Spectroscopic study of fast conformational dynamics in perdeuteriated cyclopentene and its monohydrogenated [3-1H1] and [4-1H1] derivatives

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Abstract
The fast internal ring-puckering dynamics of the monohydrogenated cyclopentene [3-1H1]- and [4-1H1]-[(Cy3H) and (Cy4H), respectively] have been studied by vibrational spectrometry from the gaseous to condensed phase. In the gas phase infrared and Raman spectra of the ring-puckering motion and of the stretching vibration of the isolated oscillator CH and its overtones show that isotopic monosubstitution in the allylic position reveals the existence of two spectroscopically distinct conformers. The conformer with a CH bond in the axial position is more stable (ΔE= 4.3 and 2 cm–1 for the 3- and 4-substituted derivatives, respectively). This energy difference increases in the stretching excited states. The vibrational origin of this asymmetry is shown. The anharmonicity of the axial and equatorial ν(CH) vibrations is nearly the same, but is higher in Cy3H than in Cy4H. In [2H7] and [2H8] cyclopentenes a new crystalline phase I′ is identified for which all the spectral data are consistent with the hypothesis of a planar ring conformation of cyclopentene. In all the other condensed phases, cyclopentene is found to retain a bent equilibrium conformation. In the crystalline phase I, the variations of the puckering angle and of the potential barrier with the number of deuterium atoms could explain the appearance of phase I′ in the multideuterated derivatives. The coalescence phenomenon observed between the two axial and equatorial ν(CH) bands from the solid I to the liquid phase is evidence of fast exchange between the two conformers with a rate of ca. 5 × 1012 s–1 at room temperature.

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