Ultraviolet absorption coefficients

The product obtained from this type of decarboxylation is reported to contain only about 5% of /ra s-stilbene.5 A sample made according to the above directions can be treated with bromine in carbon tetrachloride at room temperature in the dark to give an 80-85% yield of the d/-dibromide which arises from trans addition to cw-stilbene. The meso-dibromide, which is very soluble and easily separated, is obtained only to the extent of 10% or less. Part of this latter product may arise from the action of bromine atoms on cw-stilbene rather than from trans addition to tfnms-stilbene. The cu-stilbene prepared by this method is readily and completely soluble in cold absolute ethanol. It freezes solid at about —5°. Its ultraviolet absorption coefficient (8) is 1.10 X 104 at 274 mil and 8.7 X 103 at 294 mp, quite different from h-aws-stilbene. 

The vacuum ultraviolet absorption coefficients have been measured by Beddard et al. (90) and are shown in Fig. VII 216. 

Laufer, A.M., and J.R. McNesby, Deuterium isotope effect in vacuum ultraviolet absorption coefficients of water and methane. Canad J Chem f3, 3487, 1965. 

Note that in liquid phase chromatography there are no detectors that are both sensitive and universal, that is, which respond linearly to solute concentration regardless of its chemical nature. In fact, the refractometer detects all solutes but it is not very sensitive its response depends evidently on the difference in refractive indices between solvent and solute whereas absorption and UV fluorescence methods respond only to aromatics, an advantage in numerous applications. Unfortunately, their coefficient of response (in ultraviolet, absorptivity is the term used) is highly variable among individual components. 

The ultraviolet absorption spectrum of thiazole was first determined in 1955 in ethanolic solution by Leandri et al. (172), then in 1957 by Sheinker et al. (173), and in 1967 by Coltbourne et al. (174). Albert in 1957 gave the spectrum in aqueous solution at pH 5 and in acidic solution (NHCl) (175). Nonhydroxylic solvents were employed (176, 177), and the vapor-phase spectrum was also determined (123). The results summarized in Table 1-15 are homogeneous except for the first data of Leandri (172). Both bands A and B have a red shift of about 3 nm when thiazole is dissolved in hydrocarbon solvents. This red shift of band A increases when the solvent is hydroxylic and, in the case of water, especially when the solution becomes acidic and the extinction coefficient increases simultaneously. 

We have limited our investigations to the action of gamma-rays and fast neutrons on aromatic, alicyclic, aliphatic, and ionic compounds. The absorption coefficients for these types of radiation have an order of magnitude of lO /cm which is particularly adequate. Shallow penetrating radiations would only alter the superficial layers and would obviously not be able to affect the intensity of the quadrupole line substantially. This is for instance the case for ultraviolet light which has been shown to be unable to produce any effect on the resonance line of iodoform (CHI3 3Sg)... 

In the preceding discussion of Beer s Law, it was argued that x-ray absorption is a simpler process than the absorption of ultraviolet, visible, and infrared wavelengths. This greater simplicity becomes particularly obvious when x-ray absorption coefficients are examined. 

My interest at that time revolved around evaluating optical rotary dispersion data [12]. The paired values of optical rotation vs. wavelength were used to fit a function called the Drude equation (later modified to the Moffitt equation for William Moffitt [Harvard University] who developed the theory) [13]. The coefficients of the evaluated equation were shown to be related to a significant ultraviolet absorption band of a protein and to the amount of alpha-helix conformation existing in the solution of it. 

Details of the ultraviolet absorption maxima for simple silenes, silaaro-matics, and for some relatively stable silenes are known and have been summarized.6 The simplest silenes absorb in the region 245-260 nm, with unknown extinction coefficients but as the substituents become increasingly complex, the Xmax values of the silenes increase until, with the silene (Me3Si)2Si=C(OSiMe3)Ad, the absorption occurs at 340 nm5 with an extinction coefficient of about 7400, consistent with a tt-it transition. A few further studies of interest are summarized below. 

Table 14. Ultraviolet Absorption Maxima and Extinction Coefficients of Model...

It is often difficult to quantitate one particular amino acid in the presence of others because of chemical similarities. Interference from substances other than amino acids is also a problem in many reputedly specific methods. Ultraviolet spectroscopy is of little value in the detection of aromatic amino acids because they have similar absorbance maxima and considerably different molar absorption coefficients. 

A number of investigators have studied the effect of ozone on the ultraviolet absorption spectra of proteins and amino acids. A decrease in the absorption of 280-nm light in a number of proteins was originally reported ly Giese et aV to be a consequence of ozone exposure they suggested that this was due to an interaction of ozone with the ring structures of tyrosine and tryptophan. Exposure of a solution of tryptophan to ozone resulted in a decrease in 280-nm absorption, whereas the extinction coefficient of tyrosine increased. Similar results with tyrosine were reported by Scheel et who also noted alterations in the ultraviolet spectra of egg albumen, perhaps representing denaturation by ozone. 

In one study of the effects of additives,9 it was found that on electrochemical oxidation of rubrene, emission was seen in dimethylforma-mide, but not in acetonitrile. When water, n-butylamine, triethylamine, or dimethylformamide was added to the rubrene solution in acetonitrile, emission could be detected on simply generating the rubrene cation.9 This seems to imply that this emission involves some donor or donor function present in all but the uncontaminated acetonitrile system. The solvent is not the only source of impurity. Rubrene, which has been most extensively employed for these emission studies, is usually found in an impure condition. Because of its relative insolubility and its tendency to undergo reaction when subjected to certain purification procedures, and because the impurities are electroinactive and have relatively weak ultraviolet absorptions, their presence has apparently been overlooked, They became evident, however, when quantitative spectroscopic work was attempted.70 It was found, for example, that the molar extinction coefficient of rubrene in benzene at 528 mjj. rose from 11,344 in an apparently pure commercial sample to 11,980 (> 5% increase) after repeated further recrystallizations. In addition, weak absorption bands at 287 and 367 m, previously present in rubrene spectra, disappeared. 

In the atmosphere, light absorption in the ultraviolet region is predominantly due to 03 and this is predominantly in the stratosphere (Figs. 3.12 and 3.13). Since the absorption coefficients (cr) of 03 are reasonably well established, a variant of the Beer-Lambert law can be applied to determine how much of the incident light is absorbed by 03 ... 

The absorption spectrum data were obtained from measurements made with a Beckman Ultraviolet Spectrophotometer. The formulas employed in making the calculations use the term a, specific absorption coefficient. 

In the vacuum ultraviolet absorption bands in the region 1280 to 1600 A correspond to the fourth positive system A1 n-X L+. The absorption cross sections of this system are given in Fig. V-7. Since the widths of the CO rotational lines are much smaller than the instrumental resolution ( 10 cm" 1), it is not possible to obtain the absorption cross section of each rotational line [see Section 1-8 for details]. Thus, the cross sections shown in Fig. V-7 are much less than the true cross sections. An estimate of the integrated absorption coefficient of the (0,0) band is 1.7 x 104cm-latm-1 (899). Various electronic states and transitions are given in Fig. V-8. 

Fig. VI —I. Absorption coefficients of water in the vacuum ultraviolet region. k is given in units of atm-.1 cm"base e, 0°C. From Watanabe and Zelikotf (1016), reprinted by permission. Copyright 1953 by the American Institute of Physics.

Fig. VI-5, (a) Absorption coefficients or OCS in the region 2100 to 2500 A. k is given in units of atm 1 cm" 0°C, base e. From Ferro and Reuben (347), reprinted by permission of The Chemical Society. (b) Absorption coefficients of OCS in the vacuum ultraviolet, k is given in units of atm 1 cm 1,0°C, base e. From Matsunaga and Watanabc (670), reprinted by permission. Copyright 1967 by the American Institute of Physics.

Fig. VI-10. Absorption coefficients of NOC1 in the visible and ultraviolet region, < is given in units of 1 mol-1 cm-1, base 10, room temperature. [Goodeve and Katz (411), revised by Ballash and Armstrong (58)], reprinted by permission of Pergamon Press.

Fig. Vl-18. The absorption coefficients of Cl 2() in the visible and near ultraviolet regions, k is given in units of mm 1 cm"1, base 10, room temperature. From Goodeve and Wallace (409), reprinted by permission of the Chemical Society. 

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