Polarization, UV spectroscopy

Ftueh, J., Reiter, G., Mohwald, H., et al. Orientation change of Polyelectrolytes in linearly elongated polyelectrolyte multilayer measured by polarized UV spectroscopy. Colloid. Surf. A 415, 366-373 (2012). doi http //dx.doi.oig/10.1016. colsurfa.2012.08.070... 

In the case of synthetic optically active polymers, the intuitive meaning of a CD signal intensity is very similar to that of UV spectroscopy, with the additional dimension of the subtracted absorption between left and right circularly polarized light.37 Absorption of light obeys the Beer-Lambert law, and thus CD intensity is defined as Ae = eL - eR = (AL - AR)/cl, where Ae is the... 

The secondary /3-deuterium KIEs observed for the reaction of the same substrate with hydroxide ion and with tris(hydroxymethyl)methylamine in aqueous solution at 25°C were small, i.e. kH/kD = 1.09 0.01 and 1.10 0.01, respectively. While Kresge argued that the EIE was primarily due to hyperconjugation, the secondary /3-deuterium KIEs were attributed partly to hyperconjugation and partly to a polar (inductive) effect. The rate constants for the evaluation of both the EIE and the KIEs were determined in separate kinetic runs by following the increase in the absorbance due to the nitronate ion by UV spectroscopy. 

Tautomerism on polymer should be quite sensitive to neighbouring group effects (composition and unit distribution, steric hindrance and tacticity) and to the microenvironment polarity in solution (copolymer-solvent interactions, critical concentration c of coil interpenetration). The determination of the tautomerism constant KT=(total conjugated forms)/(keto form) in dilute (csemi-dilute (c>c ) solution from H-NMR at 250 MHz and from UV spectroscopy has been reported elsewhere (39,43). The following spectrometric data related to keto-2-picolyl and keto-qui-naldyl structures are quite illustrative ... 

There have been relatively little ultraviolet-visible (UV-Vis) spectroscopic data for 1,4-oxazines, but selected data are presented in Table 8. UV spectroscopy is important for photochromic compounds, such as spirooxazines. The UV spectra of 33 spirooxazines in five different solvents are collected in a review <2002RCR893>, and the more recently reported examples of photochromic oxazines 65, 66, 101, and 102 are shown here. It can be seen from Table 8 that both adding methoxy substituents to the oxazine and changing to a more polar solvent give a UV maximum at a higher wavelength. This solvent effect can also be seen in the case of 102, which also has important fluorescence properties, discussed in Section 8.06.12.2. 

The kinetics of the reduction of 1,2-dithiane with triphenylphosphine was studied in aqueous ethanol at various temperatures by UV spectroscopy <1991PS(60)215>. First-order kinetics was clearly observed and the reaction rate was found to depend strongly on the solvent polarity. 

From a systematic study of bichromophoric compounds 97-99, the importance of substituents and solvent polarity in intramolecular deactivation processes of photoexcited anthracenes by nonconjugatively tethered, and spatially separated, aromatic ketones in their electronic ground state is apparent. For 97a-d, in which the electron acceptor properties of the aromatic ketone moiety have been varied by appropriate p-substitution of the phenyl ring (R is methoxy, H, phenyl, and acetyl, respectively), the longest-wavelength absorption maximum band lies at 388 nm, i.e., any ground state effects of substitution are not detectable by UV spectroscopy. Also, the fluorescence spectra of 97a-d in cyclohexane are all related to the absorption spectra by mirror symmetry. However, the fluorescence quantum yields for 97a-d in cyclohexane dramatically are substituent dependent (see Table 19), ranging from 0.20 for the methoxy derivative to 0.00059 for the acetyl compound [33,109],... 

There are numerous ways to determine experimentally pK values of chemical compounds (205). Classical methods are potentiometric titration and ultraviolet (UV) spectroscopy, among others. These techniques have been widely applied for nucleobases and also for metal-nucleobase complexes. For the extremes such as negative pK values (pK < —2) of singly or multiply protonated nucleobases, or very high pK values (pK >15) for deprotonation of exocyclic amino groups of nucleobases (C, G, A), modifications have to be employed. These include the consideration of the Hammett acidity function in superacidic solvents or solvent mixtures (206), as well as extrapolative techniques according to Bunnett-Olsen and Marziano-Cimino-Passerini to be applied in polar, aprotic solvents (45, 207). 

Tautomerism in the benzopyranone 72 has been studied computationally, and tautomer 72 is calculated to be 1.4kcalmol 1 more stable than 73 <1994JCS(P2)2587>. 3-Acetyl-4-hydroxycoumarin has been studied using UV spectroscopy and by computational methods < 1997CJC365, 2000JR0793>. Tautomer 74 is favored in nonpolar solvents such as CCI4 or hexane. In polar solvents, 75 is favored. Computational studies at both semi-empirical and ab initio levels predict 74 and 75 to be of comparable stability, and significantly more stable than all other possible tautomers. 

Zero-order first and second derivative UV absorption spectra of 18 purines and pyrimidines were determined in aqueous solution at 298 K. The procedure, based on the direct comparison of the derived maximum and minimum wavelengths and of the sequences of relative amplitudes, can be used for concentrations of purines as low as 5 x 10-6 M <85MI 7li-0l>. A similar study has shown that second derivative spectra can be used for the identification of eight mixtures of purines and pyrimidines <88TAL513>. The structure and equilibrium of purine in ethanol has been studied by UV spectroscopy which indicates the existence of annular automery in purine and the presence of associated species of only one of its tautomers <88JST(174)83>. Both polarized UV and Raman... 

A2-Thiazolin-5-one may exist in three tautomeric forms (16a, b, c) the protomeric equilibrium has been studied by 1H NMR, IR and UV spectroscopies. Polar solvents favor the enolic (16a) and the mesoionic (16c) forms (Table 16), this latter form being detected by the yellow color (428 nm) which appears in polar solvents on high dilution (70TL169). 

It was observed that the liquid crystalline substances incorporated on the polyimide films which were only 0.8 nm (2 layers) thick were successfully oriented to the transfer direction without a surface rubbing treatment of the polyimide film. This observation can be explained by orientation of the polymer molecule to the film transfer direction as observed by polarized UV and IR spectroscopy. 

Charge-transfer (CT) molecular complexes of some pyrazole donors (pyrazole, 4-methylpyrazole, 3-methylpyrazole, and 3,5-dimethylpyrazole) with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) and tetracyanoethylene (TCNE) as 7t-electron acceptors have been studied in methylene chloride at 25 °C by UV spectroscopy <2002SAA1895>. CT complexes of some pyrazoles with iodine as a cr-electron acceptor and with DDQ, TCNE, and chloranil (CHL) as 7t-electron acceptors were also investigated spectroscopically <2003SPL357>. In both reports, the spectral characteristics and stability constants of the CT complexes formed were discussed in terms of the nature of donor and acceptor molecular structure, as well as in relation to solvent polarity. The thermodynamic parameters (A/7, AG, and AS) associated with CT complex formation were also examined. 

As an example of the use of two-photon spectroscopy in assigning excited states that are not observed in one-photon UV spectroscopy, the two-photon absorption spectrum of naphthalene is shown in Figure 1.17. Since for the point group all Bg states have a theoretical polarization degree of H = 3/2, the polarization measurement reveals immediately a Bg state near 42,000 cm . In the two-photon absorption spectrum this shows up only as a shoulder, whereas the maximum at 44,500 cm" can be assigned to an Ag state. Neither state is prominent in the one-photon spectrum. 

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

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