Ground and excited states of HNC, NH, and NH2 transients: Ab initio geometries, electronic structures, and molecular properties

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Geometrical and vibrational characterization of NH2(X̃ 2B1,Ã 2A1,B̃ 2B2), NH(X̃ 3Σ−,ã 1Δ, b̃ 1Σ+,Ã 3Π,c̃ 1Π,d̃ 1Σ+), HNC(X̃ 1Σ+) and hitherto experimentally unreported HNC(ã 3A′,b̃ 3A″) systems is reported by taking different sets of active electrons/orbitals in complete active space self consistent field (CASSCF) calculations. Employing CASSCF optimized geometries, the excitation energies, electric field gradients, and dipole moments for these systems have been computed at complete active space second-order perturbation (CASPT2) and multireference singles and doubles configuration interaction (MRD-CI) levels of theory. Computed field gradients are very similar at CASSCF and CASPT2 levels, but differ significantly with those obtained at MRD-CI level. The outcome of present numerical experiment is that the inclusion of excitations higher than singles and doubles and use of a very large CI space employing iterative natural orbitals is necessary to account for the anisotropy of the molecular charge distribution around any nucleus in a molecule.