Analysis of vibrational spectra of chlorotoluene based on density function theory calculations

The conformational behavior and structural stability of chlorotoluene were investigated by utilizing ab initio calculations with 6-31G* basis set at restricted Hartree-Fock (RHF) and density function theory (DFT) levels. The vibrational frequencies of chlorotoluene were computed at the RHF and DFT levels. Complete vibrational assignments were made on the basis of normal coordinate calculations for stable conformer of the molecule. RHF results without scaled quantum mechanical (SQM) force field procedure considered are in bad agreement with experimental values. Of the five DFT methods, BLYP reproduces the observed fundamental frequencies most satisfactorily with the mean absolute deviation of the non-CH stretching modes less than 10 cm(-1). Two hybrid DFT methods are found to yield frequencies, which are generally higher than the observed fundamental frequencies. When the calculated results are compared with 'experimental' frequencies, B3LYP method is found to be slightly more accurate for C-H stretching modes. The results indicate that BLYP calculation is a very promising approach for understanding the observed spectral features.