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

Abstract

Experimental investigations are reported for reactions of MO˙⁺ (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 × 10⁻¹¹ cm³ 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˙⁺via a three-centered transition structure.