Stereochemistry of 1,2-dichloroethane adsorbed on Pt(111)

The rotational isomerism of 1,2-dichloroethane (DCE, CH2ClCH2Cl) adsorbed on Pt(111) was studied in the temperature range of 35-100 K using high-resolution electron energy loss spectroscopy and metastable atom electron spectroscopy. In the coverage below monolayer the physisorbed and chemisorbed species coexist at 35 K in the gauche and slightly distorted trans form, respectively. Owing to the direct Pt-Cl interactions, the nonbonding Cl 3p states of the chemisorbed DCE are split off, giving rise to degradation in symmetry from the pure trans form (C2h). The physisorbed gauche conformers are arranged with the C2 axis parallel (or heavily tilted) to the substrate and converted irreversibly to the pseudo-trans form by heating at 70 K. In the multilayer, the trans and gauche conformers exist at 35 K, where the former population is increased with increasing layer thickness. Upon annealing the bilayer at 80 K, the irreversible conversion takes place to yield a higher population of the gauche conformer in the topmost layer. The conformational stabilities and mutual changes of DCE adsorbed on a metal surface are discussed in terms of intramolecular rotational potential.