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.

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► 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.