Quantification of the selective activation of C—C bonds in short-chain alkanes. Reactivity of ethane, propane, isobutane, n-butane and neopentane on Ir(111)

Abstract

The initial probabilities of trapping-mediated, dissociative chemisorption of the saturated hydrocarbons ¹³C-labelled ethane, propane, isobutane, n-butane, and neopentane on the close-packed Ir(111) surface have been investigated. The activation of both C—C and C—H bonds has been observed in all alkanes examined, except for ethane, for which only C—H bond activation was observed. The selective activation of C—C bonds in these alkanes has been quantified, after separating the contribution to the total reaction probability from the competing C—H bond-activation channel. A secondary deuterium kinetic isotope effect has been observed for the activation of C—C bonds upon deuteriation of the thermal methyl groups of propane and isobutane. With respect to the bottom of the physically adsorbed well for each hydrocarbon, the apparent C—C bond-activation energies (kcal mol^–1) have been determined to be 18.2 ± 0.6 (C₃H₈ and CH₃CD₂CH₃) and 18.9 ± 0.6 (C₃D₈), 17.6 ± 0.6 [(CH₃)₃CH and (CH₃)₃CD] and 18.1 ± 0.6 [(CD₃)₃CH], 17.7 ± 0.6 (n-C₄H₁₀), and 15.9 ± 0.6 [(CH₃)₄C]. The implications of the observed secondary kinetic isotope effect are discussed in terms of the deformation of the terminal methyl groups which accompanies the cleavage of the C—C bonds.