Vibrational stark effects of nitriles I. Methods and experimental results

Vibrational Stark effects, which are the effects of electric fields on vibr ational spectra. were measured for the C-N stretch mode of several small ni triles. Samples included unconjugated and conjugated nitriles, and mono- an d dinitriles. They were immobilized in frozen 2-methyl-tetrahydrofuran glas s and analyzed in externally applied electric fields using an FTIR; details of the methodology are presented. Difference dipole moments, Delta mu, equ ivalent to the linear Stark tuning rate, range from 0.01/f to 0.04/f Debye (0.2/f to 0.7/f cm(-1)/(MV/cm)) for most samples, with aromatic compounds t oward the high end and symmetric dinitriles cm toward the low end (the loca l field correction factor, f, is expected to be similar for all these sampl es). Most quadratic Stark effects are small and negative, while transition polarizabilities are positive and have a significant effect on Stark line s hapes. For aromatic nitriles, transition dipoles and Delta mu values correl ate with Hammett numbers. Symmetric dinitrile Delta mu values decrease mono tonically with increasing conjugation of the connecting bridge, Likely due to improved mechanical coupling and, to a lesser extent, an increased popul ation of inversion symmetric conformations. Delta mu values are close to th ose expected from bond anharmonicity and ab initio predictions.