Activation of X-H and X-D bonds (X = O, N, C) by alkaline-earth metal monoxide cations: experiment and theory

Experimental investigations are reported for reactions of MO center dot+ (M = Ca, Sr, and Ba) with elemental hydrides water, ammonia and methane proceeding in the gas phase at 295 +/- 3 K in helium buffer gas at a pressure of 0.35 +/- 0.01 Torr. Measurements were taken with an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer and a novel electrospray ion source/ion selection quadrupole/selected-ion flow tube/triple quadrupole (ESI/qQ/SIFT/QqQ) mass spectrometer. All three alkaline-earth metal oxide ions exclusively abstract a H-atom from the three hydrides with rate coefficients > 1 x 10 (-11) cm 3 molecule(-1) s(-1). Formation of metal hydroxide ion was followed by sequential addition of water or ammonia, but not methane. Density functional calculations have provided potential energy surfaces for the X-H bond activations leading to H-atom abstraction as well as those for O-atom transfer and H2O elimination ( with ammonia and methane). A comparison of experimental and theoretical isotope effects points toward a mechanism involving the direct atom transfer from XH and XD to O in MO center dot+ via a three-centered transition structure.