Ammonia activation by scandium(1+) and titanium(1+): electronic and translational energy dependence

Abstract

The reactions of Sc' and Ti' with ammonia are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. The effect of electronic energy for the Ti+ reactions is also probed by varying the conditions for forming Ti+. Excited doublet states of Ti' are found to be much more reactive than the a4F ground and b4F first excited states. Both metals form the MH+, MNH', and MNH2+ products, while only Ti+ forms MN+. The results are consistent with reaction that occurs primarily through a covalently bound insertion intermediate, H-M+-NHz, having a singlet and doublet spin state for M = Sc and Ti, respectively. The reactivities of the different electronic states of the reactant ions can be explained by using spin conservation concepts. The thresholds for the cross sections of the endothermic reactions are interpreted to give the 298 K bond energies of Do(Sc+-NH2) = 3.69 i 0.07 eV, Do(Sc+-NH) = 5.16 f 0.10 eV, Do(Ti+-NH2) = 3.69 f 0.13 eV, D(Ti+-NH) = 4.83 f 0.12 eV, and Do(Ti+-N) = 5.19 f 0.13 eV. The large bond strengths of the M+-NH2 and M+-NH species indicate that the lone pair electrons on the nitrogen atom are involved in the metal-ligand bond.