Reactions of Atomic Hydrogen with Surface-Oriented Even and Odd Chain Length Brominated Self-Assembled Monolayers

We have conducted a spatially-resolved and angle-resolved investigation of the on-surface products of reactive scattering from a directed H atom beam. Self-Assembled Monolayers (SAMs) of 11-bromoundecanethiol (11-Br), 12-bromododecanethiol (12-Br), decanethiol (10C), and mixtures thereof were prepared on Au(111) and reacted at room temperature with H atoms from an effusive atomic beam at 1600 K. The role of collisional orientation between the primary bromine and hydrogen atom was examined in the context of the debromination rate. Due to the odd–even effect, collisions of H atoms with the terminal bond of 11-Br occur at an average of 50° while collisions with 12-Br occur at an average of 75°. These average angles represent the included H–C–Br angle and take into account all 2π possible azimuthal orientations of the C–Br bond. Debromination reaction rates were monitored with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM analysis used SAMs composed of 95%/5% 10C/11- or 12-Br, as well as films composed purely of 11- or 12-Br, while XPS used only pure films of 11-Br, 12-Br, or 10C. The reaction rates of 11-Br and 12-Br were not found to differ in a statistically significant manner with reaction probabilities of P₁₁ = (1.00 ± 0.08)% and P₁₂ = (1.02 ± 0.08)% with respect to the average number of H collisions determined from the STM data of dilute brominated alkane films in 10C. It was observed in the STM data for pure films of brominated alkanes that debromination occurs primarily from the interior of SAM domains as well as from the domain grain boundary. The XPS data of pure 11-Br, 12-Br, and 10C films indicated that direct debromination of the terminal carbon is the primary pathway for bromine to leave the surface. It was concluded that the debromination pathway is favored by direct collisions between the H and Br atoms. The use of brominated alkanes presents itself as an effective method to passivate gold against H atoms in comparison to a fully hydrogenated SAM of equivalent length. These studies with surface-oriented molecules present a complementary route for examining geometry-constrained molecular reactivity, joining more traditional gas phase studies that have used either orienting fields or optical alignment to address similar questions.