Examination of allyl radical excited-state dynamics and ground-state vibrational frequencies by ultraviolet resonance Raman spectroscopy

Abstract

43) Analogous to those in the ' n r * states of ketones, studied via the circular polarization of the fluorescence, see: Schippers, P. H.; Dekkers, H. P. J. M. Chem. Phys. Lett. 1982.88, 512-516. Dekkers, H. P. J. M.; Moraal, P. F. InAbstract: The first Raman spectra of the allyl radical have been obtained. The intensities of the observed Raman spectra indicate excited-state dynamics consistent with a disrotary photoisomerization of the allyl radical to form a cyclopropyl radical. Prior to this work, direct examination of the photoisomerization pathway was not possible due to limitations of the techniques applied. The ground-state vibrational frequencies observed are found to be in excellent agreement with recent theoretical calculations suggesting a reassignment of the literature infrared frequencies. This work demonstrates that resonance Raman spectroscopy is a powerful method for examination of gas-phase free radicals.

Time-resolved photoelectron spectroscopy of the allyl radical: The lifetimes of the ultraviolet bands

“…Photocyclization in agreement with the Woodward-Hoffmann rules was suggested by theoretical studies 19 and resonance Raman experiments. 4 At the excitation energies present in our experiment, the cyclopropyl radical would lose a hydrogen atom and form the closed-shell molecule cyclopropene. In our recent nanosecond experiments, 21 investigating the Hatom product of the unimolecular dissociation of allyl radicals, no indication for cyclopropene formation was found.…”

“…1. The ground-state geometry is known with high accuracy from IR diode laser spectroscopy, 3 while the vibrational frequencies were determined by resonance Raman spectroscopy 4 and IR spectroscopy in cryogenic matrices. 5 The location of the electronic A state was derived from an unassigned, but well resolved absorption spectrum.…”

“…6 The UV band system starting at 250 nm was studied in detail by absorption spectroscopy, 7 multiphoton ionization, [8][9][10][11][12] and resonance Raman spectroscopy. 4,13,14 Excited state geometries were derived from partially rotationally resolved MPI spectra. 11,12 Although a high oscillator strength of 0.23-0.26 was calculated in a theoretical study for one of the electronic transitions, 15 and also measured in absorption, 16 the complete absence of fluorescence was reported, 4 indicating short lifetimes.…”

“…4,13,14 Excited state geometries were derived from partially rotationally resolved MPI spectra. 11,12 Although a high oscillator strength of 0.23-0.26 was calculated in a theoretical study for one of the electronic transitions, 15 and also measured in absorption, 16 the complete absence of fluorescence was reported, 4 indicating short lifetimes. The presumably laserlimited linewidth of 1 cm Ϫ1 in the MPI studies, however, poses a lower limit of 5 ps on the lifetime of the lower vibronic states.…”

“…Irradiation at 254 nm, 18 yielded a mixture of allene, propyne, acetylene, methane, and traces of propene, while irradiation at 400 nm supposedly yielded cyclopropyl radicals. 17 Theoretical studies 19 suggested a disrotatory photocyclization upon excitation in the UV bands, a reaction pathway that was supported by the results of a resonance Raman experiment 4,13,14 performed at 224.63 nm, but no direct information on the decay properties of the UV band system is yet available. Thus the results have to be regarded as inconclusive.…”