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

A Systematic Search for the Adsorption Motif of All Stereoisomers of Propylene Glycol on a Palladium(111) Surface for Fuel Cell Applications

Small organic molecules have been shown to produce sufficient power densities allowing them to be environmentally friendly renewable fuel sources and an important part of fuel cell research. Affiliated experimental work found propylene glycol, as a source of renewable fuel, produces viable power densities when utilized with an alkaline-acid fuel cell and a Pd(111) catalyst. There is limited theoretical work on propylene glycol's energy reaction pathway. Thus, the first step in understanding how propylene glycol reacts with the Pd(111) slab is understanding its adsorption. In this paper, we present the investigation of adsorption potential energies (APE) of propylene glycol stereoisomers (S)-propane-1,2-diol (1,2PGS), (R)-propane-1,2-diol (1,2PGR), and propane-1,3-diol (1,3PG) on Pd(111). The isomers are systematically scanned through different configurations to analyze the preferred stable orientation and positional motifs. Density functional theory (DFT) is used to optimize the molecular geometries and surface relaxations. The most stable configuration of the 1,2PG stereoisomers resulted in an APE of -0.97 eV. The most stable configuration of the 1,3PG resulted in an APE of -1.19 eV. Both the 1,2PG(S/R) and 1,3PG isomers favor a motif in which at least one hydroxyl oxygen atom interacts with the surface of the Pd(111) catalyst. The 1,2PG carbon backbone prefers to have the center carbon positioned away from the slab, while the 1,3PG prefers to have the center carbon positioned closer to the slab. The most stable 1,3PG differs from other reported 1,3PG and 1,2PG relaxed configurations in that both of the hydroxyl oxygen atoms interact with the Pd(111) surface. These results show more favorable APEs than previously reported calculations. This paper will discuss in detail the differences between the hydroxyl group motifs and their role in affecting adsorption.

Desorption trends of small alcohols and the disruption of intermolecular interactions at defect sites on Au(111)

Gold-based catalysts have received tremendous attention as supports and nanoparticles for heterogeneous catalysis, in part due to the ability of nanoscale Au to catalyze reactions at low temperatures in oxidative environments. Surface defects are known active sites for low temperature Au chemistry, so a full understanding of the interplay between intermolecular interactions and surface morphology is essential to an advanced understanding of catalytic behavior and efficiency. In a systematic study to better understand the adsorption and intermolecular behavior of small alcohols (C₁-C₄) on Au(111) defect sites, coverage studies of methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, and isobutanol have been conducted on Au(111) using ultrahigh vacuum temperature programmed desorption (UHV-TPD). These small alcohols molecularly adsorb on the Au(111) surface and high resolution experiments reveal distinct terrace, step edge, and kink adsorption features for each molecule. The hydrogen-bonded (H-bonded) networks of small alcohols on Au(111), except for 1-butanol and isobutanol, have been previously imaged on the molecular level at low temperatures by scanning tunneling microscopy. Primary C₁-C₃ alcohols exhibit planar H-bonded long extended zigzag chain networks while 2-butanol arranges in tetramer clusters of H-bonded molecules due to steric hindrance inhibiting the proximity of molecules on Au(111). Herein, the desorption energy of small primary alcohols was shown to trend linearly with increasing C₁-C₄ carbon chain length, indicating that the H-bonded molecular packing of 1-butanol resembles that of methanol, ethanol, and 1-propanol, while isobutanol and 2-butanol deviate from the trend. Butanol isomer studies allow the prediction of isobutanol long extended chains in contrast to tetramers. The distinction between the desorption of butanol isomers highlights the role of intermolecular interactions due to the difference in molecular packing structures on Au(111). Furthermore, by studying the energetics of terrace H-bonded networks in comparison with molecular adsorption at undercoordinated step edge and kink defect sites, it is shown that the contribution of stabilizing van der Waals forces to the overall adsorption energy is less for small alcohols adsorbed at kink sites (3.1 kJ mol^-1 per CH₂) and similar for those adsorbed at step edge (4.8 kJ mol^-1 per CH₂) and Au terrace sites (4.9 kJ mol^-1 per CH₂).

Assessing the effects of covalent, dative and halogen bonds on the electronic structure of selenoamides

The C–NMe2 bond rotation of a selenoamide is proposed as an experimental probe to compare different chemical interactions.

Associated molecular liquids at the graphene monolayer interface

We report molecular simulations of the interaction between a graphene sheet and different liquids such as water, ethanol, and ethylene glycol. We describe the structural arrangements at the graphene interface in terms of density profiles, number of hydrogen bonds (HBs), and local structuration in neighboring layers close to the surface. We establish the formation of a two-dimensional HB network in the layer closest to the graphene. We also calculate the interfacial tension of liquids with a graphene monolayer and its profile along the direction normal to the graphene to rationalize and quantify the strengthening of the intermolecular interactions in the liquid due to the presence of the surface.

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

As known, the electron density of covalently bound halogen atoms is anisotropically distributed, making them potentially able to establish many weak interactions, acting at the same time as halogen bond donors and hydrogen bond acceptors. Indeed, there are many examples in which the halogen and hydrogen bond coexist in the same structure and, if a correct bond analysis is required, their separation is mandatory. Here, the advantages and limitations of coupling the charge displacement analysis with natural orbital for chemical valence method (NOCV-CD) to separately analyze orthogonal weak interactions are shown, for both symmetric and asymmetric adducts. The methodology gives optimal results with intermolecular adducts but, in the presence of an organometallic complex, also intramolecular interactions can be correctly analyzed. Beyond the methodological aspects, it is shown that correctly separate and quantify the interactions can give interesting chemical insights about the systems.

Templated Growth of a Homochiral Thin Film Oxide

Chiral surfaces are of growing interest for enantioselective adsorption and reactions. While metal surfaces can be prepared with a wide range of chiral surface orientations, chiral oxide surface preparation is more challenging. We demonstrate the chirality of a metal surface can be used to direct the homochiral growth of a thin film chiral oxide. Specifically, we study the chiral "29" copper oxide, formed by oxidizing a Cu(111) single crystal at 650 K. Surface structure spread single crystals, which expose a continuous distribution of surface orientations as a function of position on the crystal, enable us to systematically investigate the mechanism of chirality transfer between the metal and the surface oxide with high-resolution scanning tunneling microscopy. We discover that the local underlying metal facet directs the orientation and chirality of the oxide overlayer. Importantly, single homochiral domains of the "29" oxide were found in areas where the Cu step edges that templated growth were ≤20 nm apart. We use this information to select a Cu(239 241 246) oriented single crystal and demonstrate that a "29" oxide surface can be grown in homochiral domains by templating from the subtle chirality of the underlying metal crystal. This work demonstrates how a small degree of chirality induced by slight misorientation of a metal surface (∼1 sites/20 nm²) can be amplified by oxidation to yield a homochiral oxide with a regular array of chiral oxide pores (∼75 sites/20 nm²). This offers a general approach for making chiral oxide surfaces via oxidation of an appropriately "miscut" metal surface.

Understanding Enantioselective Interactions by Pulling Apart Molecular Rotor Complexes

Enantioselective interactions underpin many important phenomena from biological mechanisms to chemical catalysis. In this regard, there is great interest in understanding these effects at the molecular level. Surfaces provide a platform for these studies and aid in the long-term goal of designing heterogeneous enantiospecific interfaces. Herein we report a model system consisting of molecular rotors, one intrinsically chiral (propylene oxide) and one that becomes chiral when adsorbed on a surface (propene). Scanning tunneling microscopy (STM) measurements enable the chirality of each individual molecule to be directly visualized, and density functional theory based calculations are performed to rationalize the chiral time-averaged appearance of the molecular rotors. While there are no attractive intermolecular interactions between the molecular species themselves, when mixed together there is a strong preference for the formation of 1:1 heteromolecular pairs. We demonstrate that STM tip-induced molecular manipulations can be used to assemble these complexes, examine the chirality of each species, and thereby interrogate if their interactions are enantioselective. A statistical analysis of this data reveals that intrinsically chiral propylene oxide preferentially binds one of the enantiomers of propene with a 3:2 ratio, thereby demonstrating that the surface chirality of small nonchiral molecules can be directed with a chiral modifier. As such, this investigation sheds light onto previously reported ensemble studies in which chirally seeded layers of molecules that are achiral in the gas phase can lead to an amplification of enantioselective adsorption.

Interplay between Gold(I)-Ligand Bond Components and Hydrogen Bonding: A Combined Experimental/Computational Study

The influence of weak interactions on the donation/back-donation bond components in the complex [(NHC)Au(SeU)]⁺ (NHC = N-heterocyclic carbene; SeU = selenourea) has been studied by coupling experimental and theoretical techniques. In particular, NMR ¹H and pulsed-field gradient spin-echo titrations allowed us to characterize the hydrogen bond (HB) between the -NH₂ moieties of SeU and the anions PF₆ ^- and ClO₄ ^-, whereas ⁷⁷Se NMR spectroscopy allowed us to characterize the Au-Se bond. Theoretically, the Au-Se and Au-C orbital interactions have been decomposed using the natural orbital for the chemical valence framework and the bond components quantified through the charge displacement analysis. This methodology provides the quantification of the Dewar-Chatt-Duncanson (DCD) components for the Au-C and Au-Se bonds in the absence and presence of the second-sphere HB. The results presented here show that the anion has a dual mode action: it modifies the conformation of the cation by ion pairing (and this already influences the DCD components) and it induces new polarization effects that depend on the relative anion/cation relative orientation. The perchlorate polarizes SeU, enhancing the Se → Au σ donation and the Au → C back-donation and depressing the C → Au σ donation. On the contrary, the hexafluorophosphate depresses both the Se → Au and C → Au σ donations.

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

The delicate balance between hydrogen bonding and van der Waals interactions determines the stability, structure, and chirality of many molecular and supramolecular aggregates weakly adsorbed on solid surfaces. Yet the inherent complexity of these systems makes their experimental study at the molecular level very challenging. In this quest, small alcohols adsorbed on metal surfaces have become a useful model system to gain fundamental insight into the interplay of such molecule-surface and molecule-molecule interactions. Here, through a combination of scanning tunneling microscopy and density functional theory, we compare and contrast the adsorption and self-assembly of a range of small alcohols from methanol to butanol on Au(111). We find that longer chained alcohols prefer to form zigzag chains held together by extended hydrogen bonded networks between adjacent molecules. When alcohols bind to a metal surface datively via one of the two lone electron pairs of the oxygen atom, they become chiral. Therefore, the chain structures are formed by a hydrogen-bonded network between adjacent molecules with alternating adsorbed chirality. These chain structures accommodate longer alkyl tails through larger unit cells, while the position of the hydroxyl group within the alcohol molecule can produce denser unit cells that maximize intermolecular interactions. Interestingly, when intrinsic chirality is introduced into the molecule as in the case of 2-butanol, the assembly changes completely and square packing structures with chiral pockets are observed. This is rationalized by the fact that the intrinsic chirality of the molecule directs the chirality of the adsorbed hydroxyl group meaning that heterochiral chain structures cannot form. Overall this study provides a general framework for understanding the effect of simple alcohol molecular adstructures on hydrogen bonded aggregates and paves the way for rationalizing 2D chiral supramolecular assembly.

Chirality in adsorption on solid surfaces

In the present review we survey the main advances made in recent years on the understanding of chemical chirality at solid surfaces. Chirality is an important topic, made particularly relevant by the homochiral nature of the biochemistry of life on Earth, and many chiral chemical reactions involve solid surfaces. Here we start our discussion with a description of surface chirality and of the different ways that chirality can be bestowed on solid surfaces. We then expand on the studies carried out to date to understand the adsorption of chiral compounds at a molecular level. We summarize the work published on the adsorption of pure enantiomers, of enantiomeric mixtures, and of prochiral molecules on chiral and achiral model surfaces, especially on well-defined metal single crystals but also on other flat substrates such as highly ordered pyrolytic graphite. Several phenomena are identified, including surface reconstruction and chiral imprinting upon adsorption of chiral agents, and the enhancement or suppression of enantioselectivity seen in some cases upon adsorption of enantiomixtures of chiral compounds. The possibility of enhancing the enantiopurity of adsorbed layers upon the addition of chiral seeds and the so-called "sergeants and soldiers" phenomenon are presented. Examples are provided where the chiral behavior has been associated with either thermodynamic or kinetic driving forces. Two main approaches to the creation of enantioselective surface sites are discussed, namely, via the formation of supramolecular chiral ensembles made out of small chiral adsorbates, and by adsorption of more complex chiral molecules capable of providing suitable chiral environments for reactants by themselves, via the formation of individual adsorbate:modifier adducts on the surface. Finally, a discussion is offered on the additional effects generated by the presence of the liquid phase often required in practical applications such as enantioselective crystallization, chiral chromatography, and enantioselective catalysis.

Chiral nanoscale pores created during the surface explosion of tartaric acid on Cu(111)

The autocatalytic decomposition of tartaric acid on Cu(111) exhibits unique kinetics, which are linked to a hexagonal surface structure adopted at high coverage.