Effect of Alkyl Chain-Length on Dissociative Attachment: 1-Bromoalkanes on Si(100)-c(4×2)

Chemical reactivity as a function of chain-length was investigated for three 1-bromoalkanes on silicon. The physisorption and subsequent thermal dissociative attachment of bromoethane (EtBr), 1-bromopropane (PrBr), and 1-bromobutane (BuBr) on Si(100)-c(4×2) were examined by scanning tunneling microscopy in ultrahigh vacuum from 50 to 180 K, and interpreted by ab initio theory. These 1-bromoalkanes were found to physisorb and react exclusively over the inter-row sites of Si(100)-c(4×2), with activation barriers, E_a, increasing with alkyl chain-length: E_a = 343 ± 5 meV for EtBr, E_a = 410 ± 6 meV for PrBr, and E_a = 536 ± 2 meV for BuBr. Extensive ab initio calculations gave increasing barriers along the series: E_c = 317 meV for EtBr, E_c = 406 meV for PrBr, and E_c = 430 meV for BuBr. On the basis of our calculated geometries, we interpret this dependence of thermal barrier on chain-length as due to the additional energy required with increasing chain-length in order to lift the alkyl chain away from the surface, in going from the initial physisorbed state to the reactive transition state. For BuBr, the measured E_a significantly exceeded the calculated value. This increase in effective barrier-height could be due to a “dynamical delay” in optimizing the configuration of the alkyl chain.