Laurence D. Barron: Fundamental Studies of Chirality
    The word 'chiral', meaning handed, was first introduced into science by Lord Kelvin (William Thomson), Professor of Natural Philosophy in the University of Glasgow from 1846-1899:
 
I call any geometrical figure, or group of points, chiral, and say that it has chirality, if its image in a plane mirror,  ideally realized, cannot be brought to coincide with itself.

Lord Kelvin, Baltimore Lectures, 1884
 
 
 
 


   In the early 1980s I initiated a discussion of the role of time reversal symmetry in optical activity and pointed out that time-even pseudoscalar observables are the hallmark of genuine chirality. This led me to propose the following new definition of chirality:

True chirality is shown by systems existing in two distinct enantiomeric states that are interconverted by space inversion, but not by time reversal combined with any proper spatial rotation.
This is essentially an extension of Lord Kelvin's definition to include motion. Hence the system breaks parity P but not time reversal T and so exhibits a time-invariant enantiomorphism. This was developed into the concept of 'true' and 'false' chirality which is gaining favour among stereochemists and others since it removed the confusion that existed since Pasteur's time concerning the nature of physical influences able to induce absolute enantioselection. A good example of a truly chiral influence is a static magnetic field collinear with an unpolarized light beam which is therefore able to induce absolute enantioselection in all circumstances, including a reacting system allowed to reach thermodynamic equilibrium. Absolute enantioselection using this magnetochiral influence was demonstrated experimentally in 2000 by G. L. J. A. Rikken and E. Raupach (Nature 405, 932-935), thereby confirming the value of this new definition of chirality. As well as their relevance to synthetic chemistry, these considerations are central to discussions of the origin of homochirality in the living world and the origin of life.
    An unexpected outcome of this work was the suggestion that an influence such as collinear electric and magnetic fields exhibiting false chirality associated with time-noninvariant enantiomorphism (in which P and  are broken separately but PT still holds) can induce a breakdown in microscopic reversibility in reaction processes far from equilibrium if chiral particles are involved, in analogy with the breakdown of microscopic reversibility associated with the CP violation observed in decays of the neutral K-meson.
    I was also the first to point out (from the CPT theorem) that, in view of the lifting of the degeneracy of mirror-image chiral molecules by the parity-violating weak neutral current interaction, the strict enantiomer of a chiral molecule having an energy identical with the original is that with the opposite absolute configuration but composed of antiparticles (i.e. the CP enantiomer), and extended the analysis to prove that this strict degeneracy holds even if CP is violated. These ideas reveal that CP violation in particle physics, which remains the most enigmatic of all physical phenomena discovered to date, is analogous to chemical catalysis since the CP-violating force changes the rates of particle-antiparticle processes without affecting the equilibrium thermodynamics.

Selected Publications

Molecular Light Scattering and Optical Activity. L. D. Barron (1982). Cambridge University Press, Cambridge. Second edition currently in press.

Optical activity and time reversal. L. D. Barron (1981). Mol. Phys. 43, 1395-1406.

True and false chirality and absolute asymmetric synthesis. L. D. Barron (1986). J. Am. Chem. Soc. 108, 5539-5542.

Reactions of chiral molecules in the presence of a time-noninvariant enantiomorphous influence: a new kinetic principle based on the breakdown of microscopic reversibility. L. D. Barron (1987). Chem. Phys. Lett. 135, 1-8.

Fundamental symmetry aspects of molecular chirality. L. D. Barron (1991). In New Developments in Molecular Chirality (ed. P. G. Mezey), Reidel, Dordrecht, pp. 1-55.

CP violation and molecular physics. L. D. Barron (1994). Chem. Phys. Lett. 221, 311-316.

Absolute asymmetric synthesis under physical fields: facts and fictions. M. Avalos, R. Babiano, P. Cintas, J. L. Jiménez, J. C. Palacios and L. D. Barron (1998). Chem. Rev. 98, 2391-2404.

Chirality, magnetism and light. L. D. Barron (2000). Nature 405, 895-896.

Time reversal and molecular properties. L. D. Barron and A. D. Buckingham (2001). Accs. Chem. Res. 34, 781-789.
 
 

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