Altered absorption maxima

Protonation State Changes Result in Altered Absorption Maxima Biophotonics that Result from the Photocycle of Bacteriorhodopsin... 

Protonation State Changes Result in Altered Absorption Maxima... 

Substitution of a phenyl residue in the 4-position of the selenazole ring causes only a minor alteration in the position of the absorption maximum. By contrast, the benzylidene compounds are markedly bathochromically shifted compared to the isopropylidene derivatives. Thus the absorption is caused by the whole of the conjugated system, which can be compared to that found in amino derivatives of... 

Alteration of the silver plasmon band spectrum upon electron and hole injection has been rationalized in terms of changes in the density, Ne, and conductivity, o, of the electron gas in the metal particles as described by Eqs. (16)—(18) [506]. Thus, a decrease in Ne by electron extraction from the metallic silver particles increases Xc (Eq. 16) and thereby shifting the absorption maximum (Eq. 15) of the plasmon band to a longer wavelength (Fig. 83). A decrease in Ne also decreases a (Eq. 18), which leads, in turn, to an increase of w (Eq. 17) that is, to an increase in the bandwidth of the plasmon band absorption (Fig. 83). Similarly, the increase in Ne by electron transfer to the silver colloids is paralleled by a decrease in Xc (Eq. 16) and, hence, by a decrease in Xm (Eq. 15), as seen by the shift of the plasmon absorption band to a shorter wavelength (Fig. 83). Electron donation to the silver particles also causes an increase in cr (Eq. 18)... 

We thank a referee for pointing out that instead of measuring a simple difference spectrum, a detailed factor analysis should have been performed. Although this may give more accurate values for the wavelength of the new absorption maximum, it would not alter the main conclusion that a novel absorption band is present in the hybrid photocatalysts. 

The absorption maximum of the cyanine dye can be changed by altering the number of conjugated alkene units linking the cyanine chromophores. This makes the cyanine borate photo-redox pair a so-called tunable photoinitiator, in that compounds which absorb throughout the visible and infrared spectrum can be obtained. Recently, Kabatc et al. [35] described the important features of cyanine borate photo-redox pairs (Table 2). The structures of dyes tested are shown in Figure 5. 

The spectra of nitroimidazoles show features that are altered characteristically by substituents and by pH, and which are useful for the purposes of orientation. Chromatographic, spectrophotometric, and polarographic methods have been used ° in the simultaneous determination of N-substi-tuted and N-unsubstituted nitroimidazoles. The latter react with hydroxyl ions to form yellow nitroimidazole anions. The difficulty of reduction of these anions, together with the shift in the absorption maximum to longer wavelengths, makes the analysis possible. 

Usually, from a spectral scan, the wavelength corresponding to the absorption maximum is selected. At max. sensitivity is maximum and, over a narrow region close to the maximum, the absorbance is constant. Therefore, minor wavelength differences do not alter the response, and also the absorbance at >,i ax is less sensitive to instrumental uncertainties. However, if adequate sensitivity exists, measurements can be made at min. if interfering absorbance is negligible at... 

The light absorption spectrum of the hemoglobin derivative to be chosen for a photometric determination should have a number of favorable properties. It should display a rather flat maximum. If this is not the case, slight alterations in filter or in photometer characteristics will introduce considerable errors. Furthermore, the light absorption maximum should... 

Similarly, the spectral effects of two isolated chromophores in a molecule (separated by at least two single bonds) are, m principle, independent and are additive. Hence, in the molecule CH3CH2CNS, an absorption maximum due to the CNS group occurs at 245 nm with an e of 800. In the molecule SNCCH2CH2CH2CNS, an absorption maximum occurs at 247 nm, with approximately double the intensity (e = 2000). Interaction between chromophores may perturb the electronic energy levels and alter the spectrum. 

This complex is prepared by a variation of the published method. To potassium hexachloropIatinate(IV) [Aldrich Chemical] (4 g) and Na2HP04 (8 g) in a 500-mL flask fitted with a condenser are added concentrated aqueous ammonia (25%, 75 mL) and water (120 mL). The mixture is stirred and heated to reflux, and the temperature is maintained until the suspension turns white (—10 min). Then the mixture is cooled to or below room temperature. The white precipitate is collected on a frit, washed with methanol (2 X 25 mL), and air-dried. The precipitate is dissolved in hot (—70°) 0.1 M HCI (—200 mL). The mixture is Altered, and concentrated HCI (50 mL) is added. The colorless solution is reduced in volume to —40 mL on a rotary evaporator, during which time a white precipitate forms. After cooling, this is collected by flitration on a frit, washed with ice-cold 3 M HCI (20 mL), methanol (3 X 25 mL), and diethyl ether (25 mL), and dried in a vacuum desiccator over P40,o. Yield 3.0 g (90%). Absorption maximum in water 286 nm (e 141 M cm" ). 

Besides fluorescence spectroscopy, time-resolved spectroscopy can rely on the measurement of excited (singlet or triplet) state absorption. Similarly to ground-state absorption, the spectral and absorbance properties may be altered by CyD complexation and yield information about the behavior of the complex in the excited state in addition, the time dependence (formation and decay) of the excited state absorption yields information about the kinetics and dynamics of the system. This is illustrated by the behavior of the lowest triplet state of naphthalene as measured by nanosecond spectroscopy using a Q-switched Nd YAG laser at 266 nm for excitation [21]. The triplet-triplet absorption spectra were measured in neat solvents (water and ethanol) and in the presence of a- and -CyD (Fig. 10.3.3). The spectra in ethanol and H2O had the same absorption maximum, but the transition was considerably weaker and broadened in H2O. Both CyDs induced a red shift, and a-CyD additionally narrowed the main band considerably. Fig. 10.3.4 shows the effect of a-CD concentration on the time evolution of the triplet-triplet absorption at 416 nm in the microsecond range. Triplet decay was caused by O2 quenching a detailed kinetic analysis of the time dependence yielded two main components which could be assigned to the free guest and the 1 2 complex, in full... 

The change in structure when NAD is reduced or oxidized is reflected in an alteration of its ultraviolet spectrum. The reduced form has an absorption maximum at 340nm, while the oxidized form has little absorption at this wavelength (Figure 16.2). This forms the basis for assays of biological oxidation/reduction reactions where alteration in the absorption at 340 nm can be related to changes in the concentration of NADH. 

Our problem is to determine how the changes of total and free energy, AU and A P, or, what are the same, the heat absorption at constant configuration and the maximum work, Qx and At, of an isothermal and reversible process, alter with the temperature of execution of the process. 

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