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 cyclopentene (monohydrogenated -3-h(1) and -4-h(1)). Reasonable agreement with experimental data is found. It is shown that zero-point vibrational energy effects contribute to the empirically derived puckering inversion barrier and are responsible for the asymmetry of the two potential wells in the monohydrogenated compounds, in good agreement with the previous experimental determinations. The structure of the isolated CH stretching band in the infrared (Delta v = 1 and 2) and in the isotropic Raman (Delta v = 1) spectra of gaseous monohydrogenated cyclopentenes is well reproduced by a theoretical calculation which emphasizes that the variation of the puckering potential energy function between the fundamental and the excited states of the CH stretching vibration is only due to the conformational dependence of the CH stretching energy. For the -3-h(1) compound, a linear correlation between the CH stretching 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.