VIBRATIONAL STARK SPECTROSCOPY .2. APPLICATION TO THE CN STRETCH IN HCN AND ACETONITRILE

Finite-field ab initio calculations at the MP4 and/or QCISD levels are performed on HCN and acetonitrile (CH3CN) to calculate molecular dipo le moments, polarizabilities, and hyperpolarizabilities as a function of the CN molecular vibration; these are then used to simulate the ele ctroabsorption responses of these molecules, determining, for example, the Stark tuning rate. Our theory for these responses (part 1 of this series) assumes an experimental situation in which the chromophores a re isolated and isotropic, being held firmly in a low-temperature glas s. For simplicity, we also exclude anharmonic internode couplings and neglect contributions from all but the largest tensor component of the molecular polarizabilities, etc. This work is inspired by the recent electroabsorption measurements of Chattopadhyay and Boxer on the CN st retch in p-anisonitrile. We find that, for CN stretch vibrations, solv ent effects dominate the observed electroabsorption responses, and we are able to qualitatively interpret the observed spectra.