Thermal activation of methane and ethene by bare MO ^.+ (M=Ge, Sn, and Pb): A combined theoretical/experimental study

The thermal ion/molecule reactions (IMRs) of the Group 14 metal oxide radical cations MO ^.+ (M=Ge, Sn, Pb) with methane and ethene were investigated. For the MO ^.+/CH ₄ couples abstraction of a hydrogen atom to form MOH ⁺ and a methyl radical constitutes the sole channel. The nearly barrier-free process, combined with a large exothermicity as revealed by density functional theory (DFT) calculations, suggests a fast and efficient reaction in agreement with the experiment. For the IMR of MO ^.+ with ethene, two competitive channels exist: hydrogen-atom abstraction (HAA) from and oxygen-atom transfer (OAT) to the organic substrate. The HAA channel, yielding C ₂H ₃ ^. and MOH ⁺ predominates for the GeO ^.+/ethene system, while for SnO ^.+ and PbO ^.+ the major reaction observed corresponds to the OAT producing M ⁺ and C ₂H ₄O. The DFT-derived potential-energy surfaces are consistent with the experimental findings. The behavior of the metal oxide cations towards ethene can be explained in terms of the bond dissociation energies (BDEs) of MO ⁺-H and M ⁺-O, which define the hydrogen-atom affinity of MO ⁺ and the oxophilicity of M ⁺, respectively. Since the differences among the BDEs(MO ⁺-H) are rather small and the hydrogen-atom affinities of the three radical cations MO ^.+ exceed the BDE(CH ₃-H) and BDE(C ₂H ₃-H), hydrogen-atom abstraction is possible thermochemically. In contrast, the BDEs(M ⁺-O) vary quite substantially; consequently, the OAT channel becomes energetically less favorable for GeO ^.+ which exhibits the highest oxophilicity among these three group 14 metal ions.