Vibrational Stark Effects of Nitriles I. Methods and Experimental Results

Vibrational Stark effects, which are the effects of electric fields on vibrational spectra, were measured for the C−N stretch mode of several small nitriles. Samples included unconjugated and conjugated nitriles, and mono- and dinitriles. They were immobilized in frozen 2-methyl-tetrahydrofuran glass and analyzed in externally applied electric fields using an FTIR; details of the methodology are presented. Difference dipole moments, Δμ, equivalent 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 toward the high end and symmetric dinitriles toward the low end (the local field correction factor, f, is expected to be similar for all these samples). Most quadratic Stark effects are small and negative, while transition polarizabilities are positive and have a significant effect on Stark line shapes. For aromatic nitriles, transition dipoles and Δμ values correlate with Hammett numbers. Symmetric dinitrile Δμ values decrease monotonically with increasing conjugation of the connecting bridge, likely due to improved mechanical coupling and, to a lesser extent, an increased population of inversion symmetric conformations. Δμ values are close to those expected from bond anharmonicity and ab initio predictions.