Correlation effects in the isomeric cyanides: HNC↔HCN, LiNC↔gLiCN, and BNC↔gBCN

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Abstract
Correlated values of the isomerization energy and barrier for the HNC→HCN reaction are obtained from many‐body perturbation theory, including the effects of quadruple excitations. Extended basis sets of better‐than‐triple‐zeta plus double‐polarization quality are used, as well as basis sets including counterpoise and bond‐centered orbitals. The best of these basis sets is sufficient to account for 84% of the valence correlation energy of HCN. These studies predict an isomerization energy for the HNC→HCN rearrangement to be −15±2 kcal/mole, in disagreement with a recent experimental value of −10.3±1. kcal/mole. Correlated isomerization energies of LiCN→LiNC and BCN→BNC are obtained in bases of double‐zeta plus polarization quality. In all cases, correlation stabilizes the cyanide isomer more than the isocyanide. Trends in the series R–NC⇄RCN for R=H, Li, and B are discussed.