Primary bands

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

TABLE 7.14 Primary Bands of Substituted Benzene and Heteroaromatics In methanol. 

The —I—M 3-substituted thiophenes in alcohol show only one band, and, as is found in the 2-isomer, this is displaced (with increased extinction) toward longer wavelength with increasing conjugating power of the substituent. It is probable that this is the displaced 235-m/A band of thiophene, since the spectra of 3-acetylthio-phene and 3-cyanothiophene also show a primary band at about 225 mju, in hexane solution. ... 

In alkyl aryl sulfoxides the Cotton effect corresponding to the UV primary band occurring at about 235 to 255 nm has a high molecular amplitude and the positive agn characterizes the absolute configuration R. 

TABLE 6.6 Primary Band of Substituted Benzene and Heteroaromatics... 

FTIR spectra in the near-IR region consist entirely of overtones and combinations of primary bands within the mid-IR region. For macromolecules or complex mixtures, the excessive overlapping of bands produces a diffuse absorption continuum with few characteristic features, making unequivocal band assignment practically imposssible. Thus, this spectral range has a limited use in qualitative analysis. Even so, a major asset of near-IR analysis is the ease with which reproducible spectra can be obtained by reflectance and transreflectance (a combination of transmission and reflectance) techniques in every state of aggregation without complicated sample preparation. 

Various approximations are involved in the derivation of Koopmans Theorem, so quantitative predictions of IE are not accurate, but it is of considerable use in predicting the number and type of primary bands in the PE spectrum of a molecule, though not necessarily their energy ordering. 

PE spectra can show bands corresponding to ion states that cannot be accessed from the ground-state molecular configuration by the removal of one electron. Such signals can normally be represented by removal of one electron synchronized with excitation of another they are hence referred to as many-electron processes, and the additional satellite structure accompanying the primary band as shake-up structure. A classic example of a two-electron transition is the very weak band found in the spectrum of atomic mercury for the process shown in equation (12) ... 

In some works a tendency toward convergence of the volcanogenicsedimentary and clastic-sedimentary hypotheses is noted. Belevtsev et al. (1966), who considered mainly the clastic-sedimentary hypothesis, postulate the extensive occurrence of acid waters in the Precambrian hydrosphere as the result of intensive volcanic activity. Tyapkin and Fomenko (1969) believe that the main source of iron and silica in the Precambrian was the basic rocks which were the chief constituent of the Earth s crust at that time, but that some was also derived from basaltic rocks erupted along abyssal faults and other products of basic volcanism. In this case it is impossible to deny the possibility that part of the iron and silica was supplied to the sea basins along with products of volcanic activity. In this scheme the role of volcanic activity in the formation of the BIF comes down chiefly to the creation of acid environments which promoted the leaching of iron compounds from basic rocks and its transport and subsequent accumulation. The primary banding is explained by periodic revival and extinction of volcanic activity, as a result of which the pH of the water basin varied, which ultimately led to deposition of iron or cherty sediments in turn. The periodicity of those cycles might have been of the order of several hundred years. 

The simplest aromatic compound is benzene. It has a ring current of tt electrons, which shows strong ti -> ti absorptions at 184 nm 60,000), and at 204 nm 7900). (This is called a primary band.) Benzene exhibits a low intensity band at 256 nm 200) (Known as a secondary or fine-structure band), with a series of fine-structue bands between 230 and 270 nm). Any substitution n the benzene ring, irrespective of its electronic character... 

Another approach to predicting, the Xmax of the primary band of substituted benzenes involves the use of Table 1.5. This table has been successfully used with disubstituted compounds when the following rules are used ... 

Table 1.5 Calculation of the Primary Band (71 substituted Benzenes (CH3OH solvent) Base Value 203. 5 nm...

Compound Solvent Primary band Secondary band ... 

Instrument 683 Slit W Multiplier 4 Mode DB EVALUATION TYPE 12 PRIMARY BAND ... 

Effects of substitution on the 200 m/x band of benzene have been studied in great detail by Doub and Vandenbelt10. They find that the ratio of the wavelengths of the 260 mp band (primary band) and the 200 mp band... 

The spectrum of decamethylbiphenyl is quite similar to that of hexamethyl-benzene in double concentration (Figure 8.3). The replacement of the oriho-methyl groups by bulkier groups, like the t-butyl group, results in a further decrease in intensity and increase in the fine structure of the benzenoid band at 260 m/i. In addition, the primary band also undergoes a blue-shift. The effect of increasing the size of the substituents in 2,2 -disubstituted biphenyls... 

Sangster (36) has investigated the transient spectra observed following the pulse radiolysis of degassed aqueous benzoate solutions at pH 12.8. A band at 3280 A. was ascribed to the OH radical adduct and two bands at 3220 and 4440 A. to the protonated form of the electron adduct. These wavelengths slightly differ from those given by Chutny (20) for the primary bands, viz. OH-adduct 3450 A., H-adduct 3450 A., electron adduct 3130 A., but this may arise in part from differences in pH. 

---