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

Abstract

The excited CH stretching vibrational states of gaseous cyclopentene (4-HC 5 D 7 ) have been measured from ∆V ) 3 to ∆V ) 6. Overtone spectra are modified by the large-amplitude ring-puckering motion and by a rapid energy flow to combination states. These two types of couplings with the CH stretching vibrations are explicitly modeled. The reconstruction of the spectra gives the predominant pathways of the intramolecular vibrational redistribution of the energy.

High Vibrational State Energy Redistribution in Two Deuterated Cyclopentenes

“…The theoretical approach in the analysis of the spectra has previously been described in the first papers on cyclopentene. , The difference in the present work resides in the writing of the vibrational Hamiltonian. The previous Hamiltonian was expressed in curvilinear normal coordinates.…”

“…The present work is situated in the frame of IVR studies of the methylenic group. The vibrational energy flow characteristic of this functional group has been studied in cyclic molecules such as cyclohexene 6 or monohydrogenated cyclopentene. , But the overtone spectra have been modeled in normal coordinates and thus, the anharmonic potentials describing the energy flow were not transferable from one molecule to another. In the present study, we have focused our attention on the intramolecular vibrational energy redistribution in the overtone region of CH stretches of two isopotomers of the cyclopentene molecule, one with one CHD methylene group, the cyclopentene-1,2,3,4,4,5,5- d 7 (3h 1 ) molecule and one with a CH 2 group on the same carbon, cyclopentene-4,4,5,5- d 4 ( d 4 ) (Figure ).…”

“…When there are strong Fermi resonances, the characteristic time of the energy flow has been estimated to be less than 100 fs. The adiabatic approximation has proved powerful in describing the high vibrational states of nitromethane 12,13 or cyclopentene. − In particular, it clears up the discrepancy between the spectra of the first CH stretching excited states of different isotopomers of the same molecule. Indeed, the coupling between the two motions can be described by an effective potential that contains a vibrational part corresponding to the variation of vibrational energy during internal motion. ,, For the monohydrogenated molecule, that vibrational energy breaks the symmetry of the effective potential modifying the large amplitude motion levels of the vibrationally excited molecule.…”

Method for Evaluating Vibrational Mode Assignments in Surface-Bound Cyclic Hydrocarbons Using Sum-Frequency Generation

“…In surface-bound cyclopentene, a peak is observed in the SSP-polarized spectrum at 2926 cm –1 . In contrast to the peaks observed for the other species in this frequency range, this peak most likely corresponds to a coupled motion of asymmetric stretches as many assigned spectra of cyclopentene include a peak at 2933 cm –1 assigned to an out-of-phase asymmetric stretch (A″ in C s symmetry). ,,, This particular assignment is far from established, as there are few examples in the literature of assigned vibrational spectra of cyclopentenes substituted in the 4-position, although one computational study of cylopropanecarboxaldehyde has no peak that is assigned for that frequency …”

“…In contrast to the peaks observed for the other species in this frequency range, this peak most likely corresponds to a coupled motion of asymmetric stretches as many assigned spectra of cyclopentene include a peak at 2933 cm À1 assigned to an out-of-phase asymmetric stretch (A 00 in C s symmetry). 108,109,114,115 This particular assignment is far from established, as there are few examples in the literature of assigned vibrational spectra of cyclopentenes substituted in the 4-position, although one computational study of cylopropanecarboxaldehyde has no peak that is assigned for that frequency. 110 Two more vibrations observed at slightly higher frequencies do not have PPP/SSP values that match the calculated values for an isolated asymmetric stretch in C 2v symmetry.…”