Electronic Splitting between the 2B1 and 2A1 States of the NH2 Radical

Theoretical calculations are reported for the ground and first excited states of NH2. A contracted Gaussian basis of four s, two p, and one d functions is centered on the nitrogen atom, while for hydrogen two s and one p functions are used. Both self-consistent-field (SCF) and multiconfiguration first-order wave-functions have been computed, the latter using the iterative natural-orbital method. Two new theoretical ideas were tested and found useful: (a) Bunge's partitioning of degenerate spaces and (b) a procedure for generating uniform sets of starting orbitals for multiconfiguration calculations. For the 2B1 state the SCF, CI, and experimental geometries are θ=105.4°, r=1.019 Å; θ=102.7°, r=1.055 Å; θ = 103.3± 0.5, r=1.024± 0.005 Å. The analogous results for the 2A1 state are θ=141.9°, r=0.997 Å; θ144.7°, r=1.010 Å; θ = 144± 5, r=0.97–1.00 Å. For the upper 2A1 state the barrier to linearity is 1370 cm−1 in the SCF approximation, 1030 cm−1 from the correlated wavefunctions, and 770± 100 cm−1 experimentally. The 2B1—2A1 splitting Te is predicted to be 12 800 cm−1 (SCF) and 14 500 cm−1 (CI), whereas the experimental value is thought to be ∼ 11 000 cm−1. Potential curves are shown and electronic structure considerations discussed.