Spectrophotometric studies

Walmsley, F. Aggregation in Dyes A Spectrophotometric Study, /. Chem. Educ. 1992, 69, 583. 

Still lower pHs to give a pale-yellow solution. As a result of spectrophotometric studies there is general agreement that the predominant species in the initial colourless solution is the tetrahedral V04 ion and, in the final pale-yellow solution, the angular VOz ion. In the intervening orange to red solutions a complicated series of hydrolysis-polymerization reactions occur, which have direct counterparts in the chemistries of Mo and W and to a lesser extent Nb, Ta and Cr. The polymerized species involved are collectively known as isopolymetallates or isopolyanions. The determination of the equilibria involved in their formation, as well as their stoichiometries and structures, has been a confused and disputed area, some aspects of which are by no means settled even now. That this is so is perfectly understandable because ... 

Quinoxaline-2,3-dithione (2,3-dimercaptoquinoxaline) (90) forms chelates with several transition elements and is used as a colorimetric agent for the detection and quantitative estimation of nickeT and also for the quantitative estimation of palladium. " Nickel gives a pink coloration with (90) in ammoniacal solution, and palladium an orange-red color in AA-dimethylformamide solution containing a little hydrochloric acid. Spectrophotometric studies of the chelate compounds of (90) with Ni(II), Co(II), and Pd(II) in alkaline solu-... 

Discussion. The dissociation of an acid-base indicator is well suited to spectrophotometric study the procedure involved will be illustrated by the determination of the acid dissociation constant of methyl red (MR). The acidic (HMR) and basic (MR-) forms of methyl red are shown below. 

Among the recently (as of 2003) described reactions of such quaternary salts, reduction to dihydroquinoxalines has been exemplified toward the end of Section 2.2.4 treatment of the diquaternary compound, 1,4,6,7-tetramethylquinox-alinediium bis(tetrafluoroborate) (215), with sodium iodide gave a well-characterized product that appears in X-ray studies to have the cation radical structure (216) ""° and a fundamental spectrophotometric study on the addition of hydroxyl and methoxyl ions to the 1-methylquinoxalinium cation has been reported. ... 

We passed then to a particular olefin, adamantylideneadamantane, whose reaction with Br2 had been shown to stop at the stage of bromonium ion formation because of steric hindrance to backside nucleophilic attack. An UV-Vis spectrophotometric study (ref. 10) has shown that the complicated equilibrium reported in Scheme 4 is immediately established on mixing the olefin and Br2 in DCE. Equilibrium (1) could be isolated by working at low Br2 and ten to hundred fold higher olefin concentrations. A Scott plot followed by a NLLSQ refinement of the data gave a Kf = 2.89 x 10 (4.0) M-l. It is worth noting that conductimetric measurements showed the non-ionic nature of the 1 1 adduct, consistent with a CTC intermediate, but not with a bromonium-bromide species. 

Quantitative information about the equilibrium between olefin and Br2 on the one hand and CTC s and bromonium ion species on the other (Scheme 4) has been obtained by the already mentioned UV-Vis spectrophotometric study of the adamantylideneadamantane Br2 system (ref. 10). The spectrophotometric UV-Vis data of a large set of solutions of different reagents concentrations have been used to dissect, using a program based on NLLSQ fitting procedures, the complex spectra in those of the single species present at the equilibrium, as shown in Figure 5. 

Spectrophotometric studies revealed that these hydrocarbons reversibly dissociate in DMSO and in 5 95 3-methylsulfolane-sulfolane (Miyabo et al.. 

Eehrmann R, Bjerrum NJ, Poulsen EW (1978) Lower oxidation states of sulfur. 1. Spectrophotometric study of the sulfur-chlorine system in molten sodium chloride-aluminum chloride (37 63 mol%) at 150 °C. Inorg Chem 17 1195-1200... 

From spectrophotometric studies on 0.5 M HCl solutions of plutonium containing Pu(V), Connick attempted to investigate the disproportionation reaction of this species... 

According to spectrophotometric studies of Espenson , the oxidation of vana-dium(IV) by chromic acid follows the rate law... 

Schwing-Weill, M.J. and Wechsler, S., Spectrophotometric study of carminic acid in solution application to its determination, Analysis, 14, 290, 1986. 

Douglas, W. H. (1978b). The metal oxide/eugenol cements. II. A diffuse reflectance spectrophotometric study of the setting of zinc oxide and magnesium oxide cements. Journal of Dental Research, 57, 805-9. 

Ali Z, Zuhri A (1984) Spectrophotometric studies and analytical application of Ce(III) chelates withl-(2-Pyridylazo)-2-naphthol (PAN). Monatsh Chem 115(1) 57—58... 

It is always wise to calibrate physical methods of analysis using mixtures of known composition under conditions that approximate as closely as practicable those prevailing in the reaction system. This procedure is recommended because side reactions can introduce large errors and because some unforeseen complication may invalidate the results obtained with the technique. For example, in spectrophotometric studies of reaction kinetics, the absorbance that one measures can be grossly distorted by the presence of small amounts of highly colored absorbing impurities or by-products. For this reason, when one uses indirect physical methods in kinetic studies, it is essential to verify the stoichiometry of the reaction to ensure that the products of the reaction and their relative mole numbers are known with certainty. For the same reason it is recommended that more than one physical method of analysis be used in detailed kinetic studies. 

S. Trevin, F. Bedioui, and F. Devynck, Electrochemical and spectrophotometric study of the behavior of electropolymerized nickel prophyrin films in die determination of nitric oxide in solution. Talanta 43,... 

Itaconic acid was hydrogenated rapidly to a 20% ee, and kinetic and spectrophotometric studies on this system were interpreted in terms of a mechanism involving a standard unsaturate route [cf. Eq. (5)]. The actual catalyst was thought to be HRh(DIOP)(DIOP ), where DIOP represents a monodentate DIOP with one dangling —CH2PPh2 moiety (273, 275). Rhodium(I) carbonyls (276) and ruthenium(II) complexes (90, 275) (Section III,B) containing monodentate DIOP have been isolated. 

Spectrophotometric study of the decomposition kinetics of nitronate (MeC>2C)2C=N(0)C)Me (73a) demonstrated that this compound decomposes at 25°C by a first-order reaction according to Equation 1 (Scheme 3.72) to give the... 

In an earlier spectrophotometric study of this enzyme, a red shift of the reduced nicotinamide absorbance had been noted in the difference spectrum of the binding of reduced coenzyme to the purified protein. Fisher et al.92> had pointed out that this is characteristic of most B-stereo-specific dehydrogenases, so Biellman et al. have made a successful prediction for Fisher. Fisher s suggestion that the supernatant and mitochondrial forms of malate dehydrogenase have different stereospecificities for NAD+ has not been substantiated, however 89>. 

Pyrrole [165] is the simplest aromatic ring, in which the nitrogen may be regarded as being conjugated with the ol- and 3-carbon atoms of the ring. The nitrogen lone electron pair is part of the aromatic sextet and N-protonation is not expected. Pyrrole itself is unstable in dilute acid (trimerization occurs), but in more concentrated acid solutions, in which protonation occurs, a spectrophotometric study... 

Organometallic Titaniam(iv) Compounds.—Charge-transfer interactions between TiC and aromatic hydrocarbons and fluorocarbons have been characterized by spectrophotometric studies. " The other work described here will mainly be concerned with selected aspects of the chemistry of Ti -alkyl and -cyclopentadienyl derivatives. 

Spectrophotometric studies of the reaction between Np and 1,3-dikelones in water revealed that 1 1 and 1 2 adducts were formed, except with fluorinated species which preferred only to form 1 1 complexes. The stability of Np02 complexes was lower than that of the corresponding UOj species. 

Spectrophotometric studies were conducted from pH 0.1 to 5.7. The thermodynamic constants calculated, assuming that the absorbance of AH+-AH+ was twice that of AH -AOH and that the second hydration constant (pAhi) was equal to the proton transfer constant (pA, were 1.8 (pAhi) for the first hydration reaction and 4.6 (pAThi pA) for the second, whereas the hydration and proton transfer constants calculated for malvidin 3-glucoside are 2.6 and 4.25, respectively. Based on these hydration constants, the only significant form of the methylmethine dimer at wine pH is AH+-AOH, in which one of the anthocyanin moieties is under the red flavylium form and the other one is hydrated, as predicted from mass spectrometry data. Thus, conversion of grape anthocyanins (75 to 80% colorless AOH, 20 to 25% red AH+ in wine pH range) to the methylmethine bis-anthocyanin (50% AOH, 50% AH ) may be responsible not only for a shift toward a more purple tint but also for a twofold increase in color intensity. 

Milburn and Vosburgh 16) confirmed the formation of Fe2(OH)2 in a spectrophotometric study, and their equilibrium constants log = —2.89, log / 22 =—2.58 in 3MNaC104 at 25° are in reasonable agreement with those of Hedstrom. The dimer has an absorption band at 335 ra/i, whereas both Fe + and FeOH2+ apparently have a band at 240 vcifi 17). Kinetic evidence for dimer formation emerged from a pressure jump study by Wendt in 1962 18). As expected for a polynuclear complex, the rate constant for dissociation... 

Karpinski, Z.J., Osteryoung, R.A., Spectrophotometric studies of iodine complexes in an aluminum chloride— butylpyridinium chloride ionic liquid, Inorg. Chem., 23,4561-4565,1984. 

The reduction of aromatic nitro compounds is believed to proceed to an intermediate mixture of nitroso compounds and substituted hydroxylamines which are not isolated but condense to form an azoxy compound which, in turn, is reduced to an azo compound. Contributing evidence to substantiate this mechanism is that the reduction of a mixture of two aromatic nitro compounds leads to a mixture of azo compounds consistent with that predicted if each of the nitro compounds were reduced to a nitroso compound and a hydroxylamine and these, in turn, reacted with each other in all possible combinations. This observation also implies that the bimolecular reduction of nitro compounds is practical only from the preparative standpoint for the production of symmetrically substituted azo compounds. Spectrophotometric studies of the reaction kinetics of the reduction of variously substituted nitro compounds may, however, uncover reasonable procedures for the synthesis of unsymmetrical azo compounds. 

Incident sunlight is partially absorbed by minerals on an asteroid s surface, and the fraction of light that is reflected can vary as a function of wavelength. Spectrophotometric studies of asteroids have been carried out for decades. Asteroid reflectance spectra have been compared to the spectra of meteorites measured in the laboratory (Fig. 11.3), providing a way to link some kinds of meteorites to possible parent bodies. Most measurements are made at visible wavelengths ranging from 0.4 to 1.0 pm, because the illuminating solar flux peaks... 

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