Knowledge Bank

Ab initio quadratic vibrational force fields have been calculated at the Hartree-Fock level for the trans and gouche conformers of n-butane. Gas phase Raman and infrared spectra have been observed and assigned for several isotopically substituted species. The ab initio force fields must be scaled appropriately to reproduce the frequency assignments for the observed spectra. However, the scaled force fields reported last year at this conference were not able to reproduce observed frequencies and Raman trace scattering cross sections in an entirely satisfactory manner. Since the quadratic ab initio force field calculates harmonic frequencies, scaling such a force field to reproduce observed (anharmonic) frequencies involves the drastic assumption of negligible anharmonicity. In cases where a detailed vibrational analysis is not possible, harmonic frequencies may be estimated by an empirical `harmonization' procedure. as has been done, for example, in the case of $ethane1$. The application of such a procedure in the cases of n-propane, previously investigated by $us2$, and n-butane is being investigated. Secondly, ab initio force fields which incorporate electron correlation contributions have been more successful in reproducing vibrational frequencies in many cases, especially for modes involving skeletal motions. However, the direct calculation of such force fields for n-butane is not currently feasible, especially for the lower symmetry gauche conformer. Differences between force constant matrix elements calculated for benzene with and without electron correlation were recently reported to be independent of the basis $set3$. In an extension of this idea, the two types of force fields available for $n-propane2$ may be used to estimate corrections to the n-butane force fields, in an attempt to account for the effects of electron correlation in this more complicated case. The effectiveness of these measures to obtain improved force fields for n-butane will be discussed.