Hegstrom’s 1991 Paper discussing Electron Chirality
Chemists usually think of chirality as a structural property of molecules,
defining a chiral molecule as one with a geometrical structure which is not
superimposable upon its mirror image. Less familiar is the fact that individual
electrons can be chiral, and that the existence of electron chirality gives rise to some interesting relatively new asymmetric phenomena [ 11.
In fact, all elementary particles with spin can have chiral states. It is found
that mirror-image electron states interact differently by means of weak interactions, leading to parity violation [ 1,2]. Two immediate experimental consequences of this result are
(1) an energy difference between enantiomers and
(2) the chirality of atoms and consequential optical activity and other chiral
properties of atoms. Both of these effects are very small owing to their origin
in the weak interactions. The energy difference between enantiomers has been calculated [3] to be of the order of 10e2’ a.u. and has not yet been measured.
The predicted optical activity of atoms (of the order of 10m7 rads) has now
been confirmed experimentally in several laboratories and there is no doubt
that atoms are chiral, if only weakly so (see for example ref. 4). The chirality
of an atom can be visualized by mapping its electronic current density [ 51.
Another experimental consequence of chiral electron states is a difference
in the rates of chiral electron-impact ionization of enantiomers. Chiral electrons can be prepared in the laboratory or obtained naturally from nuclear j3 decay.
According to calculations [ 61, the relative difference in the rate of ionization
ranges from roughly 1 to 10 ppm for low incident electron energies to
roughly one part in lOlo to 10” for incident energies of the order of 100 keV,
such as found in natural B emitters.
Incident chiral positrons produce similar
effects, including different rates of positronium formation in enantiomers, and
these effects are now being searched for experimentally [ 71. One reason for
interest in these phenomena, including the energy difference between enantiomers, is their possible role in the origin of biomolecular chirality [ 11.
In summary, electron chirality gives rise to optical activity in atoms, energy
differences between enantiomers, differential impact ionization of enantiomers, and related properties. Electron chirality density maps out the electron chirality per unit volume in the spatial regions in an atom, molecule or solid.
The recognition that the concept of chirality applies generally to quantum
states, even to the states of individual electrons, has opened up interesting new areas for investigating asymmetric phenomena in atoms, molecules and solids
Mobius Shaped Electrons 