A comparative study of the structure and vibrational spectra of diphenylmethane, the carcinogen 4,4′-methylenedianiline and 4,4′-methylenebis(N,N-dimethylaniline)

The structural stability of 4,4′-methylenedianiline (MDA), 4,4′-methylenebis(N,N-dimethylaniline) (MBDMA) and their parent diphenylmethane (DPM) was investigated by the DFT-B3LYP and ab initio MP2 calculations with the 6-311G^·· basis set. From the calculations the three diphenylmethanes were predicted to exist predominantly in a non-planar structure with a near C₂ molecular symmetry in agreement with X-ray studies. The CCCC angles in DPM and the two dianilines were calculated to be nearly equal of about 60° as compared to X-ray angles of 17° and 67° for MDA. The NH₂ inversion barriers in MDA was estimated to be about 1-5 kcal/mol. On comparison of the Raman spectra of the three compounds, the line intensity and position of the characteristic ring breathing mode were shown to be sensitive to the amino substitution. The vibrational wavenumbers of the optimized structures of the two dianilines were computed at the DFT-B3LYP level of theory. Tentative vibrational assignments were made on the basis of combined DFT and experimental FT-IR and FT-Raman data of MDA and MBDMA.