Decomposition of Vibrational Shifts of Nitriles into Electrostatic and Hydrogen-Bonding Effects

Infrared (IR) band shifts of isolated vibrational transitions can serve as quantitative and directional probes of local electrostatic fields, due to the vibrational Stark effect. However, departures from the Stark model can arise when the probe participates in specific, chemical interactions, such as direct hydrogen bonding. We present a method to identify and correct for these departures based on comparison of C-13 NMR chemical shifts and IR frequencies each calibrated in turn by a solvato-chromic model. We demonstrate how the tandem use of these experimental observables can be applied to a thiocyanate-modified protein, ketosteroid isomerase, and show, by comparison to structural models, that changes in electrostatic field can be measured within the complex protein environment even in the background of direct hydrogen bonding to the probe.