Finite-field ab initio calculations at the MP4 and/or QCISD levels are performed on HCN and acetonitrile (CH3CN) to calculate molecular dipole moments, polarizabilities, and hyperpolarizabilities as a function of the CN molecular vibration; these are then used to simulate the electroabsorption 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 are isolated and isotropic, being held firmly in a low-temperature glass. 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 stretch in p-anisonitrile. We find that, for CN stretch vibrations, solvent effects dominate the observed electroabsorption responses, and we are able to qualitatively interpret the observed spectra.