CALCULATION OF VIBRATIONAL FREQUENCIES OF SULFUR-COMPOUNDS USING SEMIEMPIRICAL MOLECULAR-ORBITAL METHODS (AM1, PM3, MNDO)

Vibrational frequencies of 30 sulphur-containing molecules for which detailed experimental data are available were calculated using semiempirical molecular orbital methods (AM1, PM3, MNDO). The agreement between the uncorrected calculated frequencies and the experimental data was found to be satisfactory, the calculated mean errors (AM1, 9.5%; PM3 9.0%; MNDO 12.2%) being comparable with those commonly obtained using ab initio SCF-MO calculations (7-10%). Because of the systematic nature of the errors in the calculated frequencies, simple empirical corrections significantly improve the agreement with experiment (mean errors in the corrected calculated frequencies are: AM1, 3.4%; PM3, 3.3%; MNDO, 3.5%), thus making this kind of calculation of practical value in helping the assignment of vibrational spectra of considerably large sulphur-containing molecules. Thermodynamic quantities evaluated using the semiempirical vibrational frequencies were also calculated, showing very good agreement with the experimentally available values.