Photodissociation dynamics of the 2-propyl radical, C3H7

Abstract

The photodissociation of 2-propyl leading to propene+H was investigated with nanosecond time resolution. A supersonic beam of isolated 2-propyl radicals was produced by pyrolysis of 2-bromopopane. The kinetic energy release of the H-atom photofragment was monitored as a function of excitation wavelength by photofragment Doppler spectroscopy via the Lyman-alpha transition. The loss of hydrogen atoms after excitation proceeds in alpha position to the radical center with a rate constant of 5.8x10(7) s-1 at 254 nm. Approximately 20% of the excess energy is deposited as translation in the H-atom photofragment. In contrast 1-propyl does not lose H atoms to a significant extent. The experimental results are compared to simple Rice-Ramsperger-Kassel-Marcus calculations. The possible reaction pathways are examined in hybrid density functional theory calculations.

Ultraviolet photodissociation dynamics of the n-propyl and i-propyl radicals

“…The overall ⟨ f T ⟩ of the entire distribution is 0.2, in agreement with the earlier study. 35 The peak of ∼6 kcal/mol in the P(E T ) distribution is consistent with the ∼3 kcal/mol exit channel barrier in the unimolecular dissociation to H + propene. 17 The slow component with β ∼ 0 indicates that the dissociation lifetime of the slow channel is comparable or longer than the rotational period (∼ps) of the i-propyl radical, again consistent with unimolecular dissociation of the hot radical.…”

“…35 In the photodissociation of ethyl via its 3s Rydberg state, 32-34 bimodal product translational energy release and energy-dependent angular distribution of the Hloss products were observed and attributed to two dissociation pathways. 33,34 The excited 3s Rydberg state decays via a conical intersection in a nonclassical H-bridged geometry, producing a fast and anisotropic channel with direct H-atom scission via the nonclassical H-bridged transition state and a slow and isotropic channel from unimolecular dissociation of the hot radical after internal conversion.…”

“…33 Photodissociation of the jet-cooled propyl radicals was investigated by Hatom photofragment Doppler spectroscopy. 35 The H-atom dissociation channel was examined for the i-propyl radical from 230 to 255 nm, while the n-propyl radical was found to lose very small amount of H atoms in the same range (postulated to be caused by a competing dissociation channel, such as methyl and ethylene). About 20% of the excess energy of the i-propyl radical was found to be disposed as translational energy for the H-loss product channel.…”

“…It was argued that although the activation energy for isomerization of i-propyl to n-propyl is higher than that for the dissociation channel of i-propyl, the hot i-propyl radical may still have a chance to pass the higher barrier to decompose via the methyl-loss channel. 35 The current work investigates the UV photodissociation dynamics of jet-cooled n-propyl and i-propyl radicals in the photolysis wavelength region of 230-260 nm. This study aims to probe the H-atom photodissociation of the n-propyl radical via the 3s Rydberg excited state (the broad absorption peak at 245 nm) and the i-propyl radical via the 3p Rydberg states (the 236 nm peak), respectively.…”

Energy Deposition in Planetary Atmospheres by Charged Particles and Solar Photons

“…Exceptions to this generalization include some trace species, such as ozone, which absorbs weakly in the Chappuis bands from ∼ 4500−8500 Å in the terrestrial and Martian atmospheres. Some hydrocarbon radicals, which may be found in small abundances in the middle atmospheres of the outer planets, theoretically may undergo photodissociation in the visible region to produce an energetic H atom, such as C 2 H 5 + hν → C 2 H 4 + H, for which the threshold dissociation energy (DE) is 1.65 eV (7535 Å) (e.g., Gilbert et al 1999) and the 2-propyl radical C 3 H 7 + hν → C 3 H 6 + H for which the DE is 1.536 eV (8071.9 Å) (e.g., Noller and Fischer 2007). Visible photons therefore either penetrate to the surfaces or are scattered by cloud and haze particles on all the planets and satellites that have significant atmospheres.…”