A solid-state dedicated circular dichroism (CD) spectrophotometer (J-800KCM) was designed and constructed. As a CD spectrophotometer is a polarization–modulation instrument, CD spectra are necessarily accompanied by artifacts due to macroscopic anisotropies such as linear birefringence (LB) and linear dichroism (LD) which are unique to the solid state. A photomultiplier with the least polarization characteristics and a photoelastic modulator (PEM) with the least residual static birefringence were selected for the new instrument, which was based on the electrical and optical systems of a commercially available spectrophotometer. A phased-locked loop circuit was introduced to a PEM driver, and a sample rotation holder, a stage controller, and an analyzer were installed. We have designed and built a special solid-state sample holder to enable the cancellation of artifact CD, and a lens unit for smaller samples. A set of procedures for obtaining true CD has been devised based on the Mueller matrix method, and a program was written to facilitate the otherwise cumbersome measurements. Using the J-800KCM instrument and this approach, we could obtain nearly zero CD for achiral polyvinyl alcohol (PVA) film dyed with achiral Congo Red by cancelling the substantial apparent CD arising from large macroscopic anisotropies. Experiments on chiral single crystals of α-Ni(H2O)6⋅SO4, one with larger and one with smaller LB, have established that, by our method, we could also eliminate parasitic artifacts to obtain the true CD.
The concentration and the temperature dependencies of H1 and C13 chemical shifts in NMR of aqueous acetone mixtures were studied, together with the concentration dependence of the frequency of the C–H stretching vibration of acetone in IR spectra. H1 and C13 chemical shifts were measured at 1 °C, 23 °C, and 48 °C by the external double reference method using a capillary with a blown-out sphere at the bottom for tetramethylsilane as the external reference substance. By this method, it is possible to determine the volume magnetic susceptibility of a sample solution at each temperature, for which the observed chemical shifts may be corrected exactly. Thus, we revealed the detailed electronic polarization in acetone as well as water as functions of concentration and temperature. On diluting acetone with water, the chemical shift of water protons, δH2O, is 3 ppm at the mole fraction of water Xw=0,05 and increases to the value for pure water, ca. 5 ppm, at Xw=0.96, with increasing Xw. In the region of Xw>0.96, δH2O is slightly larger than the value, indicating the presence of more polarized water species than pure water. The chemical shifts of C–H proton, δCH_3, and C–H carbon, δC_H3, also increase slightly with increasing Xw up to Xw=0.96. The frequency for the C–H vibration of acetone, νC–H, increases from the value for pure acetone, 3005 cm−1, to 3013 cm−1 at Xw=0.96, while it decreases sharply with further increase in Xw. These results of IR and NMR measurements show that the hydration of acetone accompanies electronic redistribution in the C–H bonds in cooperated with the change in the polarization of the surrounding water molecules, and that two different types of hydration of acetone are predominant in different concentration regions, Xw<0.96 and Xw>0.96. In the region of Xw<0.96, the results can be explained satisfactorily if we consider that a part of the electron about the C–H proton is pushed out into the C–H bond due to a repulsive interaction between the C–H hydrogen and water oxygen. In the region of Xw>0.96, we can interpret the results well by considering that the pushing by the water oxygen becomes strong enough to induce the polarization of the C–H bonds compared to the pushing at Xw⩽0.96. Since the polarization of the C–H bond was found to increase with decreasing temperature, the repulsive interaction seems to have the property of hydrogen bonding and to be denoted as C–H⋯OH2(⋯OH2)n, where OH2(⋯OH2)n expresses water molecules hydrogen-bonded cooperatively and responsible for the more polarized water than pure water. The ratio of water to acetone seems to be a predominant factor to cause the transition of the hydration state from the repulsive interaction to hydrophobic hydration of acetone.
91which seems to account for all the experimental observations, is summarized in Figure 12. It should, however, be realized that the scheme presented in Figure 12 is still largely phenomenological. Structure-photoreactivity correlations of the excited states are yet to be unraveled, and the role of the quartet manifold, if any, in the photochemical pathways of the higher excited states is completely unknown.Finally, the ability to monitor the absorption spectrum of D, could allow some significant observations on the energy levels of the radicals. In principle, due to parity selection rules, transitions which are forbidden for the ground state have to become symmetry-allowed in the first excited state. In the radicals for which (26) Sitzmann, E. V.; Wang, Y . ; Eisenthal, K. B. J. Phys. Chem. 1983, 87, 2283.the absorption spectra of D, were monitored, the lowest observable transition corresponds to excitation to a level which lies 5.3 eV above the ground state. Since neither experimental nor calculated information is available on the absorption spectrum of the ground state in this region, comparison at this stage between the absorption spectra of the two states is premature. , 5785-66-0; 1-( 10,l l-dihydro-5H-dibenzo[a,d]cyclohepten-5-ylidene)ethyl amidogen, 93474-28-3. 93564-50-2; Ph2CHC1, 90-99-3; Ph2CHOH, 91-01-0; PhlC(CH,)OH, Ph,CH, 4471-17-4; Ph,C., Acknowledgment. The dedicated operation of the linac by D. c-Pr)OH Abstract:The Mueller matrix approach is used to analyze the apparent circular dichroism (CD) spectra observed in cholesteric liquid crystals (CLC) which can be regarded as being built up of a large number of thin birefringent layers arranged helically. Attention is called to the artifacts resulting from coupling of CLC with nonideal optics and electronics of the CD spectropolarimeter. On the basis of the results obtained, it is concluded that "liquid crystal induced CD" cannot be related to true optical activity without careful consideration of the experiment.
An interesting paper on circular dichroism (CD) and linear dichroism ( L D ) microscopy by Livolant, Mickols, and Maestre [ (1988) Biopolymers 27, 1761-19691 has attracted our attention. They reported that they have succeeded in deconvoluting mathematically true CD and LD from signals obtained for their samples of strong and incompatible anisotropies. If this is true, then their method of mathematical deconvolution will open new possibilities in polarization-modulation spectroscopy. However, after carefully reading through their paper, we found the following obscure points in their results: ( 1 ) the hardware of their spectrometer; ( 2 ) the sensitivity matrix; ( 3 ) the method for separation of CD and LD by using two standards; ( 4 ) average CD curves; and ( 5 ) circular intensity differential scattering (CIDS) and linear intensity differential scattering (LIDS) in the wavelength range from 300 to 330 nm. Making use of the Mueller matrix analysis, we present our comments on the above subjects together with our data. In conclusion, we maintain that it is difficult to accept their claim of having a general and useful approach to separating true CD and LD from signals observed in macroscopically anisotropic samples.
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