Molecular Dynamics Simulations of Small Clusters and Liquid Hydrogen Sulfide at Different Thermodynamic Conditions

Computational analysis of vibrational frequencies and rovibrational spectroscopic constants of hydrogen sulfide dimer using MP2 and CCSD(T)

Previous studies have shown that the weakly bonded H₂S dimer demands high level quantum chemical calculations to reproduce experimental values. We investigated the hydrogen bonding of H₂S dimer using MP2 and CCSD(T) levels of theory in combination with aug-cc-pV(D,T,Q)Z basis sets. More precisely, the binding energies, potential energy curves, rovibrational spectroscopic constants, decomposition lifetime, and normal vibrational frequencies were calculated. In addition, we introduced the local mode analysis of Konkoli-Cremer to quantify the hydrogen bonding in the H₂S dimer as well as providing for the first time the comprehensive decomposition of normal vibrational modes into local modes contributions, and a decomposition lifetime based on rate constant. The local mode force constant of the H₂S dimer hydrogen bond is smaller than that of the water dimer, in accordance with the weaker hydrogen bonding in the H₂S dimer.

Effects of Hydrogen Bonding on the Rotational Dynamics of Water-Like Molecules in Liquids: Insights from Molecular Dynamics Simulations

Rotational motion of molecules plays an important role in determining NMR spin relaxation properties of liquids. The textbook theory of NMR spin relaxation predominantly uses the assumption that the reorientational dynamics of molecules is described by a continuous time rotational diffusion random walk with a single rotational diffusion coefficient. Previously we and others have shown that reorientation of water molecules on the timescales of picoseconds is not consistent with the Debye rotational-diffusion model. In particular, multiple timescales of molecular reorientation were observed in liquid water. This was attributed to the hydrogen bonding network in water and the consequent presence of collective rearrangements of the molecular network. In order to better understand the origins of the complex reorientational behaviour of water molecules, we carried out molecular dynamics (MD) simulations of a liquid that has a similar molecular geometry to water but does not form hydrogen bonds: hydrogen sulfide. These simulations were carried out at T=208K and p=1 atm (~5K below the boiling point). Ensemble-averaged Legendre polynomial functions of hydrogen sulfide exhibited a Gaussian decay on the sub-picosecond timescale but, unlike water, did not exhibit oscillatory behaviour. We attribute these differences to hydrogen sulfide’s absence of hydrogen bonding.

Cooperative nature of the sulfur centered hydrogen bond: investigation of (H2S)n (n = 2-4) clusters using an affordable yet accurate level of theory

Existing studies have shown that appreciably high level quantum chemical calculations are required to reproduce experimental energetic and geometric features of a H₂S dimer. This condition severely restricts any practical possibility of obtaining reliable results for higher order H₂S clusters. We have shown here that the binding energies calculated at the CCSD(T)/CBS level with counterpoise corrected geometries calculated at the MP2/aug-cc-pV(Q+d)Z level of theory excellently match with the experimental results for the H₂S dimer. Subsequently, the above mentioned levels of theory were used for trimers and tetramers. (H₂S)_n (n = 2-4) clusters were found to show cooperative strengthening of S-HS hydrogen bonds, which is clearly evident from the evolution of binding energies and hydrogen bond lengths, with increasing cluster size. Localized molecular orbital energy decomposition analyses have been carried out to understand how the contributions of various energy components modulate with the size of the clusters and what are their relative contributions towards the overall stabilization of the clusters. Natural bond orbital and atoms in molecules analyses were also carried out in order to look into the evolution of the electronic charge transfer and electron density topology with cluster size.