Absorption ultraviolet

The absorption spectra of tryptophan derivatives in the region 310 to 250 nm are composed of two different types of k k electronic transitions. One is the A — Lb, while the other is the A — La type. Unfortunately, the intensities and shapes of the bands have been expecially difficult to determine owing to their extensive overlap. 

Measuring the UV absorption of NF2 at 260 nm became an established method of determining NF2 concentrations. The dissociation of N2F4 and formation of NF2 (see pp. 326/31) or the kinetics of reactions involving NF2 radicals can be followed by monitoring the absorption intensity at 260 nm. 

The extinction coefficient at 260 nm has been measured repeatedly the results, however, are unsatisfactory, except possibly those of [5, 6] (and thus [1]) who obtained comparable e values by two different methods. For some years, many authors [7 to 12] fortuitously ( ) measured e values of about 540 to 600 L mol cm T The values were thought to be in good or 

Published extinction coefficients e are listed in Table 14 and details concerning the method of measurement and the derivation of the data are given in the remarks following the table. 

The discrepant e values obtained from the shock-tube experiments [9 to 11], as compared to other reported e values [1,5], can be attributed to the higher NF2 concentrations used thus 

Considering the ionization potential of NF2 (see pp. 237/8) these bands were generally assigned as the first members of Rydberg series (i.e., n = 3) leading to NFJ ( A ), specific assignments, however, were not attempted [21]. 

In reality, the inherent ultraviolet edge of a glass is rarely observed. Very small concentrations of iron and other impurities result in very intense absorption bands. Since the absorption of energy is due to the transfer of an electron from the cation to a neighboring anion, these absorptions are said to be due to a charge transfer transition and the absorption band is called a charge transfer band. These bands are so 

Considerable concern has been generated in recent years over the possible presence of carcinogenic polynuclear aromatic hydrocarbons in mineral oils. 

Detection of these chemicals by ultraviolet absorption spectroscopy (ASTM D-2269) measures the absorbance over the wavelength range of 260-350 nm in a 10-mm cell of a dimethyl suffone extract of the oil. The polynuclear aromatic hydrocarbons present in the mineral oils are concentrated. In fact, the ultraviolet absorption level corresponds, approximately, to a maximum polynuclear aromatics content of about 5 ppm. 

For oils of a similar type, ultraviolet (UV) absorptivity is a good indicator of the resistance of an oil to discoloration under exposure to artificial or natural light. Oils with low absorptivity at 260 nm have been found to impart good color stability to light-colored rubber compounds (ASTM D-2008). 

Viscosity (ASTM D-445, IP 71) is one of the most important properties to be considered in the evaluation of a mineral oil. 

Requirements for viscosity vary widely according to the user for which the oil is intended and may be as low as 4cSt or as high as 70cSt. Mineral oil for internal use generally should have high viscosity to minimize possi-bihties of leakage. 

---

Note that in liquid phase chromatography there are no detectors that are both sensitive and universal, that is, which respond linearly to solute concentration regardless of its chemical nature. In fact, the refractometer detects all solutes but it is not very sensitive its response depends evidently on the difference in refractive indices between solvent and solute whereas absorption and UV fluorescence methods respond only to aromatics, an advantage in numerous applications. Unfortunately, their coefficient of response (in ultraviolet, absorptivity is the term used) is highly variable among individual components. 

The use of more complex or more costly articles of equipment, such as catalytic hydrogenation apparatus, autoclaves, polari-meters, ultraviolet absorption spectrometers, etc., has not been described, because the type of such apparatus employed indifferent laboratories varies considerably, and students must be taught the use of their own laboratory equipment. 

Tabus XIII. Ultraviolet Absorption Maxima oi cis and trass... 

Section A,7, Applications of infrared and ultraviolet absorption spectra to organic chemistry, should provide a brief introduction to the subject. 

As expected. 2-aminothiazole is more basic (piVj, = 5.28) than thiazole (pXj = 2.52) (681. Ultraviolet absorption properties as a function of pH... 

Characteristic spectroscopic data (infrared and NMR) of 2-thiazolyl ureas are given in a recent report (484). Their characteristic ultraviolet absorption is in the 260 to 270 nm region (487). 

Typical ultraviolet absorption data for the three protomers are approximately 250 nm for 211a, approximately 322 nm for 211b. and. 370 to 415 nm for 211c (451). Ultraviolet absorption at 305 nm, which had been previously associated with A-2-thiazoline-5-one (447). is in fact typical of 5-aceioxy derivatives (451). 

The ultraviolet absorption spectra of most new thiazoles currently synthesized have been described and occasionally used for structural... 

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. 

TABLE 1-15. ULTRAVIOLET ABSORPTION SPECTRUM OF THIAZOLE IN THE VAPOUR PHASE AND IN DIFFERENT SOLVENTS... 

TABLE 1-16 ULTRAVIOLET ABSORPTION SPECTRA OF THIAZOLE AND ITS MONOMETHYL DERIVATIVES... 

TABLE 1-17. ULTRAVIOLET ABSORPTION SPECTRA OF SOME TYPICAL derivatives OFTHIAZOLE IN EtOH SOLUTION... 

Quantum chemistry methods allow the prediction of the ultraviolet transitions in good agreement with the experimental values in the case of thiazole and its three methyl derivatives (Table 1-18). A very weak absorption has been indicated at 269.5 nm that could correspond to an n- TT transition given by calculation at 281.5 nm (133). Ultraviolet absorption spectroscopy has been investigated in connection with steric interactions in the A-4-thiazoline-2-thione (74) series (181). It was earlier demonstrated by NMR technique that 4-alkyl-3 isopropyl-A-4-thiazoline-2-thiones exist in solution as equilibrium mixtures of two conformers (75 and 76), the relative populations of which vary with the size of R4 (182) for R4 = rBu the population of rotamer A is 100%, whereas for R4 = Me it is only 28%. Starting from the observed absorption wavelength for... 

As in the case of pyridine (185), the quaternization of thiazole induces a bathochromic shift of the ultraviolet absorption spectrum in ethanol the long wavelength maximum at 232.3 nm (3900) for thiazole moves to 240 nm (4200) for 3-methylthiazolium tosylate (186) (Table 1-19). 

TABLE 1-19. ULTRAVIOLET ABSORPTION SPECTRA OF THIAZOLIUM TOSYLATES IN EtOH (186) COMPARED WITH CALCULATED TRANSITION ENERGIES (187)... 

Ultraviolet absorption data of different arylthiazoles are indicated in Table III-12. 

Effect of the monochromator s slit width on noise and resolution for the ultraviolet absorption spectrum of benzene. The slit width increases from spectrum (a) to spectrum (d) with effective bandpasses of 0.25 nm, 1.0 nm, 2.0 nm, and 4.0 nm. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

Many kinds of detectors have been designed, ranging from the widely used, cheap but robust flame ionization (GC) or ultraviolet absorption type (LC) to the much more exciting and informative, if much more expensive, mass spectrometer. 

In LC, the most common means for monitoring the eluant is to pass it through a cell connected into an ultraviolet spectrometer. As substances elute from the column, their ultraviolet absorption is measured and recorded. Alternatively, the refractive index of the eluant is monitored since it varies from the value for a pure solvent when it contains organics from the column. 

---