Roaming Dissociation of Ethyl Radicals

Abstract

Previous studies on the photodissociation of C2H5 reported rate constants for H-atom formation several orders of magnitude smaller than that predicted by Rice-Ramsperger-Kassel-Marcus (RRKM) theory. This Letter provides a potential explanation for this anomaly, based on direct trajectory calculations of C2H5 dissociation. The trajectories reveal the existence of a roaming dissociation channel that leads to the formation of C2H3 and H2. This channel is found to proceed over the ridge between the transition state of H-atom elimination and that of bimolecular H-abstraction. The formed C2H3 radical can subsequently dissociate to C2H2 and a H atom; this secondary dissociation is suggested to be a potential reason for the unexpectedly slow H-atom formation observed in the photodissociation experiments.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…189 Direct dynamics calculations have variously suggested roles for a minor channel leading to electronically excited triplet C2H4(ã 3 B1u) + H products, 192 and for a roaming channel leading to H2 + C2H3 products. These C2H3 products are then proposed to release an H atom, which could offer a possible rationale for the reported slow build-up rate of the atomic products, 193 though it is difficult to reconcile the observed H atom velocity distribution with such a mechanism. 194 Finally, we note that C-C bond fission is also thermodynamically allowed when exciting in this wavelength range, but we are not aware of any reports of such product formation.…”

The photodissociation dynamics of alkyl radicals

“…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. However, the observed H-atom translational energy release does not seem to be in agreement with this pathway.…”

Semiclassical Monte Carlo: A first principles approach to non-adiabatic molecular dynamics

“…This is, presumably, an artifact of the short time approximation that we used in the derivation of Eq. (10). Indeed, it is easy to check that Eq.…”

“…Indeed, it is easy to check that Eq. (10) conserves energy approximately, up to the leading order in E = E 2 − E 1 . Furthermore, we have run the test problems considered below using condition of Eq.…”

“…5,6 The latter are characterized by different reaction pathways that lead to different reaction products. For a) serg@lanl.gov b) mozyrsky@lanl.gov example, upon absorbing a photon, a molecule may undergo radiative relaxation processes, 7 such as fluorescence, 8 and phosphorescence; non-radiative relaxation processes, such as photodissociation [9][10][11] or photoisomerization; [12][13][14][15] charge; [16][17][18][19] and energy transfer; [20][21][22] or intersystem crossing. [23][24][25] Such dynamics are obviously accompanied by electronic transitions, involving complex electron-phonon interaction 26,27 and possibly many electronic states, and therefore cannot be described within the standard adiabatic or Born-Oppenheimer approximation.…”