Nitrile Infrared Intensities Characterize Electric Fields and Hydrogen Bonding in Protic, Aprotic, and Protein Environments

Nitriles are widely used vibrational probes; however, the interpretation of their IR frequencies is complicated byhydrogen bonding (H-bonding) in protic environments. We report a new vibrational Stark effect (VSE) that correlates the electricfield projected on the-C???N bond to the transition dipole moment and, by extension, the nitrile peak area or integrated intensity.This linear VSE applies tobothH-bonding and non-H-bonding interactions. It can therefore be generally applied to determineelectricfields in all environments. Additionally, it allows for semiempirical extraction of the H-bonding contribution to the blueshiftof the nitrile frequency. Nitriles were incorporated at H-bonding and non-H-bonding protein sites using amber suppression, andeach nitrile variant was structurally characterized at high resolution. We exploited the combined information available from variationsin frequency and integrated intensity and demonstrate that nitriles are a generally useful probe for electric fields.

Automated summary

This study reports a new vibrational Stark effect that can be used to measure electric fields and reveal the electric field characteristics of nitriles through their frequency and integrated intensity.