INFRARED CIRCULAR-DICHROISM

A dissymmetric (chiral) molecule or molecular conformation—that is, one which is not superimposable on its mirror image—exhibits a differential interaction with left and right circularly polarized radiation. Circular dichroism, the difference in absorbance of left versus right circularly polarized light, can occur for both electronic and vibrational excitation of a chiral molecule. Although the infrared spectral region primarily encompasses vibrational transitions, low-lying electronic transitions in metal complexes can also occur in the infrared. This chapter uses the terms “infrared circular dichroism” (IRCD) to refer to both types of transitions and vibrational circular dichroism (VCD) to distinguish purely vibrational transitions. The first measurement of IRCD was reported in 1972 and that of VCD in 1973. Since that time, numerous advances in experimental techniques have expanded the region over which IRCD can be measured, and advances in interpretational and calculational approaches have provided a framework for correlating the observed spectra with specific molecular conformations and configurations. Metal complexes, metalloproteins, and metalloenzymes provide a particularly interesting area for IRCD investigation, because both conformational and configurational chirality can be present in the vicinity of the metal ion. © 1993, Academic Press, Inc.