Vibrational Stark Effects of Nitriles II. Physical Origins of Stark Effects from Experiment and Perturbation Models

Abstract

Vibrational Stark effects, which are the effects of electric fields on vibrational spectra, were measured previously for the C−N stretch mode of several small nitriles, yielding difference dipole moments, difference polarizabilities, and transition polarizabilities for each species [Andrews, S. S.; Boxer, S. G. J. Phys. Chem. A 2000, 104, 11 853]. This paper explains the physical origins of the observed Stark effects using two theoretical models and, in the process, computes several molecular parameters for each nitrile. A model with a single vibrational mode, developed with perturbation theory, is found to explain most of the experimental Stark effects. Because it cannot account for coupling between modes, which is ubiquitous and important for resonant vibrations and for combination mode absorption, another model is developed which considers multiple vibrational modes and three spatial degrees of freedom. It is found that difference dipole moments arise from a combination of mechanical anharmonicity and ...