Gaseous [H2, C, N]+ and [H3, C, N]+.cntdot. ions. Generation, heat of formation, and dissociation characteristics of methylenenitrenium(1+) ([H2CN]+), iminomethylene(1+) ([HCNH]+), aminomethylidene(1+) ([CNH2]+), formaldimine(1+.cntdot.) ([H2CNH]+.cntdot.), and aminomethylene(1+.cntdot.) ([HCNH2]+.cntdot.) ions