Hydrocarbons ultraviolet spectrum

The ultraviolet spectra of these compounds are similar to those of trans stilbene or of 2- and 4-stilbazole. The effect on the ultraviolet spectrum of various substituents have been found to parallel in many respects the efiects produced by the corresponding group in derivatives of aromatic hydrocarbons (142). 

There is considerable interest in establishing the location within a micelle of the solubilized component. As we have seen, the environment changes from polar water to nonpolar hydrocarbon as we move radially toward the center of a micelle. While the detailed structure of the various zones is disputed, there is no doubt that this gradient of polarity exists. Accordingly, any experimental property that is sensitive to the molecular environment can be used to monitor the whereabouts of the solubilizate in the micelle. Spectroscopic measurements are ideally suited for determining the microenvironment of solubilizate molecules. This is the same principle used in Section 8.3, in which the ultraviolet spectrum of solubilized benzene was used to explore the solvation of micelles. Here we take the hydration for granted and use similar methods to locate the solubilizate. 

A molecular orbital calculation of the ultraviolet spectrum of the quinolizinium cation has been made employing self-consistent field molecular orbitals of the parent hydrocarbon.34 Of the four band positions calculated on this basis, only two have been observed. The calculated and observed values are given in Table IV. 

Water as Reaction Medium. Being transparent to the near-ultraviolet spectrum of sunlight, pure water can serve as an inert medium in which pesticide transformations take place. For example, Henderson and Crosby (28) have shown that suspensions of the chlorinated hydrocarbon insecticide dieldrin (I), although essentially insoluble in water, undergo a photocondensation reaction to give photodieldrin (II) (Equation 2). 

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. 

Azulene. The absorption spectrum of azulene, a nonbenzenoid aromatic hydrocarbon with odd-membered rings, can be considered as two distinct spectra, the visible absorption due to the 1Lb band (0-0 band near 700 nm) and the ultraviolet absorption of the 1L0 band.29 This latter band is very similar to the long wavelength bands of benzene and naphthalene CLb) and shows the same 130 cm-1 blue shift when adsorbed on silica gel from cyclohexane.7 As in the case of benzene and naphthalene, this blue shift is due to the fact that the red shift, relative to the vapor spectra, is smaller (305 cm"1) for the adsorbed molecule than in cyclohexane solution (435 cm"1). Thus it would appear that the red shifts of the 1La band are solely due to dispersive forces interacting with the aromatic molecule, in agreement with Weigang s prediction,29 and dipole-dipole interaction is negligible. 

Ultraviolet Radiation The portion of the electromagnetic spectrum emitted by the sun adjacent to die violet end of die visible light range. Often called black light , it is invisible to the human eye but when it falls on certain surfaces it causes them to fluoresce or emit visible light responsible for the photo-oxidation of certain compounds including hydrocarbons. 

It is in this afterglow region where most of the light is emitted. In a pure hydrogen-air flame, virtually the only light emitted is from the OH radical, which is in the ultraviolet region of the spectrum, and therefore invisible. If hydrocarbons are present, then the blue, green, and yellow emissions by the radicals CH and C2 are visible, and possibly the incandescence of hot soot particles. 

The ultraviolet absorption spectrum of cycloheptanone in hydrocarbon solution is similar to that of cyclohexanone both in its position and in its diffuseness (4). 

Evidence of intramolecular overcrowding in 3,4-benzophenanthrenes, obtained from the ultraviolet absorption studies, has been discussed by Johnson (1959). It was predicted that the methylene-bridged hydrocarbon, 1,12-methylenebenzophenanthrene (85), should afford the spectrum of a 1,12-dimethylbenzophenanthrene devoid of any steric interference induced by the overcrowded methyl groups. A comparison of the ultraviolet absorption spectra of 1,12-dimethylbenzophenanthrene (83), benzophenanthrene (3), 6,7-dimethylenebenzophenanthrene (84),... 

To relate the wettability changes more firmly to the photooxidation processes and products, a detailed study was carried out with polystyrene. This polymer was selected because the formation of oxidation products in the hydrocarbon surface gave rise to large changes in wettability and because these products would be readily accessible to optical methods of analysis. The ultraviolet absorption spectrum of polystyrene shows a sharp cut-off, and the extinction coefficients for the radiation absorbed are sufficiently high that almost all of the photochemical reaction should be confined to the surface layers. 

Ultraviolet Spectrophotometry Pesticides in food, drink, and biological fluids may be detected by ultraviolet spectrophotometry, usually after a simple extraction procedure. However, care must be taken to ensure that there are no ultravioletabsorbing co-extracted compounds which could interfere. The main drawback of the method is a lack of specificity, because the UV spectrum usually only indicates the group to which a particular pesticide belongs. In addition, those pesticides which lack a chromophore, e.g. chlorinated hydrocarbons, cannot be screened by this method. Nevertheless, UV data can be useftil when used in conjimction with data derived from chrom-atographic methods. Table 2 (p. 85) gives data for those pesticides which show significmt UV absorption. 

If the solvent is to be used for ultraviolet spectroscopy, it is necessary to remove all the aromatic compounds. This may be done by shaking the hydrocarbon with a mi.xture of concentrated sulfuric and nitric acids, which will nitrate the aromatic compounds. The hydrocarbon is separated, washed with water, distilled, and passed through a column of activated alumina which will remove any residual unsaturated or nonhydrocarbon materials. The spectrum of the solvent is monitored as it passes from the column, and when significant absorption at 210 m/i is observed, the alumina is replaced. 

Atomic selenium was monitored in flashed CSe2 by kinetic absorption spectroscopy, and its rate of reaction with ethylene was measured in the temperature range 302-412 K. The rate of appearance of a new spectrum in the far ultraviolet (intense bands at 2259 and 2208 A) was symmetrical with atomic decay, and was assigned to ethylene selenide, i.e. selenirane (1) (Equations (la) and (lb)). Absorption spectra of flashed CSe2 and alkene mixtures show band systems in the 2000-2300 A region which do not occur in flashed saturated-hydrocarbon and CSe2 mixtures. The band centers obtained from their spectra are listed in Table 4. 

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