DFT theoretical study on the reaction mechanism of the nitrate radical with alkenes:: 2-butene, isobutene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene

A general mechanism for the reactions of the NO3 radical with 2-butene, isobutene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene is proposed on the basis of density functional theory (DFT) calculations. This mechanism is compared with previously reported model experimental kinetic studies at low pressures and temperatures in anaerobic conditions. In our theoretical proposal of mechanism, the initial step is the addition of the. NO3 radical to the double bond. For the systems showing different substitution on both sides of the double bond, two adducts have been obtained, one following the Markovnikov rule and the other with the anti-Markovnikov orientation. Starting from the adduct we have found that three main reaction pathways follow. The first one leads to epoxide and NO2 formation, the second to carbonyl compounds, and the third, through the cleavage of the C-C bond, to carbonyl compounds with a lower number of carbon atoms than the original substrate and NO. The theoretical proposal of mechanism leads to the following products: (a) for 2-butene, 2,3-dimethyloxirane, butanone, and ethanal; (b) for isobutene, 2-methylepoxypropane, 2-methylpropanal, butanone, propanone, and formaldehyde; (c) for 2-methyl-2-butene, 2-methylepoxybutane 3-methylbutanone, propanone, 2-dimethylpropanal, and ethanal; (d) for 2,3-dimethyl-2-butene, 2,3-dimethylepoxybutane, 3-dimethylbutanone, and propanone. In all,cases, NO2 and NO are also obtained as products. The geometry of all the involved stationary points in the potential energy hypersurface has been optimized at the DFT level with the B3LYP functional and a 6-31G* basis set. All these conformations were characterized at the same calculation level.