Electronic changes due to thermal disorder of hydrogen bonds in liquids: Pyridine in an aqueous environment

Combined Metropolis Monte Carlo computer simulation and first-principles quantum mechanical calculations of pyridine in water are performed to analyze the role of thermal disorder in the electronic properties of hydrogen bonds in an aqueous environment. The simulation uses the $\mathrm{NVT}$ ensemble and includes one pyridine and 400 water molecules. Using a very efficient geometric-energetic criterion, the hydrogen bonds between pyridine and water ${\mathrm{C}}_5{\mathrm{H}}_5\mathrm{N}---{\mathrm{H}}_2\mathrm{O}$ are identified and separated for subsequent quantum mechanical calculations of the electronic and spectroscopic properties. Statistically uncorrelated configurations composed of one pyridine and one water molecule are used to represent the configuration space of the hydrogen bonds in the liquid. The quantum mechanical calculations on these structures are performed at the correlated second-order perturbation theory level and all results are corrected for basis-set superposition error. The results are compared with the equivalent electronic properties of the hydrogen bond in the minimum-energy configuration. Charge transfer, dipole moment, and dipole polarizabilities are calculated for the thermally disordered and minimum-energy structures. In addition, using the mean and anisotropic polarizabilities, the Rayleigh depolarizations are obtained. All properties obtained for the thermally disordered structures are represented by a statistical distribution and a convergence of the average values is obtained. The results indicate that the charge transfer, dipole moment, and average depolarization ratios are systematically decreased in the liquid compared to the optimized cluster. This study quantifies, using ab initio quantum mechanics and statistical analysis, the important aspect of the thermal disorder of the hydrogen bond in a liquid system.