A New Way for IVR Induced by Fast Internal Motion As Revealed by a Jet-Cooled Spectrum of Cyclopentene

Vibrational Absorption and Circular Dichroism Studies oftrans-(3S,4S)-d6-Cyclopentene in the Gas Phase

Vibrational absorption (IR) and circular dichroism (VCD) measurements of trans-(3S,4S)-d6-cyclopentene in the gas phase were performed in the C-H, C-D, and mid-infrared regions. In this study, we report the first VCD spectra recorded at high spectral resolution (up to 0.5 cm(-1)) with a very good signal-to-noise ratio (differential absorbance lower than 5 x 10(-6)). The quality of the experimental spectra allows us the observation of the vibration-rotation structure of the bands in both absorption and VCD spectra. Experimental spectra have been compared with the density functional theory (DFT) absorption and VCD spectra, calculated using B3LYP functional and cc-pVTZ basis set for the axial, equatorial, and planar conformers. Lorentzian and PQR band profiles have been used to convert the calculated dipolar and rotational strengths. In the mid-infrared (<2000 cm(-1)) region, predicted (population-weighted) spectra were in excellent agreement with experiment, allowing the determination of the absolute configuration of this molecule. Above 2000 cm(-1), a reasonable agreement was obtained even if anharmonicity was not considered and if Fermi resonance occurs in the C-D stretching region. Finally, a more precise analysis of the absorption spectrum has been achieved by taking into account anharmonicity of the C-H stretching and its coupling with the ring-puckering motion.