Description of the Methylene Amidogene Radical and Its Anion with an Economical Treatment of Correlation Effects Using Density Functional Theory Orbitals.

The ground and low-lying excited state electronic structural properties (such as equilibrium geometries, harmonic frequencies, excitation energies, barrier energy, and so on) of the methylene amidogene radical (HCN) and its anion (HCN) have been studied using the CASCI (complete active space configuration interaction) and SSMRPT (state-specific multireference Møller-Plesset perturbation theory) methods with density function theory (DFT) orbitals. Here, the span of the active orbitals have been obtained from Kohn-Sham DFT using B3LYP exchange-correlation functionals in the CASCI (DFT-CASCI) approximation to describe nondynamic correlation associated with electronic degeneracies. The DFT-SSMRPT protocol provides an attractive way to deal with both dynamical and nondynamical correlation effects in strongly correlated systems such as HCN and HCN. The present work clearly indicates that the electronic absorption band near 35,050 cm corresponds to the B̃A ← X̃B transition. DFT-SSMRPT findings are in close agreement with high-level theoretical estimates. It is concluded that the transition at 1725 cm could be due to the CN stretching of the -HCNH isomer which is originally assigned to the CN stretch of HCN in the experiment. The present results confirm most of the previous vibrational assignments. It is not possible to definitively assign a transition to the 35,600 cm band with the present estimations, suggesting further experiment is urgently called for.