Mechanistic Aspects of Gas‐Phase Hydrogen‐Atom Transfer from Methane to [CO]^.+ and [SiO]^.+: Why Do They Differ?

Abstract

The reactivity of the two diatomic congeneric systems [CO](·+) and [SiO](·+) towards methane has been investigated by means of mass spectrometry and quantum-chemical calculations. While [CO](·+) gives rise to three different reaction channels, [SiO](·+) reacts only by hydrogen-atom transfer (HAT) from methane under thermal conditions. A theoretical analysis of the respective HAT processes reveals two distinctly different mechanistic pathways for [CO](·+) and [SiO](·+), and a comparison to the higher metal oxides of Group 14 emphasizes the particular role of carbon as a second-row p element.

Thermal hydrogen-atom transfer from methane: A mechanistic exercise

“…Adapted from Ref. [56]. requires only 63 kJ mol −1 , and is much less energy-demanding than the alternative route.…”

Temperature-Dependent Kinetics of Charge Transfer, Hydrogen-Atom Transfer, and Hydrogen-Atom Expulsion in the Reaction of CO+ with CH4 and CD4

“…22−27 Rate constant and product branching measurements have been made using selected ion flow tube (SIFT) 25 or ion cyclotron resonance (ICR) techniques. 16,27 It has been shown that the reaction proceeds at the collision rate, and in addition to the HAT pathway, charge-transfer (CH 4 + + CO) and H-atom expulsion (CH 3 CO + + H) products are found. The mechanistic details of the HAT pathway in this reaction have only very recently been tied to experimental results, utilizing ICR methods coupled with quantum chemical calculations.…”

“…7 While this indirect mechanism is not a true HAT mechanism as defined above, it represents the preferred pathway in almost all of the diatomic systems studied. 12−15 An exception is the reaction of CO + with methane, 16 an interesting case study for two reasons. First, while heavier members of carbon's group prefer the indirect pathway where the metal or metalloid facilitates HAT, CO + opts for the direct HAT mechanism.…”

“…Second, CO + represents the only cationic system that can facilitate HAT at two sites, resulting in two isomeric HAT products (HCO + and COH + ). 16 The structure and isomerization energy of these formyl isomers have received significant attention, mainly due to their importance in interstellar chemistry. 17−21 In this light, the reaction of CO + with methane has been studied by a handful of groups in the gas phase, as early as the late 1960s.…”

Activation of Methane by the Pyridine Radical Cation and its Substituted Forms in the Gas Phase

“…Bettinger et al [31] extended this research to electrophilic singlet borylnitrene compounds and found that certain borylnitrenes are good reagents for the transformation of methane. Hydrogen atom abstraction from CH 4 is considered to be a key step in the oxidative dehydrogenation and dimerization of methane [32][33][34], and abstraction by metal-free radical cation oxides, such as [P 4 O 10 ] + and [SO 2 ] + at room temperature have been reported by Schwarz et al [35,36] and de Petris et al [37]. The gas-phase experiments on methane activation are helpful in revealing the mechanistic details of the elementary steps in the condensed phase [11].…”