Theoretical prediction of the vibrational spectrum geometry and scaled quantum mechanical force field for phenylacetylene

Abstract

The optimized geometry and the complete harmonic force field of phenylacetylene have been determined by ab initio Hartree-Fock calculations, using a 4-21 Gaussian basis set. Systematic errors in the bond lengths, characteristic at this level of theory, were corrected by empirical offset values. Only at the ipso position does the equilibrium geometry obtained in this way differ significantly from the experimental one (MW; r{sub g}). A detailed analysis suggests that while the experimental ipso CCC angle may be correct, the ipso CC distance should be reconsidered. The force field was evaluated at the above geometry as a reference. To remove systematic deficiencies, the final scaled quantum mechanical (SQM) force field was obtained by applying scale factors optimized previously for benzene and acetylene. All force constants that occur both in phenylacetylene and in the isoelectronic molecule benzonitrile are very nearly the same. Frequencies calculated from the SQM force field of phenylacetylene confirm, with only a few exceptions, the published experimental assignments for all four isotopomers investigated. When the C-H (and C-D) stretching frequencies, perturbed by anharmonicity, are not considered, the average deviation between the observed and calculated frequencies is below 10 cm{sup {minus}1}. Theoretical dipole moment derivatives are discussed, and infraredmore » intensities are presented. Quartic centrifugal distortion constants, calculated also from the SQM force field, agree well with the results of an incomplete experimental study.« less