Spectrum, unpolarized absorption

Fig. 15. Infrared absorption spectra (unpolarized radiation) of epidermin films showing the progress of heat denaturation. Films were heated in hot water for various times A, not heated B, 68°C., 2 minutes C, 80°C., 2 minutes D, 100°C., 2 minutes E, 100°C., 30 minutes F, 100°C., 270 minute. Curves G is spectrum of epidermin highly stretched in saturated ammonium sulfate.

Single crystal absorption spectra (unpolarized) of [Fe(ptz)6l(BF4)2 before bleaching (b.b.) and after bleaching (a.b.) at 8K for 2 min with white light. (Reproduced from Ref. 11 with permission of the American Chemical Society.)... 

The absorption spectrum of TIN in methylcyclo-hexane/isopentane at 150 K is represented by curve I, Fig. k. Curve III shows the absorption spectrum of MT in hexane at 296 K. In unpolar solvents the intra-molecular hydrogen bond of TIN is still intact (curve... 

As an example of the VCD ability to discriminate absolute configurations, the experimental spectrum of 4-fluorophenyl-3-hydroxymethyl-l-methylpiperidine and the calculated spectra of the (3R,4R)- and (3R,4S)-isomer are presented in Fig. 8.5 [109]. An almost perfect band-to-band correspondence is evident between experimental and calculated spectra of the (3R,4S)-isomer, while the calculated spectrum of the low energy conformer of the (3R,4R)-isomer provides unrealistic VCD pattern with a poor or even no coincidence with the experiment. In the calculated spectrum of the (3R,4S)-isomer, four low-energy conformers were included in the spectral envelope. For comparison, the corresponding calculated and experimental IR absorption spectra are shown. It is evident that the variance with respect to configuration is much more pronounced in VCD spectra While the calculated spectrum for the (3R,4R)-isomer significantly differs from experimental VCD, the unpolarized absorption spectrum of the (3R,4R)-isomer exhibits some common... 

Figure 6-19. Comparison of the b- and c-polarized single crystal absorption spectra at 4.2 K with the unpolarized absorption spectrum of a polycrystalline thin film at 1.6 K. Note that the origin is also observed in the film.

Figure 1 Unpolarized absorption spectrum and VCD spectrum of camphor in CC14 solution concentration 0.6 M pathlength 151 pm resolution 4 cm-1 VCD scan time 1 hr.

MCD, in combination with the unpolarized absorption spectrum, is a powerful tool for characterizing a chromophore. The diversity of possible MCD spectral shapes provides more qualitative information than is available from absorption alone. The ratio of the magnitudes of the MCD and absorption at defined wavelengths provides an even more informative quantitative criterion. Because both MCD and absorption are proportional to the concentration of the absorbing species, their ratio is independent of concentration and, hence, is an intrinsic property of the material that is. 

Circular dichroism (CD) can be observed in the vibrational transitions of chiral molecules vibrational circular dichroism (VCD). An example of a VCD spectrum is shown in Figure 1, together with the corresponding unpolarized absorption spectrum. The sample is a 0.6 M solution of (lR,4jR)-(-i-)-cam-phor in CCI4. Here, we discuss the theoretical analysis of VCD spectra. The current state-of-the-art is illustrated in Figure 1, where VCD and absorption spectra of camphor, predicted within the harmonic approximation (HA) using ab initio density functional theory (DFT), are shown. 

Consider the integrated molar absorptivity for an S S, absorption, Je(n)dn, obtained from the appropriate segment of the uv-visible absorption spectrum (using unpolarized light and isotropic samples) and plotted as the molar decadic extinction coefficient (absorptivity), e(n) in dm moT cm , as a function of n in wavenumbers. An approximate value of the radiative rate constant for the corresponding S Sq transition may be obtained directly from ... 

ECD is basically a spectrophotometric measurement and as such is subjected to the same precautions as those applied for the collection of an ordinary UV-Vis unpolarized absorption spectrum. First, the total sample absorption must not exceed 1.5 absorbance units, with 0.8 being a recommended value. This may be achieved by adjusting either the path length (O.l-lOOmm) of the sample cuvette... 

II). From this it must be concluded that the long-wavelength absorption of curve I is due to the intra-molecular hydrogen bond of TIN in unpolar solvents as the intensity of this band is reduced in polar solvents (curve II) and disappears completely in the spectrum of MT (without intramolecular hydrogen bond), curve III. Curve IVa represents the fluorescence and IVb the phosphorescence emission of both TIN and MT in... 

Figure 4.11 Polarized absorption spectra of vanadium-bearing zoisite (modified from Faye and Nickel, 1969). The zoisite from Tanzania contains 0.012 V3+ ions per formula unit, (a) Spectrum (A) y-spectrum of unheated zoisite Spectrum (B) y-spectrum of zoisite (now tanzanite) after heating to 600 °C Spectrum (C) unpolarized spectrum of tanzanite. (b) Spectrum (A) (1-spectrum of unheated zoizite Spectrum (B) p-spectrum of tanzanite after heating to 600 °C. [Optic orientation 0C = b P = c y = a.]...

Fig. 11 The effect of polarized photolysis on the near-UV absorption of Cr(CO)5N2 in pure N2 (a) after 20 min unpolarized photolysis X = 314 nm and photolysis with X = 367 nm, (b) with vertically polarized photolysis 20 min, (c) with horizontally polarized photolysis 30 min, vertically polarized photolysis 15 min solid line) spectrum polarized vertically, broken line) spectrum polarized horizontally. Adapted from [24]...

Figure 14 shows the unpolarized absorption spectra of films of polythiophene [124] and its hexamer a-sexithienyl [125]. The hexamer solution spectrum is at almost the same energy. As in Fig. 12a, the oligomer shows structure on its rising edge, and the polymer spectrum is structureless. But in Fig. 14 the absorption thresholds differ by = 0.2 eV only one would then say that conjugation extends in PT over only about six monomers. One could show similar data for many other CPs. 

Figure 7.34. ATR unpolarized difference spectra of galena-0.01 M borate, pH 9.18, interface during positive scan. Smooth background at low frequencies (dashed lines) arises from free-hole absorption. Reference for spectrum marked by -0.40 V is spectrum measured at -0.50 V after reduction. Each of the other curves is spectrum measured at indicated potential after subtracting spectrum measured at preceding potential. Horizontal lines indicate zero absorbance. Reprinted, by permission, from I. V. Chernyshova, J. Rhys. Chem. B 105, 8178 (2001), p. 8180, Fig. 2. Copyright 2001 American Chemical Society.

As in absorption spectroscopy, instrumental polarization effects can yield unwanted artefacts and therefore it is appropriate to introduce depolarizers into the optical path before and after the sample if the aim is to monitor the true unpolarized fluorescence spectrum. Unfortunately, this will reduce the light levels com-mensurately and is therefore frequently not pursued. Other methods, involving the use of polarizers set at magic angles to minimize some unwanted polarisation effects have been devised, but are even less frequently employed. 

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