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 the basis of 131 observed IR and Raman frequencies of 6 isotopomers of C2h symmetry. This reproduces the observed vibrational frequencies with an rms error of 6 cm-1 (9.6 cm-1 for the highly anharmonic C-H/D stretches, 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 transforming them into the presently used set of internal coordinates whereby good agreement is found, especially with the purely empirical force field 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 following Pulay et al. Thereby, it is found that a scaling model with only 12 independent variables reproduces the spectra and the diagonal elements of the reference force field quite well provided that the starting Hessian is of good quality. This remains true even if only IR frequencies are used in the fitting, as it is the case in our studies on radical ions.