Quantum Vibrational Dynamics of Molecular Species Relevant to Atmospheric Chemistry and ClimateScience. Formic Acid and its Clusters with Benzene

Abstract

Vibrational dynamics of formic acid and its noncovalenly bonded complexes with benzene was studied at series of finite temperatures mimicking atmospheric conditions, as well as in the limit of 0 K. Potential energy surfaces (PES) of bare formic acid as well as of its binary noncovalent clusters with benzene were explored employing density functional theory and many-body perturbation theory. Anharmonic OH stretching frequencies of cis- and trans- conformers of formic acid at 0 K were calculated by computing the corresponding anharmonic adiabatic vibrational potentials and subsequently numerically solving the vibrational Schrödinger equations. Similar computations were carried out for the minima located on the benzene – formic acid PESs. The corresponding vibrational frequency shifts with respect to the free monomeric species were compared to the available experimental data. The dynamics of cis → trans and reverse interchange was studied on the basis of computed fully relaxed torsional potentials at the mentioned theoretical levels employing WKB semiclassical methodology. To study the finite-temperature effects on the inter- and intramolecular vibrational dynamics of the studied species, a series of ab initio molecular dynamics (MD) simulations were carried out, employing both Born-Oppenheimer molecular dynamics (BOMD) as well as the atom-centered density matrix propagation scheme (ADMP). All ab initio MD simulations were performed in the NVE ensemble. In parallel, for comparison purposes, also a series of classical Monte Carlo (MC) simulations were performed, using Coulomb + Lennard-Jones interaction potentials. Vibrational spectroscopic properties of monomeric and noncovalently bonded dimeric species were elucidated from dynamical simulations analyzing the corresponding time- correlation functions, i.e. employing time-series analytic methods. For that purpose, both velocity autocorrelation function as well as the atomic position autocorrelation function was computed and subsequently Fourier-transformed. The differences between “static” and dynamic vibrational spectra were noted and discussed.