Nitrile IR intensities directly measure electric fields in protic and non-protic environments

Abstract

Nitriles are useful vibrational probes; the interpretation of their frequencies, however, is complicated by hydrogen bonding in protic environments. Here we observed a vibrational Stark effect (VSE) which correlates electric field to transition dipole moment (and, by extension, the nitrile peak area) instead of to frequency. This VSE governs the transition dipole moment for the CO stretch in acetophenone and the CN stretch in benzonitrile, with studies performed in bulk-phase solvents. While modest for the acetophenone carbonyl CO, the VSE for the benzonitrile nitrile CN is dramatic, and a >300% increase in absolute peak area from hexanes to water.