Photodissociating methyl vinyl ether to calibrate O¿ethylene product branching and to test propensity rules for product channel electronic accessibility

These experiments investigate the photodissociation of methyl vinyl ether at 193 nm in a crossed laser-molecular beam apparatus. We observe two C–O bond fission channels, a minor channel producing CH31CH2CHO (X̃ A9) and the major channel yielding CH31CH2CHO (à A8). Some of the neutral à state vinoxy product undergoes secondary dissociation to produce ketene1H. These experiments on the photodissociation of methyl vinyl ether, which produce nascent vinoxy and methyl radicals cleanly in a one-to-one ratio, serve two purposes. First, using the measured photofragment velocities and product branching we calibrate the relative sensitivity of mass spectrometric detection to the methyl and vinoxy polyatomic radical products at the m/e515 daughter ion, taking into account the loss of neutral vinoxy to ketene1H formation. This relative mass spectrometric sensitivity calibration factor is determined to be 0.11660.022. Knowledge of this factor allows us to extract the product branching ratio between the two major competing primary product channels from the O(P)1ethylene reaction, a branching ratio that has been much disputed in the literature. Our results give a CH31HCO/H1CH2CHO product branching ratio of R50.616 .11, corresponding to 38% ~65%! branching to the CH31HCO channel. Second, we use the result that the channel producing Ã-state vinoxy dominates over the formation of ground-state vinoxy to test propensity rules being developed to help predict what product channels may be suppressed by electronically nonadiabatic effects in chemical reactions. These propensity rules discriminate between channels that are ‘‘electronically facile’’ and ‘‘electronically difficult/ prohibitive.’’ We find that a qualitative analysis of the changes in electronic configuration along the reaction coordinates for the photodissociation of methyl vinyl ether correctly predicts the dominant channel to be the production of excited state (Ã) vinoxy, the electronically facile channel. © 2001 American Institute of Physics. @DOI: 10.1063/1.1369662#