Theoretical study of the HCN–CH3 and HNC–CH3 radicals: Hydrogen and covalent bonding

The interaction between the methyl radical with hydrogen cyanide and hydrogen iso-cyanide is reported at several ab initio levels of theory. Four structures are studied: two hydrogen-bonded (H3C⋯HCN and H3C⋯HNC) and two covalent-bonded (H3C–NCH and H3C–CNH) structures. It is shown that hydrogen cyanide forms a weak hydrogen bond with the methyl radical ( D e CP = − 1.3 kcal / mol ) , while its covalent-bonded isomer is mostly unbound with respect to the fragments ground state ( D e CP = + 0.5 kcal / mol ) . The ground state of the hydrogen isocyanide complexed with the methyl radical is found to be a covalent-bonded complex ( D e CP = − 25.5 kcal / mol ) , whereas its hydrogen-bonded isomer is only a weak complex ( D e CP = − 2.2 kcal / mol ) .