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O-H · · · O versus O-H · · · S Hydrogen Bonding. 2. Alcohols and Thiols as Hydrogen Bond Acceptors Himansu S. Biswal,† Pranav R. Shirhatti, and Sanjay Wategaonkar* Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai, India 400 005 ReceiVed: March 15, 2010; ReVised Manuscript ReceiVed: May 29, 2010
In this paper, the effect of alkyl substitution at the hydrogen bond acceptor and its chain length on the strength and nature of hydrogen bonding is presented. In the present study we combine both experimental and computational methods to investigate the characteristics of O-H · · · O and O-H · · · S hydrogen bonding in the complexes of p-cresol (p-CR) with methanol (MeOH), ethanol (EtOH), methanethiol (MeSH), and ethanethiol (EtSH). The results indicate that, with an increase in the alkyl chain length, both O-H · · · O hydrogen bonding and O-H · · · S hydrogen bonding become stronger. Energy decomposition analysis emphasizes the dispersive nature of O-H · · · S hydrogen bonding. In addition, it revealed that the dispersion energy contribution in O-H · · · O hydrogen bonding increases with an increase in the alkyl chain length of the hydrogen bond acceptor. In the case of O-H · · · S hydrogen bonding, however, the dispersion energy contribution decreased from 68% for the H2S complex to 53% in the case of the MeSH complex; it remained unchanged with a further increase of the alkyl chain length. It was also observed that the red shifts in the OH stretching frequency did not correlate with the proton affinities of the O-centered acceptor vs the S-centered H-bond acceptor, in contrast with the known trend for the conventional H-bonded complexes. The IR/UV double resonance study enabled the assignments of the anti and gauche conformers of p-CR-EtOH and p-CR-EtSH. 1. Introduction In a recent paper the significant difference between O-H · · · O and O-H · · · S hydrogen bonding in the complexes of p-cresol (p-CR) with H2O/H2S was illustrated using a variety of experimental and computational methods.1 It was reported that O-H · · · S hydrogen bonding is dispersive in nature as opposed to the electrostatic nature of O-H · · · O hydrogen bonding.1-3 In this study, the effect of alkyl substitution on the hydrogen bond (HB) acceptor site (O or S atom) on the HB strength as well as on the nature of hydrogen bonding is investigated. This is the second part of the series of OH · · · SR2 (R ) radical) HB investigations that are being carried out in our laboratory.1,4-6 Alcohols such as methanol (MeOH) and ethanol (EtOH) were chosen as the O-centered HB acceptors, and thiols such as methanethiol (MeSH) and ethanethiol (EtSH) were chosen as the S-centered HB acceptors. Among the alcohols, the simplest alcohol, methanol, has been extensively characterized by various spectroscopic methods.7-10 In the case of ethanol, structural characterization7,11-18 has one more dimension due to the presence of isoenergetic anti and gauche conformers. Apart from the structural studies of these alcohols, the hydrogen-bonding interaction of these molecules with phenol is well documented in the literature.19-30 As far as alkanethiols are concerned, the effect of noncovalent interactions of the alkanethiol moiety and its conformational variants in the cysteine residue on protein folding is well recognized.31,32 There are a few experimental investigations on the structure of ethanethiol.33,34 Nonresonant two-photon pulsed field ionization photoelectron spectroscopy33,34 as well as vacuum ultraviolet mass-analyzed threshold ionization * To whom correspondence should be addressed. Phone: 91-22-22782259. Fax: 91-22-2278-2106. E-mail:
[email protected]. † Current address: Laboratoire Francis Perrin, Commissariat a` l’E´nergie Atomique (CEA), Saclay, Baˆt. 522, 91191 Gif-sur-Yvette Cedex, France.
spectroscopy33,34 gave experimental evidence for the existence of trans and gauche conformers of ethanethiol in jet-cooled conditions. However, a detailed and comparative study of the nature of hydrogen bonding in alcohols and alkanethiols has not been reported to date. In this study, the nature of OH · · · O and OH · · · S hydrogen bonding has been investigated using MeOH, EtOH, and EtSH as the respective HB acceptors. Since MeSH was not commercially available, the experimental results of MeOH could not be compared directly. Therefore, the hydrogen-bonding characteristics of p-CR-MeOH and p-CR-MeSH were compared using computational methods. The experiments were carried out in the gas phase under supersonic jet conditions. Various computational methods were used to quantify these interactions and substantiate the experimental findings. Experimental techniques such as laser-induced fluorescence (LIF), twocolor two-photon ionization (2c-R2PI) time-of-flight mass spectrometry (TOFMS), and fluorescence-detected infrared spectroscopy (FDIRS) and various ab initio computational methods such as density functional theory (DFT), second-order Møller-Plesset (MP2) perturbation theory, atoms-in-molecules (AIM) theory, natural bond orbital (NBO) analysis, and energy decomposition analyses were employed to characterize the HB interactions. 2. Experimental Details The details of the experimental setup are described elsewhere.1,35 In brief, p-cresol was evaporated at 50-60 °C and coexpanded through a 500 µm pulsed nozzle (General Valve, series 9) into a vacuum chamber using helium as the carrier gas. The molecular beam machine consisted of two 10 in. diameter differentially pumped stainless steel chambers. These two chambers were linearly connected by a skimmer located ∼25
10.1021/jp102346n 2010 American Chemical Society Published on Web 06/15/2010
O-H · · · O vs O-H · · · S Hydrogen Bonding mm downstream from the nozzle orifice. The pulsed nozzle was housed in the first chamber. The collimated beam enters the time-of-flight mass spectrometer with a 50 cm flight tube fitted with a 25 mm diameter channeltron multiplier (Dr. Sjuts Optotechnik GmbH, KBL25RS) housed in the second chamber. The output of the channeltron was sent to a digitizing storage oscilloscope (LeCroy, 9450) interfaced to a PC through a preamplifier (ORTEC, model VT120). For the R2PI experiments, a 10 Hz, nanosecond Nd3+:YAG laser (Quantel, Brilliant) pumped dye laser (Molectron, DL18P) was used to provide the fixed D0-S1 ionization source, and another Nd3+:YAG laser (Quantel, YG781C) pumped dye laser (Quantel, TDL70) was used to provide the tunable S1-S0 excitation source. The two copropagating beams were spatially and temporally overlapped and were focused onto the molecular beam using a 50 cm focal length lens. Typical pulse energies were ∼5-10 µJ for the excitation laser and ∼100 µJ for the ionization laser. The dye lasers were calibrated by means of the optogalvanic method using an Fe-Ne hollow cathode lamp. FDIRS36,37 was used to record the IR spectra of p-CR and its complexes. The fluorescence-detected experiments were carried out in the first chamber, i.e., the expansion chamber. The fluorescence was collected perpendicular to the plane defined by the laser and the molecular beam using a 50 mm focal length lens with an f1 aperture and focused by a 100 mm focal length lens onto a 1P28 photomultiplier operating in the analog mode. The laser line scatter was filtered using a WG-320 long-pass filter. The tunable IR laser was introduced 50-100 ns prior to the UV laser pulses. Whenever the IR laser was resonant with the vibrational transition of the species being probed, it depleted the population of the species in the ground state. The infrared resonances were detected as the dips in the fluorescence signal due to the population depletion. The tunable IR was generated using a 10 Hz seeded Nd3+:YAG laser (Quanta-Ray, PRO 23010) pumped dye laser (Sirah, CSTR LG 18 532). The dye laser output was mixed with the 1064 nm output of the Nd3+:YAG laser in a LiNbO3 crystal to generate the IR output by the difference frequency generation technique. The O-H stretching region and the S-H stretching region were covered using styryl-8 and styryl-9 dyes (Exciton, Inc.), respectively. The UV and the IR lasers were temporally synchronized by a master controller (SRS DG-535). The reagent p-CR was purchased from Sigma-Aldrich and used without further purification. Helium obtained from local commercial sources was used without further purification as the buffer gas. The 2-5% premixes of the other ligands (MeOH, EtOH, and EtSH) in helium were used to generate the 1:1 complexes of p-CR and the ligands. The typical backing pressure employed during the experiments was 2.5-3 atm. The typical working pressure in the source chamber was ∼6 × 10-5 Torr, and in the TOFMS chamber it was ∼2 × 10-6 Torr. 3. Computational Details The geometry optimization and the frequency calculation for the complexes were carried out at the B3LYP/aug-cc-pVDZ level of theory.38-46 The equilibrium structures were examined by harmonic vibrational frequency calculations. Single-point energy calculations at the B3LYP-optimized structures were carried out at the MP2/aug-cc-pvDZ level. The interaction energies for all the complexes were corrected for the zero-point energy (ZPE), the basis set superposition error (BSSE), and the fragment relaxation energy. All the calculations were carried out using the Gaussian 03 program suite.47 The threedimensional pictures of the complexes were generated using the ChemCraft graphics program (trial version).48
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Figure 1. Two-color R2PI spectra of (a) p-CR, (b) p-CR-MeOH, (c) p-CR-EtOH, and (d) p-CR-EtSH. The ionization laser was set at 30 770 cm-1.
The AIM49-51 theory was used to investigate the electron densities and the intermolecular hydrogen-bonding interactions. The topological properties of electron densities for the monomers and complexes at the bond critical points (BCPs) were calculated using the AIM2000 program.52 The wave functions computed at the aforementioned levels of theory were used to calculate the electron density, F(r), and Laplacian, ∇2F(r), at the bond critical points and integrated properties such as the atomic charge, q(H), atomic polarization moment, M(H), atomic volume, V(H), and atomic energy, E(H), in the atomic basin of hydrogen. To evaluate the direction and magnitude of the donor-acceptor interactions, NBO53-55 analysis was carried out using the NBO 5.0 program56 for all the complexes. The interaction energies of the complexes were decomposed into physically meaningful individual energy components57 at the HF/aug-cc-pvDZ level of theory using natural energy decomposition analysis (NEDA),58-60 Kitaura and Morokuma (KM) decomposition analysis,61 and reduced variational space self-consistent field (RVS)62 decomposition analysis. The KM and RVS decomposition analyses were performed using the Gordon and Chen63 algorithm in GAMESS.64 NEDA calculations were performed with the NBO 5.0 program54,56 linked to the GAMESS package. 4. Experimental Results 4.1. p-Cresol-MeOH Complex. Figure 1 shows the 2cR2PI spectra of p-CR (Figure 1a) and the p-CR-MeOH complex (Figure 1b). In both the cases the ionization laser energy was at 30 770 cm-1, i.e., just above the D0-S1 transition of the p-CR monomer.65 The lowest energy transition in the spectrum was observed at 34 906 cm-1 and was assigned as the band origin of the p-CR-MeOH complex. The S1-S0 band origin was red-shifted relative to that of the monomer by 425 cm-1, which is characteristic of the complexes where the phenolic OH group is the H-bond donor. The observed red shift was much larger compared to that of the p-CR-H2O complex (357 cm-1). The greater red shift suggests greater relative stabilization of the S1 state in this complex. The spectrum also shows a progression in a low-frequency (25 cm-1) intermolecular mode up to ν′ ) 4 and another low-frequency transition at 31 cm-1 from the band origin. The transition observed at 35 078 cm-1 (+172 cm-1 from the band origin) was assigned
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Figure 2. FDIR spectra of (a) p-CR and (b) p-CR-MeOH recorded while the probe laser was tuned at the band origin of the respective species.
as the intermolecular stretch (σ1). This value is comparable to that for the phenol-MeOH complex (+177 cm-1).29 The FDIR spectra of p-CR and the p-CR-MeOH complex in the S0 state are shown in Figure 2. The spectrum of the complex (Figure 2b) shows dips at 3681 and 3467 cm-1 in the OH stretch region. The dip at 3681 cm-1, which is toward the blue side of the OH stretch of the p-CR monomer, was assigned to the free hydroxyl group of MeOH in the complex. The OH frequency for the free methanol is 3686 cm-1, which gives a 5 cm-1 red shift with respect to that of the methanol monomer.9 The transition at 3467 cm-1 was assigned to the stretching frequency of the hydrogen-bonded OH of p-CR, giving a red shift of 191 cm-1 compared to that of the p-CR monomer. The red shift in the OH group belonging to p-CR indicates that it acts as the H-bond donor. This red shift is about 64 cm-1 greater than that observed for the OH stretch in the p-CR-H2O complex, and it scales well with the proton affinities (PAs) or gas-phase basicities (GBs) of H2O and MeOH. The IR data for MeSH could not be obtained because of unavailability of the MeSH compound. 4.2. p-Cresol-EtOH Complex. The 2c-R2PI spectra for the p-CR-EtOH and p-CR-EtSH complexes are depicted in Figure 1. For the p-CR-EtOH complex (Figure 1c) a lot of lowfrequency transitions were observed in the region of 34908-34970 cm-1. Figure 3 shows the 2c-R2PI spectrum of p-CR-EtOH (Figure 3a) expanded up to 150 cm-1 toward the blue side of its band origin. From the appearance of the spectrum it was difficult to say a priori as to whether the large number of transitions was due to the low-frequency modes or due to the presence of multiple conformers. The FDIR spectra were recorded while the excitation laser was tuned at the two lowest frequency transitions at 34 909 and 34 913 cm-1 and are displayed in parts b and c, respectively, of Figure 4. Both the spectra show a broad peak at 3441 cm-1, which was assigned to the OH stretch of p-CR. This was redshifted by almost 217 cm-1 compared to that for the p-CR monomer. The difference between the two FDIR spectra was in the ethanolic OH stretch region. For the 34 909 cm-1 probe frequency, the ethanolic OH stretch was observed at 3669 cm-1, and for the other excitation, it was observed at 3654 cm-1. This suggests that the two transitions in the R2PI spectrum correspond to two different conformers. The two transitions observed at 34 909 cm-1 (A) and 34 913 cm-1 (B) for p-CR-EtOH were assigned as the band origins for the two conformers giving BO
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Figure 3. 2c-R2PI spectra of (a) p-CR-EtOH and (b) p-CR-EtSH with respect to their respective band origins observed at 34 909 and 35 015 cm-1, respectively. The peaks marked A, B, C, and D correspond to different conformers (see the text).
Figure 4. FDIR spectra of (a) p-CR, (b, c) p-CR-EtOH while the probe laser was kept at positions marked as A and B in Figure 3, respectively, and (d, e) p-CR-EtSH while the probe laser was kept at positions marked as C and D in Figure 3, respectively.
red shifts of 422 and 418 cm-1, respectively. The magnitudes of the red shifts of the band origins and the red shifts in the stretching frequency of the OH group of p-CR were taken as collective evidence for the structure of the complex where the phenolic OH group acts as the H-bond donor. To sort out the rest of the spectrum, the IR/UV spectral hole burning (SHB)21,66-68 was carried out by fixing the IR laser at either of the two ethanolic OH frequencies. The corresponding spectra are shown in Figure 5. Figure 5c shows the 2c-R2PI spectrum when the IR laser was off, while parts a and b of Figure 5 show the 2c-R2PI spectra of p-CR-EtOH recorded while the IR laser was fixed at 3654 and 3669 cm-1, respectively. It can be seen that the two IR/UV SHB spectra in combination recreate the entire 2c-R2PI spectrum. Therefore, it was concluded that only two conformers were observed experimentally. 4.3. p-Cresol-EtSH Complex. The 2c-R2PI spectrum is shown in Figure 1d. The lowest energy transition in the spectrum was observed at 35 015 cm-1 with a bunch of not-so-strong
O-H · · · O vs O-H · · · S Hydrogen Bonding
Figure 5. IR/UV spectral hole burning spectrum of p-CR-EtOH while the IR laser was kept (a) at 3669 cm-1 and (b) at 3654 cm-1 and (c) with IR off.
transitions within 60 cm-1 from this electronic origin. These were attributed to the presence of multiple conformers of the p-CR-EtSH complex by analogy with the p-CR-EtOH complex. The two features labeled as C and D in the expanded R2PI spectrum (Figure 3b) were chosen for carrying out FDIRS. The lowest frequency transition (peak C) was red-shifted by 316 cm-1 with respect to that of the free monomer. The magnitude of the red shift in the band origin, although smaller compared to that of the p-CR-EtOH conformers, was indicative of the complex where the p-CR OH group acts as the H-bond donor. The FDIR spectra for p-CR-EtSH are displayed in parts d and e, respectively, of Figure 4. The FDIR spectra recorded for peaks C and D gave two distinctly different IR spectra. Unlike the p-CR-EtOH complex, the peaks due to the HB donor OH group of p-CR were observed at 3484 (red-shifted by 174 cm-1 with respect to the monomer) and 3492 cm-1 (red-shifted by 166 cm-1) for the probed transitions C and D, respectively. The red shift in the phenolic OH group clearly establishes that p-CR acts as the H-bond donor. In addition to the OH stretch, one vibronically coupled low-frequency transition was observed at 16 cm-1 to the blue side of the OH transition in each case. An attempt was made to record the IR/UV spectral hole burning spectrum by fixing the IR laser at the two observed p-CR OH stretches, viz., at 3484 and 3492 cm-1 (see Figure 4d,e). However, due to poor S/N ratio in the hole burning spectra, assigning peaks to different conformers of p-CR-EtSH could not be carried out reliably. FDIRS was attempted in the S-H stretching region; however, the IR absorption cross-section of the SH stretch of ethanethiol is too small to be detected by the FDIRS technique, and therefore, the option of detecting different conformers via the changes in the SH frequency was not available in this case. Therefore, on the basis of the two distinct FDIR spectra, it was concluded that the p-CR-EtSH complex also existed as only two conformers in the jet. 5. Computational Results 5.1. Equilibrium Geometry and Interaction Energy. The geometry optimization for all the conformers of the monomers and the complexes was done at the B3LYP level of theory. All possible structures, i.e., the XH (X ) O, S) group of the solvent molecules as well as the phenolic OH group acting as the H-bond donor, were used as the starting structures, and the ones that finally converged are presented here. In all the cases the
J. Phys. Chem. A, Vol. 114, No. 26, 2010 6947 structures with the phenolic OH acting as the donor were the most stable structures. The frequency calculations were done to ensure that all the structures were the true minima, i.e., they do not give any imaginary frequencies. Since both EtOH and EtSH have two conformers, namely, anti and gauche, when they form the hydrogen-bonded complexes, three conformers are possible for each of them. The optimized structures of the most stable conformers are depicted in Figure 6. Figure 7a shows the nomenclature scheme used for the different conformers of the EtYH (Y ) O, S) monomer, and Figure 7b shows that for the conformers of their complexes. In Figure 7b the hydrogenbonded lone pair is shown in blue color while the nonbonded lone pair is shown in red color. The two gauche conformers, gauche 1 and gauche 2 in Figure 7b, are slightly nonequivalent when they are bound to the p-CR moiety. Figure 7b also shows the optimized structures of different conformers of p-CR-EtSH; the gauche 2 conformer for the p-CR-EtOH complex could not be optimized even after a large number of optimization steps. The single-point energies of all the B3LYP-optimized structures were computed at the MP2/aug-cc-pVDZ level of theory. It is worth mentioning that, in the case of the p-CR-EtSH complex, the MP2-optimized structures did not show any O-H · · · S interaction. In this particular case the structure folds the ethyl chain over the aromatic ring and does not show the conventional hydrogen-bonded structure, but in all the other cases the MP2 level structures were similar to those obtained using the DFT method. The MP2 level calculations are known to overestimate the correlation effects, which at times give rise to some strange results. The limitations of post-Hatree-Fock methods, especially the MP2 method, are well documented in the literature.69-71 On the other hand, the recently developed dispersion-corrected density functional theory (DFT-D) has shown tremendous improvement in predicting structures and eneregtics of weakly bound intermolecular complexes.71-81 However, there is no report on the assessment of DFT-D for the O-H · · · S HB systems. With our current level of computational facilities, we are unable to perform the DFT-D level computations for these systems. Therefore, to be consistent with the other complexes, we have considered only the DFT level structures computed using a fairly large basis set. For the four complexes, i.e., the complexes with MeOH, MeSH, EtOH, and EtSH, all the geometrical parameters such as dH · · · Y (Y ) O or S), RO · · · Y, ∆rOH, the H-bond angle (θ), the angle between the H · · · Y internuclear line and the line bisecting the CYC angle (Ψ), and the H-Y-C-C dihedral angle (Φ) defining the conformers of EtOH and EtSH are listed in Table 1. For all the alcohol complexes the hydrogen bond angle (θ) was ∼173°, whereas for the thiol complexes it was ∼160°. This suggests that the O-H · · · S HB is slightly nonlinear compared to the O-H · · · O HB. The angle Ψ differed greatly for the p-CR-alcohol and p-CR-thiol complexes; i.e., it was ∼136° for the O-H · · · O and ∼94° for the O-H · · · S H-bonded complexes, respectively. For the gauche 2 conformer of p-CR-EtSH it was 101°. The computed red shifts in the O-H stretch for all the complexes follow the order p-CR-EtOH > p-CR-MeOH > p-CR-EtSH > p-CR-MeSH. The same trend was also observed in the experimentally observed red shifts, except that the computed shifts were higher than those observed experimentally. The binding energies for the different complexes computed at the MP2 level are listed in Table 2 after various energy corrections such as basis set superposition error (∆EBSSE), deformation or relaxation energy (∆Erelax), and zero point energy (∆ZPE) correction. The ZPE corrections were done using the
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Figure 6. Structures of p-CR-L (L ) MeOH, MeSH, EtOH, and EtSH) corresponding to the global minima obtained at the B3LYP/aug-cc-pVDZ level.
Figure 7. Newman projections of the anti and gauche conformers of (a) bare ethanol/ethanethiol and (b) complexes of ethanol/ethanethiol. The bottom row in trace b shows the molecular structures of the respective conformers.
B3LYP frequencies. The p-CR-EtOH complex was the most stable complex followed by the p-CR-MeOH, p-CR-EtSH, and p-CR-MeSH complexes. The interaction energy for the anti conformers was larger than that of the gauche conformers for both the p-CR-EtOH and p-CR-EtSH complexes. A relaxed potential energy surface (PES) scan was carried out at
the B3LYP/6-311++G(d,p) level to determine the barrier height for the interconversion of the conformers. Figure S1 shows the PES for both the complexes (see the Supporting Information). It was observed that the gauche conformers were thermodynamically more stable than the anti conformers in both the cases although the binding energies were higher for the anti conform-
O-H · · · O vs O-H · · · S Hydrogen Bonding
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TABLE 1: B3LYP/aug-cc-pVDZ-Computed Structural Parameters of p-CR-MeOH, p-CR-MeSH, p-CR-EtOH, and p-CR-EtSH p-CR-EtOH structural parameter
p-CR-MeSH
anti
gauche 1
anti
gauche 1
gauche 2
1.845 2.817 0.012 172.7 136.3
2.411 3.340 0.010 159.1 93.9
246
207
1.847 2.820 0.012 173.1 135.8 178.4 256
1.840 2.812 0.013 172.8 136.3 60.4 257
2.403 3.335 0.010 159.7 94.5 -177.0 219
2.410 3.344 0.010 160.4 94.0 63.8 211
2.424 3.373 0.010 164.6 101.6 -58.8 214
dH · · · Y (Å) RO · · · Y (Å) ∆rO-H (Å) θ (deg) Ψ (deg) Φ (deg) ∆νO-Ha (cm-1) a
p-CR-EtSH
p-CR-MeOH
Without any scaling factor.
TABLE 2: Binding Energies (kcal/mol) and Various Correction Terms (kcal/mol) for p-CR-MeOH, p-CR-MeSH, p-CR-EtOH, and p-CR-EtSH Complexes at the MP2 Level for the B3LYP/aug-cc-pVDZ-Optimized Structures p-CR-EtOH
p-CR-EtSH
energy term
p-CR-MeOH
p-CR-MeSH
anti
gauche 1
anti
gauche 1
gauche 2
∆EBE ∆EBSSE ∆Erelax ∆EBEBSSE ∆ZPE ∆EBEBSSE+relax+ZPE
-8.70 1.45 0.20 -7.25 1.29 -5.76
-6.93 1.77 0.13 -5.16 0.95 -4.08
-9.56 1.76 0.25 -7.79 1.21 -6.33
-9.12 1.56 0.23 -7.55 1.19 -6.14
-7.60 1.86 0.17 -5.74 0.86 -4.70
-7.07 1.76 0.14 -5.31 0.85 -4.32
-7.43 1.81 0.20 -5.62 0.86 -4.56
ers. The barrier height for the gauche to anti interconversion was 1.20 kcal/mol (420 cm-1) for the p-CR-EtOH complex, while that for the p-CR-EtSH complex was 1.48 kcal/mol (518 cm-1). 5.2. AIM Study. Atoms-in-molecules calculations were done using the ab initio wave functions computed at the MP2 level with the aug-cc-pVDZ basis set for the monomers as well as for the complexes. AIM theory takes the electron density (F) as a starting point. The topology of the electron density is generally used to determine the existence and the strength of the hydrogen bond qualitatively. One of the AIM criteria for the existence of a hydrogen bond between the donor and acceptor molecules is the presence of the (3, -1) BCP.82 Figure 8 shows the molecular graph for the p-CR-MeOH and p-CR-MeSH complexes as well as those for the different conformers of p-CR-EtOH (Figure 8c,d) and p-CR-EtSH (Figure 8e-g). In all the cases a BCP was located along the O-H · · · O or O-H · · · S interaction axis. For the p-CR-EtSH (gauche 2) conformer, a BCP was also located for the C-H · · · O type of interaction (Figure 8g). The electron density and Laplacian at this BCP were 0.0045 and 0.0207 au, respectively. The AIM criteria proposed by Popelier82 to describe and quantify a classical hydrogen bond were applied to study the O-H · · · O and O-H · · · S hydrogen-bonded complexes. All the topological parameters are listed in Table 3. The charge densities at the BCPs were ∼0.0283 and ∼0.0175 au for the alcohol and thiol complexes, respectively. These values of the electron density and its Laplacian are well within the range specified for the existence of the hydrogen bond in terms of the electron density (0.002-0.040 au) and its Laplacian (0.024-0.139 au).83 The values of the charge density and its Laplacian at the BCP for the O-H · · · S HB are smaller compared to those of the O-H · · · O HB complexes (see Table 3). The same trend was also observed for the destabilization energy, the decrease of dipolar polarization, the loss of electronic charge, and the volume of the hydrogen atom. The values of AIM topological parameters for p-CR-MeOH were very close to those of the p-CR-EtOH complex, whereas for p-CR-MeSH and p-CR-EtSH, the differences in the values were distinct. 5.3. NBO Analysis. The NBO model has been very useful in explaining hydrogen bonding in the X-H · · · Y system as the
donor-acceptor charge delocalization takes place between the lone pair of the hydrogen bond acceptor Y and the proximal antibonding σ*(X-H) orbital of the donor.84 The NBO calculation was done for the complexes and monomers at the MP2/ aug-cc-pvDZ level of theory. Table 4 lists the changes in the atomic charges on H [∆q(H)] and O or S [∆q(Y)] atoms, the electron occupancy in the lone pair orbital [δ(n(Y))] and the antibonding orbital [δ(σ*(O-H))], and the second-order perturbation interaction energy for both the O-H · · · O and O-H · · · S hydrogen-bonded complexes. The second-order perturbation (2) , due to the overlap of the lone pair (LP) orbitals energy, Eifj* and the OH antibonding orbital in the O-H · · · O hydrogenbonded complexes was greater than that of the corresponding O-H · · · S hydrogen-bonded complexes. The interaction energy increases slightly with an increase in the length of the alkyl chain for both the O and S acceptors. In both alcohol and thiol complexes of p-CR, both the lone pair orbitals of Y are involved in hydrogen bonding. The LP orbitals of the acceptor atom in the complexes are distinctly different for the O and S acceptors. In the O-H · · · O hydrogen-bonded complexes the acceptor oxygen (LP) orbitals are p and sp2 type, while in the O-H · · · S hydrogen-bonded complexes the sulfur (LP) orbitals are pure p and s type. The overlap of the p-type LP orbital with the OH antibonding orbital was greater for both the O- and S-centered complexes. However, the contribution of the second LP was more in the O-H · · · O HB complexes than that in the O-H · · · S HB complexes. The orbital overlaps are pictorially shown in Figure 9 for the p-CR-EtOH and p-CR-EtSH anti conformers. Parts a and b of Figure 9 show the sp2- and p-type acceptor oxygen (LP) orbitals, and parts c and d of Figure 9 show their overlap with the σ*(OH) orbital for the p-CR-EtOH (anti) conformer. Similarly, the sulfur (LP) orbitals (Figure 9e,f) and their overlap with the σ*(OH) orbital (Figure 9g,h) for the p-CR-EtSH (anti) conformer are also shown in Figure 9. All the values of the NBO parameters suggest that the strength of the O-H · · · S hydrogen bond in the thiol complexes of p-CR is smaller than that of the O-H · · · O hydrogen bond in its corresponding alcohol complexes. 5.4. Energy Decomposition Analysis. p-CR-MeOH, p-CR-MeSH, and the anti conformers of p-CR-EtOH and p-CR-EtSH were taken up for the comparative study of various
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Figure 8. Molecular graphs of (a) p-CR-MeOH, (b) p-CR-MeSH, (c, d) p-CR-EtOH, and (e-g) p-CR-EtSH obtained using the MP2/augcc-pVDZ wave functions. The bond critical points and ring critical points are represented by the small red balls and yellow balls, respectively.
TABLE 3: AIM Topological Parameters (au) for p-CR-EtOH and p-CR-EtSH Complexes Computed Using MP2/ aug-cc-pVDZ Wavefunctions for the B3LYP/aug-cc-pVDZ-Optimized Structuresa p-CR-EtOH AIM parameter FH · · · Y ∇2 F H · · · Y ∆qH ∆EH ∆|MH| ∆VH RH · · · BCP RY · · · BCP 〈RH · · · BCP〉b 〈RY · · · BCP〉b a
p-CR-EtSH
p-CR-MeOH
p-CR-MeSH
anti
gauche 1
anti
gauche 1
gauche 2
0.0283 0.1214 0.0550 0.0312 -0.0456 -9.0905 0.613 1.233 0.78 ( 0.08 1.29 ( 0.07
0.0173 0.0474 0.0186 0.0186 -0.0153 -4.6650 0.750 1.661
0.0283 0.1202 0.0570 0.0330 -0.0460 -9.5677 0.613 1.235
0.0286 0.1228 0.0627 0.0354 -0.0478 -10.0760 0.610 1.230
0.0177 0.0482 0.0255 0.0245 -0.0193 -5.9151 0.746 1.657
0.0174 0.0475 0.0228 0.0228 -0.0188 -5.7817 0.749 1.661
0.0171 0.0461 0.0233 0.0231 -0.0189 -5.6550 0.749 1.674
1.76 ( 0.08
b
The distances are in angstroms. The values are taken from ref 86.
energy contributions to the O-H · · · O and O-H · · · S HBs, respectively. Three different energy decomposition analysis schemes such as KM, RVS, and NEDA were used to determine the contributions of the individual energy components [electrostatic (ES), polarization (PL), charge transfer (CT)] to the total interaction energy. The total interaction energy computed at the MP2 level (∆Eint MP2) and the interaction energy computed using the NEDA, KM, and RVS energy decomposition analyses (∆EINT) are listed in Table 5. Table 5 also gives the corresponding values1 for the p-CR-H2O/H2S complexes for the sake of
comparison. The contribution of the different energy components to the total interaction energy is graphically presented in Figure 10a for the p-CR-MeOH and p-CR-MeSH complexes and in Figure 10b for the p-CR-EtOH (anti) and p-CR-EtSH (anti) complexes. For all the complexes, the dispersion energy was calculated as the difference between the BSSE-corrected total interaction energy computed at the MP2 level and that computed using the KM, RVS, and NEDA procedures. Table 5 provides the dispersion interaction energy contribution for all the complexes computed at the said three procedures. In the case
O-H · · · O vs O-H · · · S Hydrogen Bonding
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TABLE 4: Summary of NBO Analysisa p-CR-EtOH NBO parameter ∆q(H) ∆q(Y) δ(n(Y)) (LP1, LP2) δ(σ*(O-H)) (2) Eifj* (0) εj* - εi(0) (0) 〈φi(0)|Fˆks|φj* 〉
p-CR-EtSH
p-CR-MeOH
p-CR-MeSH
anti
gauche 1
anti
gauche 1
gauche 2
0.0360 -0.0199 1.9620, 1.9816
0.0144 -0.0076 1.9591, 1.9939
0.0363 -0.0203 1.9618, 1.9821
0.0380 -0.0216 1.9608, 1.9813
0.0157 -0.0111 1.9597, 1.9945
0.0163 -0.0116 1.9571, 1.9936
0.0176 -0.0118 1.9589, 1.9910
0.0258 17.28 (14.69 + 2.59) 2.89 (1.39 + 1.50) 0.184 (0.128 + 0.056)
0.02804 13.26 (12.66 + 0.60) 2.61 (1.12 + 1.49) 0.134 (0.107 + 0.027)
0.0268 17.54 (15.03 + 2.51) 2.88 (1.39 + 1.49) 0.184 (0.129 + 0.055)
0.0262 17.48 (15.33 + 2.15) 2.88 (1.40 + 1.48) 0.181 (0.131 + 0.050)
0.0292 13.99 (13.38 +0.61) 2.61 (1.12 + 1.49) 0.137 (0.110 + 0.027)
0.0283 13.61 (13.17 + 0.44) 2.61 (1.13 + 1.48) 0.132 (0.109 + 0.023)
0.0295 14.15 (13.14 + 1.01) 2.61 (1.12 + 1.49) 0.143 (0.108 +0.035)
a (2) Eifj* is in kilocalories per mole; all other values are in atomic units. The values in parentheses give the individual contributions of the non-bonding orbitals of oxygen and sulfur.
Figure 9. NBOs of the p-CR-EtOH (anti) conformer: (a) sp2-type donor NBO (LP1); (b) p-type donor NBO (LP2); overlap of the (c) sp2-type NBO (LP1) and (d) p-type NBO (LP2) with the σ*(O-H) NBO. NBOs of the p-CR-EtSH (anti) conformer: (e) s-type donor NBO (LP1); (f) p-type donor NBO (LP2); overlap of the (g) s-type NBO (LP1) and (h) p-type NBO (LP2) with the σ*(O-H) NBO.
of p-CR-alcohol complexes the ES and PL contributions are large, while for the p-CR-thiol complexes the dispersion energy is greater than all other energy contributions. The dispersion contributions for the p-CR-MeSH and p-CR-EtSH complexes were almost identical; however, the dispersion contribution for p-CR-MeOH was slightly smaller compared to that for the p-CR-EtOH complex. The trends in the dispersion contributions in the case of OH · · · O and OH · · · S bonded complexes were opposite each other; i.e., in the case of alcohol complexes the dispersion contribution increased to ∼30% from 24% upon alkyl substitution on the O atom, whereas in the case of thiol complexes it decreased to ∼50% from that for the p-CR-H2S complex, where it was about 70%.1 6. Discussion All the experimentally observed parameters such as the red shifts in the band origin and the H-bond donor O-H stretching
frequency as well as the shift in the O-H stretching frequency of the acceptor moiety for p-CR-RYH (R ) H, CH3, C2H5; Y ) O, S) are listed in Table 6. The literature data on the phenol complexes with MeOH and EtOH are also included in Table 6 for the sake of completeness. Table 6 also gives the computed normal-mode frequencies for the H-bond donor OH group and the alcoholic OH groups for the solvent molecules investigated in this work. The computed frequencies were scaled using a scaling factor to match the OH frequency of the p-CR monomer. It can be seen that the computed OH frequencies are in excellent agreement with the observed frequencies for each of the complexes. This establishes that the computed structures are quite reliable. The red shift of the H-bond donor O-H stretching frequency increases with an increase in the alkyl chain length of the acceptor molecules, but the red shift in the band origin of the complexes does not follow the same trend. The red shifts in the band origins of the p-CR-MeOH and p-CR-EtOH complexes were comparable and was actually slightly higher for the p-CR-MeOH complex. A similar trend was also reported for the phenol complexes with methanol and ethanol.19,85 The band origin shifts for the sulfur acceptors are smaller than those of their oxygen counterparts. Similarly, the red shifts of the O-H stretches in the O-H · · · S hydrogen-bonded complexes were about 80% of those of the O-H · · · O hydrogen bonded complexes. This trend is also reproduced in the computed frequencies. The larger red shift of the band origin for the O-H · · · O complexes suggests that the S1 state stabilization is greater in these complexes than the O-H · · · S complexes, while the larger red shift of the O-H stretch indicates that the O-H · · · O hydrogen bond strength is higher than that of the O-H · · · S hydrogen bond. It was noted that although the proton affinities of the sulfur-containing HB acceptors are greater compared to those of the corresponding oxygen-centered HB acceptor, the observed red shifts in the O-H stretch for the O-H · · · S hydrogen-bonded complexes were smaller than those for the O-H · · · O hydrogen-bonded complexes. This indicates that the strength of the OH · · · S HB interaction relative to that of the OH · · · O HB does not follow the acid-base formalism, at least in this particular case. The ground-state binding energies computed at the MP2 level for the p-CR-alcohol complexes were about 1.4 times higher than those of the p-CR-thiol complexes (see Table 2). This is consistent with the general perception that the OH · · · S H-bond is relatively weaker in comparison with the OH · · · O H-bond. Similar observations were also recorded for the other O-H · · · S HB complexes.1,3
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TABLE 5: Total Interaction Energy Computed at the MP2 level (∆EintMP2)a and Interaction Energy (∆EINT) and Dispersion Energy (∆Edisp) Contributions According to the NEDA, KM, and RVS Energy Decomposition Analysesb ∆EINT
∆Edisp
complex
∆EintMP2
KM
RVS
NEDA
KM
RVS
NEDA
p-CR-MeOH p-CR-MeSH p-CR-EtOH (anti) p-CR-EtSH (anti) p-CR-H2O c p-CR-H2S c
7.25 5.16 7.79 5.74 6.12 3.68
5.20 2.40 5.27 2.64 4.55 1.10
5.34 2.61 5.44 2.84 4.66 1.26
5.20 2.40 5.27 2.64 4.55 1.10
2.05 (28%) 2.76 (53%) 2.52 (32%) 3.10 (54%) 1.57 (26%) 2.58 (70%)
1.91 (26%) 2.55 (49%) 2.35 (30%) 2.90 (51%) 1.46 (24%) 2.42 (66%)
2.05 (28%) 2.76 (53%) 2.52 (32%) 3.10 (54%) 1.57 (26%) 2.58 (70%)
a The numbers in the first column are without the ZPE and fragment relaxation energy corrections (see row 4 of Table 2). b All the energy components are in kilocalories per mole. The numbers in parentheses denote the percentage of contribution of the dispersion interaction to the total interaction energy. c The values are taken from ref 1.
Figure 10. Decomposition of the interaction energy of (a) p-CR-MeOH and p-CR-MeSH complexes and (b) p-CR-EtOH (anti) and p-CR-EtSH (anti) complexes using the RVS energy decomposition scheme.
O-H · · · O vs O-H · · · S Hydrogen Bonding
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TABLE 6: Summary of the Observed Band Origins, p-CR O-H Stretching Frequencies, Alcoholic Hydroxyl O-H Stretching Frequencies, and Their Respective Red Shifts in the p-CR-L Complexes (L ) H2O, H2S, MeOH, EtOH, and EtSH) in Wavenumbersa complex
BO position
BO shift
p-CR p-CR-H2Ob p-CR-MeOH p-CR-EtOH (A) p-CR-EtOH (B) p-CR-H2Sb p-CR-MeSHc p-CR-EtSHc (C) p-CR-EtSHc (D) phenol (PH)d PH-H2Od PH-MeOHd PH-EtOH (anti)e PH-EtOH (gauche)e
35331.0 34974.0 34906.0 34908.6 34913.0 35092.0
-357.0 -425.0 -422.4 -418.0 -265.0
35014.7 35048.5 36348.0 35993.0 35933.0 35938.2 35940.2
-316.3 -282.5 0.0 -355.0 -415.0 -409.8 -407.8
(νOH)p-CR 3658 3531 3467 (3423) 3441 (3413) 3441 (3412) 3556 n.a. (3487) 3484 (3449) 3492 (3457) 3657 3524 3456 3432 3432
(∆νOH)p-CR
(νOH)R-OH
(∆νOH)R-OH
-127 -191 (-235) -217 (-245) -217 (-246) -102 - (-171) -174 (-209) -166 (-201)
3650 3681 (3662) 3669 (3660) 3654 (3645)
-7 -5 (-7) -7 (-9) -6 (-9)
-133 -201 -225 -225
3650
-7
3667 3653
-9 -7
a The frequencies given in parentheses give the scaled normal-mode frequencies computed at the B3LYP/aug-cc-pVDZ level. The scaling factor of 0.958 was used to match the computed and observed frequencies of phenolic OH in the p-cresol monomer. b Reference 1. c The SH frequencies of EtSH and MeSH were not measurable due to poor transition probabilities. d Reference 85. e Reference 27.
Another interesting feature of the p-CR-EtOH and p-CR-EtSH complexes was the presence of multiple conformers, which made the 2c-R2PI spectra congested and difficult to analyze as well as to assign. The vibrational, microwave, and electronic structure calculations of EtOH and EtSH in the supersonic jet conditions indicate the presence of anti and gauche conformers.11,16-18,34 On the basis of these data, one would expect at least two conformers of the p-CR-EtOH and p-CR-EtSH complexes. In fact, the presence of multiple conformers of p-CR-EtOH and p-CR-EtSH was confirmed from the FDIR and IR/UV SHB spectra. Two different sets of peaks were observed in the FDIR spectra of p-CR-EtOH while the excitation laser was kept at two different positions, viz., A and B (Figure 4a). The two IR spectra were different in the ethanolic O-H stretch region (Figure 5b,c). For conformer A, the ethanolic O-H stretch was observed at 3669 cm-1, while that for conformer B was observed at 3654 cm-1. Hence, the ethanolic O-H stretch of conformer A was blue-shifted (about 15 cm-1) relative to that of conformer B. The O-H stretch for the anti conformer of the ethanol monomer is about 16 cm-1 higher relative to that of the gauche conformer.7 The higher value of the O-H stretch for the anti conformer is attributed to the trans lone pair effect.12 This is due to the interaction of the electron lone pair at the oxygen atom with the methyl σ*(CH) orbital in the trans position. This interaction increases the s character of the O-H bond, thereby increasing the O-H stretching frequency.12 In the anti conformer two types of such interactions exist, while for the gauche conformer there is only one such interaction present (Figure 7a). The magnitude of the blue shift of the O-H stretch depends on the number of trans lone pair interactions. With this analogy conformer A of p-CR-EtOH was assigned as the p-CR-EtOH (anti) conformer and conformer B as the p-CR-EtOH (gauche) conformer. This is also in excellent agreement with the computed free OH frequency of the ethanol moiety in the complex. In the B3LYP frequency calculation, the ethanolic O-H stretch of the anti conformer was 16 cm-1 blue-shifted relative to that of the gauche conformer, which is very close to the experimental blue shift of 15 cm-1. However, from the available data it is not possible to assign the gauche conformer as the gauche 1 or gauche 2 conformer. Similarly, two sets of IR spectra were observed for the p-CR-EtSH complex by keeping the excitation laser at positions
C and D (see Figure 4b). For the conformer probed via transition C the O-H stretch for p-CR was observed at 3484 cm-1 (174 cm-1 red shift), and that for transition D was observed at 3492 cm-1 (166 cm-1 red shift). Unlike the p-CR-EtOH complex, the S-H stretch frequency cannot be used as a probe to assign the different conformers as the IR absorption cross section for S-H is too small to be observed. However, since the two probed transitions C and D gave two different O-H stretching frequencies, it was inferred that at least two conformers were formed under the experimental conditions. The computed red shift in the O-H stretch for p-CR-EtSH (anti) was greater compared to that for the two gauche conformers (see Table 1). Hence, the conformer probed by the C transition was assigned as the p-CR-EtSH (anti) conformer and that appearing at transition D as the p-CR-EtSH (gauche) conformer. Once again the exact assignment of the gauche 1 or gauche 2 conformer was not possible with the present experimental data. The values of the charge density and the Laplacian of the charge density at the BCP for the O-H · · · S HB was almost half of that for the O-H · · · O HB conformers (Table 3). The bond strength can be correlated to the magnitude of the charge density, and in that sense these are qualitatively consistent with the computed binding energies and the red shift of the O-H stretch. The concept of hydrogen bond radii proposed by Arunan et al.86 was used to quantify the HB strength. The authors have reported the average donor bond radii or the hydrogen bond radii (i.e., the distance between the BCP and the H atom) and the acceptor bond radii (the distance between the acceptor atom and the BCP) for a large number of acceptor molecules with a few H-bond donors which form strong, moderate, and weak hydrogen bonds. It was shown that the smaller the hydrogen bond radius, the stronger the hydrogen bond. The hydrogen bond radius in the case of the p-CR-alcohol complexes was about 1.2 times smaller than that of the p-CR-thiol complexes, suggesting that the p-CR-alcohol complexes are stronger than the p-CR-thiol complexes. This ratio correlates well with the ratio (1.25:1) of the experimentally observed red shift in the O-H stretching frequencies for the two complexes. The acceptor bond radii (RY · · · BCP) for alcohols and thiols in their respective hydrogen-bonded complexes are 1.23 and 1.66 Å, respectively, which are slightly smaller than the average values of the H2O and H2S acceptor bond radii, i.e., 〈RY · · · BCP〉 of 1.29 and 1.76 Å, respectively, suggesting that the alcohols and thiols form
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stronger hydrogen-bonded complexes relative to H2O and H2S, respectively. The NBO study suggests that nonbonding electron pairs (LP) on O and S atoms are not equivalent and both of them are involved in hydrogen bonding. In the case of alcohols one LP is a pure p-type oribital and another one is an sp2 hybrid orbital, whereas in the case of thiols one of the LPs is a pure s-type orbital and the other one is a pure p-type orbital. This gives rise to the difference in the structural parameter, angle Ψ, for the p-CR-thiol complexes with respect to that for the p-CR-alcohol complexes; it was ∼136° for the O-H · · · O and ∼94° for the O-H · · · S H-bonded complexes, respectively. This is due to the insignificant contribution from the second lone pair orbital (the s type) toward overlap with the σ*(O-H) orbital in the case of p-CR-thiol complexes. The second-order perturbation energy, E(2) ifj*, for the p-CR-alcohol complexes was greater than that for the p-CR-thiol complexes, in agreement with the relative red shifts in the O-H stretch. For all the p-CR-alcohol and p-CR-thiol complexes, various energy decomposition schemes were used in bringing out the relative contributions of various types of interactions for these complexes. All the energy decomposition analyses suggested that, in the case of O-H · · · S HBs, the dispersion interaction energy was the major contributor to the total interaction energy; however, its relative contribution decreased from 70% in the case of the p-CR-H2S complex to about 50% in the present case where one of the hydrogen atoms was substituted with an alkyl group. For the O-H · · · O complexes, however, the dispersion contribution increased from 24% in the case of the p-CR-H2O complex to 34% in the case of the p-CR-EtOH complex. It is difficult to provide a categorical explanation for the apparently opposite trends in the two series. However, a broad explanation could be that in the case of O-H · · · O complexes the presence of the inductive alkyl group itself is responsible for the increase in the dispersive component. In the case of O-H · · · S complexes, however, the diffuse orbitals on the S atom are the primary source of the dispersive contribution to the binding energy of the H-bonded complex. When one of the hydrogen atoms in H2S is replaced by an alkyl group in thiols, part of the diffuse electron density of the S atom interacts with the alkyl group, thereby reducing the dispersion component of the H-bond energy. One more thing that became apparent from the energy decomposition analysis was that the combined contribution of the CT and PL energy components increased with an increase in the alkyl chain length of both the O (H2O-MeOH-EtOH) and S (H2S-MeSH-EtSH) acceptors. The trend in the increment of the PL energy component for O acceptors was 1:1.20:1.20 (normalized with respect to H2O), and that for S acceptors was 1:1.27:1.27 (normalized with respect to H2S), while those values for the CT energy component were 1:1.14:1.19 and 1:1.17:1.72 for the O and S acceptors, respectively. This suggests that the increase in the red shift of the O-H stretch with an increase in the alkyl chain length of the respective acceptors is because of the higher contribution of the CT and PL energy components, while the total stabilization energy is the conglomeration of all the energy components. 7. Conclusions A comparative study of the p-CR-alcohol and p-CR-thiol complexes was carried out. Alcohols such as MeOH and EtOH were taken as the O-centered HB acceptor, and ethanethiol was taken as the S-centered HB acceptor. Since methanethiol was not available commercially, its complex was investigated computationally, and the results were compared. In the case of
Biswal et al. the p-CR-EtOH and p-CR-EtSH complexes the presence of two conformers was observed under the supersonic jet expansion conditions. With the help of FDIRS and IR/UV SHB spectroscopy, these conformers were assigned as the anti and gauche conformers. In both the cases the anti conformer was a more stable hydrogen-bonded complex than the gauche conformer. For all the complexes optimized structures were computed at the B3LYP level using the aug-cc-pVDZ basis set, and their reliability was established using the IR data. For all the p-CR-alcohol complexes, the red shifts in the band origin and O-H stretch were greater compared to those of the p-CR-thiol complexes. The greater red shift of the band origin for p-CR-alcohol suggests that the relative stabilization of the S1 state of the alcohol complexes is higher than that in the case of thiol complexes, and the larger red shift of the O-H stretching frequency suggests that O-H · · · O hydrogen bonding is stronger than O-H · · · S hydrogen bonding. With an increase in the alkyl chain length on the acceptor site, the hydrogen bond strength increases as inferred from the computed binding energies as well as the relative red shifts of the O-H stretch. One of the important observations from this study is that though the proton affinities of the sulfur HB acceptors are greater compared to those of the corresponding oxygen HB acceptors, the experimentally observed red shifts of the O-H stretch for the O-H · · · S hydrogen-bonded complexes are smaller than those of the O-H · · · O hydrogen-bonded complexes. Therefore, it can be inferred that O-H · · · S hydrogen bonding does not confirm the acid-base formalism; i.e., it is not predominantly electrostatic. The energy decomposition analysis shows that the dispersion energy contribution for O-H · · · S hydrogen bonding is more than that for O-H · · · O hydrogen bonding and corroborates the inference drawn above. Supporting Information Available: Relaxed potential energy surface for (a) p-CR-EtOH and (b) p-CR-EtSH complexes computed at the B3LYP/6-311++G(d,p) level of theory. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Biswal, H. S.; Shirhatti, P. R.; Wategaonkar, S. J. Phys. Chem. A 2009, 113, 5633. (2) Platts, J. A.; Howard, S. T.; Bracke, B. R. F. J. Am. Chem. Soc. 1996, 118, 2726. (3) Biswal, H. S.; Chakraborty, S.; Wategaonkar, S. J. Chem. Phys. 2008, 129, 184311. (4) Biswal, H. S.; Wategaonkar, S. J. Phys. Chem. A 2010, 114, 5947. (5) Biswal, H. S.; Wategaonkar, S. J. Phys. Chem. A 2009, 113, 12774. (6) Biswal, H. S.; Wategaonkar, S. J. Phys. Chem. A 2009, 113, 12763. (7) Bakke, J. M.; Bjerkeseth, L. H. J. Mol. Struct. 1997, 407, 27. (8) Huisken, F.; Kulcke, A.; Laush, C.; Lisy, J. M. J. Phys. Chem. 1991, 95, 3924. (9) Larsen, R. W.; Zielke, P.; Suhm, M. A. J. Chem. Phys. 2007, 126. (10) Hu, Y. J.; Fu, H. B.; Bernstein, E. R. J. Chem. Phys. 2006, 125. (11) Wolff, H.; Szydlowski, J. Can. J. Chem. 1985, 63, 1708. (12) Krueger, P. J.; Jan, J.; Wieser, H. J. Mol. Struct. 1970, 5, 375. (13) Coussan, S.; Bouteiller, Y.; Perchard, J. P.; Zheng, W. Q. J. Phys. Chem. A 1998, 102, 5789. (14) Emmeluth, C.; Dyczmons, V.; Kinzel, T.; Botschwina, P.; Suhm, M. A.; Yanez, M. Phys. Chem. Chem. Phys. 2005, 7, 991. (15) Emmeluth, C.; Dyczmons, V.; Suhm, M. A. J. Phys. Chem. A 2006, 110, 2906. (16) Gardner, E. A.; Nevarez, A.; Garbalena, M.; Herndon, W. C. J. Mol. Struct. 2006, 784, 249. (17) Scheiner, S.; Seybold, P. G. Struct. Chem. 2009, 20, 43. (18) Chen, X.; Wu, F.; Yan, M.; Li, H.-B.; Tian, S. X.; Shan, X.; Wang, K.; Li, Z.; Xu, K. Chem. Phys. Lett. 2009, 472, 19. (19) Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1982, 86, 1768. (20) Abe, H.; Mikami, N.; Ito, M.; Udagawa, Y. J. Phys. Chem. 1982, 86, 2567. (21) Lipert, R. J.; Colson, S. D. J. Phys. Chem. 1989, 93, 3894.
O-H · · · O vs O-H · · · S Hydrogen Bonding (22) Hartland, G. V.; Henson, B. F.; Venturo, V. A.; Felker, P. M. J. Phys. Chem. 1992, 96, 1164. (23) Cordes, E.; Dopfer, O.; Wright, T. G.; Muller-dethlefs, K. J. Phys. Chem. 1993, 97, 7471. (24) Gerhards, M.; Beckmann, K.; Kleinermanns, K. Z. Phys. D: At., Mol. Clusters 1994, 29, 223. (25) Schmitt, M.; Muller, H.; Henrichs, U.; Gerhards, M.; Perl, W.; Deusen, C.; Kleinermanns, K. J. Phys. Chem. 1995, 103, 584. (26) Courty, A.; Mons, M.; Dimicoli, I.; Piuzzi, F.; Brenner, V.; Millie, P. J. Phys. Chem. A 1998, 102, 4890. (27) Spangenberg, D.; Imhof, P.; Roth, W.; Janzen, C.; Kleinermanns, K. J. Phys. Chem. A 1999, 103, 5918. (28) Schmitt, M.; Kupper, J.; Spangenberg, D.; Westphal, A. Chem. Phys. 2000, 254, 349. (29) Plutzer, C.; Jacoby, C.; Schmitt, M. J. Phys. Chem. A 2002, 106, 3998. (30) Westphal, A.; Jacoby, C.; Ratzer, C.; Reichelt, A.; Schmitt, M. Phys. Chem. Chem. Phys. 2003, 5, 4114. (31) Li, H. M.; Thomas, G. J. J. Am. Chem. Soc. 1991, 113, 456. (32) Yasui, N.; Koide, T. J. Am. Chem. Soc. 2003, 125, 15728. (33) Cheung, Y. S.; Hsu, C. W.; Huang, J. C.; Ng, C. Y.; Li, W. K.; Chiu, S. W. Int. J. Mass Spectrom. Ion Phys. 1996, 159, 13. (34) Choi, S.; Kang, T. Y.; Choi, K. W.; Han, S.; Ahn, D. S.; Baek, S. J.; Kim, S. K. J. Phys. Chem. A 2008, 112, 7191. (35) Meenakshi, P. S.; Biswas, N.; Wategaonkar, S. J. Chem. Phys. 2002, 117, 11146. (36) Ebata, T.; Mizuochi, N.; Watanabe, T.; Mikami, N. J. Phys. Chem. 1996, 100, 546. (37) Ebata, T.; Fujii, A.; Mikami, N. Int. ReV. Phys. Chem. 1998, 17, 331. (38) Hobza, P.; Sponer, J. Chem. ReV. 1999, 99, 3247. (39) Shamovsky, I. L.; Riopelle, R. J.; Ross, G. M. J. Phys. Chem. A 2001, 105, 1061. (40) Bayari, S.; Saglam, S.; Ustundag, H. F. J. Mol. Struct.: THEOCHEM 2005, 726, 225. (41) Liedl, K. R.; Kroemer, R. T. J. Phys. Chem. A 1998, 102, 1832. (42) Tuma, C.; Boese, A. D.; Handy, N. C. Phys. Chem. Chem. Phys. 1999, 1, 3939. (43) Hwang, R.; Huh, S. B.; Lee, J. S. Mol. Phys. 2003, 101, 1429. (44) Wang, B. Q.; Li, Z. R.; Wu, D.; Hao, X. Y.; Li, R. J.; Sun, C. C. Chem. Phys. Lett. 2003, 375, 91. (45) Hwang, R.; Park, Y. C.; Lee, J. S. Theor. Chem. Acc. 2006, 115, 54. (46) Park, Y. C.; Lee, J. S. Bull. Korean Chem. Soc. 2007, 28, 386. (47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.1; Gaussian, Inc.: Wallingford, CT, 2005. (48) ChemCraft Web Site. http://www.softpedia.com/progDownload/ ChemCraft Download-56007.html. (49) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Clarendon Press: Oxford, U.K., 1990. (50) Bader, R. F. W. Chem. ReV. 1991, 91, 893.
J. Phys. Chem. A, Vol. 114, No. 26, 2010 6955 (51) Bader, R. F. W. In Encyclopedia of Computational Chemistry; Schleyer, P. v. R., Allinger, N. L., Gasteiger, T., Clark, J., Kollman, P. A., Schaefer, H. F. S., III, Schreiner, P. R., Eds.; John Wiley & Sons: Chichester, U.K., 1998; Vol. 1; p 64. (52) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899. (53) Weinhold, F. In Encyclopedia of Computational Chemistry; Schleyer, P. v. R., Allinger, N. L., Gasteiger, T., Clark, J., Kollman, P. A., Schaefer, H. F. S., III, Schreiner, P. R., Eds.; John Wiley & Sons: Chichester, U.K., 1998; Vol. 3; p 1792. (54) Weinhold, F.; Landis, C. R. Chem. Educ. Res. Pract. Eur. 2001, 2, 91. (55) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural Bond Orbital Donor-Acceptor PerspectiVe; Cambridge University Press: New York, 2005. (56) NBO 5.0; Glendening, J., Badenhoop, K. Reed, A. E. Carpenter, J. E. Bohmann, J. A. Morales, C. M. Weinhold, F., Eds.; Theoretical Chemistry Institute, University of Wisconsin: Madison, 2001. (57) Chalasinski, G.; Szczesniak, M. M. Chem. ReV. 2000, 100, 4227. (58) Glendening, E. D.; Streitwieser, A. J. Chem. Phys. 1994, 100, 2900. (59) Glendening, E. D. J. Am. Chem. Soc. 1996, 118, 2473. (60) Schenter, G. K.; Glendening, E. D. J. Phys. Chem. 1996, 100, 17152. (61) Kitaura, K.; Morokuma, K. Int. J. Quantum Chem. 1976, 10, 325. (62) Stevens, W. J.; Fink, W. H. Chem. Phys. Lett. 1987, 139, 15. (63) Chen, W.; Gordon, M. S. J. Phys. Chem. 1996, 100, 14316. (64) 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. J. Comput. Chem. 1993, 14, 1347. (65) Lin, J. L.; Li, C. Y.; Tzeng, W. B. J. Chem. Phys. 2004, 120, 10513. (66) Frost, R. K.; Hagemeister, F. C.; Arrington, C. A.; Schleppenbach, D.; Zwier, T. S.; Jordan, K. D. J. Chem. Phys. 1996, 105, 2605. (67) Frost, R. K.; Hagemeister, F. C.; Arrington, C. A.; Zwier, T. S.; Jordan, K. D. J. Chem. Phys. 1996, 105, 2595. (68) Mitsuzuka, A.; Fujii, A.; Ebata, T.; Mikami, N. J. Chem. Phys. 1996, 105, 2618. (69) Chalasinski, G.; Szczesniak, M. M. Chem. ReV. 1994, 94, 1723. (70) Sponer, J.; Sabat, M.; Burda, J. V.; Leszczynski, J.; Hobza, P. J. Phys. Chem. B 1999, 103, 2528. (71) van Mourik, T. J. Chem. Theory Comput. 2008, 4, 1610. (72) Antony, J.; Grimme, S. Phys. Chem. Chem. Phys. 2006, 8, 5287. (73) Morgado, C.; Vincent, M. A.; Hillier, I. H.; Shan, X. Phys. Chem. Chem. Phys. 2007, 9, 448. (74) Barone, V.; Biczysko, M.; Pavone, M. Chem. Phys. 2008, 346, 247. (75) Hohenstein, E. G.; Chill, S. T.; Sherrill, C. D. J. Chem. Theory Comput. 2008, 4, 1996. (76) Lin, I. C.; Rothlisberger, U. Phys. Chem. Chem. Phys. 2008, 10, 2730. (77) Aeberhard, P. C.; Arey, J. S.; Lin, I. C.; Rothlisberger, U. J. Chem. Theory Comput. 2009, 5, 23. (78) Johnson, E. R.; Mackie, I. D.; DiLabio, G. A. J. Phys. Org. Chem. 2009, 22, 1127. (79) Lill, S. O. N. J. Phys. Chem. A 2009, 113, 10321. (80) Foster, M. E.; Sohlberg, K. Phys. Chem. Chem. Phys. 2010, 12, 307. (81) Pavlov, A.; Mitrasinovic, P. M. Curr. Org. Chem. 2010, 14, 129. (82) Koch, U.; Popelier, P. L. A. J. Phys. Chem. 1995, 99, 9747. (83) Grabowski, S. J. J. Phys. Org. Chem. 2004, 17, 18. (84) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899. (85) Tanabe, S.; Ebata, T.; Fujii, M.; Mikami, N. Chem. Phys. Lett. 1993, 215, 347. (86) Raghavendra, B.; Mandal, P. K.; Arunan, E. Phys. Chem. Chem. Phys. 2006, 8, 5276.
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