Absorption spectra mixtures

As yet no quinazoline has been discovered which has any appreciable amount (say, 1%) of hydrated species in the neutral molecule,but several quinazolines were shown to contain a mixture of anhydrous and hydrated species in the cations. Anhydrous neutral molecules and anhydrous cations have an ultraviolet absorption spectrum of the general type C (Fig. 3) and hydrated cations, the type... 

The influence of an impurity (Y) on the absorption spectrum of a substance (X) can often be eliminated by considering derivative curves as shown in Fig. 17.15 the second-order plot of the mixture is identical with that of pure X. When the interference spectrum can be described by an nth-order polynomial, the interference is eliminated in the (n+ 1) derivative. 

Fig. 6.2.4 Change in the absorption spectrum of pholasin (14.5 p,M) caused by the luminescence reaction catalyzed by Pholas luciferase (1.1 p.M). The curve shown is the differential spectrum between a cell containing the mixture of pholasin and Pholas luciferase (0.9 ml in the sample light path) and two cells containing separate solutions of pholasin and the luciferase at the same concentrations (in the reference light path), all in 0.1 M Tris-HCl buffer, pH 8.5, containing 0.5 M NaCl. Four additions of ascorbate (3 iM) were made to the sample mixture to accelerate the reaction. The spectrum was recorded after 120 min with a correction for the base line. From Henry and Monny, 1977, with permission from the American Chemical Society.

Photoirradiation of the modified electrode with nanoclusters of Cj qN alone or the mixture of C oN and MePH afforded anodic photocurrents. The photocurrent action spectrum was in fair agreement with the absorption spectrum of the THF-H2O (2 1) mixed solution containing nanoclusters of the mixture of CfioN and MePH or C oN alone. These results strongly indicate that the photocurrents can be ascribed to photoexcitation of the nanoclusters of C qN. ... 

A method suitable for quantification of the functional class of bis(ethanol)amine antistatics, which lack UV chromophores, consists of reaction with methyl orange [53]. Atmer 163 (alkyl-diethanol amine) has been determined as a yellow complex at 415 nm after interaction with a bromophenol/cresole mixture [64]. Hilton [65] coupled extracted phenolic antioxidants with diazotised p-nitroaniline in strongly acidic medium and carried out identification on the basis of the visible absorption spectrum in alkaline solution. The antioxidant Nonox Cl in... 

The overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

Figure 6.14. The n-n region of the ultraviolet absorption spectrum of (a) compound (7), n = 1 (solid line) and (b) equal molar mixture of acetophenone and trans-/3-methylstyrene.<89) Reprinted by permission of the American Chemical Society.

The analyses of periodate and iodate mixtures is readily accomplished colorimetrically at 222.5-230 m u (in the ultraviolet region).263-273 The method utilizes very low concentrations of reactants, but is useless when the reactants or products have structures which absorb light in this region of the ultraviolet absorption spectrum. Also, certain undesirable side-reactions are catalyzed by ultraviolet light.273... 

Spectral experiments on incorporation of sodium alkylsulfonates as guest molecules into the host bilayer assembly strongly suggest formation of the void space expected in the structural model I. Cast films of 1 1 mixture of CsAzoCioN+Br and sodium n-butylsulfonate (kmax = 303 nm) or sodium propylsulfonate (kmax = 302 nm) showed a similar absorption spectrum of pure C5AzoCjoN+Br cast film (kmax = 301 nm). Longer guest molecules sodium n-pentylsulfonate (kmax = 326 nm), n-hexylsulfonate (kmax = 329 nm), and n-heptylsulfonate... 

The structure of HRP-I has been identified as an Fe(IV) porphyrin -ir-cation radical by a variety of spectroscopic methods (71-74). The oxidized forms of HRP present differences in their visible absorption spectra (75-77). These distinct spectral characteristics of HRP have made this a very useful redox protein for studying one-electron transfers in alkaloid reactions. An example is illustrated in Fig. 2 where the one-electron oxidation of vindoline is followed by observing the oxidation of native HRP (curve A) with equimolar H202 to HRP-compound I (curve B). Addition of vindoline to the reaction mixture yields the absorption spectrum of HRP-compound II (curve C) (78). This methodology can yield useful information on the stoichiometry and kinetics of electron transfer from an alkaloid substrate to HRP. Several excellent reviews on the properties, mechanism, and oxidation states of peroxidases have been published (79-81). 

In the case of benzotriazole compounds which display both absorption bands, the observed spectrum consists of the superposition of the individual spectra that correspond to the two distinct ground-state species. The absorption spectrum of TIN in methanol/dimethylsulfoxide (DMSO) solvent mixtures varies with the composition of the solvent (Figure 5) in a manner which suggests that the proportion of planar and non-planar forms of TIN is solvent dependent. 

The absorption spectrum of each resolved component of TIN in the methanol/DMSO mixtures can be calculated by virtue of the isosbestic point at 290 nm because at this wavelength the extinction coefficients of the two forms are equal (Figure 6). At 300 nm the extinction coefficient of the planar and non-planar forms is 1.5 x 104 M 1 cm 1 and 1.0 x 104 M 1 cm 1 respectively. These values are very similar to those calculated for TINS in acetonitrile/water mixtures (1.8 x 104 M 1 cm 1 and 1.0 x 104 M 1 cm 1 for the planar and non-planar forms respectively) (12). 

In methanol/DMSO solvent mixtures the fluorescence spectrum of TIN (A.max = 400 nm) displays a normal Stokes shift indicating that this emission arises from a non proton-transferred, excited state of TIN. The fluorescence excitation spectrum for this emission coincides with the absorption spectrum of the resolved non-planar species suggesting that this conformer is the ground-state precursor responsible for the observed emission. As the amount of DMSO in the mixture increases the fluorescence maximum undergoes a bathochromic shift from 415 nm in pure methanol to 440 nm in pure DMSO. 

A problem with this method is the fact that mineral oil is a mixture of covalent substances (high-molecular-weight hydrocarbons) and its characteristic absorption spectrum will be found superimposed in the spectrum of the solid analyte, as with the solvents used for liquid solutions discussed previously. However, the spectrum is a simple one (Figure 8.24) and often does not cause a significant problem, especially if the solid is not a hydrocarbon. 

FIGURE 13.9 The HPLC diode array UV absorbance detector. When a mixture component elutes from the column, not only the chromatography peak but the entire UV absorption spectrum for that component can be recorded. 

Fig. B9.3.1. A absorption spectrum of the B excitation polarization spectra of the model multi-chromophoric cyclodextrin CD7(6) and compound NAEt and CD7(6). Solvent mixture variations in the emission maximum as a func- (9 1 v/v) of propylene glycol and 1,4-dioxane at tion of the excitation wavelength (broken line). 200 K (adapted from Berberan-Santos et al.a)).

The Na+ sensor M-9 has a structure analogous to that of compound E-4, but instead of two identical pyrene fluorophores, it contains two different fluorophores with a pyrene group and an anthroyloxy group. Resonance energy transfer (see Chapter 9) from the former to the latter is then possible because of the spectral overlap between the fluorescence spectrum of the pyrene moiety and the absorption spectrum of the anthroyloxy moiety. Upon addition of Na+ to a solution of M-9 in a mixture of MeOH and THF (15 1 v/v), the fluorescence of the anthroyloxy group increases significantly compared with that of the pyrene group, which permits a ratiometric measurement. 

Furthermore, the preparation and reactions of 2-methoxythiophene were studied by Sice (70). This compound was obtained by a copper catalysed Williamson synthesis. It was also found that iodothiophene reacted readily with sodium alkoxides, whereas bromothiophene reacted slowly and chlorothiophene did not react at all. Sodium iodide accelerated the reaction of bromothiophene. The ortho, para orienting alkoxy group on carbon atom 2 increased the directive influence of the sulphur atom to the 5 position but competed with it to induce some attack on the 3 position by electrophilic reagents (nitration, acylation). The acylation of 2-methoxythiophene with stannic chloride at low temperatures furnished a mixture of two isomers. The 5-methoxy-2-acetothienone was obtained in higher yield and was identified by its ultraviolet absorption spectrum. 

One of the first applications of this chopped-beam irradiation technitriplet spectra was reported by Labhart From a knowledge of the intensity of the irradiation light, he determined the quantum yield of triplet generation to be 0.55 0.11 for outgassed solutions of 1,2-benzanthrazene in hexane at room temperature. Hunziker 32) has applied this method to the study of the gas-phase absorption spectrum of triplet naphthalene. A gas mixture of 500 torr Na, 0.3 mtorr Hg, and about 10 mtorr naphthalene was irradiated by a modulated low-pressure mercury lamp. The mercury vapor in the cell efficiently absorbed the line spectrum of the lamp and acted as a photosensitizer. The triplet state of naphthalene was formed directly through collisional deactivation of the excited mercury atoms. 

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