A scaled quantum mechanical reinvestigation of the vibrational spectrum of toluene

A recent investigation and assignment of the IR spectrum of liquid toluene between 400 and 4000 cm−1 by Keefe and coworkers [J.E. Bertie, Y. Apelblat, C.D. Keefe, J. Mol. Struct. 750 (2005) 78] has been reexamined theoretically using the scaled quantum mechanical (SQM) force field method. It is proposed that three bands - a weak, broad shoulder at 947 cm−1 assigned as a combination band, an unassigned feature at 1467 cm−1 and a medium broad band at 2979 cm−1, also assigned as a combination band - are all in fact fundamentals. Agreement with the remaining assignments is excellent, with an average deviation between the observed and theoretically predicted vibrational fundamentals of just 5.28 cm−1. The overall intensity pattern of the experimental spectrum is also well reproduced. Analysis involving free rotation of the methyl group and interpretation of the vibrational spectrum in terms of C2v symmetry for the phenyl ring and C3v for the methyl group very likely contributed to the experimental misassignments.