1-Bromo-4-ethynylbenzene on Cu(100): Adsorption, bonding structures, and reaction mechanisms

Temperature-programmed reaction/desorption, X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, near edge X-ray absorption fine structure and density functional theory calculations have been employed to investigate the adsorption, bonding structures, and reaction pathways of 1-bromo-4-ethynylbenzene (BrC₆H₄C ≡ CH, 1Br4EB) on Cu(100). This molecule is chemically adsorbed, via the ethynyl group attaching to the surface, forming two C-Cu bonds and a ~ 60° tilting aromatic plane with respect to the surface. At 370 K, dissociation of both the C-Br and CC-H bonds of 1-bromo-4-ethynylbenzene generates adsorbed intermediates of C₆H₅C₂, C₆H₄C₂H₂, and C₆H₄C₂. The C-Br breakage temperature is much higher than those (~ 240 K) observed in the cases of bromobenzene/Pt(111) and dibromopyridine/Cu(100), owing to the adsorption geometry of 1-bromo-4-ethylnylbenzene, with the C-Br pointing away from the surface. In the case of higher coverages, molecular desorption occurs near 370 K. The adsorption structures of the aromatic intermediates of C₆H₄CHCH, C₆H₄CCH₂, C₆H₄CH₂C, C₆H₄C₂, and C₆H₅C₂ are calculated and compared. They are stable up to ~520 K and decompose to form H₂ at higher temperatures (~520–870 K). No hydrocarbon products, such as C₂H₂, C₂H₄, C₆H₆, C₆H₅CCH, or C₆H₅CHCH₂, are measured.