Surfaces achiral molecules

When a molecule is adsorbed on a surface, the symmetry of the combined adsorbate-substrate system is very likely to be reduced compared to that of the isolated gas-phase species or the bare adsorption site. This raises the possibility that, if mirror planes present in the isolated achiral molecule and those at the relevant... 

Chirality at surfaces can be manifested in a number of forms including the intrinsic chirality of the surface structure and even the induction of chirality via the adsorption of achiral molecules onto achiral surfaces. The ability of STM to probe surfaces on a local scale with atomic/molecular resolution has revolutionized the understanding of these phenomena. Surfaces that are globally chiral either due to their intrinsic structure or due to the adsorption of chiral molecules have been shown by STM to establish control over the adsorption behavior of prochiral species. This could have profound consequences for the understanding of the origin of homochirality in life on Earth and in the development of new generations of heterogeneous chiral catalysts that may, finally, make a substantial impact on the pharmaceutical industry. 

Interestingly, it is found that chiral exhibition is not simply limited to systems in which chiral molecules are adsorbed at achiral surfaces (i.e., adsorption of (R, R)-7A on the Ni or Cu surface) but it can also be displayed in systems where no initial chirality is present, i.e., from the adsorption of achiral molecules at achiral surfaces [203-211], Raval and coworkers [203] reported on the adsorption of succinic acid on Cu(l 1 0) and compared the results with those found for (R,R)-TA. Structurally, succinic acid is very similar to TA, with the only difference being that the two hydroxyl groups present in TA are replaced by hydrogen atoms, leading to a consequent loss of both chiral centers (Figure 14.7). 

According to the above criteria, the boundary between the regions R and L must be just the subspace of the achiral molecules. On the other hand, the boundary between two regions of the A-space must necessarily be (n—l)-dimensional. Thus, the requirements can only be satisfied if the subset of the achiral molecules is a ( —l)-dimensional hypersurface, or a set of such surfaces. 

Recent work reveals a much more complex adsorption phase diagram than was first suspected [28]. For the purposes of highlighting the creation of chirality from the adsorption of achiral molecules on achiral surfaces only the (9 0,1 1), (9 0,-1 1) and p(4x2) phases will be discussed [26]. 

Humblot V, Raval R (2005) Chiral metal surfaces from the adsorption of chiral and achiral molecules. Appl Surf Sci 241 150... 

The mechanism of conversion of an achiral molecule into a chiral one was found by Kuhn in circular dichroism spectra. The type of natural quartz used, amethyst crystals, contained colloidal inclusions of Fe. The circular dichroism spectra revealed bonds involving Fe. It would be of importance from that point of view if the films of metals removed from surfaces of quartz crystal would show circular dichroism spectra, too. 

Achiral Molecules on Achiral Surfaces - The Racemic Reaction... 

The asymmetric polymerization in crystalline architectures provides an excellent environment to conduct the absolute asymmetric synthesis of polymers, and also provides an effident route for the ampHfication of chirality. Mirror-symmetry breaking might occur either through total asymmetric transformations, either in enantiomorphous crystals that have self-assembled from achiral molecules, or within racemic crystalline architectures which are delineated by chiral rims or surfaces when one of the chiral faces is blocked by an interface. The self-assembly of nonracemic mixtures into a mixture comprising eutectic compositions of a racemic compound and an enantiomorphous assembly, followed by asymmetric transformation, provides a series of thermodynamically controlled, alternative routes for the effident ampHfication of homochirality. 

As mentioned already, achiral molecules can also be employed to obtain chiral surfaces. Wang et studied the self-assembly of an achiral aldehyde-substituted oligo (p-phenylene vinylene) (OPV3-CHO) at the air/graphite interface (Figure 12a) focusing on the structure and... 

Although the examples presented in the previous paragraph illusfrate that achiral molecules do express chirality on confinement into a 2D plane, globally the surface still remained achiral. Nevertheless, it is possible to induce homochirality in such systems in a number of ways. Following is a brief account on two different approaches. Berg and Patrick demonsttated that mirror symmetry can... 

Many other interesting examples of spontaneous reflection symmetry breaking in macroscopic domains, driven by boundary conditions, have been described in LC systems. For example, it is well known that in polymer disperse LCs, where the LC sample is confined in small spherical droplets, chiral director structures are often observed, driven by minimization of surface and bulk elastic free energies.24 We have reported chiral domain structures, and indeed chiral electro-optic behavior, in cylindrical nematic domains surrounded by isotropic liquid (the molecules were achiral).25... 

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