Molecular chirality

The investigation of chiral molecular phenomena, and associated technique development, thus hnds potentially signihcant practical applications to place alongside the fundamental interest of this topic. This chapter will examine the recently investigated phenomenon of photoelectron circular dichroism (PECD) that arises from a dissymmetry in the angular distribution of photoelectrons... 

Consider a threaded rod, representing a molecular enantiomer, that lies away from an observer. If the observer reaches out and spins a nut on the rod clockwise with his right hand, the nut will travel forward, away from the observer, and will shortly fly off the rod. Here, the angular momentum imparted to the nut (electron) by the observer s hand (photon) causes it to be ejected in a specific direction from the rod (molecular enantiomer) in the observer s reference frame. This is mediated by the interaction between the chiral thread of the rod and nut (the chiral molecular potential). If the rod is turned through 180° and the action repeated, the nut (electron) still departs in the same direction, away from the observer. Hence, the orientation of the rod (molecule) in the observer s frame does not alter the direction in which the nut (electron) is ejected. 

The continuum electron-phase shifts induced by the short-range scattering off the chiral molecular potential are most conveniently introduced by a third choice of continuum function, obtained by diagonalizing the K-matrix by a transformation U, resulting in a set of real eigenchannel functions (apart from normalization) [41] ... 

It may be worthwhile to compare briefly the PECD phenomenon discussed here, which relates to randomly oriented chiral molecular targets, with the likely more familiar Circular Dichroism in the Angular Distribution (CDAD) that is observed with oriented, achiral species [44 7]. Both approaches measure a photoemission circular dichroism brought about by an asymmetry in the lab frame electron angular distribution. Both phenomena arise in the electric dipole approximation and so create exceptionally large asymmetries, but these similarities are perhaps a little superficial. 

For the carbonyl carbon Ij core level ionization, excellent quantitative agreement of the b parameters is found, both between the alternative calculations and between either calculation and experiment (see Section VLB.I). Given the spherical, therefore achiral, nature of the initial orbital in these calculations, any chirality exhibited in the angular distribution must stem from the final-state photoelectron scattering off the chiral molecular ion potential. Successful prediction of any non-zero chiral parameter is clearly then dependent on a reliable potential model describing the final state. At this level, there is nothing significant to choose between the potential models of the two methods. 

Unfortunately, experimental difficulties precluded measurements closer to threshold, and the B-spline calculation also does not properly span this near threshold region down to the onset [57]. However, the general trend rising above 5 eV is for the dichroism to become attenuated, easily rationalized as the ejected photoelectron displaying less sensitivity to the chiral molecular potential as it acquires more energy. 

In literature, SOHNCKE space-group types are often termed chiral space groups , which is not correct. Most chiral molecular compounds do not crystallize in a chiral (enantiomorphic) space group. For details see [86]. 

Leigh DA, Perez EM (2006) Dynamic Chirality Molecular Shuttles and Motors. 265 185-208... 

The Rosetta mission with its planned landing on a comet, with analysis of cometary material (see Sect. 3.2), should provide more information on the occurrence of chiral molecular species in the cosmos (Adam, 2002). The GC-MS apparatus installed in the robotic lander RoLand is also able to separate and analyse chiral organic molecules (Thiemann and Meierhenrich, 2001). 

A new chiral benzothiazine ligand 205 was synthesized by Harmata and co-workers <06JOC3650>. It could be converted into a chiral molecular receptor 207 in a simple way. This chiral species 207 could be used as a new class of chiral molecular tweezers. The synthesis of 205 commenced with the protection of the commercially available compound 202, which was then coupled with f7tj-sulfoximinc 77b using the one-pot, one-operation procedure <99AG(E)2419> affording enantiomerically pure benzothiazine 204. This was followed by deprotection to produce benzothiazine 205 in good yield. 

The following sections describe examples of the application of these approaches to the study of chiral molecular recognition in thin film systems, using the battery of techniques outlined above. 

Figure 17 shows the 11/A isotherms of racemic and enantiomeric films of the methyl esters of 7V-stearoyl-serine, -alanine, -tryptophan, and -tyrosine on clean water at 25°C. Although there appears to be little difference between the racemic and enantiomeric forms of the alanine surfactants, the N-stearoyl-tyrosine, -serine, and -tryptophan surfactants show clear enantiomeric discrimination in their WjA curves. This chiral molecular recognition is first evidenced in the lift-off areas of the curves for the racemic versus enantiomeric forms of the films (Table 2). As discussed previously, the lift-off area is the average molecular area at which a surface pressure above 0.1 dyn cm -1 is first registered. The packing order differences in these films, and hence their stereochemical differentiation, are apparently maintained throughout the compression/expansion cycles. 

All of the experiments in pure and mixed SSME systems, as well as in the Af-stearoyltyrosine systems, have one common feature, which seems characteristic of chiral molecular recognition in enantiomeric systems and their mixtures enantiomeric discrimination as reflected by monolayer dynamic and equilibrium properties has only been detected when either the racemic or enantiomeric systems have reverted to a tightly packed, presumably quasi-crystalline surface state. Thus far it has not been possible to detect clear enantiomeric discrimination in any fluid or gaseous monolayer state. 

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