Achiral surface

Only for achiral surfaces does the last tensor element vanish altogether. Equation (4) retains a similar form but now accommodates a new tensor element. To date, very few experimental works have been reported on chiral surfaces, although the nonlinear effects are expected to be rather large [51]. 

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. 

Figure 9.9 Simulated normalized line shapes of -polarized (a-c) and p-polarized (if-/) second-harmonic signals for quarter waveplate measurements (a) and (if) hypothetical achiral surface (hs = 0.5 fp = 0.75, gp = —0.5), (b) and (if) hypothetical chiral surface with in-phase chiral coefficient (fs = 0.75, hs = 0.5 fp = 0.75, gp = —0.5, hp = 0.25), (c) and (/) hypothetical chiral surface with out-of-phase chiral coefficient ( fs = 0.75 0.25i, hs = 0.5 fp = 0.75, gp = —0.5, hp = 0.25z). Upper (solid line) and lower (dashed line) sign in expansion coefficients correspond to two enantiomers. Rotation angles of 45° and 225° (135° and 315°) correspond to right-hand (left-hand) circularly polarized light and are indicated for one of enantiomers with open and filled circles, respectively.

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). 

R-3-Methylcyclohexanone desorption reveals the same types of easily assigned desorption features on many of the other chiral and achiral surfaces in the stereographic triangle [23]. 

Fig. 4.10 Scanning tunneling micrograph of a Cu(lOO) surface modified by the adsorption of L-lysine at 430 K. The adsorption of L-lysine results in the formation of a homochiral set of (3,1,17) microfacets thus, imprinting the chirality of the adsorbate into an achiral surface. Reprinted with permission from [37]. Copyright 2000 American Chemical Society...

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]. 

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

Davankov and other researchers made substantial contributions to impart the three-point interaction model with modern interpretation [24-26]. As pointed out by Davankov et al., it is required (but not necessarily sufficient) for the chiral selector to recognize the enantiomers to have at least three configuration-dependent active points, which are different in nature, on both chiral selector and enantiomer molecules. The active points on chiral selector must be complementary to and be able to simultaneously interact with those on enantiomer molecules. It is possible that two of the three required interactions can be repulsive if the third one is strong enough to promote the formation of diastereomeric associates between chiral selector and selectand [25]. Davankov et al. used the left- and right-hand model to vividly demonstrate that with the assistance of achiral surface, two-point or even one-point interaction is sufficient for chiral recognition. They treated these cases as expansion of TPI model rather than contradictions to it and asserted that the model is also applicable to CSPs based on proteins and polysaccharides. In some instances, achiral elements, such as solvent molecules and sorbent surfaces, may also participate in the chiral recognition process [24, 25]. 

---