Dissociation Pathways of the CH₂CH₂ONO Radical: NO₂ + Ethene, NO + Oxirane, and a Non-Intrinsic Reaction Coordinate HNO + Vinoxy Pathway

We first characterize the dissociation pathways of BrCH₂CH₂ONO, a substituted alkyl nitrite, upon photoexcitation at 193 nm under collision-free conditions, in a crossed laser–molecular beam scattering apparatus using vacuum ultraviolet photoionization detection. Three primary photodissociation pathways occur: photoelimination of HNO, leading to the products HNO + BrCH₂CHO; C–Br bond photofission, leading to Br + CH₂CH₂ONO; and O–NO bond photofission, leading to NO + BrCH₂CH₂O. The data show that alkyl nitrites can eliminate HNO via a unimolecular mechanism in addition to the commonly accepted bulk disproportionation mechanism. Some of the products from the primary photodissociation pathways are highly vibrationally excited, so we then probe the product branching from the unimolecular dissociation of these unstable intermediates. Notably, the vibrationally excited CH₂CH₂ONO radicals undergo two channels predicted by statistical transition-state theory, and an additional non-intrinsic reaction coordinate channel, HNO elimination. CH₂CH₂ONO is formed with high rotational energy; by employing rotational models based on conservation of angular momentum, we predict, and verify experimentally, the kinetic energies of stable CH₂CH₂ONO radicals and the angular distribution of dissociation products. The major dissociation pathway of CH₂CH₂ONO is NO₂ + ethene, and some of the NO₂ is formed with sufficient internal energy to undergo further photodissociation. Nascent BrCH₂CHO and CH₂Br are also photodissociated upon absorption of a second 193 nm photon; we derive the kinetic energy release of these dissociations based on our data, noting similarities to the analogous photodissociation of ClCH₂CHO and CH₂Cl.