OBSERVATION OF THE DEPHASING OF THE CN STRETCHING VIBRATION IN LIQUID NITRILES BY FEMTOSECOND TIME-RESOLVED COHERENT ANTI-STOKES-RAMAN SCATTERING

Femtosecond time-resolved coherent anti-Stokes Raman scattering has been observed for the C=N stretching at about 2250 cm-1 of alkanenitriles (CnH2n+1CN) and alpha,omega-alkanedinitriles (NCCnH2nCN). The vibrational dephasing of the C=N stretching of alkanenitriles becomes faster with the molecular size, which is reflected in the increasing density of vibrational states at the C=N stretching energy level. Addition of methanol in alkanenitriles accelerates the dephasing, indicating the importance of intermolecular hydrogen bonding as a factor which causa vibrational dephasing. Intermolecular interactions in neat alkanenitriles seem to play a basic role in the dephasing of the C=N stretching. The vibrational dephasing of smaller alpha,omega-alkanedinitriles like NCCH2CN is significantly faster than that of the corresponding alkanenitriles. As the dinitrile becomes larger, the dephasing rate approaches that of the corresponding alkanenitrile. These observations imply that the intramolecular coupling between the symmetric and antisymmetric C=N stretching vibrations in smaller dinitriles accelerates the dephasing of the C=N stretching vibration.