Diffuse absorption bands

Nitrous acid exhibits diffuse absorption bands in the region 3000 to 1000 A. These bands are ascribed to the XA" A transition [King and Moule (567)]. The absorption cross sections in the region 2000 to 4000 A have been measured by Cox and Derwent (249) and are given in Fig. VII-7. 

Like peroxidase, catalase forms both primary and secondary complexes with methyl and ethyl hydroperoxide (Chance, 73). The primary complexes are green having a diffuse absorption band in the red starting at 670 mju. The secondary complexes are red and have absorption maxima in the visible region at 572 and 536 mu. The catalase-ethyl hydroperoxide complex found by Stern (74) had maxima at approximately these wavelengths and was thus the secondary complex. The Soret bands of the primary complexes are similar in shape to that of the free enzyme but are shifted toward the red by several millimicrons. At... 

A system of diffuse absorption bands (2 = 280 to 320 nm) has been used to detect NClg radicals formed in the rapid reaction ... 

Water incorporated into fused silica is of considerable importance because the resulting silanol groups affect the NIR transmission of silica optical fibers and other optical components. Silanols are discussed in the OH section. In mixed glasses, such as those containing borosilicates, aluminosilicates, and so on, associated water OH groups give rise to diffuse absorption bands. The absorption bands of water molecules on silica surfaces have been described by Klier et al. " ... 

In a general way, the frequency domains of coordinated groups obtained in the crystalline phases can be used in order to determine the coordination numbers of certain cations in corresponding glasses. However, this kind of analysis must be conducted with caution coupling between different coordinated groups and wide, diffuse absorption bands, mainly in the low-frequency spectral region, reduces considerably the efficiency of the method. 

Photoexcited fluorescence from spread monolayers may be studied [158,159] if the substance has both a strong absorption band and a high emission yield as in the case for chlorophyll [159]. Gaines and co-workers [160] have reported on the emission from monolayers of Ru(bipyridine)3, one of the pyridine ligands having attached C g aliphatic chains. Ruorescence depolarization provides information about the restriction of rotational diffusion of molecules in a monolayer [161], Combining pressure-area... 

The skeleton vibrations. C3NSX, CjNSXj. C NSXY, or C NSXj (where X or Y is the monoatomic substituent or the atom of the substituent which is bonded to the ring for polyatomic substituents), have been classified into suites, numbered I to X. A suite is a set of absorption bands or diffusion lines assigned, to a first approximation, to a same mode of vibration for the different molecules. Suites I to VIII concern bands assigned to A symmetry vibrations, while suites IX and X describe bands assigned to A" symmetry vibrations. For each of these suites, the analysis of the various published works gives the limits of the observed frequencies (Table 1-29). 

The EMIRS and SNIFTIRS methods provide the IR vibrational spectra (really the difference spectra - see later) of all species whose population changes either on the electrode surface or in the electrical double layer or in the diffusion layer in response to changing the electrode potential. Spectra will also be obtained for adsorbed species whose population does not change but which undergo a change in orientation or for which the electrode potential alters the Intensity, the position or shape of IR absorption bands. Shifts in band maxima with potential at constant coverage (d nax 6 very common for adsorbed species and they provide valuable information on the nature of adsorbate/absorbent bonding and hence also additional data on adsorbate orientation. 

Connors and Jozwiakowski have used diffuse reflectance spectroscopy to study the adsorption of spiropyrans onto pharmaceutically relevant solids [12]. The particular adsorbants studied were interesting in that the spectral characteristics of the binary system depended strongly on the amount of material bound. As an example of this behavior, selected reflectance spectra obtained for the adsorption of indolinonaphthospiropyran onto silica gel are shown in Fig. 1. At low concentrations, the pyran sorbant exhibited its main absorption band around 550 nm. As the degree of coverage was increased the 550 nm band was still observed, but a much more intense absorption band at 470 nm became prominent. This secondary effect is most likely due to the presence of pyran-pyran interactions, which become more important as the concentration of sorbant is increased. 

The utilization of IR spectroscopy is very important in the characterization of pseudopolymorphic systems, especially hydrates. It has been used to study the pseudopolymorphic systems SQ-33600 [36], mefloquine hydrochloride [37], ranitidine HC1 [38], carbovir [39], and paroxetine hydrochloride [40]. In the case of SQ-33600 [36], humidity-dependent changes in the crystal properties of the disodium salt of this new HMG-CoA reductase inhibitor were characterized by a combination of physical analytical techniques. Three crystalline solid hydrates were identified, each having a definite stability over a range of humidity. Diffuse reflectance IR spectra were acquired on SQ-33600 material exposed to different relative humidity (RH) conditions. A sharp absorption band at 3640 cm-1 was indicative of the OH stretching mode associated with either strongly bound or crystalline water (Fig. 5A). The sharpness of the band is evidence of a bound species even at the lowest levels of moisture content. The bound nature of this water contained in low-moisture samples was confirmed by variable-temperature (VT) diffuse reflectance studies. As shown in Fig. 5B, the 3640 cm-1 peak progressively decreased in intensity upon thermal... 

Fig. 5 A, diffuse reflectance IR spectra of SQ-33600 displaying the absorption band due to bound water of hydration after exposure to different relative humidity conditions of (a) initial sample containing 3.5% water, (b) 6%, (c) 15%, (d) 31%, (e) 43%, (f) 52%, (g) 70%, and (h) 84%. B, variable-temperature diffuse reflectance IR spectra of SQ-33600 initially containing 3.5% water.

As will be shown later, the surface coverages of CO vary with distance into the pellet during CO adsorption and desorption, as a result of intrapellet diffusion resistances. However, the infrared beam monitors the entire pellet, and thus the resulting absorption band reflects the average surface concentration of CO across the pellet s depth. Therefore, for the purpose of direct comparison between theory and experiment, the integral-averaged CO coverage in the pellet... 

Analogously, when a colourless aqueous solution of quinolinium chloride (Q+ CP) is mixed with an almost colourless aqueous solution of Na+V(CO)p the well-formed dark green crystals of the vanadate salt precipitate immediately. In each case, the spontaneous separation of the highly coloured salts is made even more dramatic by the absence of colour in the aqueous mother liquors throughout the course of precipitation. The quantitative effects in these coloured salts are observed as broad absorption bands in the spectral region between 350 to beyond 700 nm when they are dissolved in dichloromethane, or in the diffuse reflectance spectra of the crystalline salts. The correspondence of the band maximum (Act) and... 

Let us consider the tetravalent lanthanide ions first. Because this valency is hard to realize, the number of investigations is restricted. Jorgensen and Rittershaus 3) described the diffuse reflection spectra of Pr + and Tb + in Th02 and Y2O3 and Blasse and co-workers those in Zr02 [4). These spectra show strong absorption bands in the visible which were ascribed to c.t. transitions. 

The absorption bands of goethite arise, as do those of the other FeOOH polymorphs, from Fe-OH and Fe-O vibrations. There are 36 possible Fe-O vibrations and 12 hydroxyl vibrations. Of these, 12 Fe-O and 5 hydroxyl vibrations (all B type) are infrared active, although not all of these are observed experimentally (Table 7.2). The same bands are detected whether the sample is examined by transmission, diffuse reflec-... 

Diffused reflectance spectra of the vPs/AIPO -S samples of different contents are shown in Fig. 7. By increasing vp, content, the absorption band appeared in the range between 400 and 550 nm, which gave rise to the formation of crystalline vp. Therefore, as evidenced by infrared measurements, it is concluded that the crystalline phase is formed on the AiP04-5 surface after the formatbn of surface vanadium species containing V in a tetrahedral symmetry. The diffused reflectance spectmm of VAPO -5 [7,8] was similar to that of calcined VjOj/AIPO -S. 

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