Structure and properties of chiral modifiers (R)-(+)-1-(1-naphthyl)-ethylamine and (S)-(-)-1-(2-naphthyl)-ethylamine on Pd(1 1 1) using infrared spectroscopy

Abstract

(R)-(+)-1-(1-naphthyl)-ethylamine (R-1-NEA) and (S)-(-)-1-(2-naphthyl)-ethylamine (S-2-NEA) were studied on Pd(111) in ultrahigh vacuum using reflection–absorption infrared spectroscopy (RAIRS) to measure the structure, bonding and orientation using the frequency shifts and surface infrared selection rules. Temperature-programmed RAIRS experiments of NEA multilayers showed that the heat of adsorption of 2-NEA (73 kJ/mol) was>1-NEA (67 kJ/mol) resulting in 2-NEA forming ordered films when heated. Both NEA isomers adsorbed at ∼ 160 K adopted flat-lying geometries at the lowest coverages with the naphthyl ring parallel to the surface with an almost vertical C-CH3 group. However, both isomers tilted along the × axis due to surface crowding as the first layer coverage increases and then also tilted along the y axis when the second layer was populated. Adsorption above the multilayer desorption temperatures resulted in the formation of flat-lying species for both 1-NEA and 2-NEA at all coverages.