An Ab initio study of the hydrogen addition to methyl isocyanide (CH3NC) and methanediazonium ion (CH3NN+): a case of a bifurcating point on the potential energy surface

Abstract

Results of ab initio calculations on the reaction pathways for the hydrogen addition to methanediazonium ion (CH₃NN⁺) and methyl isocyanide (CH₃NC) at both central and terminal atoms as well as for the stereomutation of α-adducts are reported. Structures of points on the energy surfaces were determined at the UHF/3-21 G level while their relative energies were estimated at UMP4SDQ/6-31 G**. Thermochemical properties were also computed. The α-addition of H˙ either to the terminal nitrogen of CH₃NN⁺ or to the carbon atom of CH₃NC is not stereospecific. Only a single transition structure can be located in each case. At the transition state, the substrate is only marginally bent. Soon after the transition state is passed, a bifurcation of the addition pathway occurs leading to the formation of both cis- and trans-isomers of the adduct. In both cases, the trans-adduct is more stable than the cis; the cis–trans stereomutation takes place via nitrogen inversion of the methyl group and requires only 5–6 kcal mol^–1 therefore the trans-isomer should be the major adduct. The β-addition to central nitrogen exhibits, in both cases, an appreciably larger energy barrier which precludes the formation of the β-adduct.