Bond dissociation energy and electronic spectroscopy of Cr(NH) and its isotopomers.

The electronic spectra of Cr(NH), Cr(ND), and Cr(NH) have been measured from 14 200 to 17 400 cm using photodissociation spectroscopy. Transitions are predominantly observed from the A ground state, in which the Cr has a 3 electronic configuration, to the E (Π) state (3 4). There is extensive vibronic structure in the spectrum due to a long progression in the Cr-N stretch and transitions to all six spin-orbit levels in the upper state. The spin-orbit splitting in the excited state is observed to be A' = 39 cm. For the lowest spin-orbit level, the Cr-N stretching frequency in the excited state is 343 cm, with an anharmonicity of 4.2 cm. The E (Π) origin is predicted to lie at T = 14 697 cm. The first peak observed is due to v' = 1, so the observed photodissociation onset is thermodynamic rather than spectroscopic, giving D(Cr-NH) = 14 830 ± 100 cm (177.4 ± 1.2 kJ/mol) and D(Cr-ND) = 15 040 ± 30 cm (179.9 ± 0.4 kJ/mol). The E (Π) state of Cr(NH) is ∼2740 cm less strongly bound than the ground state, and the Cr-N bond length increases by 0.23 ± 0.03 Å upon electronic excitation. Calculations at the time-dependent density functional theory (M06) and equations of motion coupled cluster, with single and double excitations (EOM-CCSD) level fairly accurately predict the energy and vibrational frequency of the excited state. Multi-reference configuration interaction calculations show how the spin-orbit states of Cr(NH) evolve into those of Cr + NH.