A study of glycine adsorption on a Cu {110} surface using reflection adsorption infrared spectroscopy

The chemisorption of glycine (NH2CH2COOH) and its fully deuterated analogue d(5)-glycine, vacuum deposited on a clean Cu{110} surface, have been investigated using reflection absorption infrared spectroscopy (RAIRS). At room temperature (300 K) a saturated monolayer is dissociatively adsorbed producing glycine in its anionic form; a glycinate species. The orientation of the adsorbed species changes as the coverage increases with bonding occurring initially via equivalent oxygen atoms of the carboxylate group, orientated broadly perpendicular to the surface, similar to the bonding found in simple carboxylic acids. At saturation coverage, at 300 K, a different orientation of the carboxylate group is observed with the carboxylate binding in a more unidentate fashion. The IR spectra are then very similar to those of solid copper glycinate. Low energy electron diffraction (LEED) shows a poor (3 x 2)g pattern. On annealing this fully covered surface to 420 K, there is no significant desorption and the IR spectra show dramatic changes in relative band intensities with the spectra becoming more similar to those obtained at low coverage. The (3 x 2)g LEED pattern sharpens considerably. At this stage, we suggest that anionic glycine (glycinate) species are adsorbed with the O-2-C-C-N backbone essentially parallel to the surface with interadsorbate bonding dominated by CH ... O and N-H ... O hydrogen bonds similar to those found in bulk glycine. The (3 x 2) structure is a compromise between optimal adsorbate site, intermolecular hydrogen bonding and maximum adsorbate density. With the substrate held just below room temperature, multilayers of the zwitterionic glycine form which desorb in two stages with the second layer leaving the surface at a higher temperature than the other layers. At Liquid nitrogen temperatures (85 K), some of the glycine is present on the surface in its acid form, as evidenced by the presence of a carbonyl (C=O) stretch. (C) 1998 Elsevier Science B.V. All rights reserved.