Optical Rotation of Achiral Compounds

Abstract

Oriented achiral molecules and crystals with D(2d) symmetry or one of its non-enantiomorphous subgroups, S(4), C(2v), or C(s), can rotate the plane of transmitted polarized light incident in a general direction. This well-established fact of crystal optics is contrary to the teaching of optical activity to students of organic chemistry. This Minireview gives an overview of the measurement and calculation of the chiroptical properties of some achiral compounds and crystals. Methane derivatives with four identical ligands related by reflection symmetry are quintessential optically inactive compounds according to the logic of van't Hoff. Analysis of the optical activity of simple achiral compounds such as H(2)O and NH(3) provides general aspects of chiroptics that are not readily broached when considering chiral compounds exclusively. We show here, through the use of group theoretical arguments, the transformation properties of tensors, and diagrams, why some achiral, acentric compounds are optically active while others are not.

Asymmetric Autocatalysis Induced by Chiral Crystals of Achiral Tetraphenylethylenes

“…Chiral crystallization of achiral organic compounds has been one of the important candidates for the origin of chirality (Kahr and Gurney 2001;Sakamoto 1997;Matsuura and Koshima 2005;. Stereospecific reactions using enantiomorphous crystals as reactants in the solid state have been reported to form enantioenriched organic compounds (Penzien and Schmidt 1969;Green et al 1979;Claborn et al 2008). However, this process does not increase the molar amount of chirality.…”

Inducing chirality ex nihilo by an electric field

“…In our set of compounds, almost all different combinations were observed. For CH 4 and CCl 4 , there is no significant effect on the magnitude and no change in sign. A change of sign while maintaining the original value is observed for ethylene, and sign reversal and a significant change of magnitude were demonstrated for glycine and CH 2 ClF.…”

“…Our effect is also similar but distinct from the well-known optical activity of certain achiral molecules in a definite orientation relative to the incident light. 4 In contrast, the external electric field will be seen here to induce a tangible change in the compound itself that renders it chiral regardless of its orientation with respect to the incident light.…”

Amplification of chirality as a pathway to biological homochirality

“…So far, several chiral factors have been proposed as the origins of the chirality of organic compounds, including circularly polarized light (CPL) [11][12][13], inorganic crystals such as quartz [14,15] and sodium chlorate [91,92], enantiomorphous organic crystals formed from achiral organic molecules [93][94][95][96] and spontaneous absolute asymmetric synthesis [97]. The enantiomeric imbalances that were induced by these proposed mechanisms have usually been very small; therefore, a suitable amplification process is required to reach homochirality of biological organic compounds.…”