Quantifying Local Electrostatics through Vibrational Lifetimes in Solution: Methyl Thiocyanate as a Model Nitrile Probe

Frequency shifts from vibrational Stark effect (VSE) are widely used to probe local electric fields, while alternative observables are less explored. Here, we show that vibrational lifetimes of nitrile groups provide a sensitive probe of local electrostatics through their influence on vibrational energy relaxation pathways. Using femtosecond infrared pump-probe spectroscopy, the C≡N stretching vibration of methyl thiocyanate (MeSCN) was systematically investigated in polar and nonpolar solvents. A nearly 4-fold variation in the CN vibrational lifetime is observed, exhibiting a strong linear correlation with local electric fields. In contrast, the transition dipole moment of the CN vibration varies only ∼1.5-fold, highlighting the enhanced sensitivity of vibrational lifetimes. This behavior is attributed to electric-field modulation of anharmonic couplings and dephasing line widths. These results establish MeSCN as a model thiocyanate system for correlating local electrostatics with vibrational energy relaxation and provide a new dynamical observable for probing complex chemical environments.