Large dissymmetries

The rotational strength calculated for I is as large as that of a butadiene twisted by 20°. In II, with an out-of-plane methyl, R increases by a factor of about 2. This shows that the contributions to R of dissymmetric substituents of chiral cisoid dienes may be comparable to and even outweigh the contributions arising from the intrinsic dissymmetry of the chromophore. 

The dissymmetry method is useful especially if the instrument does not afford facilities for a wide angular scan of scattered intensities. For large particles the scattering envelope is not symmetrical and, as already indicated in Fig. 1, the forward scatter is larger than that in the backward direction. Hence the dissymmetry Z is greater than unity, where... 

A large number of diaryl ketone substrates, including those listed in Table 9.3, have been reduced with high enantioselectivity with the protocol described here. Unlike analogous chemical catalysts, the commercially available biocatalysts displayed no dependence on ortho substitutions or electronic dissymmetry, and produced diaryl methanols with good to excellent ee values in nearly all cases. 

When the particles are large enough to display angular dissymmetry, Eq. (9) is only valid when 0 = 0, as P(0) = 1 for the angle. The experimental scattering intensities can only be analyzed as a function of the observation angle as it is impossible to measure the scattered intensity at zero angle [9]... 

The class of cubane clusters (4w) is large and includes complexes of metals distributed throughout the Periodic Table, with diverse bridging ligands X and with 0-4 terminal ligands L. There is substantial variability of shape, symmetry and dimensions of the central core apart from dissymmetry due to partial substitution of M, X or L, the concentric interpenetrating M4 and X4... 

A chiral substance is defined by the International Union of Pure and Applied Chemistry (lUPAC) as one that interacts differently with left and right circularly polarized light. Two types of molecular optical activity are recognized inherent dissymmetry characterized by large rotational strengths and inherently symmetrical, but asymmetrically perturbed, molecules for which rotational strengths are less by a factor of a thousand or so. 

In 1964, Cahn, Ingold, and Prelog (see p. 130) proposed that chemists use the terms chiral and chirality as defined by Kelvin. Based on the Greek word for hand (cheir), chirality means handedness, in reference to that pair of non-supcrimposablc mirror images we constantly-have before us our two hands. There has been wide-spread acceptance of Kelvin s terms, and they have largely displaced the earlier dissymmetric and dissymmetry (and the still earlier—and less accurate— asymmetric and asymmetry ), although one must expect to encounter the older terms in the older chemical literature. 

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