Chiral organic molecular structures supported by multilayer surfaces

TL;DR

This study numerically investigates how achiral acetanilide molecules become chiral on multilayer boron nitride surfaces, forming various hydrogen-bonded structures depending on their isomeric composition, with implications for 2D crystal formation and melting behavior.

Contribution

It demonstrates the emergence of chirality and complex secondary structures in acetanilide molecules on multilayer surfaces, revealing how isomeric composition influences structure formation and melting scenarios.

Findings

Homochiral molecules form stable secondary structures like arcs, circles, and spirals.

Racemic mixtures do not form regular secondary structures, only random chains.

Coverage density affects melting behavior and layer shifting at high temperatures.

Abstract

We study numerically the dynamics of acetanilide (ACN) molecules placed on a flat surface of a multilayer hexagonal boron nitride structure. We demonstrate that the ACN molecules, being achiral in three dimensions, become chiral after being placed on the substrate. Homochirality of the ACN molecules leads to stable secondary structures stabilized by hydrogen bonds between peptide groups of the molecules. Numerical simulations of systems of such molecules reveal that the structure of the resulting hydrogen-bond chains depends on the isomeric composition of the molecules. If all molecules are homochiral (i.e. only one isomer is present), they form secondary structures (chains of hydrogen bonds in the shapes of arcs, circles, and spirals). If the molecules at the substrate form a racemic mixture, then no regular secondary structures appear, and only curvilinear chains of hydrogen bonds of random shapes can emerge. A hydrogen-bond chain can form a straight zigzag only if it has an alternation of isomers. Such chains can create two-dimensional (2D) regular lattices, or 2D crystals. The melting scenarios of such 2D crystals depend on density of its coverage of the substrate. At 25% coverage, melting occurs continuously in a certain temperature interval. For a complete coverage, melting occurs at 415-470 K due to a shift of 11% of all molecules into the second layer of the substrate.