1,2-Dibromoethane on Cu(100): bonding structure and transformation to C2H4.

Temperature-programmed reaction/desorption, mass spectrometry, reflection-absorption infrared spectroscopy, x-ray photoelectron spectroscopy, and density functional theory calculations have been employed to explore the reaction and bonding structure of 1,2-C(2)H(4)Br(2) on Cu(100). Both the trans and gauche conformers are found to dissociate by breaking the C-Br bonds on clean Cu(100) at 115 K, forming C(2)H(4) and Br atoms. Theoretical investigations for the possible paths of 1,2-C(2)H(4)Br(2) → C(2)H(4) + 2Br on Cu(100) suggest that the barriers of the trans and gauche molecules are in the ranges of 0-4.2 and 0-6.5 kcal/mol, respectively. The C-Br scission temperature of C(2)H(4)Br(2) is much lower than that (~170 K) of C(2)H(5)Br on Cu(100). Adsorbed Br atoms can decrease the dissociation rate of the 1,2-C(2)H(4)Br(2) molecules impinging the surface. The 1,2-C(2)H(4)Br(2) molecules adsorbed in the first monolayer are structurally distorted. Both the trans and gauche molecules exist in the second monolayer, but with no preferential adsorption orientation. However, the trans molecule is the predominant species in the third or higher layer formed at 115 K. The layer structure is not thermally stable. Upon heating the surface to 150 K, the orientation of the trans 1,2-C(2)H(4)Br(2) molecules in the layer changes, leading to the rotation of the BrCCBr skeletal plane toward the surface normal on average and the considerable growth of the CH(2) scissoring peak. On oxygen-precovered Cu(100), decomposition of 1,2-C(2)H(4)Br(2) to form C(2)H(4) is hampered and no oxygenated hydrocarbons are formed. The presence of the oxygen atoms also increases the adsorption energy of the second-layer molecules.