The infrared and Raman spectra of 2,3-, 2,4-, and 2,5-dimethylbenzonitriles are recorded in the regions 400 to 4000 cm−1 and 100 to 4000 cm−1, respectively. Depolarization measurements are also undertaken. Assuming a Cs point group for all the molecules, vibrational analysis is carried out. The frequencies of the umbrella vibration (mode 11) are calculated, and the calculated values agree very well with the observed values.
T. V. K. Sarma, "Infrared and Raman Spectra of 2,3-, 2,4-, and 2,5-Dimethylbenzonitriles," Appl. Spectrosc. 40, 933-939 (1986)