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, iso butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene is proposed on the bas is of density functional theory (DFT) calculations. This mechanism is compa red with previously reported model experimental kinetic studies at low pres sures 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 o f the double bond, two adducts have been obtained, one following the Markov nikov rule and the other with the anti-Markovnikov orientation. Starting fr om the adduct we have found that three main reaction pathways follow. The f irst one leads to epoxide and NO2 formation, the second to carbonyl compoun ds, and the third, through the cleavage of the C-C bond, to carbonyl compou nds 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 isobuten e, 2-methylepoxypropane, 2-methylpropanal, butanone, propanone, and formald ehyde; (c) for 2-methyl-2-butene, 2-methylepoxybutane 3-methylbutanone, pro panone, 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 stat ionary points in the potential energy hypersurface has been optimized at th e DFT level with the B3LYP functional and a 6-31G* basis set. All these con formations were characterized at the same calculation level.