Nitrile infrared intensity is more sensitive to local electric field change than its frequency: Application to probe protein hydration dynamics

The stretching vibration of a nitrile (C≡N) group gives rise to a sharp and strong infrared (IR) absorption band. Because the frequency (νCN) and molar extinction coefficient at νCN (εCN) of this vibrotational mode depend not only on the parent molecule but also on interactions that can affect the electronic distribution of the C≡N triple bond, it has found broad utility as an IR probe of the local environment of various chemical and biological systems. However, most previous studies only used the νCN to extract the information of interest. Herein, we show that the integrated molar extinction coefficient [∫ενdν] of the C≡N band has a stronger dependence on local electric field than its frequency and, hence, can be used to probe smaller environmental changes, as demonstrated in a proof-of-principle application. In addition, we find that for a series of C≡N IR bands that are measured with different aromatic nitriles or in different solvents, the νCN exhibits a linear correlation with the square root of the frequency normalized ∫ενdν, indicating that the underlying IR transition dipole moment is linearly dependent on the electronic interaction between the C≡N triple bond and the parent aromatic molecule.