A possible role for triplet H2CN+ isomers in the formation of HCN and HNC in interstellar clouds

Abstract

The structures and energies of the lowest triplet states of four isomers of H2CN+ have been determined by self-consistent field and configuration interaction calculations. When both hydrogen atoms are attached to the nitrogen atom, H2NC+, the molecule has its lowest triplet state energy, which is 97.2 kcal mol−1 above the energy of the linear singlet ground state. The structure has C2v symmetry, with an HCH bond angle of 116.8°, and bond lengths of 1.009 Å (H–N) and 1.268 Å (N–C). Other isomers investigated include the H2CN+ isomer at 104.7, the cis-HCNH+ isomer at 105.3, and the trans-HCNH+ isomer at 113.6 kcal mol−1. The H2CN+ isomer has an unusual ’’carbonium nitrene’’ structure, with a C–N bond length of 1.398 Å. It is suggested that the triplet H2NC+ isomer may play a role in determining the relative yields of HCN and HNC from the reaction of C+ and NH3. Specifically, a triplet path is postulated in which C+ and NH3 yield the triplet H2NC+ isomer, which then yields the singlet H2NC+ isomer by phosphorescent emission. Because this emission removes a large amount of energy, the singlet H2NC+ isomer may have insufficient energy to isomerize to the linear singlet ground state. Subsequent dissociative recombination would yield the HNC isomer exclusively.