Photodissociation of methyl nitrate in a molecular beam

Abstract

The photodissociation of methyl nitrate, CH30N02, has been investigated by photofragment translational spectroscopy. At the photolysis wavelength of 193 nm the predominant primary decay (-70%) involves the fission of the weak CH3O-NO2 bond (Di - 41 kcal/mol) to yield CH3O + NO2 fragment pairs which show a bimodal translational energy distribution. The component consisting of the fast fragment pairs has an average translational energy (ET) = 41 kcal/mol and a recoil anistropy 6 = 0.9, while the slow component is characterized by (ET) = 19 kcal/mol and 0 = 0. A significant portion of the NO2 fragments of the slow component is subject to a unimolecular decay to NO + O(3P). An additional primary decay route leads to the formation of methyl nitrite, CH3ON0, and atomic oxygen O(lD) with (ET) = 6 kcal/mol and /3 = 1.2. This competing reaction requires fission of the strong N-0 bond (@,I - 118 kcal/mol) and its occurrence indicates an initial localization of the photoexcitation on the NO2 moiety in the parent molecule. At higher laser fluence secondary photodissociation of the primary fragments CH3ONO was observed. Photolysis at 248 nm is shown to produce CH30 + NO2 fragment pairs exclusively, with (ET) = 17 kcal/mol. Nitric oxide (NO) and nitrogen dioxide (NOz), collectively denoted as NO,, play a central role in the catalytic and photochemical processes of the atmo~phere.'-~ The most important precursor and product species of atmospheric NO, are nitric acid (HN03), nitrate ion (NOj) in aerosols, and peroxyacetic nitric anhydride (PAN). These compounds and NO, account for most of what is often referred to as reactive oddnitrogen, NO,. Recent atmospheric measurements3q4 indicate that in some cases 742% of the NO, consists of relatively stable organic species, most notably the alkyl nitrates RON02, R = CH3, CzH5, etc. These compounds are formed as minor products in the reaction of organic peroxy radicals with NO and are removed both by reaction with OH radicals and by photolysis with light in the near-UV region. The relative importance of the two destruction mechanisms depends on the atmospheric conditions and on the substituent group R, with the photolytic decay route being most important in the case of methyl nitrate, CH30N02.3 From this point of view the photodissociation mechanism of alkyl nitrates merits attention.