Linear and Circular Dichroism in the Vacuum Ultraviolet Region

Circular dichroism spectroscopy has been extended to the VUV region through the introduction of the photo-elastic modulator with a calcium fluoride optical element. Spectroscopic and stereochemical applications of the technique to chiral molecules in the vapour and in the condensed phase are reviewed, together with related linear dichroism studies of orientated molecular systems. 1 . Introduction. Polyatomic molecules with only a pure rotational point symmetry exhibit a mirrorimage, type of structural isomerism, as was first recognized by Pasteur [I] from a correlation between the enantiomorphous crystal forms of optical isomers and their respective positive and negative circular birefringence, i.e. optical rotation, in the visible region when randomly oriented in the fluid phase. Pasteur termed a structure which is not superposable upon its mirror-image dissymmetric, although the term chiral, introduced by Kelvin [2], is now the more commonly employed. Circular birefringence in the transparent wavelength regions implies the differential absorption of leftand right-circularly polarized radiation at higher frequencies and the expected effect, the Circular Dichroism (CD) of chiral molecules in solution, was discovered by Cotton [3] in 1895. It was only some seventy years later, however, that CD measurements became routinely practicable [4], following pioneer studies of the Cotton effect, notably by Lowry [5], Kuhn [6], and Mathieu [7], which demonstrated the potential of the CD technique in applications to spectroscopic and stereochemical problems. The basic limitation of CD measurements on randomly-oriented chiral molecules is the generallysmall magnitude of the CD relative to the average light-absorption, expressed by Kuhn's dissymmetry factor [6], g = ( E ~ ~~)/i3, where E expresses the extinction coefficient. The 9-factor of chiral molecules in the fluid phase has a magnitude which, at most, corresponds to the ratio of molecular dimensions to the wavelength of the light absorbed, rarely exceeding 10and often several orders of magnitude smaller. The g-factor becomes large, of the order of unity, for axial propagation of the radiation through chiral molecules oriented in a uniaxial single crystal belonging to one of the enantiomorphous space groups, or in a cholesteric mesophase. The corresponding Linear Dichroism (LD) ratio, r = (&Il E ~ ) ~ Z , is similarly large for anisotropic molecules well-oriented in a single crystal with lower than cubic symmetry, or in a nematic mesophase, or in an oriented film. However, LD measurements on solutions of anisotropic molecules with only a small degree of statistical orientation, such as that produced by the application of an electric field in the case of dipolar solutes, or by streaming the solution in the case of long-chain oligomeric solutes, involve small r-factors and are attended by the same metrological problems as those encountered in determining the g-factor of chiral molecules in the gas phase or isotropic solutions. 2. Instrumentation. The classical CD studies generally were carried out by separate measurements of the sample-absorption of leftand of right-circularly polarized light followed by subtraction [5-71. The first-reported CD spectrum in the vacuum ultraviolet region (VUCD), that of S-(+)-3-methylcyclopentanone (I), was obtained by this method, employ-