UV absorption bands

On the basis of the studies described in the preceding chapters, we anticipated that chelation is a requirement for efficient Lewis-acid catalysis. This notion was confirmed by an investigation of the coordination behaviour of dienophiles 4.11 and 4.12 (Scheme 4.4). In contrast to 4.10, these compounds failed to reveal a significant shift in the UV absorption band maxima in the presence of concentrations up to one molar of copper(ir)nitrate in water. Also the rate of the reaction of these dienophiles with cyclopentadiene was not significantly increased upon addition of copper(II)nitrate or y tterbium(III)triflate. 

Table 10 UV Absorption Bands of the Simple Monocyclic Azines ...

Applications Applications of UV/VIS spectrophotometry can be found in the areas of extraction monitoring and control, migration and blooming, polymer impregnation, in-polymer analysis, polymer melts, polymer-bound additives, purity determinations, colour body analysis and microscopy. Most samples measured with UV/VIS spectroscopy are in solution. However, in comparison to IR spectroscopy additive analysis in the UV/VIS range plays only a minor role as only a limited class of compounds exhibits specific absorption bands in the UV range with an intensity proportional to the additive concentration. Characteristic UV absorption bands of various common polymer additives are given in Scheirs [24],... 

The photolysis of matrix-isolated di-f-butyldiazidosilane in 3-MP at 77 K or in argon matrix at 10 K yields several products, the JV,AT-di-f-butylsi-lane diimine,41 among others. This pale yellow compound is characterized by UV absorption bands at 240 nm and 385 nm. 

A UV-visible absorption spectrum of a 10 (J.M ethanol solution of a model compound (DAFc in Fig. 5) for the D moiety is shown together with that of the A-S-D triad in Fig. 16. The absorption bands between 200 and 300 nm can be clearly seen for DAFc. Qualitatively, absorption spectra for A-S-D triads in the LB films and in the 10 pM ethanol solution are similar. It is interesting to note, however, that the relative intensities of the acylated perylene band around 450 nm against the 200-300 nm UV absorption bands are different between the LB films and the ethanol solution. This difference can be attributed to orientation of the perylene moiety in the LB films in the same way as in the antenna LB films reported previously [38]. 

Most UV absorption bands correspond to transitions of electrons from ra->7i, or n o molecular orbitals. Besides aromatic compounds, organic functional groups such as carbonyl, carboxylic, amido, azo, nitro, nitroso, and ketone groups have absorbance in the UV region. 

Photolytic. The UV absorption band for dichlone is 330 nm (Gore et al., 1971). Irradiation of dichlone in a variety of organic solvents (benzene, isopropanol, ethanol) using UV light produced dehalogenated compounds. In the absence or presence of oxygen, 2-chloro-l,4-naphthoquinone,... 

UV absorption bands (Figure 2.2) are characterised by the wavelength of the absorption maximum ( max ) Th values of s associated with commonly... 

UV absorption bands have fine structure due to the presence of vibrational sub-levels, but this is rarely observed in solution due to collisional broadening. As the transitions are associated with changes of electron orbitals, they are often described in terms of the orbitals involved, e.g. 

Table 2.2 UV Absorption Bands in Common Carbonyl Compounds...

Table 2.3 UV Absorption Bands in Common Benzene Derivatives...

Table 6.2. Long wavelength uv absorption bands of benzene radical-cations in water, determined by pulse-radiolysis techniques.

Chlorine dioxide readily degrades in aqueous solutions imder ultraviolet light. It has a broad UV absorption band with a maximum near 360 nm and a molar extinction coefficient of about 1,150 (M X cniy (Aieta and Berg 1986). It is postulated that the reaction in solution proceeds as in the gas phase, to give CIO and O. The initial photodissociation reaction is followed by rapid dark and light reactions to produce the products, chlorate (CIO3 ), hypochlorite (OCl ), and chloride (Cl ) (Zika et al. 

Very intense and sharp near-UV absorption bands due to radical ions of polysilanes [53a,b] and polygermanes [53c] were observed by nanosecond pulse radiolysis. Broad visible and IR absorption spectra due to the radical ions of polysilyne [54] and polygermyne [54] were also observed. Very systematic pulse radiolysis studies on many different kinds of polysilanes [55] have been made by our improved nanosecond pulse radiolysis system over a wide range of... 

The application of standardized UV (or UV-Vis) spectroscopy has for years been used in analyses of flavonoids. These polyphenolic compounds reveal two characteristic UV absorption bands with maxima in the 240 to 285 and 300 to 550 nm range. The various flavonoid classes can be recognized by their UV spectra, and UV-spectral characteristics of individual flavonoids including the effects of the number of aglycone hydroxyl groups, glycosidic substitution pattern, and nature of aromatic acyl groups have been reviewed in several excellent books. ... 

In a recent paper, the interaction of various simple flavonoids with an anionic surfactant, sodium dodecyl sulfate (SDS) in aqueous solution, has been studied through absorption spectroscopy as a function of the concentration of the surfactant above and below the critical micelle concentration.The approximate number of additive molecules (flavonoids) incorporated per micelle was estimated at a particular concentration of SDS. Incorporation of flavonoids in micelles shifted the UV absorption bands toward higher wavelengths, and the bathochromic shifts also depended upon the nature of the surfactant head group. 

Figure 5.12 Chiral self-aggregation of porphyrin compounds (adapted from Ribo et al., 2001, with kind permission), (a) The monomer and J-aggregates structures. (b). The outcome of rotating directions of the flask in the rotary evaporator, clockwise (CW) and anticlockwise (ACW), on the preparation of aggregates by concentration of a monomeric solution of the porphyrin. The corresponding CD spectra, showing the chirality signature, and the UV absorption bands of the J-aggregates are also shown. Notice that the two UV spectra are identical, whereas the two CD spectra are opposite to each other.

In spite of its long-assumed intermediacy in several reactions, no carbon-substituted silylene was directly observed for many years. In 1979, however, Drahnak et al. detected a broad ultraviolet (UV) absorption band (3-niax = 450 nm) after the photolysis of dodecamethylcyclohexasilane (6) in 3-methylpentane. This band was assigned to dimethylsilylene (5). Many different approaches to this intermediate, either photochemicaUy or thermally, were examined (Scheme 14.6). ... 

Analysis of the data of Table 1 verifies the same finding that the shift of the near-UV absorption band of singlet arylnitrenes correlates with the shift of the intense near-UV absorption band of triplet nitrenes. Furthermore, the ort/io-substituents influence the absorption spectra of singlet and triplet phenylnitrenes more significantly than do para-substituents. 

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