Vibrational analysis and valence force field for nitrotoluenes, dimethylanilines and some substituted methylbenzenes

The Fourier transform infrared (FTIR) and Raman spectra of 2-amino-4-nitrotoluene; 2-amino-5-nitrotoluene; 2,4-dimethylaniline; 2,5-dimethylaniline; 2,6-dimethylaniline; 1,2,4-trimethylbenzene; 1,3,5-trimethylbenzene and pentamethylbenzene have been recorded in the range 4000–400 cm−1 and 4000–30 cm−1, respectively. A normal coordinate analysis was carried out for both in-plane and out-of-plane vibrations of these molecules using an 81-parameter modified valence force field. The force constants were refined using 369 frequencies of eight molecules in the overlay least-squares technique. The reliability of the force constants was tested by making a zero-order calculation for both in-plane and out-of-plane vibrations for five related molecules. The potential energy distributions and eigen vectors calculated in the process were used to make unambiguous vibrational assignments of all the fundamentals.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Four hundred and forty-four valence force constants for eight molecules are reduced to 81 using 369 frequencies. ► Vibrational assignment is made for all the fundamentals using eigen vectors and PED. ► The average error between observed and calculated frequencies is 13 cm−1. ► Transferability of 81-element valence force field has been demonstrated for related molecules. ► The dispersions of force constants are small in these molecules.