Vibrational Energy Relaxation of Deuterium Fluoride in -Dichloromethane: Insights from Different Potentials.

Vibrationally excited deuterium fluoride (DF) formed by fluorine atom reaction with a solvent was found (, , 347, 530) to relax rapidly (less than 10 ps) in acetonitrile- (CDCN) and dichloromethane- (CDCl). However, insights into how CDCl facilitates this energy relaxation have so far been lacking, given the weak interaction between DF and a single CDCl. In this work, we report the results of reactive simulations with a two-state reactive empirical valence bond (EVB) potential to study the energy deposited into nascent DF after transition-state passage and of nonequilibrium molecular dynamics simulations using multiple different potential energy functions to model the relaxation dynamics. For these second simulations, we used the standard Merck molecular force field (MMFF) potential, an MMFF-based covalent-ionic empirical valence bond (EVB) potential (EVB), a newly developed potential [referred to as MMFF(rDF)] which extends upon the MMFF potential by making the DF/CDCl interaction depend on the value of the D-F bond stretching coordinate and by taking the anisotropic charge distribution of the solvent molecules into account, the polarizable atomic multipole optimized energetics for biomolecular applications (AMOEBA) potential, and the quantum mechanics/molecular mechanics (QM/MM) potential. The relaxation is revealed to be highly sensitive to the potential used. Neither standard MMFF nor EVB reproduces the experimentally observed rapid relaxation dynamics, and they also fail to provide a good description of the interaction potential between DF and CDCl as calculated using CCSD(T)-F12. This is attributed to the use of a point-charge model for the solute and to failing to model the anisotropic electrostatic properties of CDCl. The MMFF(rDF), AMOEBA, and QM/MM potentials all reproduce the CCSD(T)-F12 two-body DF---CDCl interaction potential rather well but only with the QM/MM approach is fast vibrational relaxation obtained (lifetimes of ∼288, ∼186, and ∼8 ps, respectively), which we attribute to differences in the solute-solvent local structure. With QM/MM, a unique "many-body" interaction pattern in which DF is in close contact with two solvent Cl atoms and more than three solvent D atoms is found, but this structure is not seen with other potentials. The QM/MM dynamics also display enhanced solute-solvent interactions with vibrationally excited DF that induce a DF band redshift and hence a resonant overlap with solvent C-D modes, which facilitate the intermolecular energy transfer. Our work also suggests that potentials used to model energy relaxation need to capture the fine structure of solute-solvent interactions and not just the two-body part.