A Classical Trajectory Study of the Dissociation and Isomerization of C₂H₅

Abstract

Motivated by photodissociation experiments in which non-RRKM nanosecond lifetimes of the ethyl radical were reported, we have performed a classical trajectory study of the dissociation and isomerization of C2H5 over the energy range 100-150 kcal/mol. We used a customized version of the AIREBO semiempirical potential (Stuart, S. J.; et al. J. Chem. Phys. 2000, 112, 6472-6486) to more accurately describe the gas-phase decomposition of C2H5. This study constitutes one of the first gas-phase applications of this potential form. At each energy, 10,000 trajectories were run and all underwent dissociation in less than 100 ps. The calculated dissociation rate constants are consistent with RRKM models; no evidence was found for nanosecond lifetimes. An analytic kinetics model of isomerization/dissociation competition was developed that incorporated incomplete mode mixing through a postulated divided phase space. The fits of the model to the trajectory data are good and represent the trajectory results in detail through repeated isomerizations at all energies. The model correctly displays single exponential decay at lower energies, but at higher energies, multiexponential decay due to incomplete mode mixing becomes more apparent. At both ends of the energy range, we carried out similar trajectory studies on CD2CH3 to examine isotopic scrambling. The results largely support the assumption that a H or a D atom is equally likely to dissociate from the mixed-isotope methyl end of the molecule. The calculated fraction of products that have the D atom dissociation is ∼20%, twice the experimental value available at one energy within our range. The calculated degree of isotopic scrambling is non-monotonic with respect to energy due to a non-monotonic ratio of the isomerization to dissociation rate constants.

Site-specific hydrogen-atom elimination in photoexcited ethyl radical

“…Fast H atoms are thus produced by direct dissociation on the 3s state, while some population goes into the ground state forming slow H atoms. Theoretical studies including ab initio molecular dynamics employing surface hopping have also been performed supporting the existence of these two mechanisms although some small discrepancies exist concerning the unimolecular dissociation leading to slow H atoms 20,21. Besides, Matsugi22 performed a classical trajectory study revealing in particular the existence of a minor roaming pathway in ground state ethyl leading to C 2 H 3 + H 2 followed by secondary dissociation of C 2 H 3 into C 2 H 2 + H.…”

“…Among them, the ethyl radical, one of the simplest hydrocarbon radicals, constitute a prototype of alkyl radical. Numerous investigations have been performed on its thermal decomposition9–13 as well as on its UV-VUV excitation and photodissociation dynamics, from both experimental14–19 and theoretical approaches 20,21…”

The 3s versus 3p Rydberg state photodissociation dynamics of the ethyl radical

“…Among them, the ethyl radical, one of the simplest alkyl radicals, constitutes a prototype for such studies and several theoretical and experimental investigations have been reported focusing in particular on its thermal decomposition [2][3][4][5][6] and on its UV-VUV photoexcitation and further dissociation. [7][8][9][10][11][12][13][14][15] A first broad absorption band centered at ∼245 nm and assigned to excitation to the 3s Rydberg state characterizes the absorption spectrum of the ethyl radical, while a second band centered at ∼200 nm is attributed to the population of the 3p Rydberg state 16 . A first study on the photodissociation of the ethyl radical was carried out in the early 1990s by Koplitz and coworkers measuring the intensity of the Doppler profile of atomic hydrogen and deuterium from photoexcited ethyl and its isotopologues (i.e.…”

The photodissociation dynamics of alkyl radicals

“…30,31 The results were confirmed by ab initio molecular dynamics surface hopping calculations that reproduced the site-selectivity, 32 but they were questioned again by a more recent classical trajectory study. 33 Matsugi identified a roaming pathway in trajectory calculations on ethyl, leading to vinyl + H 2 . 34 He suggested that part of the previously observed H-atoms might originate from the photodissociation of vinyl.…”