the cp-MP2 number was of the order of 72%. Similar observations were made in regard to those computed using the M06-2X functional. The LC-ωPBE functional did a fairly good job with respect to the oxygen containing solvents but was consistently poor in regard to the sulfur containing solvents. It is well known that phenol is a weak acid and generally acts as HB donor. p-Fluorophenol is no exception to this. Upon electronic excitation, electron density from the O−H bonding orbital is partially transferred to the aromatic ring, resulting in weakening of the bond. Therefore, the O–H bond becomes more acidic in S1 state compared to the S0 state and that is why phenols are also called photoacids. Electronic excitation increases the permanent dipole moment of the phenolic O−H group; hence electrostatic interaction with the solvents becomes more favorable in S1 state. Given this and 21 ACS Paragon Plus Environment

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the fact that the conventional H-bonds are predominantly electrostatic in nature, the general observation has been that the band origin of the S1 state red shifts in wavelength in the case of O−H···X type H-bonds involving phenols and its derivatives as H-bond donor. The observed red shifts in the band origin transitions in all the complexes presented here indicate that these complexes are of the O−H···A type. The excitation spectra of the complexes show that the BO transitions in the case of the oxygen containing solvents are more red shifted compared to their sulfur analogs. Since the S complexes are stabilized largely by the dispersion interaction, their relative stabilization in the S1 state is less compared to O bound complexes. With alkyl substitution on the oxygen or sulfur atoms in the H-bond accepting solvents, the excitation spectra became richer in the number of transitions. The IR-UV hole burning spectra in all the cases confirmed that these were arising due to the FC active low frequency vibrational modes. In all the cases, wherever observed, up to three fundamental low frequency modes were observed. From the computed normal mode frequencies, the most likely modes in the observed frequency range were the out-of-plane and in-plane bending modes and the torsion mode, in the order of increasing frequency. We qualitatively assign the three modes listed in Table 1 as A, B, and C to these three intermolecular modes, respectively. It is meaningless to try to assign them categorically to a specific mode for two reasons; one, the computed modes are for the S0 state and two, because these are highly anharmonic modes and the level of computations presented here predicts only the harmonic modes. Interestingly, the excitation spectra of the S bound complexes were richer compared to their O bound analogs indicating greater changes in the geometry of the former complexes in their S1 state along the corresponding normal modes.64 Apart from the red shifts in the band origins, the IR spectroscopic data on all the complexes indicated that the O–H stretching frequency in the complexes was red shifted compared to that in the monomer. Conventionally, the red shift of the X–H stretching frequency in the X−H···Y H-bonded complex is taken as an indicator of its strength, i.e., greater the red shift stronger is the H-Bond. The red shift in phenolic O–H frequency indicates that in these complexes, FP acts as O–H donor and A (O/S) as an acceptor. In the case of the O bound complexes the red shifts in O–H frequency were 140, 206, 258, and 333 cm-1 for H2O, MeOH, Me2O, and THF, respectively and those for the S bound 22 ACS Paragon Plus Environment

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complexes were 115, 270, 304 cm-1 for the H2S, Me2S, and THT, respectively. In the case of the O containing solvents, an excellent linear correlation (R2 = 0.9391) was observed between red shift of O−H stretching frequency (∆νO−H) and proton affinity (PA) as displayed in Figure 9a.

In the

case of the sulfur based solvents also the correlation using the three experimental data points was very good (Figure 9b). In the case of MeSH, the red shift read out from plot 9b was 202 cm-1. This is in good agreement with the red shift

Figure 9: The correlation plots of red shift in O−H frequency (∆νO−H) vs. proton affinity (PA) for the O–H···O and O–H···S bound complexes.

predicted at the cp-MP2 level if one considers the trend in the predicted vs. observed red shifts for

the Me2S and THT complexes (i.e., 153 + 38 = 191 cm-1). It has been already shown that for the complexes of phenol with various oxygen and nitrogen based solvents, the O−H red shift correlates well with the proton affinity (PA) of the acceptor.37 The present set of experiments extends this trend to the sulfur containing solvents even though these complexes have the dispersion interaction as the major component of their binding energy. The only caveat is that the O and S based solvents do not fall on the same regression plot, i.e., they follow two distinctly different series. The binding energies computed at the cp-MP2/aug-cc-pVDZ level (Table 3) also show a good correlation with the proton affinities of the solvent, albeit there are two distinctly different linear correlation plots (Figure 10). For instance, although the proton affinities of H2O Figure 10: The correlation plots of dissociation energy (D0) vs. proton affinity (PA) for the O–H···O and O–H···S bound complexes.

and H2S are quite comparable (165.2 vs. 168.5

kcal/mol), with that for the latter being actually slightly greater than the former, the binding energy for the FP−H2S complex is quite smaller than that for the FP−H2O complex (3.01 vs. 4.99 kcal/mol). 23 ACS Paragon Plus Environment

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This is also corroborated by the QTAIM analysis which predicts the ρ values at the BCPs of the FP−H2S and FP−H2O complexes as 0.0136 a.u. and 0.0251 a.u., respectively.

This seemingly

apparent discrepancy arises due to several factors. The basic difference between O and S is the difference in their electronegativity (0.86 on Pauling scale) and the van der walls radii (0.28 Ǻ). These affect the electric dipole moment and the dispersion contribution of H2O and H2S. Although the proton affinities of H2O (165.2 kcal/mol) and H2S (168.5 kcal/mol) are quite comparable, there is significant difference between the dipole moment of H2O (1.85 D) and H2S (0.97 D) and it is the dipole−dipole interaction which provides such stronger stability to H2O compared to H2S. Similar arguments can be used to explain the observed trend in the relative stability of the complexes of the analogous pairs of oxygen and sulfur containing solvents. In the case of MeOH and MeSH, the dissociation energies significantly increase and they were 6.59 and 5.23 kcal/mol, respectively. Similar increment was also observed in ρ values at BCPs of the FP−MeOH (0.0292 a.u.) and FP−MeSH (0.0160 a.u.) complexes. The proton affinities of MeOH and MeSH are 180.3 kcal/mol and 184.9 kcal/mol, respectively, which is a significant enhancement compared to previous set of acceptors. On the contrary, the dipole moment of MeOH (1.70 D) is smaller than that of H2O whereas MeSH has much higher dipole moment (1.52 D) than that of H2S. Hence, the higher proton affinity of MeOH and MeSH account for the substantial increase in the stabilization whereas the relative dipole moments justify the comparable stability of their complexes. For the remaining two pairs, namely the Me2O/Me2S and THF/THT complexes, there is monotonous increase in their proton affinities as well as the dipole moments and the binding energies. It is important to highlight that for similar set of acceptors, i.e., oxygen vs. sulfur containing acceptors, the dissociation energy, D0 of the complexes increases with increase in PA of the acceptor. Figure 10a depicts the excellent linear correlation (R2=0.9604) between the D0 and PA for the O bound complexes.

A different and slightly better linear correlation (R2=0.9746) was observed

between D0 and PA for the S bound complexes as shown in Figure 10b. These important correlations connect the molecular property with the bulk property. The question to ask is whether there is any unified correlation. The binding energy, proton affinity are all thermodynamic quantities which are 24 ACS Paragon Plus Environment

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the net result of several fundamental interactions, such as charge-dipole, dipole-dipole, dipoleinduced dipole as well as higher order multipole interactions. And of course in addition to all this the all-pervading dispersion interaction cannot be ignored as it is apparent in many weakly bonded systems.

The answer to the question

raised above can be found if we can examine the Figure 11: Correlation plot of (Ect+Eex) (kcal/mol) vs. red shift in O−H frequency (∆νO−H) for the O–H···O and O– H···S bound complexes.

individual components of the net interaction somehow. As we mentioned earlier the NEDA

was carried out to extract various components of the interaction energy, EintNEDA. The important components such as the electrostatic (Ees), induced polarization (Epol), charge transfer component (Ect), core interaction energy (Ecore), etc. are given in Table S-28. The dispersion interaction was estimated as the difference between the EintNEDA and the dissociation energy computed at the MP2 level (EintMP2). Various correlations were tried and the one that best correlates the red shifts in the O−H stretching frequencies for the complete set of complexes (both oxygen and sulfur based solvents) was with the sum of Ect and Eex. An excellent unified linear correlation plot (R2=0.9910) between ∆νO−H and (Ect+Eex) as displayed in Figure 11. This is quite interesting and intuitive because the O−H red shift in the complex necessarily has to come from the charge transfer from the lone pair(s) of the acceptor or any other part of the acceptor molecule and the anti-bonding orbital of the X−H donor. Addition of the exchange component of the energy improved to correlation. Unlike the charge transfer component the exchange energy does not have any physical analog. The origin of the exchange component is the antisymmetric nature of wave function and its extent would depend on the exchange integral of the interacting orbitals.

The observed excellent correlations between the

aforementioned properties of the non-covalently bound complexes and the PA which refers to the ability of the molecule to covalently bind to a proton, indicate that there exists a good symptomatic connection between the covalent properties and the intermolecular non-covalent interactions. 25 ACS Paragon Plus Environment

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The binding energy or the proton affinity of the acceptor molecule could be larger due to several other contributions, including the dispersion, but the red shift in the O−H stretching frequency does not necessarily have to scale with it. So far for all the conventional hydrogen bonds, necessarily involving the first row elements, the X−H red shift did correlate with the PA of the solvents, but it became apparent from this study that such correlation does not hold across the periods of the periodic table. In the light of this the unified correlation pointed out here is a significant finding. The NEDA analysis further showed that the dispersion interaction is dominant in the S bound complexes. Among the four complexes, H2S shows the least dispersion interaction (53%) whereas in the case of the rest of the acceptors dispersion contributes more that 70% to the total stabilization energy. The diffuse d orbitals of S are the source of such high dispersion contribution. Apart from the H2S complex, the rest of the S complexes have highly folded structure in which alkyl group tends to fold over the aromatic part of the HB donor, thereby increasing the dispersion interaction. Finally it can be concluded that just as in the case of the O−H···O interaction, which is more electrostatic in nature, O−H···S interaction where dispersion is the dominant interaction also follows acid-base formalism.

Concluding Remarks The HB complexes of FP with a set of four acceptors containing O and S have been characterized completely by applying various experimental techniques such as 2c-R2PI, RIDIR, and IR-UV double resonance spectroscopy along with the necessary ab initio computations.

The

magnitudes of red shift in the phenolic O−H frequency obtained by RIDIR and in band origins of the S1-S0 transition confirm that in all these complexes phenolic O−H acts as a HB donor and O/S acts as an acceptor. The IR-UV double resonance spectroscopy shows that there exists only single conformer for each complex. Ab initio computations at the cp-MP2-aug-cc-pVDZ level also show that the O−H···A type conformations were the most stable ones and that agrees with the experimental observations. The amount of red shift in S1−S0 band origin was relatively higher for O acceptor compared to S acceptor. This was attributed to the lesser electrostatic component in the O−H···S type interaction. Structurally the S bound complexes were found to have folded structures (low Ψ value) 26 ACS Paragon Plus Environment

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relative to O bound complexes. The computed binding energies and the observed O−H frequency shifts scaled with the proton affinity for both the O and S series of solvents, hence conforming to the acid-base formalism for the O−H···S interaction. The binding energy of the FP−H2S complex was half as that of the FP−H2O complex, however, as the alkyl substitution on the central atom increased the relatively stability of the S bound complexes increased. The energy decomposition analysis (NEDA) rendered the dispersion interaction as the dominant component in stabilizing the S bound complexes and eventually its contribution was as large as >70% in case of MeSH, Me2S and THT complexes. From the QTAIM analysis it was quite obvious that for the acceptors of the same family, the ρ and ∇2ρ at BCPs increased with the dissociation energy of the complexes. The most important finding was that despite the different nature of the interaction stabilizing the complex, there existed an excellent linear correlation of red shift in O−H frequency and the dissociation energy with PA for both O and S centered complexes, although the each series of acceptors followed a separate correlation plot.

However, an examination of the NEDA results indicated an excellent unified

correlation (R2= 0.9910) between the sum (Ect+Eex) and ∆νO−H. This is for the first time such correlation is reported for the O−H···X interaction, where X spans across two periods.

Supporting Information Available: Individual 2c-R2PI, IR-UV hole burning spectra, RIDIR spectra of FP−Me2S, FP−THF, FP−THT by exciting at multiple transitions of the 2c-R2PI spectra, molecular graph obtained from QTAIM, NBO orbital diagrams, hybridization of HB acceptor, optimized coordinates of all the conformers, and the details of NEDA. 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]; Phone: +91-22-2278-2259 Present Address †

Institute of Physical Chemistry, Georg-August University of

Goettingen, D-37077 Goettingen, Germany. Notes The authors declare no competing financial interest.

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Figure Captions

Figure 1: 2c-R2PI excitation spectra of (a) FP, (b) FP−H2O, and (c) FP–H2S. Transitions within the black rectangles are artifacts due to the baseline distortion caused by strong transitions of the monomer at those frequencies and not real features of the complexes.

Figure 2: 2c- R2PI excitation spectra of (a) FP–MeOH, (b) FP–Me2O, and (c) FP–Me2S complexes.

Figure 3: 2c-R2PI excitation spectra of (a) FP–THF, and (b) FP–THT complexes. Transitions within the black rectangles are artifacts due to the baseline distortion caused by strong transitions of the monomer at those frequencies and not real features of the complexes.

Figure 4: RIDIR spectra of (a) FP, (b) FP–H2O, (c) FP–MeOH, (d) FP–Me2O, (e) FP–THF, (f) FP– H2S, (g) FP–Me2S, and (h) FP–THT recorded by probing them at their respective band origins except for the FP–Me2S complex which was probed at 34745 cm-1, i.e., at the first strongest transition adjacent to the band origin at 34725 cm-1. Numbers in parenthesis indicate the shifts with respect to the IR transition observed for the monomer.

Figure 5: The optimized structure of FP−Me2S at cp-MP2/aug-cc-pVDZ level. The atom number 23 is the dummy (D) one such that the line C14D23 bisects the ∠C19A14C15 (A=O/S); θ = ∠O7H13A14; ψ =

∠H13A14D23; Φ = Dihedral angle formed by C4C1O7H13; Φ' = Dihedral angle formed by C4C1O7A14.

Figure 6: Side view of minimum energy conformers of (a) FP−H2O, (b) FP−MeOH, (c) FP−Me2O, (d) FP−THF, (i) FP−H2S, (j) FP−MeSH, (k) FP−Me2S, and (l) FP−THT complex; Top view of minimum energy conformers of (e) FP−H2O, (f) FP−MeOH, (g) FP−Me2O, (h) FP−THF, (m) FP−H2S, (n) FP−MeSH, (o) FP−Me2S, and (p) FP−THT complex.

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Figure 7: The correlation plots of electron density at the BCP (ρH···A) vs. the dissociation energy (D0) kcal/mol for the O–H⋅⋅⋅O and O–H⋅⋅⋅S bound complexes. Figure 8: Percentage contribution of NEDA interaction energy (EintNEDA) and dispersion interaction (EintDISP) to the total interaction energy (EintMP2)

Figure 9: The correlation plots of red shift in O−H frequency (∆νO−H) vs. proton affinity (PA) for the O–H⋅⋅⋅O and O–H⋅⋅⋅S bound complexes.

Figure 10: The correlation plots of dissociation energy (D0) vs. proton affinity (PA) of O–H⋅⋅⋅O and O–H⋅⋅⋅S bound complexes.

Figure 11: Correlation plot of (Ect+Eex) (kcal/mol) vs. red shift in O−H frequency (∆νO−H) for O– H⋅⋅⋅O and O–H⋅⋅⋅S bound complexes.

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Table Captions Table 1: Summary of the observed fundamental normal modes in the 2c-R2PI and RIDIR experiments. All numbers are in cm-1. *O–H frequencies computed at the cp-MP2/aug-cc-pVDZ level scaled by a factor of 0.9613.

Table 2: Dissociation energy (D0) including zero point energy (∆ZPE) contribution and basis set superposition error (BSSE) correction for various possible conformations computed using aug-ccpVDZ basis.

Table 3: Red shift in phenolic Ο−Η frequency (∆νΟ−Η) and dissociation energy (D0) including zero point energy contribution computed by various methods at aug-cc-pVDZ level on counterpoise corrected surface. Scaling factors for frequencies are 0.9613, 0.9377, 0.9388, 0.9423, and 0.9586 for MP2, ωB97X-D, LC-ωPBE, M06-2X, and B3LYP respectively. For the ∆νΟ−Η, the numbers in the parenthesis represent deviation from the observed frequency shifts and for the dissociation energies (D0) the numbers in the parentheses represent the % deviation from the D0 value at the cp-MP2 level. Table 4:

Summary of ab-initio computations, QTAIM and NBO analysis. Optimization and

frequency calculations were performed at the cp-MP2/aug-cc-pVDZ level. Frequencies are obtained by scaling the computed frequency with factor 0.9613. Dissociation energy (D0) was calculated after adding zero point energy. QTAIM and NBO analysis was performed upon that optimized geometry at aug-cc-pVDZ basis set. Summary of 2c-R2PI and RIDIR experiments and comparison with computed O−H frequencies.

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Table of Contents (TOC) Image

Unified correlation for O–H⋅⋅⋅ ⋅⋅⋅O ⋅⋅⋅S ⋅⋅⋅ and O–H⋅⋅⋅ ⋅⋅⋅ hydrogen bonding interaction: The red shift in phenolic O−H frequency (∆νΟ−Η) was determined by resonant ion-dip infrared spectroscopy for the 1:1 complexes of p-Flurophenol with various O and S centered solvent. The dissociation energies of these complexes were computed at MP2 level on counterpoise corrected surface using aug-cc-pVDZ basis. A unified correlation for both the O and S centered hydrogen bonded system was obtained between the ∆νΟ−Η vs. the sum of charge transfer (Ect) and exchange (Eex) contribution to the dissociation energy of the complex.

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(42) M. J. Frisch, G. W. T., H. B. Schlegel, G. E. Scuseria, ; M. A. Robb, J. R. C., G. Scalmani, V. Barone, B. Mennucci, ; G. A. Petersson, H. N., M. Caricato, X. Li, H. P. Hratchian, ; et. al. Gaussian 09; Revision C.01; Gaussian, Inc., Wallingford, CT, 2010. (43) Chai, J. D.; Head-Gordon, M. Long-Range Corrected Hybrid Density Functionals with Damped Atom-Atom Dispersion Corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615-6620. (44) Vydrov, O. A.; Scuseria, G. E. Assessment of a Long-Range Corrected Hybrid Functional. J. Chem. Phys. 2006, 125, 074106. (45) Zhao, Y.; Truhlar, D. G. Density Functionals with Broad Applicability in Chemistry. Accounts Chem Res 2008, 41, 157-167. (46) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0,Theoretical Chemistry Institute, University of Wisconsin, Madison 2001. (47) AIM 2000 Version 2.0 ed. 2002. (48) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General Atomic and Molecular Electronic-Structure System. J. Comput. Chem. 1993, 14, 13471363. (49) Varsanyi, G. Assignments of Vibrational Spectra of Seven Hundred Benzene Derivatives. 1974, Wiley: New York. (50) Zhang, B.; Li, C. Y.; Su, H. W.; Lin, J. L.; Tzeng, W. B. Mass Analyzed Threshold Ionization Spectroscopy of p-Fluorophenol Cation and the p-Fluoro Substitution Effect. Chem. Phys. Lett. 2004, 390, 65-70. (51) Wategaonkar, S.; Doraiswamy, S. Laser-induced Fluorescence Spectroscopy of jet cooled p-Aminophenol. J Chem Phys 1996, 105, 1786-1797. (52) Chakraborty, S.; Misra, P.; Wategaonkar, S. Zero Kinetic Energy Spectroscopy of Hydroquinone-water (1 : 1) complex: A Probe for Conformer Assignment. J Chem Phys 2007, 127, 124317. (53) Liptak, M. D.; Gross, K. C.; Seybold, P. G.; Feldgus, S.; Shields, G. C. Absolute pKa Determinations for Substituted Phenols. J. Am. Chem. Soc. 2002, 124, 6421-6427. (54) Fujimaki, E.; Fujii, A.; Ebata, T.; Mikami, N. Autoionization-Detected Infrared Spectroscopy of Intramolecular Hydrogen Bonds in Aromatic Cations. I. Principle and Application to Fluorophenol and Methoxyphenol. J. Chem. Phys. 1999, 110, 4238-4247. (55) Gerhards, M.; Unterberg, C. IR Double-Resonance Spectroscopy Applied to the 4Aminophenol(H2O)1 Cluster. Appl Phys a-Mater 2001, 72, 273-279. (56) Zielke, P.; Suhm, M. A. Raman Jet Spectroscopy of Formic Acid Dimers: Low Frequency Vibrational Dynamics and Beyond. Phys. Chem. Chem. Phys. 2007, 9, 4528-4534. (57) Courty, A.; Mons, M.; Dimicoli, I.; Piuzzi, F.; Brenner, V.; Millie, P. Ionization, Energetics, and Geometry of the Phenol-S Complexes (S = H2O, CH3OH, and CH3OCH3). J Phys Chem A 1998, 102, 4890-4898. (58) Bader, R. F. W.; Nguyendang, T. T. Quantum-Theory of Atoms in Molecules Dalton Revisited. 1981, 14, 63-124. (59) Bader, R. F. W. A Quantum-Theory of Molecular-Structure and Its Applications. 1991, 91, 893-928. (60) Koch, U.; Popelier, P. L. A. Characterization of C-H-O Hydrogen-Bonds on the Basis of the Charge-Density. J. Phys. Chem. 1995, 99, 9747-9754. (61) Boys, S. F.; Bernardi, F. Calculation of Small Molecular Interactions by Differences of Separate Total Energies - Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553-566. (62) Salvador, P.; Szczesniak, M. M. Counterpoise-Corrected Geometries and Harmonic Frequencies of N-Body Clusters: Application to (HF)n (n=3,4). J. Chem. Phys. 2003, 118, 537-549. (63) Valiron, P.; Mayer, I. Hierarchy of Counterpoise Corrections for N-Body Clusters: Generalization of the Boys-Bernardi Scheme. Chem. Phys. Lett. 1997, 275, 46-55. (64) Platts, J. A.; Howard, S. T.; Bracke, B. R. F. Directionality of Hydrogen Bonds to Sulfur and Oxygen. J. Am. Chem. Soc. 1996, 118, 2726-2733. 35 ACS Paragon Plus Environment

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O-H···S hydrogen bonds conform to the acid-base formalism.

Hydrogen bonding interaction between the ROH hydrogen bond donor and sulfur atom as an acceptor has not been as well characterized as the O-H···O inte...
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