Fast beam photodissociation of the CH2NO2 radical

Abstract

The photodissociation of the nitromethyl radical, CH2NO2, has been studied using a fast beam photofragment translational spectrometer. In these experiments, a fast beam of mass selected, internally cold nitromethyl radicals is formed via negative ion photodetachment of CH2NO−2 and subsequently dissociated. The recoiling photofragments are detected in coincidence using a microchannel plate detector equipped with a time- and position-sensing anode. Two dissociation product channels are observed at each of three dissociation wavelengths investigated in the range 240–270 nm and are identified as (I) CH2NO2→CH2NO+O and (II) CH2NO2→H2CO+NO. In marked contrast to the ultraviolet photodissociation of CH3NO2, no evidence is found for simple C–N bond fission to give (III) CH2NO2→CH2+NO2. Translational energy and angular distributions were obtained for the two observed channels. The translational energy distribution of channel (I) peaks at only 5–8 kcal/mol, while the distribution for channel (II) peaks at ∼60 kcal/mol. The angular distributions for both channels are largely isotropic. The nature of the electronic excitation and dissociation dynamics are considered at length. The upper state in the electronic transition is assigned to the 1 2B1 state. Results of attempts to model various aspects of the dissociation dynamics as statistical processes on the ground state surface indicate this mechanism is very unlikely. Instead, both dissociation channels are believed to occur primarily on excited state surfaces, and mechanisms for these processes are proposed.