Shift hydroxyl band

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

Table 4.6 Hydroxyl band shifts measured by low-temperature CO adsorption for various...

By measuring the shifts of the various hydroxyl bands of the zeolite, a direct measure of the relative acid site strengths can be made without the need for thermal desorption. Table 4.6 lists the measured hydroxyl band shifts for a variety of hydroxyl groups on different zeolites using low temperature CO adsorption. This data indicates that there is indeed a difference in the intrinsic acid strength of the bridging hydroxyl groups in different zeolites as well as in the same zeolite structure with different framework aluminum content. 

Kasha s tests for identification of n- n and r. tz transitions. Solvent perturbation technique is a useful way to identify transitions as ->it or 7t for complex molecules. While comparing bands of different orbital promotion types in hydroxylic solvents such as water and ethanol with those in hydrocarbon or nonpolar solvents, if the band shifts towards the high frequency or shorter wavelength side (blue-shift) then the transition is probably n -> w. If there is a small red shift, the transition is likely to be it - The effect of solvents on the n - w transition in acetone and pyrimidine is shown in the Table 3.3. 

Figure 5.9 Total content and absolute surface concentrations of the two different hydroxyl types, according to the band shift model ( total OH + free OH bridged OH).

The increased reactivity of alkoxysilanes in the reaction with isolated silanol groups can be explained by the higher proton affinity of the oxygen atom of the Si-O-Si group, compared to the Cl-atom in the Si-Cl group. This is evidenced by the observation that the physisorption of an alkoxysilane on a silica surface results in a free hydroxyl band shift of approximately 300 cm 1, whereas the physisorption of a chlorosilane induces a shift of only 100 cm 1.89... 

Fig. 7.3 shows the second-derivative spectra of control caeruloplasmin and of caeruloplasmin (20 mg/ml) treated with hydroxyl radicals generated from 0.5 mM Cu(II) and 5 mM hydrogen peroxide. These spectra were obtained for samples in 2H20. Control caeruloplasmin has bands near 1637 cm-1 indicating -structure with a relatively weaker band near 1652 cm-1 due to a small amount of a-helix/random coil structure. On exposure of caeruloplasmin to the hydroxyl radical-generating system, this band shifts to 1656 cm-1, indicative of a transition to the disordered conformation, with no major shift in the original band indicative of the -structure. Thus FTIR-spectroscopy allows sensitive demonstration of the loss of the small amount of a-... 

Interestingly, reduction of benzoquinone on a hydridic surface occurs only via the valence band, a result associated with the strong cathodic flat-band shift of the latter surface compared with the hydroxylated surface, as seen in Fig. 44(c). 

Incorporation of Zr shifts hydroxyl bands to higher frequency and makes them more intense. It implies that a part of the hydroxyls is bound with zirconium cations, since the hydroxyl band positions is rather close to those observed partially stabilized zirconias [15]. The appearance of bands at - 3780 cm implies the emergence of terminal hydroxyls. For... 

In this context, interesting results were reported by Bjorgen et al. (321). After low-temperature adsorption of 1-butene on HBEA, these authors observed a red shift of the 3610 cm band (bridging hydroxyls) by —500 cm and assigned this phenomenon to formation of a hydrogen bond between the OH group and 1-butene. Simultaneously, a shift by... 

The first and still only clue for a difference between the chemical structure of chondroitin 4-sulfate and chondroitin 6-sulfate was supplied by studies of the infrared spectra. An adsorption band corresponding to an axial sulfate group was observed in the 840 to 850 wave number region for chondroitin 4-sulfate, whereas this band shifted to the 815 to 825 wave number region for the 6-isomer, corresponding to an equatorial position. Since the only hydroxyl group, in equatorial position and available for... 

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. 

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