Ultraviolet absorption bands

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... 

The Reich test is used to estimate sulfur dioxide content of a gas by measuring the volume of gas required to decolorize a standard iodine solution (274). Equipment has been developed commercially for continuous monitoring of stack gas by measuring the near-ultraviolet absorption bands of sulfur dioxide (275—277). The deterrnination of sulfur dioxide in food is conducted by distilling the sulfur dioxide from the acidulated sample into a solution of hydrogen peroxide, foUowed by acidimetric titration of the sulfuric acid thus produced (278). Analytical methods for sulfur dioxide have been reviewed (279). 

Kuokkanen (1986, 1987 a, 1991) supported the proposal of Nakazumi et al. (1983) based on kinetic and spectrophotometric comparisons of arenediazonium salt solutions in the presence of 18-crown-6 and pentaglyme. He also extended the systematic work on complex formation of benzenediazonium salts, substituted in the 2-position, and in the presence of 15-crown-5 (Kuokkanen, 1990 Kuokkanen et al, 1991). He discovered a useful way to differentiate between the two types of complexes in Scheme 11-2. Increasing the relative concentration of the host compound shifts the ultraviolet absorption band of both types of complex hypsochromically, whereas the NN stretching frequencies are significantly increased only in the case of insertion complexes. ... 

My interest at that time revolved around evaluating optical rotary dispersion data [12]. The paired values of optical rotation vs. wavelength were used to fit a function called the Drude equation (later modified to the Moffitt equation for William Moffitt [Harvard University] who developed the theory) [13]. The coefficients of the evaluated equation were shown to be related to a significant ultraviolet absorption band of a protein and to the amount of alpha-helix conformation existing in the solution of it. 

Photodissociation of the water molecule is a model system for both experimental and theoretical studies. Extensive experimental and theoretical studies have been performed on this system during the last few decades. Excitation in its longest wavelength ultraviolet absorption band around 150-200nm leads to the lowest excited singlet state Dissociation... 

Using the simple analysis outlined in Section 8.1, it is possible to predict the effects a substituent will have on the observed ultraviolet absorption bands of benzene. As was done before, substituents as well as their effects can be classified into four major groups. 

The C state has attracted much attention from spectroscopists because the state was believed to be the upper state of the ultraviolet absorption band which can help establish the accurate dissociation energy of the ground state of H2 [82]. Therefore, several attempts have been made to obtain the complete PEC for the C state [53,76,83,84]. The calculations by Browne revealed that there exists a suspicious maximum at i = 8a.u. which is about 160 cm above the dissociation limit [76]. This observation was consistent with the conclusion of Herzberg and Monfils that a maximum might exist in the vicinity of / — 13 a.u. [82]. The subsequent calculations by Kolos and Wolniewicz [53], and Namioka [77] refined the barrier to be about 105.5 cm at i = 9 a.u. The computed PEC for the C state and the fitted spectroscopic parameters (r, v, v Xe, a ) in the present study agree well with the experiment, as demonstrated by Table 8. 

Table III. Assignment of Ultraviolet Absorption Bands in Irradiated Polyethylene (11)...

Principal Ultraviolet Absorption Bands of Isatogens18 and Indolones49 50... 

Sukhorukov et al. 99 measured the displacement of the near ultraviolet absorption bands of several cytosines upon transition from the neutral to the ionic form and evaluated the ionization constants in the excited state of the compounds. They concluded that on excitation there was no considerable shift in the tautomeric equilibrium of the forms 2 and 6 (in the excited state) = 1.55 x 104]. They also... 

In the vacuum ultraviolet absorption bands in the region 1280 to 1600 A correspond to the fourth positive system A1 n-X L+. The absorption cross sections of this system are given in Fig. V-7. Since the widths of the CO rotational lines are much smaller than the instrumental resolution ( 10 cm" 1), it is not possible to obtain the absorption cross section of each rotational line [see Section 1-8 for details]. Thus, the cross sections shown in Fig. V-7 are much less than the true cross sections. An estimate of the integrated absorption coefficient of the (0,0) band is 1.7 x 104cm-latm-1 (899). Various electronic states and transitions are given in Fig. V-8. 

To clarify the effect of substituents we will discuss the spectrum of 4-nitro-benzenol, even though the compound has no value as a dye. It is a pale yellow compound ( raax 320 nm) with an ultraviolet absorption band tailing into the visible, as in Figure 28-8. Its close relatives are benzene, benzenol, and nitrobenzene ... 

TABLE 6. Molecular refraction and ultraviolet absorption bands of common substituents... 

In the second method measurements were made of the variation with temperature (25-150°C) of the intensity of an ultraviolet absorption band centered at 2600 A (100, 237). It was shown that this band is associated with the NF2 radical and that the intensity is proportional to its concentration. The heat of dissociation found was 90.8 kJ mol-1. In the third method the height of a peak due to the NF2 radical in the electron paramagnetic resonance spectrum was measured over the temperature range 70-180°C. The heat of dissociation found was 89.9 + 6.7 kJ mol-1 (237). 

McGrath and Morrow " studied the reactions of both 0( D) and 0( P) with cyanogen at room temperature by flash photolysis. The 0( D) was produced by the photolysis of ozone. The reaction was monitored by absorption spectroscopy. At first, they attributed the previously unidentified ultraviolet absorption bands at 3250-3330 A to the fulminate radical (CNO), concluding that the initial step of the reaction was... 

The highly levorotatory y-hexaacetate would have to be a structural isomer of the two anomeric a and /3 hexaacetates. It was pointed out that the aldehyde structure XLIII was improbable from the mode of its formation moreover it was excluded by the fact that the compound failed to give rise, on treatment with thiosemicarbazide, to the ultraviolet absorption band at 2700 A characteristic of tiuosemicarbazones. Since ring isomerism in the N-methyl-L-glucosamine portion or a 1,3-orthoacetate structure did not seem probable, the 1,6-furanose structure (XLIV) was tentatively proposed for this compound. 

Concluding this section all that one can say is that we found no relationship between anesthetic potency and either the ionization potentials or the frequency of the lowest ultraviolet absorption band. The observation that replacement of a fluorine atom by a hydrogen usually lowers the IP is probably of some value. However, as was pointed out above this could only indicate the possibility of charge transfer interaction if the electron affinities followed the same trend. Unfortunately these have not been determined and the variations in the frequencies of the broad UV bands are too irregular to draw conclusions. It seems that there exists an indirect relationship between the acidity of these molecules and their IPs and what counts is their proton donor ability connected with the acidic hydrogen as has been concluded from the infrared studies described in previous sections. 

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