Benzene chromophore

UV-VIS Just as with arylammes (Section 22 20) it is informative to look at the UV VIS behavior of phenols m terms of how the OH group affects the benzene chromophore... 

A comparison of the thus calculated with the measured specific rotations of the 0th- to 4th-generation dendrimers of this kind gave a close resemblance, with a curve, approaching asymptotically a limiting value (Fig. 26). It was also shown that the shape of this curve was independent of solvent, concentration and temperature. This was not the case when CD spectra of these dendrimers were compared (Fig. 27) in solvents such as CH2C12 and f-butyl methyl ether a constant rise of the Cotton effect was observed, which correlates with the increasing amount of benzene chromophores in the dendrimers. However, in the... 

Pseudopterosin X (1) was isolated as a yellow colored gum. The UV spectrum of 1 showed maximum absorption at 280 nm due to the presence of a highly substituted benzene chromophore [10], Its IR spectmm displayed intense absorption bands at 3,470 (OH), 2,904 (CH), 1,705 (C = O), 1,595 (C = C) and 1,100 (C-0) cm . The high-resolution electron-impact mass spectmm (HREIMS) of 1 showed M+ at m/z 474.2622, and this mass provided molecular formula indicating the presence of nine double bond equivalents in 1. The C-NMR chemical shift assignments of 1 are shown around stracture 1. On the basis of the detailed NMR studies and comparison with the reported pseudopterosins in the literature and L-xylose [3-5], stmcture 1 was proposed for this new natural product. 

Figure 4.10 shows the UV absorption spectra of a solution of procaine in 0.1 M HCl and O.IM NaOH. In procaine, the benzene chromophore has been extended by addition of a C = O group and under acidic conditions, as in Figure 4.10, the molecule has an absorption at 279 nm with an A (1%, 1 cm) value of 100. In addition to the extended chromophore, the molecule also contains an auxochrome in the form of an amino group, which under basic conditions has a lone pair of electrons that can interact with the chromophore producing a bathochromic shift. Under acidic conditions the amine group is protonated and does not function as an auxochrome but when the proton is removed from this group under basic conditions a bathochromic shift is produced and an absorption with A, max at 270 nm with an A (1%, 1 cm) value of 1000 appears. 

Many chromophores are suitable for use in the exciton chirality method. One of the features required for such a chromophore is its planarity or near-planarity. Nonplanar (inherently dissymmetric) chromophores would contribute to the CD spectra by other mechanisms. The other limiting factor is the position of the transition in the spectral region studied. For example, the 1B transition in the alkyl-substituted benzene chromophore appears near the short-wavelength recording limit around 200 nm, making its use in the exciton chirality method less attractive. Furthermore, the direction of polarization of the lB transition in alkyl-substituted benzene derivatives is not readily determined. In such cases calculation of the rotatory strength is more reliable than qualitative analysis. 

Based on the CD-spectra of various optically active carbophanes, a sector rule for the correlation of the sign of the 1Lb-Cotton effect of the benzene chromophore with the absolute chiralities of these phanes (as well as for chiral dihydro-9,10-ethano-anthracenes) has been proposed 63). 

Effect of Conjugation on Electronic Absorption by the Benzene Chromophore... 

The data of Table 22-3 show the effect on the benzene chromophore of this type of substituent —the substituent often being called an auxochrome.2 This term means that, although the substituent itself is not responsible for the absorption band, it shifts the absorption of the chromophoric group, in this case the benzene ring, toward longer wavelengths. The auxochromic groups usually increase the intensity of the absorption also. 

Disrotatory closure of the substituted benzene to produce a Dewar benzene is photo-chemically allowed, as is, of course, the reverse process. However, because benzene is conjugated, it absorbs UV light at longer wavelengths than the Dewar benzene isomer. Therefore, it is possible to selectively excite the benzene chromophore and produce the less stable Dewar isomer. In this particular case the rm-butyl groups favor the reaction because they destabilize the benzene isomer somewhat, owing to steric hindrance. Because the two adjacent im-butyl groups in the Dewar isomer do not lie in the same plane, this steric strain is decreased in the product. Because of this steric effect and the forbidden nature of the conversion back to benzene, the Dewar isomer is relatively stable. However, when it is heated to 200°C, it is rapidly converted to the benzene isomer, probably by a nonconcerted pathway. 

Characterisation of vesicles was achieved using a combination of methods, including photon-correlation spectroscopy, video-enhanced and cryo-electron microscopy. Measurements of the cmc of the surfactants (in the absence of salt) were made using uv-visible spectrophotometry and electrical conductivity (k). For cmc measurements, there is a convenient change in the extinction coefficient of the benzene chromophore at 262 nm. The onset of vesicle formation, and hence the determination of the esc, can be measured by 90° scattering and 180° optical turbidity measurements at 300 nm. 

Since the end of the seventies, interest in cyclotriveratrylene has moved towards the use of its cone shaped structure for applications in various fields, including investigations of the electronic transitions of the benzene chromophore via UV and CD spectroscopy, studies in the area of host-guest chemistry, synthesis of new types of liquid crystals, and searches for new three-dimensional organic charge-transfer materials. These works have been made possible because efficient synthetic... 

Very interesting effects on eukaryotic cells result after treatment with ansamycins containing a benzene chromophore, especially with compounds of the maytan-sine group which are the only ansamycins found so far to occur in plants. 

More numerous, however, are aromatic compounds with an effectively planar n system, the optical activity of which comes from chiral perturbations. Benzene derivatives such as phenylalanine (3) are particularly important. The Lb band of the benzene chromophore (A = 260 nm) is neither magnetically nor electrically allowed. The symmetry selection rule may be broken by vibronic interactions or due to substituents. The interpretation of the observed rotatory strength is not easy, but empirical sector rules have been proposed. (Cf. Charney, 1979 Pickard and Smith, 1990.)... 

Often, however, it is sufficient to distinguish between o and tc orbitals in order to decide whether a barrier is likely to occur. For instance, the T, surface of toluene along the path of nuclear geometries that leads to dissociation to CjHjCHj- + H- can be viewed as originating from interaction of a locally excited configuration of the benzene chromophore... 

Benzoid Chromophobe. The spectrum of the aromatic parent, benzene (Figure 2), displays considerable fine structure, a property which is not shared to the same extent with many of its derivatives. The three-bonded spectrum (248, 254, and 260 /x) of benzene will be considered as one chromophore. Benzene absorbs at 184 fi (Om 60,000), 203.5 fi (Om 7400), and 254 fi (Om 204) in hexane (4). These maxima are considered as the 7T 7T bands of the benzene chromophore. Increasing alkyl substitution causes a bathochromic shift of the 254-/i band, an eflFect which reaches its maximum at tetrasubstitution. New intense bands appear in the spectrum of benzoid compounds upon introduction of a substituent... 

The X max of 242, 247, 252, 258, 26k, and 267 nm are typical of fine structure due to isolated benzene chromophore. Using the spectrum and the molar concentration of propoxyphene hydrochloride (1.94 x 10"3 moles/liter), the following table can be constructed. 

Table 3 Properties of quadrupolar bis(styryl)benzene chromophores...

Because the substituted benzene chromophores absorb in the 205- to 280-nm range and have low e values, the solvents used must be transparent down to at least 250 nm. This requirement is unnecessary for the more conjugated naphthalene and anthracene chromophores. The usual polar, nucleophilic solvents that have been used to observe ions and ion-derived products are various alcohols, water, or water mixed with a cosolvent such as dioxane for solubility reasons. Recently, and particularly for the observation of the intermediate carbocations by laser flash photolysis (LFP) methods, the strongly ionizing (high Tore values) but weakly nucleophilic Oow N values) alcohols, 2,2,2-trifluoroethanol (TFE) and l,l,l,3,3,3,-hexafluoro-2-propanol (HFIP), have been more commonly used. A limited list of polar solvents and their properties is given in Table 2 [23,24]. 

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