Enantioselectivity in a chemical reaction arises when a system is placed under a chiral influence. One of the first “chiral influences” to be used to induce asymmetry in a chemical reaction was circularly polarized light (CPL). An enantiomer of a chiral molecule will preferentially absorb one helicity of circularly polarized light (CPL). In pioneering work in the 1930s, Kuhn used this phenomenon to generate an enantiomeric excess (ee) from an initially racemic mixture by enantioselective photodecomposition.(1) Unfortunately the circular dichroism (CD) in the absorption cross-section is very small, with asymmetry factors (g) ≪ 10−3, consequently enantioselective photodecomposition is very inefficient, with ee of typically <3% being generated after 99.9% photodecomposition.(2) Although extremely inefficient, nonthermal enantioselective chemistry still interests researchers because of the suggestion that CPL is a chiral influence, which could have contributed to the evolution of chemical asymmetry in nature.(3) Low energy electrons (<5 eV) are far more prevalent in the natural environment than CPL, since they can be generated by solar or cosmic radiation through ionization, and as a consequence are significantly more likely to initiate reactions. Indeed low energy electrons are believed to be responsible for inducing much of the chemistry in the interstellar medium. However, it has until now been believed that since unpolarised electrons are not intrinsically chiral, they could not initiate asymmetric chemistry.
- electron beam
- surface chemistry