Anisotropy macroscopic

In the general case, the average anisotropy has to be determined from Eq. (10) and Eq. (11) by numerical iteration. Explicit solutions can be obtained in the limiting cases of a vanishing or dominating macroscopic anisotropy Ku. The results are... 

It is, however, important to point out that extreme caution should be exercised with cast film VCD measurements because additional macroscopic anisotropies such as LBs may be introduced in the film making process, causing severe artifacts in the observed VCD spectra [57]. For example, Merten et al. found that the VCD measurements of solid polymer films were severely contaminated with LBs [58]. They and others have developed procedures to remove such effects by measuring the film sample under several orientations [58, 59]. 

When chirality is involved, information on solid-state structures and supra-molecular properties must be obtained by solid-state circular dichroism (CDf spectroscopy, as certain characteristics may be lost upon dissolution. However extreme care is required to obtain artifact-free solid-state CD spectra. This is because CD spectra in the solid state (except for special homogeneous cases [9,10]) are inevitably accompanied by parasitic signals that originate from thd macroscopic anisotropies of a sample such as LD (linear dichroism) and LB (linear birefringence) [11-16]. We have been working in the field of solid-state chirality for the last 30 years and recently developed a novel universal chiroptical spectrophotometer, UCS J-800KCM, for the measurement of true CD and circular birefringence (CB) spectra in the solid state [17]. 

In microcrystallines where crystals are randomly oriented, the macroscopic anisotropies can be cancelled, and thus in principle their CD can provide reasonable solid-state spectra. However, artifacts of different types may exist in such cases (see V.B). Films usually exhibit large macroscopic anisotropies. 

As the CB signal is usually 102 to 103 times smaller than LB and LB signals, and thus is buried under large macroscopic anisotropy signals, it is difficult to detect. However, we have found that the following set of measurements makes this possible. At its (+) extremum position during sample rotation, a signal 2co becomes... 

As optic and crystal axes are common in the case of uniaxial crystals, singlecrystal CD experiments are generally limited to these kinds of crystals. For cases where moderate macroscopic anisotropies exist, these can be studied on the UCS J-800-KCM as is described in V.A.3. 

Most organic compounds crystallize in the monoclinic or orthorhombic crysta systems, which contain substantial macroscopic anisotropies, and thus the single crystal CD technique cannot be applied, although our method of measuremei may be useful if the macroscopic anisotropies are not very large. An alternative... 

This chapter has described the principles and application of solid-state CD. Solid-state CD can offer valuable information about the supramolecular nature of compounds and solid-state reactions, but it must b kept in mind that these suffer from substantial macroscopic anisotropies. We have shown that CD measurements in the solid state must be accompanied by measurement of corresponding LB and LD to evaluate the magnitudes of macroscopic anisotropies. We have developed an instrument and methods for obtaining the true CDs and CBs even for samples with modest macroscopic anisotropies. The technique is applied to solid-state photoreactions as well. 

The growth of fiberlike aggregates of optically active molecules [48], like those encountered with some optically active organogelators, is known to be accompanied by important CD modifications. CD data, free of linear dichroic effects, so as to avoid the contribution of macroscopic anisotropy, reveal chirality effects typical of helical structures. The electronic absorption spectra of the gelators indicate the wavelengths at which the electronic transitions occur and where dichroic effects have to be sought (unless selective reflection components... 

The dependence of macroscopic anisotropy on pore diameter and Ni content is analyzed. Approach to magnetic saturation curves and exchange constant of nanocrystalline Fei.xNix(P) columns deposited into porous polycarbonate matrix are investigated. 

Most organic compounds crystallize in the monoclinic or orthorhombic crystal systems, which contain substantial macroscopic anisotropies, and thus the singlecrystal CD technique cannot be applied, although our method of measurement may be useful if the macroscopic anisotropies are not very large. An alternative way to obtain crystalline-specific information is to examine the microcrystalline state. Measurements can be usually made either in nujol mull or KBr disc form, where microcrystals are dispersed randomly either in nujol or in a KBr microcrystalline matrix. The method was developed and applied to inorganic complexes by Mason [34], Bosnich [35], and Kuroda [9,10], and since then its application to metal complexes has been carried out sporadically [36,37]. Recently, the technique has become popular in the field of organic chemistry as well, probably stimulated by our work [38]. An alternative technique recently developed by us is diffuse reflectance CD (DRCD) which will be briefly described in V.B.2. 

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