Vibrational frequencies for polyatomic molecules. I. Indole and 2,3-benzofuran spectra and analysis

A method is presented that can predict the vibrational frequencies of two and three ring polycyclic molecules to an accuracy sufficient for identifying fundamental vibrations. The scaled internal coordinate method of Pulay and co-workers is applied to force constants calculated with the Austin Method 1 semiempirical Hamilitonian. The technique was applied to benzene, furan, and pyrrole to generate a set of scale factors that would allow the a priori prediction of the vibrational spectra for benzofuran and indole. The predicted frequencies had root mean squared deviations of 15–23 cm−1 from the observed frequencies. Using the indole and benzofuran predictions, the vibrational assignments were made and ideal gas entropies were calculated. These spectroscopic entropies agreed with the calorimetrically determined entropies.