An ab initio calculation of the vibrational force field and spectrum of pyrazole

Abstract

The complete harmonic vibrational force field of pyrazole has been computed at the Hartree—Fock level with the 4–21 basis set of Gaussian orbitals. The harmonic force field was scaled by using 6 scale factors previously derived by fitting the computed force field of benzene to the observed benzene vibrational spectrum. The resulting scaled force field was then used to predict the vibrational spectrum of pyrazole. The frequency predictions have also been done by using the 9 scale factors (including those for the NH motions) previously optimized for imidazole. The predicted values for the fundamentals ν 12 and ν 19, for which no experimental data are available, are totally consistent in the above two procedures. In order to obtain the best possible force field, a new set of 10 scale factors, some of which turned out to be identical with those from imidazole, was derived by fitting the computed spectrum of pyrazole to the observed pyrazole spectrum. Dipole-moment derivatives were also calculated and used to predict infrared intensities which are compared with experimental values.