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.