Interpretation of the vibrational spectra of matrix-isolated uracil from scaled ab initio quantum mechanical force fields

The vibrational spectrum of uracil trapped in an argon matrix has been interpreted based on ab initio Hartree–Fock SCF calculations with a split-valence 4?21 basis set. The directly computed theoretical general valence force field was scaled with empirical scale factors in order to correct for the systematic errors originating in the limitation of the theoretical model. Scale factors transferred from related molecules provided a priori prediction of fundamental frequencies and intensities, permitting several corrections to be proposed for earlier assignments. Using the observed spectrum with the few altered assignments, a new set of scale factors was optimized to give the best force field available from combined consideration of the experimental and the theoretical data. For unknown reasons, the out-of-plane force field predicted a spectrum agreeing slightly less well with experiment than did the in-plane force field. However, the overall agreement between theory and experiment provided additional support for the assumptions involved in the method. The computed force fields were compared with others available from previous work. The comparison demonstrated the importance of expanding the energy surface around the true energy minimum and of using a proper scaling procedure. Previous scaled CNDO /2 calculations were found to be surprisingly good despite the large corrections required and the fact that they were made at an incorrect geometry.