Three Lowest-Lying Electronic States of NH2

Abstract

The three lowest-lying electronic states, X̃ 2B1, à 2A1, and B̃ 2B2, as well as the lowest linear 2Π stationary point of NH2 have been investigated systematically using ab initio electronic structure theory. The SCF, CASSCF, CISD, CASSCF-SOCI, CCSD, and CCSD(T) levels of theory have been employed to determine total energies, equilibrium structures, and physical properties including dipole moments, harmonic vibrational frequencies, and infrared intensities of NH2. According to the instability analysis of the reference SCF wave functions, physical properties of the three lowest-lying equilibrium states of NH2 may be obtained correctly in the variational sense with all wave functions employed in this study. The lowest linear stationary point (2Π) possesses two distinct imaginary vibrational frequencies along the bending coordinate, indicating a strong Renner−Teller interaction. The predicted geometries and physical properties of the two lowest states of NH2 are in good agreement with available experimental results. At the CCSD(T) level of theory with the largest basis set, the triple-ζ (TZ) plus triple polarization with two sets of higher angular momentum and two sets of diffuse functions [TZ3P(2f,2d)+2diff], the à 2A1 state of NH2, with a large bond angle of 144.9°, is predicted to lie 32.1 kcal/mol (1.39 eV, 11 200 cm-1) above the ground state. This is in excellent agreement with the experimental T0 value of 31.80 kcal/mol (1.379 eV, 11 122.6 cm-1). The second excited state (B̃ 2B2) possesses an acute bond angle of 49.3° and is determined to lie 100.1 kcal/mol (4.34 eV, 35 000 cm-1) above the ground state. The classical (and effective) barriers to linearity for the X̃ 2B1 and à 2A1 states were predicted to be 11 870 (12 310) cm-1 and 720 (790) cm-1, which are again in good accord with the experimentally estimated values of 12 024 cm-1 (X̃ 2B1) and 730 cm-1 (à 2A1).