Proline-Derived Structural Phases on Cu{311}

Structural phases formed by adsorption of l-proline onto a Cu{311} surface in ultra-high vacuum were investigated using reflection–absorption infrared spectroscopy, low-energy electron diffraction and scanning tunnelling microscopy. An ordered structural phase formed by self-assembly of l-prolinate with (2,1;1,2) periodicity, and a transition from pure μ3 bonding to a mixture of μ3 and μ2 bonding with increasing exposure at 300 K, were observed. This behaviour has broad parallels with that previously seen with alaninate and glycinate on Cu{311}, but the detailed correlation between structure and bonding, and their evolution during subsequent annealing, are markedly different for prolinate as compared to alaninate and glycinate. At annealing temperatures around 480–490 K, a new structural phase with (5,3;4,6) periodicity emerges. We tentatively attribute this to pyrrole-2-carboxylate, formed by dehydrogenation and aromatization of the pyrrolidine ring of prolinate. The observation of equal areas of the two possible mirror domains associated with the two possible adsorbate–substrate bonding enantiomers implies a prochiral intermediate.