Conformational pseudopolymorphism and orientational disorder in two-dimensional alkyl carbamate crystals

The structures of self-assembled physisorbed monolayers of alkyl carbamates were examined with atomic detail by scanning tunneling microscopy at the liquid-solid interface. Through systematic variation of molecular structure, the factors determining the two-dimensional crystal packing and dynamics of alkyl carbamate monolayers were isolated. Two different conformational pseudopolymorphs on the surface were observed and their order of stability was varied by changing the length of the alkyl groups. The relative size of the two alkyl groups in a molecule affected the frequency of orientational flipping within a column, which in turn, exerts an influence on the relative orientation of the two-dimensional crystalline domains. These phenomena were explained on the basis of the preferred hydrogen-bonding geometry of the carbamate functional group and the different degree of van der Waals interaction for each form.