Extinction coefficient, hydroxyl

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. 

Ionization of the phenol hydroxyl group in tyrosine shifts the 277-nm absorption peak to 294 nm and the 223-nm peak to 240 nm. The molar extinction coefficient for the peak of the lower energy band increases from about 1350 M l cm-1 to about 2350 M em-1 and for the higher energy band from about 8200 M cm-1 to about. 1,000 M l cm-1.113 141 In addition, the lower energy absorption band of tyrosine shows vibrational structure that is lost upon ionization of the phenol side chain. 

The commonest modern method for determining the degree of hydration is to measure the intensity of the broad n- carbonyl absorption band at about 280 m/x, which disappears on hydration. Early measurements (Schou, 1926, 1929 Harold and Wolf, 1929, 1931) show considerable discrepancies, but the results of later workers are in reasonable agreement. The main uncertainty lies in the value to be assigned to the maximum extinction coefficient of the unhydrated carbonyl compound, which varies between 12 and 80 for different compounds. This is commonly taken as the value measured in a non-hydroxylic solvent such as hexane or cyclohexane, but this is not strictly valid, since the intensities of n-n- transitionsvary somewhat with the solvent (see e.g. Dertooz and Nasielki, 1961) moreover, since the shape of the band and the value of e are also solvent-dependent it may make some difference whether the extinction coefficients are compared at the same wavelength, at the respective maxima, or in terms of the band area. Special difficulties arise... 

The carboxyl function does absorb ultraviolet radiation, but the wavelengths at which this occurs are appreciably shorter than for carbonyl compounds such as aldehydes and ketones, and, in fact, are out of the range of most commercial ultraviolet spectrometers. Some idea of how the hydroxyl substituent modifies the absorption properties of the carbonyl group in carboxylic acids can be seen from Table 18-2, in which are listed the wavelengths of maximum light absorption (Amax) and the extinction coefficients at maximum absorption (emax) of several carboxylic acids, aldehydes, and ketones. 

The mechanism of 4-chloroaniline photochemistry was independently studied by Guizzardi et al. in organic solvents they reached very similar conclusions [57]. These authors pointed out that the aminophenyl cation has a triplet-diradical character which fully explains its reactivity in organic solvents [57]. However, in aqueous solutions the cation reacted with hydroxyl ions with a rate constant of 3.1 x 1010 M s, which can only be interpreted as a deprotonation reaction [55]. The carbene 4-iminocyclohexa-2,5-dienylidene thus must exist in aqueous solutions, even though its properties have not yet been characterized. This is partly due to an expected low extinction coefficient, similar to the neutral anilino radical [55]. Following these arguments, the primary pathways of 4-chloroaniline photolysis in polar solvents may be pictured as shown in Scheme 7. 

The most recent value for the extinction coefficient of the benzyl radical (in cyclohexane solution) is 1.2 X 104 M"1 cm."1 at 317 m/x and 6 X 10PM"1 cm."1 at 360 m/x (16). Other recent values are 1.9 X 104M 1 cm."1 at 318 mp (20) and 1.8 X 1(PM"1 cm."1 at 320 m/x (17). If we assume that similar extinction coefficients apply in aqueous solution, that the hydroxyl adducts of styrene and a-methylstyrene have the same extinction coefficient as benzyl, and that no other species absorb at these wavelengths, we would estimate that about 20-40% of the OH radicals add to the -position of the vinyl group for styrene and 15-30% for a-methylstyrene. 

Rate constants for methanol and ethyl alcohol relative to those for benzoate ion, phenylacetate ion and p-nitrobenzoate ion are shown in Table III. Each value in the table consists of experiments at five separate concentration ratios. The random uncertainty in each value is less than 10%. In determining these rate constants from optical density ratios it was necessary to make a small correction for the contribution to the optical density by the H-adduct free radical. The molar extinction coefficients at 340-350 m/x for the H-adduct and OH-adduct are similar for benzoic acid (22) and were assumed to be comparable for the other two aromatic ions in the table. The correction is necessary since the rate constants for the reaction of hydrogen atoms with the alcohols used are two orders of magnitude lower than the rate constants for hydrogen atom addition to the aromatic ring, while the analogous hydroxyl rate constants are roughly comparable. 

The concentration of hydroxyl groups formed was estimated by comparing the intensity of the 0-H stretching bands centered at 3250 cm to the intensity of the ester carbonyl stretch band at 1735 cm in the untreated film. The molar extinction coefficients for the hydroxyl and carbonyl stretch bands were calculated from the model studies. 

Solomon (16,has uset a different method to obtain extinction coefficients. Essentially, total hydrogen content from elemental analysis and hydroxyl content from measurements of the area of the 0-H stretching band near 3450 cm were used in conjunction with the peak areas of aliphatic and aromatic bands to obtain a plot from which extinction coefficients can be determined. In principle, this approach appears to be sound, but there are a number of problems. One difficulty, discussed above, is general to all infrared methods that have been employed so far what errors are introduced by summing peak areas over a number of bands, each of which has an individual extinction coefficient, and essentially averaging such coefficients for the total area Other problems involve the correct use of curve resolving techniques and the measurement of hydroxyl groups, which we will now consider in more detail. 

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