Chiro-optics of achiral compounds
Chirality is not a necessary condition for the presence of chiro-optical properties in some oriented media. The non-enantiomorphous point symmetries D2d, S4, C2i, and CS also admit optical activity. The optical rotation tensors of some achiral crystals were determined and interpreted with respect to...
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| Ngôn ngữ: | en_US |
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University of Washington
2007
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| Truy cập trực tuyến: | http://ir.vnulib.edu.vn/handle/123456789/1546 |
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| Tóm tắt: | Chirality is not a necessary condition for the presence of chiro-optical properties in some oriented media. The non-enantiomorphous point symmetries D2d, S4, C2i, and CS also admit optical activity. The optical rotation tensors of some achiral crystals were determined and interpreted with respect to molecular and electronic structure. The isomorphous crystals (D2d) of the Group 14 tetraphenyls (C(C6H5)4, Si(C6H5)4, Ge(C6H5)4, Sn(C6H5)4, Pb(C6H5)4) were compared and interpreted according to a classical dipole-dipole interaction theory for crystals. Pentaerythritol (C(CH2OH)4) crystals (S4) were measured to facilitate comparison with computations for a single molecule according to semi-classical quantum theory. The computational effort was extended to water (C2v), the simplest common molecule that supports natural optical rotation, for which the generation of optical rotation according to a modified Rosenfeld equation was interpreted in terms of classical electrodynamics. Although the sign of optical rotation or circular dichroism in achiral materials does not distinguish absolute configuration as for chiral point symmetries (0, T, Dn, CO, it does establish the absolute direction of the axes.
As an alternative to measuring optical rotation and circular dichroism in molecular crystals wherein intermolecular contributions can be significant, dyed crystals were investigated. The guest molecules are then not only well separated from one another, but are well-oriented within individual sectors of the host crystal. These dyed crystals gave strong chiro-optical signals that were consistent with the host symmetry, but were inconsistent with an intrinsic effect when the wave vector was reversed. The observed optical effects were given the phenomenological names anomalous azimuthal rotation when measured by incident linearly polarized light and anomalous circular extinction when measured by differential circularly polarized light. The new experimental effects are sensitive to the sense of inclination (clockwise or counterclockwise) of the embedded dipoles from the eigenmodes of the host. The signals therefore provide information about the absolute orientation of the adsorbate, a supramolecular stereochemical quality that cannot be distinguished by polarized absorption measurements alone. |
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