PNNL

Abstract

High resolution infrared absorption spectra have been analyzed for two bicyclo[1.1.1]pentane isotopologues, C5H8 (-d0) and C5H7D (-d1), where in the latter the D-atom replaces a hydrogen on the C3 symmetry axis such that the molecular symmetry is reduced from D3h to C3v. Two (a2") parallel bands, ?17 and ?18, of bicyclopentane-d0 were studied and the former was found to be profoundly affected by Coriolis coupling with the nearby (e') perpendicular band, ?11. Weaker coupling was observed between the ?18 band and the nearby ?13(e') band, for which fewer transitions could be assigned. For bicyclopentane-d1, the ?5 parallel band was also studied along with the nearby ?15(e') band to which it is coupled through a similar type of Coriolis resonance. For both isotopologues, quantum calculations (B3LYP/cc-pVTZ) done at the anharmonic level were very helpful in unraveling the complexities caused by the Coriolis interactions, provided that care is taken in identifying the effect of any Coriolis resonances in the theoretical values of aB and q rovibrational parameters. The ground state B0 constants were found to be 0.2399412(2) and 0.2267506(11) cm-1 for the -d0 and -d1 isotopologues. The difference yields an Rs substitution value of 2.0309(2) ? for the position of the axial H atom relative to the -d0 center of mass, a result in good accord with a corresponding Ra value of 2.044(6) ? from electron diffraction data. For both isotopologues, the theoretical results from the quantum calculations are in good agreement with all corresponding values determined from the spectra.