Ab Initio Investigations of the Electronic Structure of HeNH + and HeNH 2+

Abstract

Electronic structures of HeNH⁺ and HeNH²⁺ were investigated using the all-electron coupled cluster single, double and triple excitation (CCSD(T)_AE) method coupled with an augmented correlation-consistent polarised core valence triple zeta basis set (aug-cc-pCVTZ). For HeNH⁺ and HeNH²⁺, the CCSD(T)_AE/aug-cc-pCVTZ model yielded cations of C_s symmetry with ² A″ and ¹ A′ ground electronic states possessing optimised geometrical parameters {r_N-H; r_N-He; θ_H-N-He} of {1·064 Å, 1·517 Å, 88·5°} and {1·192 Å, 1·309 Å, 96·2°} respectively. Of all the linear isomeric forms possible for these two cations, the only converged minimum energy structures were for the ⁴Σ^- states of HeHN⁺ and HeNH⁺ yielding optimised structural parameters of {1·093 Å, 2·403 Å, 180·0°} and {1·117 Å, 1·364 Å, 180·0°} respectively. These two linear structures were calculated to be 9·6 and 18·6 kJ mol^-1 respectively above the C_s structure. The calculated harmonic frequencies were real for all these states and their magnitudes were sensitive to the incorporation of electron correlation.