Theoretical study of the mechanism of the addition of diazomethane to ethylene and formaldehyde. Comparison of conventional ab initio and density functional methods

The 1,3-dipolar cycloaddition reactions of diazomethane with ethylene and f ormaldehyde as well as the nitrogen elimination reactions from the cycloadd ucts have been studied using density functional and conventional ab initio methods. The exothermicity of the reactions is underestimated by DFT method s with respect to CCSD(T) due to an overestimation of the C-N dissociation energy of diazomethane. For the cycloaddition reactions all methods lead to similar transition state geometries, and the potential energy barriers com puted using DFT methods are similar to the CCSD(T) ones. On the other hand, for the nitrogen elimination reactions transition state geometries and ene rgy barriers are more dependent on the level of calculation. The results ob tained show that for the reaction between diazomethane and ethylene the pyr azoline intermediate is more stable than the reactants and that the Gibbs e nergy barrier for nitrogen elimination is larger than the barrier correspon ding to its formation. On the contrary, for formaldehyde the kinetically mo st favorable cycloadduct, 1,2,3-oxadiazoline, is less stable than the react ants and has a lower barrier for nitrogen elimination.