The structure and energetics of the C3N+ ion

Abstract

Ab initio calculations of the relative energies for various structures and electronic states of the C₃N⁺ ion have been performed and the results used to interpret the C₃N⁺ ionization efficiency curve measured by the ‘near-monochromatic’ electron-impact ionization of cyanoacetylene, HC₃N. The calculated lowest-energy isomer, which is reconciled with the experimental appearance energy of the C₃N⁺ ion at 17.78 ± 0.08 eV, is a ³Σ^– state of the CCCN⁺ isomer, corresponding to an experimentally determined C₃N⁺ enthalpy of formation of 1850 kJ mol^–1. A sharp break in the experimental ionization efficiency curve 0.9 eV above the appearance energy is in accord with the formation of a cyclic C₃N⁺ isomer which is calculated to lie 1.0 eV above the ground state. There is no conclusive experimental support for the electron-impact formation of the isocyano CCNC⁺ isomers, which are calculated to lie 0.15 and 0.50 eV above the ground state.