Thermal Activation of Methane and Ethene by Bare MO.+ (M=Ge, Sn, and Pb): A Combined Theoretical/Experimental Study

摘要

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.+/CH₄ 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 C₂H₃. and MOH+ predominates for the GeO.+/ethene system, while for SnO.+ and PbO.+ the major reaction observed corresponds to the OAT producing M+ and C₂H₄O. 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(CH₃H) and BDE(C₂H₃H), 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.