Stabilising an unstable conformer: 1,2-dichloroethane on clean and chlorinated Cu(111)

Line-of-sight temperature programmed desorption (LOSTPD) is used to measure the bonding of 1,2-dichloroethane (DCE) to clean and chlorinated, (root 3x root 3)R30 degrees-Cl, Cu(111). A monolayer of DCE on clean Cu(lll) has an activation energy for desorption of similar to 43 kJ mol(-1) at zero coverage and exhibits an increase in activation energy for increasing coverage due to attractive lateral interactions, which is a feature of the trans conformation of DCE on this surface. Monolayer adsorption of DCE on the chlorinated surface is less stable with an activation energy for desorption of similar to 40 kJ mol(-1), the activation energy decreasing with increasing coverage, implying repulsive interactions between adsorbed DCE molecules. Such behaviour is consistent with the adsorbed DCE adopting a gauche conformation. The postulated layer of gauche DCE on the halogenated surface causes the second layer of DCE to grow in a strained, unstable structure, such that the second layer desorbs with a smaller activation energy, similar to 35 kJ mol(-1) than normal multilayers of DCE ice, similar to 38 kJ mol(-1), which grow on the clean copper surface; a somewhat surprising observation. When this second unstable layer of DCE saturates, it spotaneously reverts to the normal DCE ice structure, causing the TPD peak to jump to a higher temperature. (C) 1999 Published by Elsevier Science B.V. All rights reserved.