UV photodissociation dynamics of allyl radical by photofragment translational spectroscopy

Abstract

Photodissociation of the allyl radical, CH2CHCH2, has been studied using the method of molecular beam photofragment translational spectroscopy following excitation to the (C) over tilde(2 B-2(1)) and (A) over tilde(1 B-2(1)) states by 248 and 351 nm photons. Two different primary channels have been detected following 248 nm excitation: H-atom loss (84%) and CH3 elimination (16%). From the product translational energy distribution and polarization dependence studies, dissociation processes from the ground-state C3H5 potential energy surface are inferred for both wavelengths. At 248 nm there may also be a contribution to the H-atom loss channel from predissociation by a higher electronically excited state. Rice-Ramsperger-Kassel-Marcus (RRKM) calculations show that the formation of cyclopropene is not important, while formation of allene and methylacetylene from dissociation of 1- and 2-propenyl radicals are important reaction pathways at both wavelengths. Translational energy distributions peaking well away from zero provide evidence for CH3 elimination directly from an allylic structure through a four-member cyclic transition state. (C) 1998 American Institute of Physics. [S0021-9606(98)02137-0]

Photochemistry of C3Hphydrocarbons in Titan’s stratosphere revisited

“…A.5. Photodissociation in the 403 nm band leads to H-atom loss only (Stranges et al 1998), and mainly to CH 2 CCH 2 + H formation (Castiglioni et al 2006;Szpunar et al 2002). At 248 nm the H atom loss channel has been found to be equal to 84% with the CH 3 + C 2 H 2 channel equal to 16% (corrected to 5% subsequently Stranges et al 2008).…”

Primary and Secondary Dissociation from Allyl Iodide Excited at 193 nm: Centrifugal Effects in the Unimolecular Dissociation of the Allyl Radical

“…The presence of allyl radicals that do not dissociate despite having internal energies greater than 60 kcal/ mol could be reconciled if the barrier to dissociation of the allyl radical were more than 15 kcal/mol higher than the literature value, but this is unlikely because there are two independent calculations in the literature that place the barrier between 59 and 63 kcal/mol. 4,7 Clearly, we are observing allyl radicals that are stable to secondary dissociation even though they have internal energies well above the barrier height.…”

“…Several studies have detailed the dynamics of vibrationally hot allyl radicals formed via UV excitation of ground-state allyl radicals. 3-5,7a,25 Stranges et al 4 used photofragment translational spectroscopy to study the photodissocation dynamics of the allyl radical produced through the pyrolysis of allyl iodide. Photolysis at 248 and 351 nm excited the allyl radical to the C (2 2 B 1 ) and à (1 2 B 1 ) states, which then relaxed to the ground electronic state through internal conversion.…”

“…Despite its role as a precursor for the allyl radical, [1][2][3][4][5][6][7][8][9][10][11] little is known about the photochemistry of allyl iodide. There are two UV spectra in the literature, both of which display a strong, broad, structureless absorption band peaking near 200 nm.…”

Rotationally resolved photoelectron spectroscopic study of the Jahn–Teller effect in allene

“…[4][5][6][7][8][9] Allene is also the smallest member of the cumulene family and one of the simplest representatives of the D 2d ͑M͒ molecular symmetry group. The X + 2 E electronic ground state of the cation exhibits the E ͑b 1 b 2 ͒ Jahn-Teller ͑JT͒ effect 10,11 encountered only in molecules with one or more 4n-fold axes of symmetry: A strong distortion along the torsional mode 4 ͑b 1 symmetry͒ is revealed in the He I photoelectron spectrum by a long progression in that mode.…”