EVALUATION OF HARMONIC FORCE-FIELDS FROM LIMITED SPECTROSCOPIC INFORMATION AND CALCULATED HESSIAN MATRICES - BUTADIENE

A fully optimized harmonic force field was obtained for butadiene on t he basis of 131 observed IR and Raman frequencies of 6 isotopomers of C2h symmetry. This reproduces the observed vibrational frequencies wit h an rms error of 6 cm-1 (9.6 cm-1 for the highly anharmonic C-H/D str etches, 4.0 cm-1 for the other modes) and probably represents the best force field which can be obtained for this molecule within a harmonic approximation. It is compared to those obtained earlier by transformi ng them into the presently used set of internal coordinates whereby go od agreement is found, especially with the purely empirical force fiel d evaluated by Furukawa et al. In a second step, the Hessian matrices obtained from different quantum mechanical procedures are transformed into symmetry coordinates and subjected to various scaling models foll owing Pulay et al. Thereby, it is found that a scaling model with only 12 independent variables reproduces the spectra and the diagonal elem ents of the reference force field quite well provided that the startin g Hessian is of good quality. This remains true even if only IR freque ncies are used in the fitting, as it is the case in our studies on rad ical ions.