Determination of the hydroxy nitrate yields from the reaction of C₂-C₆ alkenes with OH in the presence of NO

The yields of hydroxy nitrates from the reaction of selected C₂-C₆ alkenes with OH in the presence of NO were measured at 296 ± 3 K in a 9600 L photochemical smog chamber. Hydroxyl radicals were produced from the photolysis of isopropyl nitrite in the presence of NO. The loss of the alkene was followed using gas chromatography. The hydroxy nitrate products were determined using a combination of capillary chromatography and an organic nitrate specific chemiluminescence detector. The yield of hydroxy nitrates was observed to increase with the size of the precursor alkene as follows: ethene (0.86%), propene (1.5%), 1-butene (25%), cis-2-butene (3.4%), and 1-hexene (5.5%). Previous studies involving the production of alkyl nitrates from alkanes show a similar trend, but the yields reported here are a factor of 2-3 lower than for the corresponding simple alkylperoxy radical. The impact of a β-hydroxy group on the nitrate yield is examined using an ab initio molecular orbital study. It indicates that a hydrogen-bonded peroxy nitrite intermediate is formed, which results in a decrease in D₀(O-O) for the peroxy linkage of about 8 kJ/mol. This would be expected to effectively decrease the organic nitrate yield. The implications of these findings for the organic nitrate path as an atmospheric NO_x removal mechanism are discussed.