Article pubs.acs.org/JPCA

Acid−Base Formalism in Dispersion-Stabilized S−H···Y (YO, S) Hydrogen-Bonding Interactions Aditi Bhattacherjee,†,∥ Yoshiyuki Matsuda,*,‡,§ Asuka Fujii,‡ and Sanjay Wategaonkar*,† †

Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki-Aoba, Aoba-ku, Sendai 980-8578, Japan § Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai 980-8576, Japan ‡

S Supporting Information *

ABSTRACT: The role of sulfhydryl (S−H) group as hydrogen bond donor is not as well studied as that of hydroxyl (O−H). In this work we report on the hydrogen-bonding properties of S−H donor in 1:1 complexes of H2S with diethyl ether (Et2O), dibutyl ether (Bu2O), and 1,4-dioxane (DO). The complexes were prepared in supersonic jet and investigated using infrared predissociation spectroscopy based on VUV photoionization detection. The IR spectra of all the complexes showed the presence of a broad, intensity-enhanced, and red-shifted hydrogen-bonded S−H stretching transition. The S−H stretching frequency was red-shifted by 46, 63, and 49 cm−1 in H2S−Et2O, H2S−Bu2O, and H2S−DO complexes, respectively, suggesting that all the complexes are S−H···O bound. Computationally, two different S−H···O bound structures, namely, “coplanar” and “perpendicular”, were obtained as the minimum energy structures for these complexes at the MP2/6-311++G** level, with the former being the global minimum. However, with Dunning-type basis sets (aug-cc-pVDZ and aug-ccpVTZ) only the perpendicular structures were found to be stable at the MP2 level. The large widths of the bound S−H stretch observed in the experimental spectra (fwhm of 35 to 80 cm−1) were attributed to inhomogeneous broadening due to multiple conformations of the alkyl chains in the coplanar and perpendicular structures populated in the jet. The frequency shifts in the hydrogen-bonded S−H stretching mode as well as the bond dissociation energies of all S−H···Y (YO,S) complexes of H2S, which includes the H2S dimer and H2S−methanol (H2S−MeOH) complexes reported in our previous work (ChemPhysChem 2013, 14, 905−914), were found to scale linearly with the proton affinity of the acceptor molecule. In this regard the S−H group, like O−H, is found to conform to the widely accepted acid−base nature of hydrogen-bonding interactions.

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of the Met side chain.22 Besides, the smallest sulfur containing molecule, namely, H2S has also been identified an important biological regulator and signaling agent associated with innumerable physiological processes23−30 and pathological conditions.31,32 Recent studies also reveal that although hydrogen-bonding interaction involving S as acceptor is primarily dominated by dispersive forces, these are not any weaker than interactions involving O, which have a stronger electrostatic component.6,33 One of the earliest systematic investigations of hydrogen bonding involving S−H as donor was reported on a set of thiophenols in solutions of carbon tetrachloride, chloroform, dioxane, acetonitrile, acetone, and benzene.34,35 The red shift in the S−H stretching frequency in all solvents with respect to the monomeric stretching frequency, on the order of 15 to 50 cm−1, was presented as evidence of hydrogen bonding of the S−H···O, S−H···N, and S−H···π type. A new red-shifted feature was also reported to appear with increasing concen-

INTRODUCTION Sulfur-centered hydrogen bonding has generated much interest in the past few years. The low electronegativity of sulfur (2.58 on the Pauling scale) was thought to make it a weak contender in hydrogen-bonding interactions compared to oxygen (3.44 on the same scale).1 Nevertheless, sulfur is a key player in numerous hydrogen bonded (H-bonded) systems spanning molecular complexes,2−7 organic crystals,8−15 and peptides.16,17 Its role in stabilization of protein structures and selective binding of ligands at the active site of proteins is profound.18,19 It is a constituent atom of two naturally occurring amino acids, cysteine and methionine. The S−H group in cysteine exhibits dual H-bonding property; that is, it can function as both donor and acceptor of H-bonds.20,21 Microwave spectroscopy of neutral cysteine in the gas phase showed the presence of six conformers, each characterized by the presence of intramolecular H-bonds such as S−H···N, S−H···O, and N−H···S, which involve the S−H group as both donor and acceptor.21 Methionine on the other hand, has a sulfur atom that can act only as an acceptor. IR spectroscopy of two methionine containing dipeptides in the gas phase revealed the presence of strong N−H···S H-bonds, which could induce the local folding © XXXX American Chemical Society

Received: December 7, 2014 Revised: January 21, 2015

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are directly ionized by a single photon of VUV light. The resultant mass-selected ion current is monitored by a microchannel plate detector mounted at the end of a Wiley− McLaren type time-of-flight mass spectrometer,68 while the IR laser is scanned. In the event that the IR frequency is in resonance with any of the vibrational levels of the complex it predissociates, resulting in depletion of the ion signal. Such depletions are signatures of the frequency of the normal modes of the complex in the ground state. The IR spectra of the complexes were recorded in the regions of S−H and C−H fundamental stretching frequencies. The complexes of H2S with Et2O, Bu2O, and DO were prepared by the supersonic jet expansion technique.69 A 5% mixture of H2S in helium was made to flow over a reagent bottle containing the ether and allowed to expand through a 500 μm orifice into a chamber with base pressure 1.3 × 10−6 Torr. The skimmed beam was irradiated with VUV laser and a tunable IR laser. VUV light of 118 nm was generated by focusing the third harmonic (355 nm) of an Nd:YAG laser (Continuum Surelite SL III-10) in a static gas cell filled with 200 Torr of a 10:1 mixture of Ar and Xe. The IR beam was generated by difference frequency generation of the second harmonic of an Nd:YAG laser (Continuum PL8000) and the fundamental output of a dye laser (Continuum ND 6000, DCM dye and R640 dye mixture). The two beams were spatially overlapped in a counter propagating manner. The IR pulse arrived ∼20 ns prior to the VUV pulse. The experimental spectra were compared with the computed ones obtained for the equilibrium structures optimized at the MP2 level using both Pople-type (6-311++G**) and Dunningtype (aug-cc-pVDZ and aug-cc-pVTZ) basis sets. MP2 level was chosen for the calculations because it accounts for dispersion forces that are known to be the dominant component in hydrogen-bonding interactions involving sulfur.70 The equilibrium structures were examined by the harmonic vibration frequency analysis to obtain the H-bonded stretching frequencies. The dissociation energies were obtained by correcting for the zero-point energy as well as the basis set superposition error.71 Furthermore, the MP2 energies at the DZ-TZ level were extrapolated to the complete basis set using the two point extrapolation formula of Helgaker et al. to determine the binding energy at the complete basis set limit.72 The ab initio calculations were performed using the Gaussian 09 suite of programs.73 Electron density topology maps for the optimized geometries were obtained using Bader’s quantum theory of atoms in molecules (QTAIM) theory.74 The charge density ρ(r) and the Laplacian of the charge density ∇2ρ(r) were determined at the bond critical point using the AIM2000 package.75 The delocalized natural bonding orbitals leading to the formation of the H-bond were generated using NBO analysis to obtain a physically intuitive picture of the interaction.76 The programs NBO 5.0 and NBOView were used for this purpose.77 Energy decomposition was carried out with natural energy decomposition analysis (NEDA) to find the relative contribution of different components to the overall binding energy.62 NEDA calculations were carried out with the NBO 5.9 program linked to the GAMESS package.78

tration in CCl4, which was attributed to either S−H···S or S− H···π intermolecular hydrogen bonding. S−H···Y hydrogen bonding is not uncommon in crystals as well.9,36 In a crystal structure data analysis, Allen and coworkers found that 70.3% of the S−H donors that occur with neighboring acceptor groups such as O, S, N, Cl, and F participate in hydrogen bonding.37 In the gas phase, reports are fewer and mostly of S acceptor in aromatic systems,5−7 haloforms,38 and amino acids.22 Very few experimental reports are available of SH as hydrogen-bond (H-bond) donor in the gas phase.6,39−43 Besides, there are a few computational studies on the nature of S−H acting as donor toward O,44,45 S,45 N,46 and π47−52 acceptors. The electrostatic model of the H-bonds entails that the strength of the interaction increases with increasing proton affinity of the acceptor for a given H-bond donor.53 In the case of strong H-bond donors such as O−H and N−H, spectral parameters such as the red shift in the electronic/vibrational frequencies that reflect the H-bond strength are empirically seen to scale with the proton affinity of the acceptor or pKa of the donor.54−61 This is the well-known acid−base formalism of hydrogen-bonding interactions. However, it is now recognized that the energy associated with the H-bond is a result of contributions from several components such as electrostatics, polarization, exchange repulsion, charge transfer, and dispersion.62 The role of dispersion interactions in stabilizing Hbonds involving sulfur cannot be overemphasized; in the case of several molecular complexes it accounts for >50% of the overall binding energy.6,40,60,63 The relative contribution of electrostatics in SH versus OH donors is elegantly captured in vibrational Stark spectroscopy measurements that reveal frequency shifts in O−H···π H-bonds to scale linearly with the electric field strength,64 whereas nonlinear effects come into play in S−H···π interactions,65 consistent with the polarizabilities of the X−H bonds. It is of interest to determine the behavior of a weakly acidic donor such as S−H with respect to the varying proton affinity of the acceptor, especially because such hydrogen-bonding interactions are expected to be predominantly dispersion-stabilized. So far, the S−H···Y hydrogen bonds have been mainly studied for carbonyl group in ketones, carboxylate, and amide acceptors37,66 in cold matrices. One of the limitations of such studies is that many a times the shifts in the SH stretching frequencies40 due to H-bonding are of similar magnitude as the matrix-induced shifts. Apart from the importance of studying the H-bonding interaction of SH group previously outlined, the fundamental interest in the hydrogen bonding properties of H2S stems from its similarity with water as well as its representation of the smallest molecular prototype for a donor/ acceptor H-bonding functionality built on sulfur. Here we report vibrational spectroscopy of S−H···O hydrogen-bonded complexes of H2S with a set of ethers in gas phase. The ether molecules chosen for this study were diethyl ether (Et2O), dibutyl ether (Bu2O), and 1,4-dioxane (DO). With the help of our previous work on (H2S)2 and H2S−MeOH complexes,40 this article seeks to characterize the response of the donor S−H group in H2S to the varying proton affinity of the acceptor solvent molecule in S−H···Y (YO, S) H-bonded complexes.

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RESULTS AND DISCUSSION a. IR Predissociation Spectra. The top trace (a) in Figures 1−3 shows the IR predissociation spectra of H2S−Et2O, H2S− Bu2O and H2S−DO complexes, respectively. A strong and broad ion signal dip was identified a little below 2600 cm−1 in

METHODS The H-bonded complexes of H2S with the ethers were studied by VUV-ionization-detected IR predissociation spectroscopy (VUV-ID-IRPDS).67 In brief, in this technique, neutral clusters B

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bonding interaction. Because of the large width and associated noise in the H-bonded S−H stretching peak profiles, they were fitted to a Gaussian function to smoothen the features and obtain the central S−H stretching frequencies. The fitting rendered 2575, 2558, and 2572 cm−1 as the H-bonded S−H stretching frequencies for the H2S−Et2O, H2S−Bu2O, and H2S−DO complexes, respectively, which were red-shifted with respect to the mean of the free ν1 and ν3 S−H stretches of H2S monomer by 46, 63, and 49 cm−1, respectively. The full width at half-maximum (fwhm) of the peaks were 46, 72, and 39 cm−1 for the H2S−Et2O, H2S−Bu2O, and H2S−DO complexes, respectively. In comparison with the fwhm of 13 cm−1 observed for the bound S−H stretch of the H2S dimer,40 the higher fwhm in the aforementioned complexes reflect inhomogeneous broadening of the bound S−H stretching transition, which may arise due to the presence of multiple species with different orientations of the free alkyl chains (in case of Et2O and Bu2O) with respect to the intermolecular H-bond associated with two distinctly different conformers, vide infra. The limitation of the VUV single-photon-ionization-based spectroscopic techniques is that they lack preselection of a specific conformer through S1−S0 excitation. Therefore, the resulting IR spectra may contain inhomogeneously broadened features due to all possible conformers of the molecule/complex present in the beam. The peaks between 2800 and 3050 cm−1 in the spectra arise from the C−H stretches of the alkyl groups of the ether molecules. The relatively large width of the C−H stretching frequencies in the H2S−Bu2O complex compared to the other two is due to overlapping transitions owing to the large number of C−H oscillators present in Bu2O (18) in comparison with Et2O (10) and DO (8). The transitions due to the free S−H stretch could not be observed in our experiments due to its very low oscillator strength, which is clearly reflected in the computed vibrational spectra as a small feature close to 2600 cm−1 (traces b and c of Figures 1−3). b. Computed Structures, Binding Energies, And Vibrational Frequencies. The minimum energy structures of the H2S−ether complexes were optimized at the MP2 level using Pople-type (6-311++G**) and Dunning-type (aug-ccpVDZ) basis sets. With Pople-type basis set two types of structures, namely, coplanar and perpendicular, were obtained, with the former one being the global minimum. With the Dunning-type basis set, only the perpendicular structure was obtained. Figure 4 shows both the minimum energy structures obtained at MP2/6-311++G** along with the scheme used for numbering the atoms. In the coplanar conformer, the S−H···O H-bond in the complex was found to be nearly coplanar with the Cα−O−Cα′ group of the ether molecule, whereas in the perpendicular conformer the H-bond orientation was perpendicular. This is evident from the (S)H−O−Cβ−Cα dihedral angles (H16−O1−C8−C2 for H2S−Et2O, H28−O1−C8−C2 for H2S−Bu2O) and H15−O9−H13−C2 dihedral angle for H2S−DO listed in Table 1. The dihedral angles for the H2S− Et2O, H2S−Bu2O, and H2S−DO complexes were 178.9, 179.8, and 171.5° for the coplanar structures and 108.3, 105.4, 139.6° for the perpendicular structures, respectively. Vibrational frequency analysis confirmed that both the coplanar and perpendicular structures were minimum energy structures. Table 1 lists the relevant geometrical parameters for the coplanar and perpendicular structures that are illustrated in Figure S1 in the SI. The (S)H···O (r) and S···O (R) distances were slightly greater (∼0.02 to 0.07 Å) in the coplanar structures of the H2S−Et2O and H2S−Bu2O complexes

Figure 1. (a) IR predissociation spectrum of H2S−Et2O complex in comparison with the computed IR stick spectra of the (b) coplanar and (c) perpendicular structures computed at the MP2/6-311++G** level and scaled by a factor of 0.927. Inset shows optimized structures corresponding to the computed IR spectra.

Figure 2. (a) IR predissociation spectrum of H2S−Bu2O complex in comparison with the computed IR stick spectra of the (b) coplanar and (c) perpendicular structures computed at the MP2/6-311++G** level and scaled by a factor of 0.927. Inset shows optimized structures corresponding to the computed IR spectra.

Figure 3. (a) IR predissociation spectrum of H2S−DO complex in comparison with the computed IR stick spectra of the (b) coplanar and (c) perpendicular structures computed at the MP2/6-311++G** level and scaled by a factor of 0.927. Inset shows optimized structures corresponding to the computed IR spectra.

all of the complexes. The symmetric and antisymmetric stretches of H2S monomer in the gas phase have been reported to occur at 2614 and 2628 cm−1, respectively.79−81 The ion depletion peaks below 2600 cm−1 were attributed to the Hbonded S−H stretching frequency in the complexes. The appearance of a red-shifted S−H stretching frequency with respect to the free S−H stretch in H2S monomer in all three complexes is an indicator of an S−H···O type of hydrogenC

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Figure 4. Front and side views of the optimized structures of the S−H···O bound complexes of H2S−Et2O, H2S−Bu2O, and H2S−DO obtained at the MP2/6-311++G** level. The atom numbering scheme is also indicated.

Table 1. Geometrical Parameters of the Coplanar Structures (I) and the Perpendicular Structures (II) Obtained at MP2/6-311+ +G**a H2S−Et2O

H2S−Bu2O

H2S−DO

geometrical/topological parameters

coplanar (I)

perpendicular (II)

coplanar (I)

perpendicular (II)

coplanar (I)

perpendicular (II)

r(H···O) (Å) R(S···O) (Å) ΔrS−H (Å) observed νS−H (cm−1) computed νS−H (cm−1) observed ΔνS−H (cm−1) computed ΔνS−H (cm−1) intensity enhancement ratio α (deg) β (deg) H−O−C/H−C dihedral (deg) ρ(r) ∇2ρ(r)

2.1290 3.4461 0.0059

2.1016 3.3889 0.0084

2.1125 3.4343 0.0066

2.0935 3.3679 0.0092

2.0944 3.3539 0.0059

2.1029 3.4045 0.0071

2523 (52)

2549 (9)

2516 (42)

2558 (14)

2575 2556 (19)

2558

46 65 90 166.7 164.1 178.9 0.0169 0.0638

2572

63 98 102 159.1 89.5 108.3 0.0209 0.0649

72 94 167.9 166.8° 179.8 0.0175 0.0663

2542 (30) 49

105 96 156.6 84.0 105.4 0.0213 0.0664

63 87 154.6 140.4 171.5 0.0192 0.0681

79 93 162.3 120.6 139.6 0.0201 0.0649

ΔrS−H = increase in the S−H bond length. Computed νS−H is the scaled S−H stretching frequency (scaling factor 0.927), and the number in parentheses indicates its deviation from the observed frequency. Δν = red shift in the H-bonded S−H stretching frequency. Intensity enhancement ratios of the bound S−H stretch are relative to the symmetric S−H stretch in H2S at the same level. Dihedral angles specified include H16−O1− C8−C2 for H2S−Et2O, H28−O1−C8−C2 for H2S−Bu2O, and H15−O9−H13−C2 for H2S−DO. QTAIM topological parameters electron density ρ(r) and its Laplacian ∇2ρ(r) at the (3,−1) BCP. a

the coplanar structures, while the corresponding angles were 89.5, 84.0, and 120.6°, respectively, for the perpendicular ones. The salient difference between the two structures was the relative increase in the S−H bond length upon H-bond formation and the resultant red shift in the bound S−H stretching frequency. The S−H bond was lengthened by 5.9 to 6.6 mÅ upon H-bond formation for the coplanar geometry, and the concomitant red shift in S−H stretching frequency was on the order of ∼63−72 cm−1. In the case of the perpendicular geometry the increase in S−H bond length (ΔS−H) was almost 1.5 times that for the coplanar complexes (7.1−9.2 mÅ).

compared to the perpendicular ones. For the H2S−DO complex, the (S)H···O (r) distance was marginally higher (0.008 Å) in the perpendicular complex. In both conformers, the (S)H···O distances were in the range of 2.094 to 2.129 Å, which were much shorter than the sum of the van der Waals radii of H and O (2.72 Å) atoms.82 The S−H−O bond angle (α) is fairly linear in both the structures and seen to lie between 155 and 168°. The angle (β) between the donor S−H bond and the angle bisector of the Cα−O−Cα′ bond of the acceptor moiety was found to be 164.1, 166.8, and 140.4° in the H2S− Et2O, H2S−Bu2O, and H2S−DO complexes, respectively, for D

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The Journal of Physical Chemistry A Table 2. ZPE- and BSSE-Corrected S−H···O Bond Dissociation Energies (kcal mol−1) of the (I) Coplanar and (II) Perpendicular Structures Computed at Various Levels of Theorya H2S−Et2O

a

H2S−Bu2O

H2S−DO

level/basis set

I

II

I

II

I

II

MP2/6-311++G** MP2/aug-cc-pVDZ MP2/aug-cc-pVTZ complete basis set

1.56 0.76* 2.17* 3.38

1.45 2.65 3.14 3.87

1.82 0.12* 2.15* 3.63

1.69 3.03 3.56 4.23

1.50 0.47* 1.98* 3.16

1.29 2.16 2.65 3.20

Values marked * correspond to single-point energies computed for the corresponding coplanar geometry obtained at the MP2/6-311++G** level.

0.927 was applied to match the stretching frequencies of the H2S monomer computed at the same level of theory with its experimentally known values.40 While both the structures corresponded reasonably well in the C−H region, the bound S−H frequency was found to be slightly more red-shifted in the case of the perpendicular structure compared to the coplanar one. Taking all three complexes into account, the mean absolute deviation of the computed frequencies from the experimentally observed values was found to be 14 cm−1 for the coplanar structures and 41 cm−1 for the perpendicular structures. The difference between these two numbers is smaller than the fwhm of the observed S−H stretching transitions. It must be noted that frequency mismatch by tens of cm−1 are routinely encountered in electronic structure calculations. This uncertainty and the large fwhm of the bonded S−H stretches in the observed spectra (35 to 80 cm−1) rendered the categorical assignment of the structures difficult. Furthermore, because the dissociation energies of the two conformers obtained at the CBS limit were within a very small margin (≤0.6 kcal mol−1), it is likely that both conformers could exist in the beam. The inhomogeneous broadening owing to the presence of multiple conformations due to (a) coplanar and perpendicular structures of the complexes and (b) multiple orientations of the methylene groups of the alkyl chains could be a possible explanation for the large fwhm of the H-bonded S−H stretches. c. QTAIM, NBO, and NEDA Analyses. QTAIM analysis revealed the presence of a (3,−1) bond critical point (BCP) along the S−H···O bond path for all of the complexes, confirming the presence of H-bond. The relevant topological parameters obtained from QTAIM analysis are summarized in Table 1. The electron densities at the BCP for all S−H···O bound complexes were within the acceptable range for the existence of H-bond.83,84 Figure S5 in the Supporting Information shows the molecular graphs obtained upon QTAIM analysis of the wave functions furnished for the coplanar and perpendicular conformers at the MP2/6-311+ +G** levels. The electron densities at the BCP were 0.0169/ 0.0209, 0.0175/0.0213, and 0.0192/0.0201 for the coplanar/ perpendicular structures of H2S−Et2O, H2S−Bu2O, and H2S− DO, respectively. NBO analysis revealed that the primary orbitals leading to the S−H···O H-bond interaction in these complexes include the lone pair (LP)-containing orbitals of the acceptor O and the antibonding orbital (BD*) of the donor S−H bond. The LPcontaining orbitals have been labeled LP1 and LP2 depending on their state of hybridization; LP1 was found to reside in a ∼sp1.5 orbital, whereas the LP2-containing orbital was exclusively or close to exclusively p in character. The overlapping natural orbitals in the H-bonded complexes obtained at the MP2/6-311++G** level are illustrated in Figure 5, and the relevant parameters are given in Table 3.

This resulted in greater red shifts in the computed S−H stretching frequency upon hydrogen bonding (∼79 to 105 cm−1) compared to those predicted for the coplanar structures. The hundred-fold enhancement in the IR absorption cross sections reaffirms the S−H···O H-bonding interactions in these complexes. As previously mentioned, only the perpendicular structures were obtained for all three complexes at the MP2/ aug-cc-pVDZ level, the geometrical parameters for which are provided in the Supporting Information (Table S1) for the sake of completion. In these structures, the H···O (r) and the S···O (R) bond distances were much shorter compared to those of the perpendicular structure obtained using the Pople basis set. Also, the predicted red shifts in the S−H stretching frequency were almost two times those predicted for the coplanar structures (MP2/6-311++G**) and much higher than those experimentally observed (by 70 to 105 cm−1). These structures will not be discussed any further for two reasons. First, the IR spectra were in poor agreement with those predicted at this level; second, it predicts only the perpendicular structure, which excludes the possibility of the coplanar conformer, as predicted by the Pople’s triple-ζ basis set. Although it is known that the aug-cc-pVDZ basis set is better suited for noncovalently bonded complexes, it must be noted that it was not able to reproduce the observed IR spectra in this case. Table 2 summarizes the dissociation energies of the S−H···O bound complexes computed at the MP2 level with 6-311+ +G**, aug-cc-pVDZ, and aug-cc-pVTZ basis sets. The dissociation energies calculated using the Dunning-type basis sets were used to extrapolate them to the CBS limit. For the coplanar structures, single-point energies were calculated using the Dunning-type basis sets for the geometry optimized at the MP2/6-311++G** level. The single-point energies are marked by * in Table 2. At the MP2/6-311++G** level, the relative stability of the coplanar structure with respect to the perpendicular was higher by ∼0.1 to 0.2 kcal mol−1. When the MP2 single-point energies were extrapolated to the CBS limit, the stability ordering of the two types of structures was reversed by the energy gap ∼0.04 to 0.6 kcal mol−1. To locate the two conformers on the energy landscape, the relaxed potential energy surface was scanned at both MP2/6-311+ +G** and MP2/aug-cc-pVDZ levels. Results of the analysis are presented in the Supporting Information (Section S.1, Figures S2−S4) and will not be discussed here. However, it must be noted that the binding energy values were unable to establish any of the structures as the undisputed global minimum. Structural assignment of H-bonded conformers is usually based on two considerations; their relative dissociation energies and agreement between the observed and computed IR spectra. The traces (b) and (c) of Figures 1−3 show the computed IR spectra of the coplanar and perpendicular conformers obtained at the MP2/6-311++G** level, which furnished both the structures as the minimum energy structures. A scaling factor of E

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Figure 5. Front and side views of the NBOs participating in the S−H···O H-bonding interaction in the complexes of H2S−Et2O, H2S−Bu2O, and H2S−DO. The NBOs were computed for the wave functions of the coplanar and perpendicular structures evaluated at the MP2/6-311++G** level. Results of the analysis are provided in Table 3

Table 3. Summary of the NBO Analyses for the (I) Coplanar and (II) Perpendicular Structures Obtained at the MP2/6-311+ +G** Level for the H2S−Et2O, H2S−Bu2O, and H2S−DO Complexesa complex

a

structure

H2S−Et2O

I II

H2S−Bu2O

I II

H2S−DO

I II

interacting orbitals LP(1) LP(2) LP(1) LP(1) LP(2) LP(1) LP(1) LP(2) LP(1)

O1 O1 O1 O1 O1 O1 O9 O9 O9

→ → → → → → → → →

BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1) BD*(1)

hybridization of interacting lone pair

second-order perturbation energy

ΔBO

1 1.40

4.46 4.40 1.26 4.76 4.54 1.15 4.20 2.56 2.91

0.0056 0.0075

H8−S9 H8−S9 H8−S9 H8−S9 H8−S9 H8−S9 H15−S16 H15−S16 H15−S16

sp p s1p1.42 s1p1.43 p s1p1.45 s1p1.36 s1p58.62 s1p1.53

0.0060 0.0078 0.0054 0.0064

ΔBO denotes the change in bond order of the H-bond donor in the complexes.

Table 4. Summary of the Energy Decomposition Analysis Based on the Optimized Structures of the H2S Dimer, H2O Dimer, H2S−MeOH, H2S−Et2O, H2S−Bu2O, and H2S−DO Complexes Obtained at the MP2/6-311++G** Levela complex

electrical (ES + POL + SE)

charge transfer (CT)

core (EX + DEF − SE)b

interaction energy (NEDA)

interaction energy (MP2)

dispersion

% dispersion

H2S−H2S H2O−H2O H2S−MeOH H2S−Et2O (I) H2S−Et2O (II) H2S−Bu2O (I) H2S−Bu2O (II) H2S−DO (I) H2S−DO (II)

−3.46 −10.66 −8.53 −9.30 −10.77 −9.84 −11.44 −9.60 −9.40

−3.37 −8.17 −8.05 −11.31 −13.71 −11.75 −14.12 −10.04 −11.47

6.74 15.23 15.57 20.24 24.81 21.26 26.13 19.13 20.91

−0.09 −3.60 −1.01 −0.37 0.33 −0.33 0.57 −0.51 0.04

−0.87 −4.44 −2.61 −2.84 −2.67 −3.12 −2.97 −2.63 −2.35

−0.78 −0.84 −1.60 −2.47 −3.00 −2.79 −3.54 −2.12 −2.39

89 19 61 87 113 89 119 81 102

a

Net interaction energy was partitioned into charge transfer (CT), electrostatics (ES), polarization (POL), exchange (EX), and dispersion (DP) components. In the case of the complexes with ethers, the first row (I) represents the coplanar structures whereas the second row (II) represents the perpendicular structures. bSE: self-energy term; DEF: deformation term.

While the coplanar structures showed only one lone pair orbital of O (LP1) interacting with the BD* of the donor S−H bond, the perpendicular structures showed both LP1 and LP2 to be involved in the interaction. The perpendicular structures yielded nearly 1.2 to 1.3 times higher second-order interaction energies compared to the coplanar structures. The bond order of the S−H bond was seen to decrease upon hydrogen bonding in both cases, and the decrease was found to be marginally

greater in the case of the perpendicular structure (∼0.0010 to 0.0019) compared to the coplanar. This was consistent with the higher red shift in the computed SH stretching frequency in the perpendicular structure. For various S−H···O bonds investigated in this work the dispersion component was calculated by subtracting the total interaction energy obtained using NEDA from the BSSEcorrected energy obtained at the MP2 level. The NEDA F

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type of structure. The S−H···O bound complexes showed increasing contribution of dispersion stabilization in the order MeOH < DO < Et2O < Bu2O, which is in agreement with the relative solvent polarizability. d. Acid−Base Formalism. A final point concerning the sulfur-centered H-bonds is how well the strength of the SH···Y H-bond correlates with the proton affinity of the solvents. For polar bonds such as N−H and O−H a good empirical correlation of the associated red shift and binding energies with the proton affinity of Y has indeed been observed.56−58,60 It is also known that the strength of H-bond in X−H···Y is reflected in the red shift observed in the X−H stretching frequency upon H-bonding.54,85,86 Figure 7 shows the correlation plots of the observed red shifts in the S−H stretching frequency and the dissociation energies (CBS limit) with the proton affinity of the H-bond acceptor for the H2S dimer, H2S−MeOH, H2S−Et2O, H2S−Bu2O, and H2S−DO complexes. The correlation between the SH red shift and the proton affinity is reasonable (linear correlation coefficient of 0.8886). Regarding the correlation between dissociation energies and proton affinity two points can be made. First, the correlation is very good (correlation coefficient of 0.9738) for the entire set of acceptors, that is, both the S and O atom containing solvents. Second, if one excludes the H2S dimer complex (in which the acceptor atom is sulfur) from the fit then the correlation for the set of O containing solvents becomes excellent (0.9989). This is consistent with the observation previously reported that the correlations of the dissociation energies and the red shifts with the proton affinity are acceptor-atom-specific; that is, for the oxygen- and sulfur-containing solvents, two separate correlations were observed.60 This was attributed to the fact that the oxygen-centered H-bonds were dominated by the electrostatic interaction, as opposed to the sulfur centered H-bonds, which are dispersion-dominated. In the present context, data are not available for more S−H···S-bonded complexes to establish this trend. Nevertheless, the correlations observed in this work echo the similarity in behavior of a weakly acidic S−H donor with conventional H-bond donors such as O−H and N−H; that is, the S−H···Y interaction also conforms to the acid−base formalism despite having a large contribution from the dispersion interaction.

interaction energy has contributions from various interactions such as electrical, charge transfer, and exchange repulsion, which are also given in Table 4. It can be seen that dispersion interaction has the stabilizing effect in the case of the perpendicular structures, that is, without its contribution the complex would be unstable. This is reported as >100% of the total binding energy in Table 4. In the case of the coplanar structures also the dispersion component is high and corresponds to ∼80% of the total binding energy. The relative contribution of electrostatics (broadly speaking of the net NEDA interaction energy) versus dispersion in the S−H···Y (YO, S) complexes is illustrated in Figure 6. The values for

Figure 6. Contributions of net NEDA interaction energy (inclusive of electrostatics, polarization, charge transfer, and exchange repulsion) and dispersion interaction toward the total binding energy of the H2S dimer, H2O dimer, and coplanar and perpendicular conformers of the H2S−Et2O, H2S−Bu2O, and H2S−DO complexes evaluated at the MP2/6-311++G** level of theory. The solid bars represent the percentage dispersion contribution, whereas the checkered bars represent the percentage NEDA interaction energy; negative contribution indicates destabilizing effect.

the water dimer at the same level of theory are also included for comparison. It can be seen that whereas the water dimer receives 81% of its stability from electrostatic interactions, the H2S dimer has the largest contribution from the dispersion energy (89%), even though both possess an open-chain linear

Figure 7. (a) Plot of the red shifts in S−H stretching frequency in S−H···Y (YS, O) H-bonded systems with the proton affinity of the acceptor; linear correlation coefficient = 0.8886. The error bars denote the fwhm of the H-bonded S−H stretching transition. (b) Plot of the dissociation energies (CBS limit) with the proton affinity of the acceptor; linear correlation coefficient = 0.9738 for the entire set of acceptors (thick line). The plot with only the O-atom-based acceptor solvents (i.e., without the H2S dimer) gave a better fit (dotted line); linear correlation coefficient = 0.9989. In the case of the H2S−ether complexes for which both the coplanar and perpendicular structures may exist, the mean of their total CBS limit energies were plotted. G

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being largely stabilized by the dispersion interaction. With the growing literature on the importance and omnipresence of hydrogen-bonding interactions involving S, we expect a molecular level understanding of such interactions to assist in bioengineering, pharmaceutical, and therapeutic applications.

This observation can be interpreted on the basis that the proton affinity of an atom/molecule is a combined effect of its electronegativity and polarizability. An indirect evidence of this is the increasing proton affinity of the rare gas atoms with increasing size (and hence polarizability). Another example is the higher gas-phase proton affinity of H2S (705 kJ mol−1) and MeSH (773.4 kJ mol−1) compared to H2O (691 kJ mol−1) and MeOH (754.3 kJ mol−1), respectively, despite the lower electronegativity of S (2.58) compared to O (3.44).87 Similarly, gas-phase measurements on the proton affinities of amines establish the order NH3 < CH3NH2 < (CH3)2NH < (CH3)3N as the methyl groups render the base more polarizable by an approaching proton.88 Such an alkyl substituent effect on proton affinity is also found in the case of O-based acceptors.89,90 In conventional hydrogen bonding, which involves sharing a proton between two electronegative atoms, a major component of the interaction strength is due to electrostatics and charge transfer. The dispersion interaction is another key contributor that scales with the polarizability of the acceptor atom/molecule, leading to stronger interaction. For weakly polarized bonds such as S−H, even though the partial positive charge on the H atom is less, the proton can effectively polarize the diffuse electron density of the acceptor atom/ group. Therefore, the more polarizable the acceptor molecule (read proton affinity) the stronger the H-bonding interaction. In such interactions the red shifts in the XH stretching frequency arise due to the orbital interactions between the lone pair orbitals of the acceptor atom and the antibonding XH orbital, and the overall H-bonding strength is dominated by the dispersion interaction. Hence it is not surprising that even for dispersion-dominated S−H···Y hydrogen-bonding interactions the interaction strength can be effectively modulated by varying the proton affinity of the acceptor (or pKa of the donor), and this effect manifests as a good linear correlation of the red shift in S−H stretching frequency with the proton affinity of the acceptor.

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ASSOCIATED CONTENT

S Supporting Information *

Geometrical parameters, relaxed potential energy surface (PES) scans, and QTAIM analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Y.M.: Tel: 81-22-795-6573. Fax: 81-22-795-6785. E-mail: [email protected]. *S.W.: Tel: 91-22-2278-2259. Fax: 91-22-2278-2106. E-mail: [email protected]. Present Address

∥ A.B.: Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partly supported by the Grant-in-Aid for Scientific Research (Project No. 26108504 on Innovative Area [2507] from MEXT Japan and No. 26288002 from JSPS).

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REFERENCES

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CONCLUSIONS The characteristics of H-bonded complexes of H2S with a set of ethers (Et2O, Bu2O, and DO) with varying proton affinities were investigated using the VUV-ID-IRPDS technique. The observed S−H stretching transition was red-shifted in all of the complexes, indicating that they were bound by S−H···O Hbond. The IR transitions were also found to be uncharacteristically broad. Computationally, two types of minimum energy structures, namely, coplanar and perpendicular, were obtained for the S−H···O bound complexes at the MP2/6-311++G** level. Because the binding energies of the two structures were found to be comparable and the harmonic vibrational frequencies were also similar, it is likely that both structures exist in the molecular beam, leading to inhomogeneous broadening of the H-bonded S−H stretching transitions. Energy decomposition analysis showed that the H-bonded complexes of H2S are predominantly stabilized by dispersion interactions. The magnitudes of the red shift and the computed binding energies (at the CBS limit) of the complexes were seen to scale linearly with the proton affinity of the acceptor. These observations along with our previous work on the S−H···Y (YO, S) bonded complexes of H2S dimer and H2S−MeOH complex40 suggest that the S−H donor in H2S shows no different behavior from stronger H-bond donors like O−H and N−H in regard to the acid−base formalism of H-bonds despite H

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DOI: 10.1021/jp511904a J. Phys. Chem. A XXXX, XXX, XXX−XXX