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 center dot+ (M = Ge, Sn, Pb) with methane and ethene were investigated. For the MO center dot+/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 center dot+ 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 center dot and MOH+ predominates for the GeO center dot+/ethene system, while for SnO center dot+ and PbO center dot+ 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 center dot+ exceed the BDE(CH3-H) and BDE(C2H3-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 center dot+ which exhibits the highest oxophilicity among these three group 14 metal ions.