Adsorbate Alignment in Surface Halogenation: Standing Up is Better than Lying Down**

Almost half a century ago, it was demonstrated that an aligned gaseous reagent molecule had a reactivity for abstractive halogen-atom transfer that depended on the direction of approach of the alkali metal atom that abstracted the halogen. At surfaces, adsorbates are commonly aligned, offering opportunities to examine the effect of molecular alignment on surface reaction. Molecules usually adsorb in a horizontal (h) state at low coverages, maximizing their interaction with the underlying surface, and in morenearly vertical (v) states at increased coverages. Recently, Dougherty et al. noted a change in electronic properties between “standing-up” (v) and “lying-down” (h) pyridine on Cu(110). Studies in this laboratory of the adsorption and reaction of haloaromatics and haloalkanes at silicon surfaces, using scanning tunneling microscopy (STM), provided evidence of vertical, v, and horizontal, h, states of single adsorbates at Si(111)-7 7; examples being 1-bromopropane, 1-bromopentane, 1-chlorododecane, and 1-bromododecane. Experimentally, the assignment of adsorbate alignment (v vs h) is based on contrasting surface mobilities (v more mobile than h) and sometimes differing STM imaging (v higher than h). The ratio of v to h varied with deposition rate f] and surface temperature. 7] Both reactive pathway and reaction rate are different for v and h states. (We earlier referred to v reaction as “daughtermediated” or “direct”, h reaction as “parent-mediated” or “indirect”.) The v reaction path frequently resulted in only the halogen-atom bound to the surface, whereas h occasionally gave halogen-atom and organic radical both bound to the surface. For 1-bromododecane on Si(111)-7 7, measurements of thermal reaction showed v reacted more rapidly than h, in qualitative accord with the computation of the thermal reaction of CH3Br brominating the same Si(111)-7 7 surface which showed horizontal (h) physisorbed CH3Br converting to a more-nearly v configuration during the approach to the transition state. A requirement that h adsorbate “stand-up” (v) en route to the transition state was apparent for a series of 1-bromoalkanes reacting by dissociative attachment on Si(100)-c(4 2); the activation energy for bromination increased with alkyl chain length in parallel with the computed energy required to go from h to v. Herein we use STM to identify the v and h states of 1bromoalkanes with different alkyl chain lengths, 1-bromopropane (PrBr) and 1-bromopentane (PeBr), on Si(111)-7 7. The v-states have higher mobility and greater height in STM topographs (Supporting Information, Figure S3). For PrBr and PeBr, we measured the rates of thermal reaction for v and h. For PeBr, we calculate, ab initio, adsorptions and reactions for both v and h, explaining the enhanced reactivity for v compared with h. Qualitatively the explanation accords with our earlier rationale. The freedom of movement for v (as compared with h, bound by its alkyl chain to the surface) permits it to achieve the most favorable C Br Si alignment and Br Si separation for reaction. Early experiments indicated that the optimal alignment for the C Br bond being broken and the Br Si bond being formed was collinear, permitting a vectorial description of the dynamics. A priori calculation for two separate examples (one earlier, and one given herein), referenced in the theory part of the present work, support the requirement for near-collinearity in the transition state between the bond broken and that formed, that is, in C Br Si. We also find the Br Si separation in the transition state has the bromine atom in the adsorbate proximal to its final Br Si bonding distance. This required alignment and proximity is more readily achieved by the mobile v. We also report energy thresholds and yields for the electron-induced reaction of the v and h states of PrBr and PeBr on Si(111)-7 7. For both PrBr and PeBr, the threshold voltage for v is about + 1 V less than for h, while electron yields for v were about 1000 greater than for h. The resemblance of this finding for relative reactivity of v and h in electron-induced reaction to that obtained for v and h in thermal reaction (as noted in our earlier work) would be understandable if in the electron-induced reaction the system returned rapidly to the ground electronic state encountering a similar energy barrier on the ground potential-energy surface as in thermal reaction. The difference in electron yield suggests an enhanced transition probability to the anionic [*] Dr. K. Huang, Dr. I. R. McNab, Prof. Dr. J. C. Polanyi, Dr. J. (S. Y.) Yang Lash Miller Chemical Laboratories, Department of Chemistry and Institute of Optical Sciences, University of Toronto 80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada) E-mail: jpolanyi@chem.utoronto.ca Homepage: http://www.utoronto.ca/jpolanyi