A highly captivating idea is to investigate the potential of a magnetic field to be involved in a photochemical reaction pathway as the means of producing enantiomer enhancement. The magnetic field of Earth or any interstellar magnetic field might then be considered important to the origin of life's molecular asymmetry.
Let us begin with a very general but simple question: "Are there any known interactions between a magnetic field and physico-chemical processes?" Thiemann (1984) answered this question with a clear "yes". He outlined several examples from the domain of magnetochemistry, starting with Zeeman-splitting of spectral lines [used in the analytical tool nuclear magnetic resonance (NMR) spectroscopy] and covering paramagnetic spin resonance, the Faraday effect [leading to magnetic optical rotation dispersion (MORD) and magnetic circular dichroism (MCD)], and others. But all this, of course, does not answer the question of magnetic fields inducing stereoselective effects in chemical reactions. Since the magnetic field alone is defined as a pseudo-chiral force, we have to think at least about a combination of two pseudo-chiral fields to expect a chiral effect on chemical structures.1
According to Thiemann (1984), an immediate and simple approach to the problem lies in the following idea: if we fix a symmetric and achiral substance such as benzene between the poles of a strong magnet, we observe a Cotton effect and a rather strong magnetic circular dichroism, due to the Faraday effect (see Chap. 2). The benzene becomes pseudo-optically active. The effect is perfectly symmetric, i.e., it changes sign, upon reversal of the field. If we then switch the magnet off, the effect, manifested by the MCD signal, disappears immediately. Would it be possible to "freeze this optical activity", i.e., to transform this pseudo-chirality into a "true", i.e., permanent, chiral structure? Can a magnetochiral anisotropy - an effect linking chirality and magnetism - give rise to an enantiomeric excess in a photochemical reaction driven by unpolarized light in a parallel magnetic field? From whatever point one is looking at it, the research in this area covers a vast field and deserves attention. Hypotheses and rather wild speculations on such a magnetochiral interaction have always been around in the literature, but only since the advent of a few fundamental papers by Lawrence Barron of the University of Glasgow has a solid foundation, and hence a firm justification, for such endeavours been laid down. Barron (1994a, 1994b) and others (Rhodes and Dougherty 1978; Rikken and Raupach 1998) pre
1 The definition of true versus false chirality lies not in the scope of this book and can be found in Barron (2004).
dicted theoretically that an enantiomeric enhancement could indeed be induced in chiral or prochiral racemic systems by the interaction with two "pseudo-chiral" fields such as ««polarized electromagnetic radiation and a parallel magnetic field.
A few experimental trials have been published based on hypotheses of this kind, making use of an interaction of magnetic fields in asymmetric chemical reactions. Among these, some did not yield the wanted results (see Teutsch and Thiemann 1986, 1990), and some were greeted with enthusiasm due to their rather dramatic positive results, but were soon withdrawn from the panel because Zadel et al. (1994) admitted to having intentionally manipulated the results (see also Chap. 1).
Until recently, however, convincing experimental verifications for this theorem, though searched for intensively, was hard to find. The first evidence was provided in June 2000 by reports on the first photoresolution of a chiral system, a CrIII-tris-oxalato complex in aqueous solution. Rikken and Raupach (2000) published the sound experimental observation of a magneto-optical effect that provided the link between magnetism and chirality. According to their report, the sign of the enan-tiomeric excess in the octahedral complex was determined by the relative orientation of the light beam to a fixed magnetic field.
The CrIII-tris-oxalato complex was chosen for this study because this chiral octahedral complex is unstable in aqueous solution, continuously dissociating and reforming. Interestingly, the dissociation-process is accelerated by the absorption of light. In 2000, the CrIII-tris-oxalato complex was irradiated by Rikken and Raupach with an unpolarized laser beam (sic!) oriented in parallel to a static magnetic field (Fig. 6.2). Here, a small excess of one enantiomer in the order of e.e. = 1.0 x 10-5 was observed. Reversing the magnetic field direction resulted in the detection of an equal concentration of the mirror-image enantiomer.
The observed process consists of a polarization independent refractive index difference between left- and right-handed systems, which is proportional to B x k, where k is the wavevector of the light and B is the magnetic field. In the above photochemical reaction with unpolarized light in a magnetic field, this magnetochiral anisotropy was shown to give rise to a small enantiomeric excess linear in B x k. The absolute magnetochiral dichroism MD value is regarded as the difference between the absorption coefficient of unpolarized light parallel to a static magnetic field en and the absorption coefficient of unpolarized light antiparallel to a static magnetic field en Eq. 6.1. The claimed effect based on magnetochiral dichroism yielded relatively low enantiomeric excesses and required a high magnetic field of about 15 T, compared with the terrestrial field of 5 • 10-5 T.
Soon after publication of these original data, questions concerning the magne-tochiral anisotropy arose: does the magnetic field convert the unpolarized electromagnetic radiation into circularly polarized light due to the well-known phenomenon of the Faraday rotation? And would this circularly polarized light be able to introduce an enantiomeric excess into the chiral octahedral complex? Is it possible to explain the described and now observed magneto-optical effect by the overlap of two well-known physico-chemical effects, namely the mentioned Faraday rotation
Fig. 6.2 Magneto-optical effect: scheme to generate an enantiomeric enrichment in a chiral organic molecule in solution by performing a photoreaction with unpolarized light in a static magnetic field. In the upper image, the unpolarized light beam hv passes the aqueous solution in parallel to a static magnetic field In the lower image, the unpolarized light passes antiparallel to the magnetic field B^. The change of the light's orientation provokes the opposite enantiomeric excess in the dissolved chiral molecules (see Barron 2000a). Illustration: Stephane Le-Saint, University of Nice-Sophia Antipolis
Fig. 6.2 Magneto-optical effect: scheme to generate an enantiomeric enrichment in a chiral organic molecule in solution by performing a photoreaction with unpolarized light in a static magnetic field. In the upper image, the unpolarized light beam hv passes the aqueous solution in parallel to a static magnetic field In the lower image, the unpolarized light passes antiparallel to the magnetic field B^. The change of the light's orientation provokes the opposite enantiomeric excess in the dissolved chiral molecules (see Barron 2000a). Illustration: Stephane Le-Saint, University of Nice-Sophia Antipolis and the asymmetric interaction of circularly polarized light with matter? In fact, Rikken and Raupach (1998) and Barron (2000a, 2000b) carefully distinguish between two mechanisms that might introduce an enantiomeric excess into racemic organic compounds or into prochiral educts: A 'cascaded' and a 'pure' mechanism.
1. 'Cascaded' mechanism. The magnetic field 'converts' the unpolarized light first into circularly polarized light as a result of the Faraday effect and the obtained circularly polarized light interacts with the molecule producing circular dichro-ism, which is well known to result in an excess of one of the enantiomers. The whole procedure is called a 'cascaded' mechanism. This sequence of effects was, however, calculated to be too small to end in the observed enantiomeric excesses, because a magnetic field of 1 T may introduce an enantiomeric excess of only e.e. = 5 x 10-7.
2. 'Pure' mechanism. Due to this, a so-called 'pure' mechanism for the observed magnetochiral anisotropy was taken into consideration, capable of producing much larger enantiomeric excesses, because it involved less electric dipole interactions. This means that the observed magneto-chiral dichroism is a phenomenon distinct from Faraday rotation.
For the above-described experiment, Rikken (2004) concluded that the cascading mechanism did not contribute significantly and that the true magnetochiral effect dominated the cascaded one. In other systems, however, he admitted that the cascading mechanism might be dominant.
In future experiments, the applications of enantioselective magnetochiral photochemistry to other chiral systems with higher relevance to prebiotic systems might provide additional convincing arguments that this effect played an active role in the origin of biomolecular homochirality. According to Barron (2000a), magne-tochiral dichroism is - in addition to circularly polarized light and the electroweak interaction - a serious candidate for the source of handedness in biomolecules. This model, however, has not yet won wide acceptance in explaining biomolecu-lar homochirality.
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