Infrared Spectroscopy of Three-Dimensional Self-Assembled Monolayers:  N-Alkanethiolate Monolayers on Gold Cluster Compounds

Transmission infrared spectroscopy has been used to probe the structure of alkanethiolate monolayers adsorbed onto nanometer-sized gold clusters. The alkyl chain lengths vary between propanethiolate and tetracosanethiolate; specifically the C3, C4, C5, C6, C7, C8, C10, C12, C16, C20, and C24 alkanethiolates have been examined as solid suspensions in KBr pellets. It has been found that the smaller chain lengths (C3, C4, and C5) are relatively disordered, with large amounts of gauche defects present, and thus most resemble the free alkanes in the liquid state. The longer length alkanethiolates are predominantly in the all trans zigzag conformation. There are detectable amounts of near surface gauche defects, the amount of which decreases with increasing chain length, and a reasonably high percentage of end-gauche defects, the relative amount of which increases with increasing chain length. Internal gauche defects cannot be detected. A model is proposed to explain these observations, and the data are compared with that collected for alkanethiolates self-assembled onto the more traditional two-dimensional systems.