Sanjay Wategaonkar

Sulfur, Not Too Far Behind O, N, and C: SH···π Hydrogen Bond

We report hydrogen-bonded complexes of H(2)S with indole and 3-methyl indole stabilized by the S-H...pi interaction. It is interesting to discover that although sulfur and its hydrides are known as poor hydrogen-bond donor/acceptors, sulfur is not too far behind oxygen, nitrogen, and carbon in regard to forming the pi-type hydrogen bonds. This report also extends the scope of our earlier studies from sigma-type hydrogen-bonded complexes of sulfur (O-H...S and N-H...S sigma-type hydrogen-bonded complexes) to pi-type hydrogen-bonded complexes of sulfur (S-H...pi pi-type hydrogen-bonded complexes). The experiments were carried out using the supersonic jet expansion technique, and the complexes were probed using laser-induced spectroscopy such as laser-induced fluorescence (LIF), resonant two-photon inonization (R2PI), and fluorescence dip infrared spectroscopy (FDIRS). The FDIR spectroscopy revealed that while there was no shift in the N-H stretch, the S-H stretch was red shifted by about 21 cm(-1). For the H(2)O complexes of indole and 3-methylindole, however, there was a significant red shift in the N-H stretch. These observations suggest that H(2)O forms a NH...O type complex, whereas H(2)S prefers to form a SH...pi type complex. The experimental results were complemented by ab initio calculations and energy decomposition analysis. The binding energies for both the sigma-type and pi-type hydrogen-bonded M.L complexes (M = indole and 3-methylindole; L = H(2)O and H(2)S) were calculated by extrapolating MP2 interaction energies to the complete basis set limit. The calculated M.H(2)S (sigma-type) interaction energy (2.74 kcal/mol) was considerably smaller than that of the M.H(2)S pi-type hydrogen-bonded complex (4.89 kcal/mol), which is exactly opposite of the trend found for the M.H(2)O complexes. This is consistent with the experimental observations. Comparison of the S-H...pi interaction with the other type of X-H...pi (X = C, N, and O) shows that the S-H...pi interaction is the strongest among them. In all of the pi-type HB complexes, the dispersion energy component has significant contribution to the total binding energy.

O−H···O versus O−H···S Hydrogen Bonding I: Experimental and Computational Studies on the p-Cresol·H2O and p-Cresol·H2S Complexes

The weak hydrogen bonding ability of sulfur-containing hydrides makes it difficult to study their complexes and has not been characterized experimentally so far. In this work, the hydrogen-bonded complexes of H(2)S and H(2)O with p-cresol (p-CR) were studied using a variety of techniques such as two-color resonant two-photon ionization (2c-R2PI) spectroscopy, single vibronic level fluorescence (SVLF) spectroscopy, resonance ion dip infrared spectroscopy (RIDIRS), and fluorescence dip infrared spectroscopy (FDIRS), with an aim of comparing the nature and strength of their respective hydrogen bonding abilities. The intermolecular stretch (sigma) and the shift in the O-H stretching frequency of p-CR in the complex were taken as the measures of the O-H...O and O-H...S hydrogen bonding strength. The experiments were complemented by the ab initio calculations, atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analyses carried out at different levels of theory. The experimental data indicates that in the p-CR x H(2)S complex, the phenolic OH group acts as a hydrogen bond donor, and sulfur as the acceptor. Further, it indicates that the p-CR x H(2)S complex was about half as strong as the p-CR x H(2)O complex. The AIM and NBO analyses corroborate the experimental findings. The energy decomposition analyses for the O-H...S hydrogen bond in the p-CR x H(2)S complex reveal that the dispersion interaction energy has the largest contribution to the total interaction energy, which is significantly higher than that in the case of the p-CR x H(2)O complex.

O−H···O versus O−H···S Hydrogen Bonding. 2. Alcohols and Thiols as Hydrogen Bond Acceptors

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 H(2)S 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.

Laser‐induced fluorescence spectroscopy of jet cooled p‐aminophenol

The laser‐induced fluorescence spectra of p‐aminophenol both in excitation and emission have been studied in a supersonic jet apparatus. The characterization of the observed spectra was done by comparison with other related substituted anilines and the IR data available in the literature. The excitation spectrum resembles that of aniline with optical activity mainly confined to 6a, 1, and the NH2 inversion mode. In addition, the C–X in‐plane bending mode 9b was also found to be optically active. The 6a mode dominates in most of the dispersed fluorescence spectra and shows a strong Franck–Condon activity. Unlike other similar molecules, the Δv=0 transitions were weak in the single vibronic level fluorescence spectra of 6a1 and 11, which has been qualitatively explained in terms of Franck–Condon analysis. The onset of intramolecular vibrational redistribution occurs at 1135 cm−1, which is much higher than many substituted anilines. The van der Waals complexes viz. p‐aminophenol–Ar1 and p‐aminophenol–Ar2 wer...

Jet spectroscopy of p-methoxyaniline and intramolecular vibrational relaxation dynamics in the p-alkoxyaniline series

The vibronic spectroscopy of p-methoxyaniline (p-anisidine) in the S1 and S0 states has been studied using laser induced fluorescence in a supersonic jet apparatus. The band origin is found at 31 581 cm−1. Vibrational modes 6a, 6b, 1, I, and 10b are found to be active in the excitation and emission spectrum and their frequencies are 395, 565, 821, 668(v=2), and 499(v=2) cm−1 in the excited state and 426, 642, 845, 472(v=2), and 525(v=2) cm−1 in the ground state, respectively. The intramolecular vibrational relaxation (IVR) dynamics discerned from the single vibronic level fluorescence (SVLF) spectra is compared with p-aminophenol and p-ethoxyaniline, the other two members of the p-alkoxyaniline series. It is postulated that the IVR dynamics in this series is dependent on the changes in the electronic structure in the excited state rather than the increase in the density of states alone.

Experimental evidence of O–H—S hydrogen bonding in supersonic jet

Experimental evidence is presented for the O-H-S hydrogen bonding in the complexes of simple model compounds of methionine (dimethyl sulphide) and tyrosine (phenol, p-cresol, and 2-naphthol). The complexes were formed in the supersonic jet and were detected using resonantly enhanced multiphoton ionization spectroscopy. In all the complexes, the band origins for the S(1)-S(0) electronic transition were redshifted relative to that of their respective monomers. The resonant ion depletion IR spectra of all the complexes show redshifts of 123-140 cm(-1) in the O-H stretching frequency, indicating that the OH group acts as the hydrogen bond donor and sulfur as an acceptor. The density functional theory calculations also predict the stable structures in support of this and predict the redshifted O-H stretching frequency in the complex. The atoms-in-molecules and natural bond orbital calculations confirm the O-H-S hydrogen bonding interaction. The significant finding of this study is that the magnitudes of redshifts in the O-H stretch in the O-H-S hydrogen bonded complexes reported here are comparable to those reported for the O-H-O hydrogen bonded complexes where H(2)O acts as the H-bond acceptor, which suggests that the OH-S interaction is perhaps as strong as the OH-O interaction. To the best of our knowledge, this is the first such report on the O-H-S hydrogen bonded complexes.

The Intermolecular SH⋅⋅⋅Y (Y=S,O) Hydrogen Bond in the H2S Dimer and the H2S–MeOH Complex

The nature of the S−H⋅⋅⋅S hydrogen-bonding interaction in the H2 S dimer and its structure has been the focus of several theoretical studies. This is partly due to its structural similarity and close relationship with the well-studied water dimer and partly because it represents the simplest prototypical example of hydrogen bonding involving a sulfur atom. Although there is some IR data on the H2 S dimer and higher homomers from cold matrix experiments, there are no IR spectroscopic reports on S−H⋅⋅⋅S hydrogen bonding in the gas phase to-date. We present experimental evidence using VUV ionization-detected IR-predissociation spectroscopy (VUV-ID-IRPDS) for this weak hydrogen-bonding interaction in the H2 S dimer. The proton-donating S−H bond is found to be red-shifted by 31 cm(-1) . We were also able to observe and assign the symmetric (ν1 ) stretch of the acceptor and an unresolved feature owing to the free S−H of the donor and the antisymmetric (ν3 ) SH stretch of the acceptor. In addition we show that the heteromolecular H2 S-MeOH complex, for which both S−H⋅⋅⋅O and O−H⋅⋅⋅S interactions are possible, is S-H⋅⋅⋅O bound.

Blue shifted hydrogen bond in 3-methylindole·CHX3 complexes (X = Cl, F)

Although the first experimental report on the blue shifted hydrogen bond in gas phase appeared about a decade ago, not many examples have been reported in the literature thereafter. Computational studies on systems exhibiting such blue shifted hydrogen bond however have been abundant since then. Many of these theoretical predictions remain to be verified experimentally. In this work we present an example of blue shifted hydrogen bond observed in the weakly bound complexes of 3-methylindole and CHX(3) (X = F, Cl). The complexes were prepared using the supersonic jet expansion method and studied using the laser spectroscopic methods. The key findings were that these complexes exhibit C-Hpi type hydrogen bonding interaction and the CH is bound to the phenyl part of the aromatic plane. The CH stretch was found to be blue shifted by 2 and 16 cm(-1) in the case of CHCl(3) and CHF(3), respectively. Ab initio calculations along with atoms-in-molecule analysis and natural bond orbital analysis support the experimental findings. The computed results at the DFT/MP2 level also indicated that the IR intensity of the H-bond donor CH-stretch increases by two to three orders of magnitude for the CHCl(3) complex whereas for the fluoroform complex the same decreases by an order of magnitude, which are consistent with the trend reported in the case of C-HO type of blue shifting hydrogen bonds.

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

Hydrogen bonding interaction between the ROH hydrogen bond donor and sulfur atom as an acceptor has not been as well characterized as the O-H···O interaction. The strength of O-H···O interactions for a given donor has been well documented to scale linearly with the proton affinity (PA) of the H-bond acceptor. In this regard, O-H···O interactions conform to the acid-base formalism. The importance of such correlation is to be able to estimate molecular property of the complex from the known thermodynamic data of its constituents. In this work, we investigate the properties of O-H···S interaction in the complexes of the H-bond donor and sulfur containing acceptors of varying proton affinity. The hydrogen bonded complexes of p-Fluorophenol (FP) with four different sulfur containing acceptors and their oxygen analogues, namely H2O/H2S, MeOH/MeSH, Me2O/Me2S and tetrahydrofuran (THF)/tetrahydrothiophene (THT) were characterized in regard to its S1-S0 excitation spectra and the IR spectra. Two-color resonantly enhanced multiphoton ionization (2c-R2PI), resonant ion-dip infrared (RIDIR) spectroscopy, and IR-UV hole burning spectroscopic techniques were used to probe the hydrogen bonds in the aforementioned complexes. The spectroscopic data along with the ab initio calculations were used to deduce the strength of the O-H···S hydrogen bonding interactions in these system relative to that in the O-H···O interactions. It was found that, despite being dominated by the dispersion interaction, the O-H···S interactions conform to the acid-base formalism as in the case of more conventional O-H···O interactions. The dissociation energies and the red shifts in the O-H stretching frequencies correlated very well with the proton affinity of the acceptors. However, the O-H···S interaction did not follow the same correlation as that in the O-H···O H-bond. The energy decomposition analysis showed that the dissociation energies and the red shifts in the O-H stretching frequencies follow a unified correlation if these two parameters were correlated with the sum of the charge transfer and the exchange component of the total binding energy.

OH···X (X = O, S) hydrogen bonding in thetrahydrofuran and tetrahydrothiophene

In this article, hydrogen bonding interaction between p-cresol (p-CR) and cyclic ether, tetrahydrofuran (THF) and thioether, tetrahydrothiophene (THT) has been investigated. Two-color resonantly enhanced two-photon ionization in conjunction with the fluorescence detected IR (FDIR) spectroscopy was used to record the changes in the OH stretching frequency in these complexes. The FDIR spectra showed existence of a single conformer of the p-CR·THF and two conformers of the p-CR·THT complex. With the help of computed IR spectra and atoms-in-molecules analysis, the two conformers of p-CR·THT were assigned as the complex of p-CR with THT (C(2))/THT (C(S)). The redshift of OH stretching frequency for the p-CR·THF complex was greater compared to those for the conformers of the p-CR·THT complex. The binding energies of the p-CR·THF and p-CR·THT complexes were computed to be 7.42 and 6.15 kcal/mole. These were of the same order as those for the acyclic analogs, diethylether (DEE), and diethylsulfide (DES), of the solvent molecules under investigation. Although the DEE and THF consist of same number of carbon atoms, the dispersion energy contribution was much higher (43%) for DEE than that for THF (30%). In the case of sulfur analogs, however, it was similar (~50%) in the case of both DES well as THT complexes. All the computed H-bond indicators for these two complexes nicely correlate with the observed redshift of the O-H stretch.