Large intramolecular energy flow in vibrational overtone spectra of cyclohexene-3,3,6,6-d4

Abstract

The vibrational structure of CH stretching states in gas-phase cyclohexene-3,3,6,6-d4 was studied using FTIR spectroscopy in the range 1200-1 1500 cm-1 and intracavity dye laser photoacoustic spectrometry in the range 12900-16000 cm-l. The structure was modeled using an effective vibrational Hamiltonian which describes the Fermi resonance couplings of the C H stretching states with suitable low-frequency vibrations. Some conclusions are made on the possible ways of intramolecular vibrational redistribution of the energy (IVR) on the two methylenic groups for wavenumbers below 1 1 000 cm-I in connection with the existence of near-infrared circular dichroism in monoterpenes.

Theoretical and experimental studies for the interpretation of vibrational circular dichroism spectra in the CH-stretching overtone region

“…35, but agrees with an ab initio study by Polavarapu et al, 43 by which it was found that the calculated CH-bond lengths are more dependent from the conformation than from the distance from the C=O; in passing, we observe that a discrepancy between the findings of infrared and NIR data was pointed out in Ref. 36, in commenting the results for cyclohexene and cyclopentene. The same can be said for cyclohexanone, where the experimental accuracy in the determination of and is not as high as for cyclopentanone, since the assignment and deconvolution of individual bands is poorer: indeed, the values for the parameters at the ␣-positions reported in Table 1 are determined from weak features and shoulders of perhydrocyclohexanone spectra.…”

“…Since the same signals are present for cyclopentanone-2,2,5,5-d 4 and cyclohexanone-2,2,6,6-d 4 , with the same frequencies and with an intensity decrease equal to the number of deuterium atoms replacing the hydrogens, 12,13 we assign the two signals to axial and equatorial CH-bond stretchings in either ␣, ␤, and ␥ (for cyclohexanone) positions. The equatorial CH-bond stretchings have higher frequencies and intrinsic intensities, as has been observed in cyclohexene, 36 and as has been recognized and rationalized to some extent in a variety of ring molecules, comprising cyclohexane, 41 dioxane, tetrahydropyrane, and piperidine. 42 The previous overtone regions ⌬v = 2 and ⌬v = 3 (Figs.…”

“…46 For these reasons we think that the values found in Ref. 36 for cyclohexene apply to (R)-limonene and to (3R)-methylcyclopentanone.…”

Ring Puckering and CH Stretching Spectra. 3. High Vibrational States of Gaseous Monohydrogenated Cyclopentene-4-h1

“…There are two kinds of off-diagonal couplings: the Fermi resonance couplings which are deduced from the first-order Hamiltonian 25 and the interbond coupling between the CH and CD stretchings.…”

“…When the movement is slow enougth as in cyclohexene, there is no competition between the two phenomena, and the vibrational energy dynamics is essentially governed by the strong Fermi resonance couplings. 25 For more rapid motions, the two couplings have to be treated together. 27 For that purpose, the adiabatic approximation has proved to be a powerful tool to decouple the fast vibration from the slow large-amplitude motion in the first excited CH stretching state 28 as well as in higher overtones.…”