Photoproduct channels from BrCD _2CD _2OH at 193 nm and the HDO + vinyl products from the CD _2CD _2OH radical intermediate

摘要

We present the results of our product branching studies of the OH + C _2D _4 reaction, beginning at the CD _2CD _2OH radical intermediate of the reaction, which is generated by the photodissociation of the precursor molecule BrCD _2CD _2OH at 193 nm. Using a crossed laser-molecular beam scattering apparatus with tunable photoionization detection, and a velocity map imaging apparatus with VUV photoionization, we detect the products of the major primary photodissociation channel (Br and CD _2CD _2OH), and of the secondary dissociation of vibrationally excited CD _2CD _2OH radicals (OH, C _2D _4/CD _2O, C _2D _3, CD _2H, and CD _2CDOH). We also characterize two additional photodissociation channels, which generate HBr + CD _2CD _2O and DBr + CD _2CDOH, and measure the branching ratio between the C-Br bond fission, HBr elimination, and DBr elimination primary photodissociation channels as 0.99:0.0064:0.0046. The velocity distribution of the signal at m/e = 30 upon 10.5 eV photoionization allows us to identify the signal from the vinyl (C _2D _3) product, assigned to a frustrated dissociation toward OH + ethene followed by D-atom abstraction. The relative amount of vinyl and Br atom signal shows the quantum yield of this HDO + C _2D _3 product channel is reduced by a factor of 0.77 ± 0.33 from that measured for the undeuterated system. However, because the vibrational energy distribution of the deuterated radicals is lower than that of the undeuterated radicals, the observed reduction in the water + vinyl product quantum yield likely reflects the smaller fraction of radicals that dissociate in the deuterated system, not the effect of quantum tunneling. We compare these results to predictions from statistical transition state theory and prior classical trajectory calculations on the OH + ethene potential energy surface that evidenced a roaming channel to produce water + vinyl products and consider how the branching to the water + vinyl channel might be sensitive to the angular momentum of the β-hydroxyethyl radicals.