Structures and energies of C2NH +2 isomers

Abstract

Ab initio calculations of the structures and relative energies of the isomers of C₂NH⁺₂ show three low-energy isomers. The lowest-energy form is a cyclic isomer with two higher-energy stable linear forms, H₂CNC⁺ and H₂CCN⁺. The relative energies calculated at the MP4SDQ/6-311G^** level of theory are 0, 60 and 52 kJ mol^–1, respectively. Interconversion between the cyclic isomer and the linear isocyano isomer H₂CNC⁺ involves an intermediate structure calculated using CISD energies to be 141 kJ mol^–1 above the cyclic structure and interconversion between the cyclic and cyano isomer involves an intermediate 226 kJ mol^–1 above teh cyclic isomer. Vibrational frequencies and infrared intensities have been calculated for the three stable isomers at the SCF DZP level of theory. The theoretical energies are compared with experimental measurements for the formation of the C₂NH⁺₂ ion from CH₃CN and CH₃NC which provided evidence for two isomers separated in energy by 51 ± 10 kJ mol^–1, that from CH₃NC being the cyclic isomer and that from CH₃CN being the linear isomer.