Vibrational interactions of acetonitrile: Doubly vibrationally resonant IR-IR-visible four-wave-mixing spectroscopy

The origin of the vibrational coupling that was observed between the CC and CN stretching modes of acetonitrile by doubly vibrationally enhanced (DOVE) IR-IR-Vis four-wave-mixing (IIV-FWM) spectroscopy is investigated by various ab initio calculations including DFT(B3LYP), HF, and MP2 methods with the same baisis set, 6-311++G**. The linear and nonlinear susceptibilities of the combination bands and cross peaks are numerically calculated and compared with the experimental values, and the agreement between ab initio results and experiments are quantitative. By separately analyzing the contributions from each coherence pathway to the vibrational coupling of the CC and CN stretching modes, a quantitative understanding of the DOVE IIV-FWM signals is possible. Although the direct coupling of the CC and CN stretching modes by mechanical and electric anharmonicity coupling is sizable, the CH bending and CH stretching modes are also involved in the vibrational coupling between CC and CN stretching modes as promoting modes. The numerically simulated two-dimensional (2D) DOVE spectrum for a CH3CN sample is presented and compared with experiment. It is found that the interference among distinctive pathways plays a central role in describing the distorted, asymmetric shape of the 2D DOVE spectrum. In addition, the IIV-FWM cross peak associated with the vibrational coupling between the CH and CN stretching mode is also calculated and its magnitude is compared with that of the CC and CN stretching modes. (C) 2002 American Institute of Physics.