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.

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.

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.

In this work we have determined dissociation energies of O-H···S hydrogen bond in the H2S complexes of various phenol derivatives using 2-color-2-photon photofragmentation spectroscopy in combination with zero kinetic energy photoelectron (ZEKE-PE) spectroscopy. This is the first report of direct determination of dissociation energy of O-H···S hydrogen bond. The ZEKE-PE spectra of the complexes revealed a long progression in the intermolecular stretching mode with significant anharmonicity. Using the anharmonicity information and experimentally determined dissociation energy, we also validated Birge-Sponer (B-S) extrapolation method, which is an approximate method to estimate dissociation energy. Experimentally determined dissociation energies were compared with a variety of ab initio calculations. One of the important findings is that ωB97X-D functional, which is a dispersion corrected DFT functional, was able to predict the dissociation energies in both the cationic as well as the ground electronic state very well for almost every case.

Dissociative electron attachment (DEA) to acetone is studied in terms of the absolute cross section for various fragment channels in the electron energy range of 0-20 eV. H(-) is found to be the most dominant fragment followed by O(-) and OH(-) with only one resonance peak between 8 and 9 eV. The DEA dynamics is studied by measuring the angular distribution and kinetic energy distribution of fragment anions using Velocity Slice Imaging technique. The kinetic energy and angular distribution of H(-) and O(-) fragments suggest a many body break-up for the lone resonance observed. The ab initio calculations show that electron is captured in the multi-centered anti-bonding molecular orbital which would lead to a many body break-up of the resonance.

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Structure of a prototypical acrylic polymer (Poly(methylmethacrylate): PMMA)/water interface is elucidated at the molecular level by heterodyne-detected sum-frequency generation. Two distinct OH’s of interfacial water are found at the interface:...

The hydrogen-bonded chain of water molecules is known to play a very important role in proton/H transfer in chemistry and biology. This kind of water chain mainly starts from a conventional hydrogen bond (HB) donor/acceptor site. Here, we report the experimental evidence of water chain formation on an unconventional C–H HB donor site in 1,2,4,5-tetracyanobenzene (TCNB). Laser-induced fluorescence (LIF) spectra and fluorescence dip IR (FDIR) spectra of 1:n (n = 1–3) clusters of TCNB with water prepared in a supersonic molecular jet are presented. Quantum chemical calculations of several intuitive conformers of 1:n (n = 1–3) clusters were performed, and the computed IR spectra were compared with the experimental FDIR spectra in order to get the structural information on the experimentally observed clusters. We find that the first water molecule binds to the C–H moiety of TCNB (1:1 cluster) and the subsequent water molecules form a water chain (1:2 and 1:3 clusters) that constitutes a bridge between the C–H and the proximal −CN moiety of TCNB, forming a cyclic ring structure. NBO calculations show that upon addition of water molecules significant change in the charge distribution takes place, mainly on the atoms which are involved in cyclic ring structure. This redistribution of charges causes a cooperativity effect in the higher water clusters of TCNB.