Quantum chemistry studies of far-infrared spectra of aromatic urethanes

Restricted Hartree-Fock and density function calculations (B3LYP), using 6-311++G(d,p), have been used to investigate the far infrared spectra of aromatic urethanes, synthesized on the basis of 2,4-and 2,6-toluene diisocyanate (2,4-TDI, 2,6-TDI), and the spectrum of ethylphenylurethane. It is shown, that the region of frequencies of 100-200 cm−1 is associated primarily with torsional vibrations of methyl groups. For almost all studied urethanes, the bands are observed in the region 385-340 cm−1, which is associated with in plane deformations of angles CCNC, COC and CNC of the urethane groups according to the calculations. The bands, observed at 300-320 and 260-280 cm−1, are assigned to in plane and out of plane deformations of the urethane skeleton, which are mixed with vibrations of methyl group connected to the benzene ring.