Polarized UV spectra

Absorption Spectrometry The amount of vibrational fine structure in the UV absorption spectrum of a compound in solution is a function of the interaction between solvent and solute. The extent of interaction between solvent and solute increases with increasing solvent polarity, leading to decreasing fine structure. As the micelle is characterised by regions of different polarity, UV spectra have been used to obtain information on the environment of the solubUisate in the micelle. 

It is normally not possible to detect the carbocation intermediate of an SnI reaction directly, because its lifetime is very short. However, in the case of 3,4-dimethoxy-diphenylmethyl acetate (7) and certain other substrates in polar solvents it was possible to initiate the reaction photolytically, and under these conditions the UV spectra of the intermediate carbocations could be obtained, providing additional evidence for the SnI mechanism. 

It should be noted that for C6F13NC8 a remarkably anisotropic orientation of the chromophore in the film plane was observed in the polarized UV-vis. spectra at the normal incidence using the rays with electric vectors perpendicular and parallel to... 

Figure 24 (a) Polarized UV-vis. spectra for C6F13NC8 films at the noemal incidence electric vectors perpendicular (--) and parallel (---) to the mono-... 

Spectra as in Fig. 3.17 can also be obtained as a function of polar and azimuthal angle, and with polarized UV light, enabling one to probe band structures in all directions [18]. In this chapter we limit ourselves to angle-integrated measurements of the electron density of states. 

The Lewis basicity of the solvents was found to be more important for the solvatochromism than the solvent polarity, since such Si... solvent and Si... F interactions are competitive in the presence of solvents with donor atoms. In contrast, the analogous non-fluorinated polysilane, poly(methyl-/z-propylsilylene), revealed a disordered conformation in both coordinating and non-coordinating solvents. Additionally, the UV spectra of various molecular weight fractions of 87 showed an unusual molecular weight dependency an isosbestic point is apparent, suggesting an equilibrium between globule- and rod-like conformations at room temperature, which was also evident from the... 

There have been relatively little ultraviolet-visible (UV-Vis) spectroscopic data for 1,4-oxazines, but selected data are presented in Table 8. UV spectroscopy is important for photochromic compounds, such as spirooxazines. The UV spectra of 33 spirooxazines in five different solvents are collected in a review <2002RCR893>, and the more recently reported examples of photochromic oxazines 65, 66, 101, and 102 are shown here. It can be seen from Table 8 that both adding methoxy substituents to the oxazine and changing to a more polar solvent give a UV maximum at a higher wavelength. This solvent effect can also be seen in the case of 102, which also has important fluorescence properties, discussed in Section 8.06.12.2. 

Figure 11. (Left) CD and UV spectra of 1 in methanol. (Right) Rotational and dipole strengths and polarization of transition of 1 calculated by the n-SCF-CI-DV MO method. Symbols and mean perpendicular and parallel, respectively, to the C2 axis. Reprinted with permission from J. Am. Chem. Soc. 109, 1661 (1987).

Marked differences in the UV spectra of 1-alkyl- and l-acyl-dibenz[f>,/]azepines support the view that the TV-acyl derivatives are best described as the polar canonical dibenz-azepinium mesomers (32), in which the nitrogen lone pair is delocalized onto the oxygen,... 

The correlations shown on Figs. 1 and 2 are particularly remarkable when other factors which influence the spectra are considered. A primary complication is the effect of solvent polarity. Ideally, UV spectra should be recorded in nonpolar hydrocarbon solvents to minimize the effect of... 

UV data for carbazole and its substituted and reduced derivatives have been tabulated (71PMH(3)115). More recently, solution and solid-state polarized reflection spectra for carbazole have been subjected to detailed analysis and the observed states assigned with the aid of calculations by the RPA methodology (76BCJ3382). 

The UV spectra of the coloured aryl-substituted alkylidenecycloproparenes show long-wavelength absorption maxima, the positions of which are solvent dependent and in accord with the measured polarities (Table 4, Section II.G)153-157-158,294 297. Removal of the... 

Fig. 4 (a) Polar extracts of fish pan fried and barbecued for 9 min. Pan frying at 200°C produced more MelQx (peak B) than barbecuing for the same time at 270°C (peak C). MelQx peak C (< 1 ng/g) illustrates the DL of the method. Online recorded UV spectra from MelQx peaks are shown at the right, (b) Polar and apolar extracts of fish barbecued per side at 270°C. These samples contained detectable PhIP in the polar extract (left) and AaC, norharman, and harman in the apolar extract (right). (From Ref. 177.)... 

Experimental evidence for this interpretation is given by ESR spectroscopy, and the UV spectra indicate that MPT is localized in a highly polar region of the aggregate, that is now the anionic head groups of the surfactant. 

The UV spectra of 9-(phenylamino)tetrahydro-4f/-pyrido[ 1,2-a] pyrimidin-4-ones 23 indicated the presence of an imine-enamine type of tautomerism in solution. A carboxyl or ester group in position 3 (R1 = COOR3 R3 = H, Et) and a methyl group in position 6 (R = Me) shifted the equilibrium toward the enamine form in a polar solvent [85JCS(P1) 1015]. For the imine tautomer, UV maxima at 235-250 nm and 280-300 nm and for the enamine absorbances at 260-265 nm and 355— 365 nm are characteristic. 

By comparing with corresponding spectra of ion pairs in different solvents one obtains information on the local environment at the micellar surface73. (The polarity was expressed in terms of a so-called effective dielectric constant). Amphiphiles with a benzene ring also show an UV absorption74-76. UV spectra of solubilized species like benzene, naphthalene and pyrene have been extensively studied and compared with spectra of the compound in reference solvents to provide an estimate of the polarity in the vicinity of the solubilizate. 

---