G2 calculations on the thermochemistry of MgCn, MgNC and related species

Abstract

The G2 (Gaussian-2) procedure has been used to characterize enthalpies of formation of MgCN, MgNC, and their ionized, hydrogenated and protonated forms. In keeping with previous theoretical studies, a discrepancy of 85 kJ mol^–1 is observed between the G2 (and G2Q) Mg—NC bond dissociation energy and the sole experimental value for this quantity. Barriers to isomerization of Mg(CN) and Mg(CN)⁺ are explored. Among ionized structures, the T-shaped isomer of Mg(CN)⁺ is calculated to be lower in energy than the linear MgCN⁺ and MgNC⁺ isomers: this is very similar to previous findings for the isoelectronic species NaCN. Implications for MgCN and MgNC formation within interstellar clouds and circumstellar shells are also discussed. A mechanism for formation of MgCN and MgNC by a common pathway is presented and assessed against observations of MgCN/MgNC distribution within the circumstellar envelope of IRC + 10 216.