Infrared and Raman spectra of bicyclic molecules using scaled noncorrelated and correlated ab initio force fields

This paper reports the application of a scaled ab initio calculated harmoni c force field to predict the frequencies, infrared intensities, Raman inten sities, and depolarization ratios of benzofuran, benzothiophene, indole, be nzothiazole, and benzoxazole. The theoretical calculations were made using the Hartree-Fock HF/3-21G* and HF/6-31G* basis sets and density-functional theory (DFT)B3-LYP/6-31G* levels. The equilibrium calculated force constant s are scaled according to the method of Pulay and compared with the experim entally determined frequencies, intensities, and depolarization ratios to a ssess the accuracy and fit of the theoretical calculation. Methods for quan titative comparison of intensities were developed. The double numerical dif ferentiation algorithm of Komornicki and McIver was analyzed and used to ca lculate the Raman intensities for the (DFT)B3-LYP/6-31G* model. The (DFT)B3 -LYP/6-31G* model is approaching the harmonic limit in the planar and nonpl anar refinement of these bicyclics with wave number fits of 5 and 4 cm(-1), respectively. It reduces the need for scale factors and increases their tr ansfer accuracy, largely because the scale factors values cluster near unit y. The Komornicki and McIver algorithm is still a viable method for calcula ting Raman intensity information for methods that do not have analytic rout ines programmed. The main shortcoming to this method may lie in the tighter self-consistent field (SCF) convergence criterion possibly needed to calcu late Raman intensities for the totally symmetric modes of large molecules. The (DFT)B3-LYP/6-31G* model was superior for calculating the planar intens ities, but equal to the HF methods for predicting the nonplanar intensities . (C) 1999 American Institute of Physics. [S0021-9606(99)30112-4].