Activation of CH 4, C 2H 6, C 3H 8, and c-C 3H 6 by gas-phase Pd + and the thermochemistry of Pd-ligand complexes

Abstract

The reactions of Pd + with CH 4, C 2H 6, C 3H 8, and c-C 3H 6 have been studied using guided ion beam mass spectrometry. It is found that elimination of CH 4 and H 2 are the dominant processes at low energies in the c-C 3H 8 reaction system, whereas only dehydrogenation is important in the c-C 3H 6 reaction system, and no exothermic channels are observed in the CH 4 and C 2H 6 systems. At high energies, CC bond cleavage processes dominate the product spectrum for the reactions of Pd + with the c-C 3H 6 reaction system, whereas both CC and CH cleavage processes are comparably important in the alkane systems. Modeling of the endothermic reaction cross-sections yields the 0 K bond dissociation energies (in eV) of D 0(Pd +CH 2) = 2.95 ± 0.05, D 0(Pd +CH 3) = 1.88 ± 0.10, D 0(Pd +C 2H 3) = 2.56 ± 0.09, and D 0(Pd +C 2H 5) = 2.41 ± 0.05. Observations of exothermic reactions set lower limits of D 0(Pd +C 2H 4) > 0.70 eV, D 0(Pd +C 3H 5) ≥ 3.40 ± 0.09 eV, and D 0(Pd +C 3H 6) > 1.12 eV. The reactions of Pd + with hydrocarbons can be largely explained by initial H − or R − abstraction at low kinetic energies, with H and R abstraction more important at elevated kinetic energies.