DETAILED QUANTUMMECHANICAL CALCULATIONS OF MOLECULAR VIBRATION FREQUENCIES OF BENZENE, NAPHTHALENE, ANTHRACENE AND PHENANTHRENE - COMPARISON OF SEVERAL QUANTUMCHEMICAL METHODS

The reliability of the MNDO/PM3 semiempirical self-consistent field molecular orbital (SCF-MO) method for the calculation of fundamental vibration frequencies, and the effect of scaling are studied. The vibration frequencies of naphthalene were calculated by various semiempirical SCF-MO methods like MNDO/PM3, AMI, and MINDO/3. Results from the ab initio STO-3G model are also included for comparison. The MNDO/PM3 frequencies and geometries are in better agreement with experiment than the other methods. The errors in the MNDO/PM3 frequencies of benzene, naphthalene, anthracene and phenanthrene are mostly characteristic for specific types of vibration. The mean percentage deviation is 5.5% for all the modes of the molecules under investigation. A scaling procedure leads to frequency errors only within 2.8%. This agreement with experiment offers hope that MNDO/PM3 calculations may usefully contribute to the determination of harmonic frequencies. Calculations of that type may be of practical use in assignments of vibrational frequencies, especially for large molecules.