Fourier Transform Microwave Spectroscopy: Handedness Caught by Rotational Coherence

摘要

That molecular structure can be chiral, that is, exhibit handedness, is of profound importance to chemistry and biology: As with our left and right hands, chiral molecules exist as otherwise equivalent mirror images of the opposite chirality. Such stereoisomers are called enantiomers. The detection and identification of enantiomers relies on the concept that the mirror symmetry of enantiomers is broken by exposure to circularly polarized radiation where the electromagnetic field vector rotates either clockwise or counterclockwise once per wavelength. This symmetry breaking is the foundation of three widely used forms of "chiroptical" spectroscopies employing ultraviolet/visible light or infrared radiation and involving electronic or vibrational transitions: Optical rotation measures the difference in index of refraction for left-circularly polarized (LCP) and right-circularly polarized (RCP) radiation; Circular dichroism measures the difference in the absorption of LCP and RCP radiation. Also Raman optical activity can be used as a chiroptical technique to determine the excess of one enantiomer over the other in a mixture or the absolute configuration, that is, specific handedness, of a given enantiomer under investigation. The optical activity can be positive or negative depending on whether the electric and magnetic dipole transition moments point into the same or opposite halves of a sphere centered on the molecule.