RING PUCKERING AND CH STRETCHING SPECTRA .1. THEORETICAL-STUDY OF GASEOUS MONOHYDROGENATED CYCLOPENTENES 3-H(1) AND 4-H(1)

Ab-initio calculations have been performed at the HF/6-31G* level to study the conformational dependence of the geometry parameters and of the vibrational frequencies of two hydrogen/deuterium isotopomers of c yclopentene (monohydrogenated -3-h(1) and -4-h(1)). Reasonable agreeme nt with experimental data is found. It is shown that zero-point vibrat ional energy effects contribute to the empirically derived puckering i nversion barrier and are responsible for the asymmetry of the two pote ntial wells in the monohydrogenated compounds, in good agreement with the previous experimental determinations. The structure of the isolate d CH stretching band in the infrared (Delta v = 1 and 2) and in the is otropic Raman (Delta v = 1) spectra of gaseous monohydrogenated cyclop entenes is well reproduced by a theoretical calculation which emphasiz es that the variation of the puckering potential energy function betwe en the fundamental and the excited states of the CH stretching vibrati on is only due to the conformational dependence of the CH stretching e nergy. For the -3-h(1) compound, a linear correlation between the CH s tretching and ab-initio CH bond length yields an excellent fit to the observed spectra. For the -4-h(1) compound, some modifications must be considered to get a correct fit of the experimental spectra.