EVB and polarizable MM study of energy relaxation in fluorine–acetonitrile reactions

Abstract

The H-abstraction reaction between fluorine atom and deuterated acetonitrile (CD₃CN) is highly exothermic and the resulting deuterium fluoride (DF) molecule is formed with a significant amount of energy that requires several picoseconds to relax into the solvent environment. Previous empirical valence bond (EVB) modelling work (D. R. Glowacki, et al., J. Chem. Phys., 2015, 143, 044120) showed that reproducing the experimental relaxation timescale is quite sensitive to the potential energy surface (PES) used, and the physical effects responsible for cooling were not fully clear. Here, we study the rate of cooling on two new carefully designed PESs, and by comparison to behaviour on other PESs, this provides additional insight into these effects. The first PES is a MMFF (Merck Molecular Force Field) based covalent-ionic two-state EVB model constructed utilizing the valence-bond resonance structures of DF, which is shown to give a good description of the PES for interaction of hydrogen fluoride through hydrogen bonding with one acetonitrile molecule, but performs relatively poorly in predicting the vibrational relaxation rate in bulk solvent. The second new PES uses the polarizable AMOEBA force field formalism, and describes both the DF–acetonitrile dimer PES and the rate of vibrational cooling very well, with good computational efficiency. Comparison of those PESs shows that as well as a good description of the non-bonded interactions in the DF–acetonitrile dimer, successful prediction of cooling dynamics requires a good description of many-body effects involving the supramolecular complex formed by DF, the H-bonded CD₃CN and nearby solvent molecules.