Reaction Pathways of Propenoyl Chloride (CH₂═CH-C(═O)Cl) on Cu(100) and O/Cu(100)

Reflection-absorption infrared spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reaction/desorption, with the aid of density-functional-theory calculations, have been employed to investigate the adsorption and multiple reaction routes of propenoyl chloride (CH₂═CH-C(═O)Cl) on Cu(100) and oxygen-covered Cu(100) (O/Cu(100)). Upon adsorption of propenoyl chloride on O/Cu(100), nucleophilic attack at the C═O by adsorbed O occurs promptly at 100 K, forming adsorbed CH₂═CH-COO (propenoate) and Cl. No other intermediates are generated from the reaction of the adsorbed CH₂═CH-C(═O)Cl and O. On Cu(100), the theoretical study predicts that propenoyl chloride (trans or cis form) is adsorbed, with the terminal CH₂ attaching to the surface. With respect to free CH₂═CH-C(═O)Cl, the adsorbed trans molecule has a relatively larger elongation in the C-Cl bond, in contrast to the greater variations in the bond lengths of CH₂═CH-C═O for the adsorbed cis molecule. Breakage of the C-Cl bond is found to be the first reaction step of propenoyl chloride on Cu(100), which can occur at 100 K. Following dechlorination, two surface species of CH₂═CH-C(═O)- and −CH₂-CH═C═O (acyl- and ketene-type intermediates) are produced. Formation of the −CH₂-CH═C═O likely originated from the adsorption structure of propenoyl chloride, with close contact between the CH₂ group and the surface. The −CH₂-CH═C═O species further undergoes dimerization to form adsorbed 3-methylene-4-ethenylidene-oxetan-2-one, instead of a cyclobutane-1,3-dione. CH₂═CH-C(═O)-, −CH₂-CH═C═O, and the oxetan-2-one dimer coexist on the surface at 200 K. Another species of CH₂═CH-COO is also produced possibly from further reactions of CH₂═CH-C(═O)- and the oxetan-2-one dimer at 300 K. Recombination of −CH₂-CH═C═O to form the oxetan-2-one dimer is shown to be an electronic favorable process, without steric hindrance.