Magneto-infrared spectra of matrix-isolated NiH and NiH2 molecules and theoretical calculations of the lowest electronic states of NiH2

Two vibronic transitions, 2Δ3/2(v=0)←2Δ5/2(v″=0) and 2Π3/2(v=0)←2Δ5/2(v″=0) at 923 and 2560 cm−1, respectively, were observed for NiH in solid argon (and krypton) at 4 K. These Ω=3/2←Ω=5/2 transitions were shifted and broadened by magnetic fields of up to 4 T. Also, its ground state vibrational frequency has been observed in neon, argon, and krypton matrices. Bands in the 600–2000 cm−1 region were assigned as frequencies of the NiH2 (NiD2, NiHD) molecule. Contrary to earlier ab initio calculations, this assignment implies that the molecule is strongly bent in its ground electronic state. This discrepancy was explored theoretically by considering the lowest triplet (linear) and singlet (bent) states of NiH2. The relative energies of the two states have not been satisfactorily resolved, but the calculated structural and vibrational properties of the singlet state are in good agreement with the observations.