The interplay of covalency, hydrogen bonding, and dispersion leads to a long range chiral network: The example of 2-butanol

Abstract

The assembly of complex structures in nature is driven by an interplay between several intermolecular interactions, from strong covalent bonds to weaker dispersion forces. Understanding and ultimately controlling the self-assembly of materials requires extensive study of how these forces drive local nanoscale interactions and how larger structures evolve. Surface-based self-assembly is particularly amenable to modeling and measuring these interactions in well-defined systems. This study focuses on 2-butanol, the simplest aliphatic chiral alcohol. 2-butanol has recently been shown to have interesting properties as a chiral modifier of surface chemistry; however, its mode of action is not fully understood and a microscopic understanding of the role non-covalent interactions play in its adsorption and assembly on surfaces is lacking. In order to probe its surface properties, we employed high-resolution scanning tunneling microscopy and density functional theory (DFT) simulations. We found a surprisingly rich degree of enantiospecific adsorption, association, chiral cluster growth and ultimately long range, highly ordered chiral templating. Firstly, the chiral molecules acquire a second chiral center when adsorbed to the surface via dative bonding of one of the oxygen atom lone pairs. This interaction is controlled via the molecule's intrinsic chiral center leading to monomers of like chirality, at both chiral centers, adsorbed on the surface. The monomers then associate into tetramers via a cyclical network of hydrogen bonds with an opposite chirality at the oxygen atom. The evolution of these square units is surprising given that the underlying surface has a hexagonal symmetry. Our DFT calculations, however, reveal that the tetramers are stable entities that are able to associate with each other by weaker van der Waals interactions and tessellate in an extended square network. This network of homochiral square pores grows to cover the whole Au(111) surface. Our data reveal that the chirality of a simple alcohol can be transferred to its surface binding geometry, drive the directionality of hydrogen-bonded networks and ultimately extended structure. Furthermore, this study provides the first microscopic insight into the surface properties of this important chiral modifier and provides a well-defined system for studying the network's enantioselective interaction with other molecules.

Chirality at two-dimensional surfaces: A perspective from small molecule alcohol assembly on Au(111)

“…The DFT calculated structure for the hydrogen bonded tetramer shows that each 2-BuOH molecule again binds to preferred Au atop sites, with the OH group almost parallel to the surface while the hydrocarbon tail tilts away from the surface. 20 Similar to the achiral C 1 -C 3 alcohols investigated before, point chirality develops when the chiral 2-BuOH molecule adsorbs on the Au atom via the oxygen lone pair. As such, the 2-BuOH molecules can have both intrinsic and surface-bound chiral centers.…”

“…Contrary to the 3-fold symmetry of the Au(111) surface, it was found that the formation of tetramers was more energetically preferred than dimers and trimers. 20 DFT calculations also found that hydrogen bonded tetramers were equally as stable as hydrogen bonded hexamers but hexameric units were not observed in the surface coverages explored. The DFT calculated structure for the hydrogen bonded tetramer shows that each 2-BuOH molecule again binds to preferred Au atop sites, with the OH group almost parallel to the surface while the hydrocarbon tail tilts away from the surface.…”

“…Thus, in this case, hydrogen bonding and vdW forces are probed for different molecular sizes, providing new insight on these important interactions in nature and on their interplay in stabilizing molecular systems. [17][18][19][20][21][22] Often distinct from bulk material properties, chirality can be important and even the basic 'rules' of hydrogen bonded systems at surfaces can be different, necessitating continued research. 23…”

Disentanglement of orthogonal hydrogen and halogen bonds via natural orbital for chemical valence: A charge displacement analysis

“…Obviously, Δ E int depends on all the interactions, therefore in this case the disentanglement of XB results to be particularly interesting. Moreover, Δ E int takes into account also the electrostatic interaction between the fragments, which always plays an important role in HBs and XBs . Interestingly, the trend of the dissociation energies is slightly different: −20.3, −18.3, and −11.6 kcal/mol for NIS, NBS, and NCS, respectively.…”

Associated molecular liquids at the graphene monolayer interface

“…As regards the liquid-vapor interface, we also note a slight orientation of alcohol molecules parallel to the interface to a lesser degree than water. Let us recall that even in bulk conditions, alcohol molecules form chainlike structures [85][86][87][88] which are less cohesive than the 3D-HB network of water.…”