Bond dissociation energy and electronic spectroscopy of Cr+(NH3) and its isotopomers.

The electronic spectra of Cr+(NH3), Cr+(ND3), and Cr+(15NH3) have been measured from 14 200 to 17 400 cm-1 using photodissociation spectroscopy. Transitions are predominantly observed from the 6A1 ground state, in which the Cr+ has a 3d 5 electronic configuration, to the B ̃ 6E (Π) state (3d 44s). 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 Aso' = 39 cm-1. For the lowest spin-orbit level, the Cr-N stretching frequency in the excited state is 343 cm-1, with an anharmonicity of 4.2 cm-1. The 6E (Π) origin is predicted to lie at T0 = 14 697 cm-1. The first peak observed is due to v' = 1, so the observed photodissociation onset is thermodynamic rather than spectroscopic, giving D0(Cr+-NH3) = 14 830 ± 100 cm-1 (177.4 ± 1.2 kJ/mol) and D0(Cr+-ND3) = 15 040 ± 30 cm-1 (179.9 ± 0.4 kJ/mol). The 6E (Π) state of Cr+(NH3) is ∼2740 cm-1 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+(NH3) evolve into those of Cr+ + NH3.