Computational study of the linear proton bound ion-molecule complexes of HCNH+ with HCN and HNC

This work reports the results of a high level ab initio study of the linear proton bound ion-molecule complex of HCNH(+) with HCN and its isomer HNC. The energetics, equilibrium geometries, and predicted equilibrium rotational constants of three strongly interacting ion-molecule complexes are reported from calculations performed at the coupled-cluster calculations including singles, doubles, and perturbative triple excitations (CCSD(T))∕aug-cc-pVnZ (n = 2-5) level of theory. Harmonic vibrational frequencies from calculations performed at the CCSD(T)∕aug-cc-pVnZ (n = 2-4) level of theory are presented. Additional calculations are performed at the CCSD(T)-F12b∕VnZ-F12 level of theory, and the associated energetics, equilibrium geometries, and equilibrium spectroscopic properties are reported. Anharmonicity is treated with the vibrational configuration interaction method, and the predicted anharmonic vibrational frequencies are reported. The results of these calculations show that of the four possible linear interactions of HCNH(+) with HCN and HNC, there are three strongly interacting proton bound complexes. Further, the study presents results that the fourth possible interaction provides the basis for a novel HNC to HCN isomerization pathway in the interstellar medium.