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

Hydrogen Bonding with Polonium

Hydrogen bonding (H-bonding) with heavier chalcogens such as polonium and tellurium is almost unexplored owing to their lower electronegativities, providing us an opportunity to delve into the uncharted territory of...

Comparing C2=O and C2=S Barbiturates: Different Hydrogen-Bonding Patterns of Thiobarbiturates in Solution and the Solid State

Carbonyl-centered hydrogen bonds with various strength and geometries are often exploited in materials to embed dynamic and adaptive properties, with the use of thiocarbonyl groups as hydrogen-bonding acceptors remaining only scarcely investigated. We herein report a comparative study of C2=O and C2=S barbiturates in view of their differing hydrogen bonds, using the 5,5-disubstituted barbiturate B and the thiobarbiturate TB as model compounds. Owing to the different hydrogen-bonding strength and geometries of C2=O vs. C2=S, we postulate the formation of different hydrogen-bonding patterns in C2=S in comparison to the C2=O in conventional barbiturates. To study differences in their association in solution, we conducted concentration- and temperature-dependent NMR experiments to compare their association constants, Gibbs free energy of association ∆Gassn., and the coalescence behavior of the N-H‧‧‧S=C bonded assemblies. In Langmuir films, the introduction of C2=S suppressed 2D crystallization when comparing B and TB using Brewster angle microscopy, also revealing a significant deviation in morphology. When embedded into a hydrophobic polymer such as polyisobutylene, a largely different rheological behavior was observed for the barbiturate-bearing PB compared to the thiobarbiturate-bearing PTB polymers, indicative of a stronger hydrogen bonding in the thioanalogue PTB. We therefore prove that H-bonds, when affixed to a polymer, here the thiobarbiturate moieties in PTB, can reinforce the nonpolar PIB matrix even better, thus indicating the formation of stronger H-bonds among the thiobarbiturates in polymers in contrast to the effects observed in solution.

A unified cost-effective method for the construction of reliable potential energy surfaces for H2S and H2O clusters

A DFT-based methodology has been used to construct the potential energy surface of H2S clusters up to pentamers. Geometrical parameters and energetics show very good agreement with the existing experimental and high-level theoretical results. Distinct stable conformers of three dimers, six trimers, eleven tetramers and twenty-three pentamers have been identified. Both S-HS H-bond and SS interactions are identified in dimers, trimers and pentamers, while no SS interactions could be found in any of the 11 tetramer conformers. The binding energies of the most stable dimer, trimer, tetramer and pentamer are -1.66, -5.21, -8.57 and -12.54 kcal mol-1, respectively. The PES has been found to be exceedingly flat and the energy gap between the most and the least stable conformers was found to be only 0.09, 2.13, 1.65 and 1.13 kcal mol-1, from the dimer to the pentamer, respectively. The proposed method has also been used for water clusters up to the pentamer. The results obtained were found to agree closely with the existing results. Only one conformer was found for the water dimer, whereas four, five and fifteen conformers were obtained for the trimer, tetramer and pentamer, respectively. Atoms in molecular calculations were found to corroborate with the geometric and energetic results for both clusters.

O-H stretching frequency red shifts do not correlate with the dissociation energies in the dimethylether and dimethylsulfide complexes of phenol derivatives

In this perspective, we present a comprehensive report on the spectroscopic and computational investigations of the hydrogen bonded (H-bonded) complexes of Me2O and Me2S with seven para-substituted H-bond donor phenols. The salient finding was that although the dissociation energies, D0, of the Me2O complexes were consistently higher than those of the analogous Me2S complexes, the red-shifts in phenolic O-H frequencies, Δν(O-H), showed the exactly opposite trend. This is in contravention of the general perception that the red shift in the X-H stretching frequency in the X-HY hydrogen bonded complexes is a reliable indicator of H-bond strength (D0), a concept popularly known as the Badger-Bauer rule. This is also in contrast to the trend reported for the H-bonded complexes of H2S/H2O with several para substituted phenols of different pKa values wherein the oxygen centered hydrogen bonded (OCHB) complexes consistently showed higher Δν(O-H) and D0 compared to those of the analogous sulfur centered hydrogen bonded (SCHB) complexes. Our effort was to understand these intriguing observations based on the spectroscopic investigations of 1 : 1 complexes in combination with a variety of high level quantum chemical calculations. Ab initio calculations at the MP2 level and the DFT calculations using various dispersion corrected density functionals (including DFT-D3) were performed on counterpoise corrected surfaces to compute the dissociation energy, D0, of the H-bonded complexes. The importance of anharmonic frequency computations is underscored as they were able to correctly reproduce the observed trend in the relative OH frequency shifts unlike the harmonic frequency computations. We have attempted to find a unified correlation that would globally fit the observed red shifts in the O-H frequency with the H-bonding strength for the four bases, namely, H2S, H2O, Me2O, and Me2S, in this set of H-bond donors. It was found that the proton affinity normalized Δν(O-H) values scale very well with the H-bond strength.

Observation of an Unusually Large IR Red-Shift in an Unconventional S-H···S Hydrogen-Bond

The S-H···S non-covalent interaction is generally known as an extremely unconventional weak hydrogen-bond in the literature. The present gas-phase spectroscopic investigation shows that the S-H···S hydrogen-bond can be as strong as any conventional hydrogen-bond in terms of the IR red-shift in the stretching frequency of the hydrogen-bond donor group. Herein, the strength of the S-H···S hydrogen-bond has been determined by measuring the red-shift (∼150 cm^-1) of the S-H stretching frequency in a model complex of 2-chlorothiophenol and dimethyl sulfide using isolated gas-phase IR spectroscopy coupled with quantum chemistry calculations. The observation of an unusually large IR red-shift in the S-H···S hydrogen-bond is explained in terms of the presence of a significant amount of charge-transfer interactions in addition to the usual electrostatic interactions. The existence of ∼750 S-H···S interactions between the cysteine and methionine residues in 642 protein structures determined from an extensive Protein Data Bank analysis also indicates that this interaction is important for the structures of proteins.

Adsorption of sulfur mustard on clean and water-saturated ZnO(101¯0): Structural diversity from first-principles calculations

We investigate the adsorption of a chemical warfare agent, namely sulfur mustard (SM), on clean and water-saturated ZnO(101¯0) surfaces using density functional theory calculations to understand the first step of its efficient neutralization to less toxic chemical compounds. We determine the relative stability of various SM conformers adsorbed at different sites on both ZnO surfaces. The unique hydrogen bonding patterns obtained for the idealized clean and the more realistic water-saturated ZnO surface are analyzed and their influence on the stability of the SM@ZnO structures is demonstrated. We find that absolute values of the calculated binding and interaction energies are significantly higher for the clean than for the water-saturated ZnO surface due to the formation of Cl⋯Zn and S⋯Zn contacts. The high adsorptive reactivity of the clean ZnO surface is also evident from the strong structural changes of the initial local energy minimum gas-phase conformations of the SM molecules upon adsorption. This phenomenon is not observed for the water-saturated ZnO surface, which has almost no impact on the SM conformation after adsorption, leaving it as it exists in the gas phase. The insights from the results obtained provide a missing piece toward the understanding of the complex mechanism of SM neutralization on ZnO surfaces.

The Prodigious Hydrogen Bonds with Sulfur and Selenium in Molecular Assemblies, Structural Biology, and Functional Materials

Hydrogen bonds (H-bonds) play important roles in imparting functionality to the basic molecules of life by stabilizing their structures and directing their interactions. Numerous studies have been devoted to understanding H-bonds involving highly electronegative atoms like nitrogen, oxygen, and halogens and consequences of those H-bonds in chemical reactions, catalysis, and structure and function of biomolecules; but the involvement of less electronegative atoms like sulfur and selenium in H-bond formation establishes the concept of noncanonical H-bonds. Initially belittled for the "weak" nature of their interactions, these perceptions have gradually evolved over time through dedicated efforts by several research groups. This has been facilitated by advancements in experimental methods for their detection through gas-phase laser spectroscopy and solution NMR spectroscopy, as well as through theoretical predictions from high level quantum chemical calculations.In this Account, we present insights into the versatility of the sulfur and selenium centered H-bonds (S/SeCHBs) by highlighting their multifarious applications in various fields from chemical reactions to optoelectronic properties to structural biology. Our group has highlighted the significance and strength of such H-bonds in natural and modified biomolecules. Here, we have reviewed several molecular assemblies, biomolecules, and functional materials, where the role of these H-bonds is pivotal in influencing biological functions. It is worth mentioning here that the precise experimental data obtained from gas-phase laser spectroscopy have contributed considerably to changing the existing perceptions toward S/SeCHBs. Thus, molecular beam experiments, though difficult to perform on smaller model thio- or seleno-substituted Molecules, etc. (amides, nucleobases, drug molecules), are inevitable to gather elementary knowledge and convincing concepts on S/SeCHBs that can be extended from a small four-atom sulfanyl dimer to a large 14 kDa iron-sulfur protein, ferredoxin. These H-bonds can also tailor a fascinating array of molecular frameworks and design supramolecular assemblies by inter- and intralinking of individual "molecular Lego-like" units.The discussion is indeed intriguing when it turns to the usage of S/SeCHBs in facile synthetic strategies like tuning regioselectivity in reactions, as well as invoking phenomena like dual phosphorescence and chemiluminescence. This is in addition to our investigations of the dispersive nature of the hydrogen bond between metal hydrides and sulfur or selenium as acceptor, which we anticipate would lead to progress in the areas of proton and hydride transfer, as well as force-field design. This Account demonstrates how ease of fabrication, enhanced efficiency, and alteration of physicochemical properties of several functional materials is facilitated owing to the presence of S/SeCHBs. Our efforts have been instrumental in the evaluation of various S/SeCHBs in flue gas capture, as well as design of organic energy harvesting materials, where dipole moment and polarizability have important roles to play. We hope this Account invokes newer perspectives with regard to how H-bonds with sulfur and selenium can be adequately adopted for crystal engineering, for more photo- and biophysical studies with different spectroscopic methods, and for developing next-generation field-effect transistors, batteries, superconductors, and organic thin-film transistors, among many other multifunctional materials for the future.

C-HS interaction exhibits all the characteristics of conventional hydrogen bonds

This is a tale of a pair of a hydrogen bond donor and acceptor, namely the CH donor and sulphur acceptor, neither of which is a conventional hydrogen bond participant. Sulfur (S), being less electronegative (2.58) compared to its first row analogue oxygen (3.44), has not been considered as a potential HB acceptor for a long time. The C-HY (Y = HB acceptor) interaction has its own history of exhibiting omnidirectional shifts in the CH stretching frequency upon complex formation. Therefore, a systematic investigation of the C-HS interaction was the primary goal of the work presented here. Together with gas-phase vibrational spectroscopy and ab initio quantum chemical calculations, the nature and strength of the C-HS hydrogen bond (HB) have been investigated in the complexes of 1,2,4,5-tetracyanobenzene (TCNB) with various sulfur containing solvents. Despite the unconventional nature of both HB donor and HB acceptor (C-H and S, respectively), it was found that the C-HS hydrogen bond exhibits all the characteristics of the conventional hydrogen bond. The binding strength of the C-HS H-bond in these complexes was found to be comparable to that of the conventional hydrogen bonds. The unusual stabilities of these HBs have been mainly attributed to the attractive dispersion interaction.

Absorptive Desulfurization of Model Biogas Stream Using Choline Chloride-Based Deep Eutectic Solvents

The paper presents a synthesis of deep eutectic solvents (DESs) based on choline chloride (ChCl) as hydrogen bond acceptor and phenol (Ph), glycol ethylene (EG), and levulinic acid (Lev) as hydrogen bond donors in 1:2 molar ratio. DESs were successfully used as absorption solvents for removal of dimethyl disulfide (DMDS) from model biogas steam. Several parameters affecting the absorption capacity and absorption rate have been optimized including kinds of DES, temperature, the volume of absorbent, model biogas flow rate, and initial concentration of DMDS. Furthermore, reusability and regeneration of DESs by means of adsorption and nitrogen barbotage followed by the mechanism of absorptive desulfurization by means of density functional theory (DFT) as well as FT-IR analysis were investigated. Experimental results indicate that the most promising DES for biogas purification is ChCl:Ph, due to high absorption capacity, relatively long absorption rate, and easy regeneration. The research on the absorption mechanism revealed that van der Waal interaction is the main driving force for DMDS removal from model biogas.

Water-Mediated Selenium Hydrogen-Bonding in Proteins: PDB Analysis and Gas-Phase Spectroscopy of Model Complexes

High-resolution X-ray crystallography and two-dimensional NMR studies demonstrate that water-mediated conventional hydrogen-bonding interactions (N-H···N, O-H···N, etc.) bridging two or more amino acid residues contribute to the stability of proteins and protein-ligand complexes. In this work, we have investigated single water-mediated selenium hydrogen-bonding interactions (unconventional hydrogen-bonding) between amino acid residues in proteins through extensive protein data bank (PDB) analysis coupled with gas-phase spectroscopy and quantum chemical calculation of a model complex consisting of indole, dimethyl selenide, and water. Here, indole and dimethyl selenide represent the amino acid residues tryptophan and selenomethionine, respectively. The current investigation demonstrates that the most stable structure of the model complex observed in the IR spectroscopy mimics single water-mediated selenium hydrogen-bonded structural motifs present in the crystal structures of proteins. The present work establishes that water-mediated Se hydrogen-bonding interactions are ubiquitous in proteins and the number of these interactions observed in the PDB is more than that of direct Se hydrogen-bonds present there.

Thiols as Hydrogen Bond Acceptors and Donors: Spectroscopy of 2-Phenylethanethiol Complexes

Evidence and understanding of sulfur-centered hydrogen bonding, especially where the donor is a thiol, lags far behind that for conventional OH interactions. To help address this deficiency, conformer specific IR spectra of 2-phenylethanethiol (PET) and associated 1:1 solvent complexes have been measured in SH, OH, and CH stretch regions using resonant-two-photon-ionization (R2PI) and IR-UV ion dip spectroscopic techniques. The aromatic and aliphatic CH stretch regions show signature differences between anti and gauche conformers. Supported by ab initio calculations, a PET-water cluster with an OH···S arrangement and a PET-diethyl ether cluster expressing an SH···O interaction were identified. The SH stretch band of the SH···O complex is red-shifted and undergoes significant intensity enhancement compared to the bare molecule, which is characteristic of hydrogen bonding. These findings offer insight into the nature of the thiol functional group as a potential hydrogen bond donor and acceptor.

The nature of selenium hydrogen bonding: gas phase spectroscopy and quantum chemistry calculations

Subsequent to the recent re-definition of hydrogen bonding by the IUPAC committee, there has been a growing search for finding the presence of this ever interesting non-covalent interaction between a hydrogen atom in an X-H group and any other atom in the periodic table. In recent gas phase experiments, it has been observed that hydrogen bonding interactions involving S and Se are of similar strength to those with an O atom. However, there is no clear explanation for the unusual strength of this interaction in the case of hydrogen bond acceptors which are not conventional electronegative atoms. In this work, we have explored the nature of Se hydrogen bonding by studying indoledimethyl selenide (indmse) and phenoldimethyl selenide (phdmse) complexes using gas phase IR spectroscopy and quantum chemistry calculations. We have found through various energy decomposition analysis (EDA) methods and natural bond orbital (NBO) calculations that, along with electrostatics and polarization, charge transfer interactions are important to understand Se/S hydrogen bonding and there is a delicate balance between the various interactions that plays the crucial role rather than a single component of the interaction energy. An in-depth understanding of this type of non-covalent interaction has immense significance in biology as amino acids containing S and Se are widely present in proteins and hence hydrogen bonding interactions involving S and Se atoms contribute to the folding of proteins.

Improved Description of Sulfur Charge Anisotropy in OPLS Force Fields: Model Development and Parameterization

The atomic point-charge model used in most molecular mechanics force fields does not represent well the electronic anisotropy that is featured in many directional noncovalent interactions. Sulfur participates in several types of such interactions with its lone pairs and σ-holes. The current study develops a new model, via the addition of off-atom charged sites, for a variety of sulfur compounds in the OPLS-AA and OPLS/CM5 force fields to address the lack of charge anisotropy. Parameter optimization is carried out to reproduce liquid-state properties, torsional and noncovalent energetics from reliable quantum mechanical calculations, and electrostatic potentials. Significant improvements are obtained for computed free energies of hydration, reducing the mean unsigned errors from ca. 1.4 to 0.4-0.7 kcal/mol. Enhanced accuracy in directionality and energetics is also obtained for molecular complexes with sulfur-containing hydrogen and halogen bonds. Moreover, the new model reproduces the unusual conformational preferences of sulfur-containing compounds with 1,4-intramolecular chalcogen bonds. Transferability of the new force field parameters to cysteine and methionine is verified via molecular dynamic simulations of blocked dipeptides. The study demonstrates the effectiveness of using off-atom charge sites to address electronic anisotropy, and provides a parametrization methodology that can be applied to other systems.

Spectroscopic Evidences for Strong Hydrogen Bonds with Selenomethionine in Proteins

Careful protein structure analysis unravels many unknown and unappreciated noncovalent interactions that control protein structure; one such unrecognized interaction in protein is selenium centered hydrogen bonds (SeCHBs). We report, for the first time, SeCHBs involving the amide proton and selenium of selenomethionine (Mse), i.e., amide-N-H···Se H-bonds discerned in proteins. Using mass selective and conformer specific high resolution vibrational spectroscopy, gold standard quantum chemical calculations at CCSD(T), and in-depth protein structure analysis, we establish that amide-N-H···Se and amide-N-H···Te H-bonds are as strong as conventional amide-NH···O and amide-NH···O═C H-bonds despite smaller electronegativity of selenium and tellurium than oxygen. It is in fact, electronegativity, atomic charge, and polarizability of the H-bond acceptor atoms are at play in deciding the strength of H-bonds. The amide-N-H···Se and amide-N-H···Te H-bonds presented here are not only new additions to the ever expanding world of noncovalent interactions, but also are of central importance to design new force-fields for better biomolecular structure simulations.

Matrix isolation FTIR study of hydrogen-bonded complexes of methanol with heterocyclic organic compounds

Hydrogen bonded complexes of heterocyclic compounds with methanol were studied using matrix isolation FTIR spectroscopy and theoretical calculations.

Acid-Base Formalism Extended to Excited State for O-H···S Hydrogen Bonding Interaction

Hydrogen bond can be regarded as an interaction between a base and a proton covalently bound to another base. In this context the strength of hydrogen bond scales with the proton affinity of the acceptor base and the pKa of the donor, i.e., it follows the acid-base formalism. This has been amply demonstrated in conventional hydrogen bonds. Is this also true for the unconventional hydrogen bonds involving lesser electronegative elements such as sulfur atom? In our previous work, we had established that the strength of O-H···S hydrogen bonding (HB) interaction scales with the proton affinity (PA) of the acceptor. In this work, we have investigated the other counterpart, i.e., the H-bonding interaction between the photoacids with different pKa values with a common base such as the H2O and H2S. The 1:1 complexes of five para substituted phenols p-aminophenol, p-cresol, p-fluorophenol, p-chlorophenol, and p-cyanophenol with H2O and H2S were investigated experimentally and computationally. The investigations were also extended to the excited states. The experimental observations of the spectral shifts in the O-H stretching frequency and the S1-S0 band origins were correlated with the pKa of the donors. Ab initio calculations at the MP2 and various dispersion corrected density functional levels of theory were performed to compute the dissociation energy (D0) of the complexes. The quantum theory of atoms in molecules (QTAIM), noncovalent interaction (NCI) method, natural bonding orbital (NBO) analysis, and natural decomposition analysis (NEDA) were carried out for further characterization of HB interaction. The O-H stretching frequency red shifts and the dissociation energies were found to be lower for the O-H···S hydrogen bonded systems compared to those for the O-H···O H-bound systems. Despite being dominated by the dispersion interaction the O-H···S interaction in the H2S complexes also conformed to the acid-base formalism, i.e., the D0 and the O-H red shift scaled with the pKa of the donor, similar to that observed in the O-H···O interaction. However, the two classes of H-bonds follow different correlations. In addition we also discuss the nuances associated with the similarity and differences in the hydrogen bonding properties of the two classes in the ground electronic state as well as in the excited state.

Structural flexibility of the sulfur mustard molecule at finite temperature from Car-Parrinello molecular dynamics simulations

Sulfur mustard (SM) is one of the most dangerous chemical compounds used against humans, mostly at war conditions but also in terrorist attacks. Even though the sulfur mustard has been synthesized over a hundred years ago, some of its molecular properties are not yet resolved. We investigate the structural flexibility of the SM molecule in the gas phase by Car-Parrinello molecular dynamics simulations. Thorough conformation analysis of 81 different SM configurations using density functional theory is performed to analyze the behavior of the system at finite temperature. The conformational diversity is analyzed with respect to the formation of intramolecular blue-shifting CH⋯S and CH⋯Cl hydrogen bonds. Molecular dynamics simulations indicate that all structural rearrangements between SM local minima are realized either in direct or non-direct way, including the intermediate structure in the last case. We study the lifetime of the SM conformers and perform the population analysis. Additionally, we provide the anharmonic dynamical finite temperature IR spectrum from the Fourier Transform of the dipole moment autocorrelation function to mimic the missing experimental IR spectrum.

Critical Assessment of the Strength of Hydrogen Bonds between the Sulfur Atom of Methionine/Cysteine and Backbone Amides in Proteins

Gas-phase vibrational spectroscopy, coupled cluster (CCSD(T)), and dispersion corrected density functional (B97-D3) methods are employed to characterize surprisingly strong sulfur center H-bonded (SCHB) complexes between cis and trans amide NH and S atom of methionine and cysteine side chain. The amide N-H···S H-bonds are compared with the representative classical σ- and π-type H-bonded complexes such as N-H···O, N-H···O═C and N-H···π H-bonds. With the spectroscopic, theoretical, and structural evidence, amide N-H···S H-bonds are found to be as strong as the classical σ-type H-bonds, despite the smaller electronegativity of sulfur in comparison to oxygen. The strength of backbone-amide N-H···S H-bonds in cysteine and methionine containing peptides and proteins are also investigated and found to be of similar magnitudes as those observed in the intermolecular model complexes studied in this work. All such SCHBs also confirm that the electronegativities of the acceptors are not the sole criteria to predict the H-bond strength.

Interaction of aromatic compounds with xenon: spectroscopic and computational characterization for the cases of p-cresol and toluene

We have investigated noncovalent interactions of two aromatic compounds (toluene and p-cresol) with Xe atoms by using infrared spectroscopy in a Ne matrix and quantum chemical calculations. The present results show that the methyl group of these molecules is a sensitive probe of the interaction with Xe. We have used the molecules with the deuterated methyl group, possessing a relatively simple spectrum, which allows us to detect characteristic vibrational shifts in the complexes, in which a Xe atom interacts with the aromatic π electron system (π structure). For the p-cresol···Xe complex, we also observed evidence of the 1:1 H-bonded structure. The amount of the H-bonded structure of the cresol···Xe complex is relatively small, which agrees with the calculated interaction energies (stronger interaction for the π structure). The bands of the 1:1 complexes of p-cresol and toluene with Xe appear at low Xe concentration and their intensities relative to the monomer bands are nearly proportional to the Xe/Ne concentration ratio. For the p-cresol-Xe system, additional OH stretching bands appear at higher Xe concentrations, which are suitable for the complexes with several Xe atoms. The π structures studied in this work can probably be formed in the case of aromatic amino acids, for which these simple aromatic compounds are useful models.

Intra-residue interactions in proteins: interplay between serine or cysteine side chains and backbone conformations, revealed by laser spectroscopy of isolated model peptides

Intra-residue interactions play an important role in proteins by influencing local folding of the backbone. Taking advantage of the capability of gas phase experiments to provide relevant information on the intrinsic H-bonding pattern of isolated peptide chains, the intra-residue interactions of serine and cysteine residues, i.e., OH/SH···OC(i) C6 and NH(i···)O/S C5 interactions in Ser/Cys residues, are probed by laser spectroscopy of isolated peptides. The strength of these local side chain-main chain interactions, elegantly documented from their IR spectral features for well-defined conformations of the main chain, demonstrates that a subtle competition exists between the two types of intra-residue bond: the C6 H-bond is the major interaction with Ser, in contrast to Cys where C5 interaction takes over. The restricted number of conformers observed in the gas phase experiment with Ser compared to Cys (where both extended and folded forms are observed) also suggests a significant mediation role of these intra-residue interactions on the competition between the several main chain folding patterns.

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.

Sulfur, not too far behind O, N, and C: SH...pi 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.

Study of electronic structure and dynamics of interacting free radicals influenced by water

We present a study of electronic structure, stability, and dynamics of interaction and recombination of free radicals such as HO(2) and OH influenced by water. As simple model calculations, we performed ab initio and density functional calculations for the interaction of HO(2) and OH in the presence of water cluster. Results indicate that a significant interaction, overcoming the repulsive Columbic barrier, occurs at a separation distance between the radicals of 5.7 A. This confirms early predictions of the minimum size of molecular dianions stable in the gas phase. It is well known that atomic dianions are unstable in the gas phase but molecular dianions are stable when the size of the molecule is larger than 5.7 A. Ab initio molecular dynamics calculations with Car-Parrinello scheme show that the reaction is very fast and occurs on a time scale of about 1.5 ps. The difference in stability and dynamics of the interacting free radicals on singlet and triplet potential energy surfaces is also discussed.