Photofragment spectroscopy and dynamics of NiOH⁺ and NiOH⁺(H₂O)

Photodissociation of thermalized NiOH⁺ and NiOH⁺(H₂O) are examined over the visible and near-ultraviolet using time-of-flight mass spectrometry. Branching ratios and photodissociation cross sections are monitored as a function of laser wavelength. Loss of OH is the only dissociation channel observed in NiOH+, whereas loss of H₂O and OH are present, and competitive, in the photodissociation of NiOH⁺(H₂O). The photofragmentspectrum of NiOH⁺ shows absorption bands at ∼300 and ∼400 nm. An almost identical spectrum is observed for the net photodissociation of NiOH⁺(H₂O). Spectroscopic thresholds taken from the photofragment spectra have been corrected to yield upper limits to the 0 K bond energies for the ions: D°₀(Ni⁺-OH) ≤ 271 ± 9 kJ/mol, D°₀(NiOH⁺-H₂O) ≤ 271 ± 9 kJ/mol, and D°₀(Ni⁺H₂O-OH) ≤ 334 ± 5 kJ/mol. Calculated B3LYP values are consistent with the experimental upper limits, but suggest that the true D°₀(NiOH⁺-H₂O) and D°₀(Ni⁺H₂O-OH) lie significantly below these upper limits. Photodissociation of NiOH⁺(H₂O) near 300 nm leads to greater OH production than RRKM calculations predict, suggesting that excitation in this region leads to direct Ni⁺-OH bond cleavage.