Ab initio electronic structure of HCN− and HNC− dipole-bound anions and a description of electron loss upon tautomerization

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The binding of an excess electron to HCN and HNC was studied at the coupled cluster level of theory with single, double, and noniterative triple excitations and with extended basis sets to accommodate the loosely bound excess electron. The HCN molecule, with a dipole moment of 3.05 Debye, binds an electron by 10 cm−1, whereas the HNC tautomer possesses a similar dipole moment (3.08 Debye) and binds the electron by 43 cm−1. The electronic stability of the anionic system along the minimum energy HCN→HNC tautomerization path has been investigated, and it was concluded that the excess electron autodetaches during the tautomerization. Unusually large electron correlation energy contributions to the total electron binding energy were found and are discussed.