Second primary band

Researchers have observed that the primary and secondary bands in the spectra of polynuclear aromatic hydrocarbons shift to longer wavelength. In fact, even the second primary band, which appears at 184 nm for benzene, is shifted to a wavelength within the range of most UV spectrophotometers, This band lies at 220 nm in the spectrum of naphthalene. As the extent of eonjugation increases, the magnitude of the bathochromic shift also increases. 

There are three dipole allowed singlet states in the region of the second absorption band, IA2", 3E and 4Ef, all of which possess approximate 3d Rydberg character (Table 24). Experimental and calculated oscillator strength ( xp = 0.04,/theor 0.01 for all three states) differ, which could not be resolved. The principal transition of the third absorption has frequently been assigned to the transition 3e —> la/, but theoretical values for the energy of the state, for which the transition 3e 1 a is the primary contribution, are 2-3 eV high-... 

Primary and secondary amides show a second strong band in the 1600—1500 cm" region which is absent from the spectra of tertiary amides and also from those of small ring cyclic lactams under normal conditions. The origins of this band are different in the two cases. Nevertheless, the band is referred to generally as the amide II band. 

Here, A is the nearly isotropic nuclear coupling constant, I is the nuclear spin (Iun = I), and m is the particular nuclear spin state. It may be observed that the zero field splitting term D has a second-order effect which must be considered at magnetic fields near 3,000 G (X-band). In addition to this complication nuclear transitions for which Am = 1 and 2 must also be considered. The analysis by Barry and Lay (171) of the Mn2+ spectrum in a CsX zeolite is shown in Fig. 35. From such spectra these authors have proposed that manganese is found in five different sites, depending upon the type of zeolite, the primary cation, and the extent of dehydration. 

The study of pseudoisocyanine absorption, LD and CD, in the presence of DNA revealed the appearance of new bands in addition to the monomer one in CD and LD spectra [36]. Increase in dye to DNA concentration ratio results in the generation of primary short-wavelength, and then long-wavelength CD and LD bands. The first one was attributed by authors to the dimer between the partly intercalated pseudoisocyanine molecule and another one bound to it, while the second corresponded to the J-aggregates of pseudoisocyanine molecules formed in the presence of DNA. 

The second derivative of a spectrum has been frequently used to identify individual bands underlying composite spectral features [67]. The local minima of the second derivative provide excellent approximations of band center positions. The presence of spectroscopic noise is the primary difficulty encountered with this approach. Noise interferes with the enumeration of bands as well as the accurate identification of the band center positions. A procedure has been developed for A kxv i- icxv) vvhich starts with second derivative information, provides a sequence of more and more refined maps, and results in a final estimate of the 2D tracks [68]. 

Such normal vibration analyses have been applied to the spectra of macromolecules to only a limited extent. In the first place, the only structure which has been analyzed in detail is that of the planar zig-zag chain of CHg groups, i.e., polyethylene. Neither substituted planar zig-zag chains nor the helical chain structures characteristic of many polymers [Bunn and Holmes (28)] have been submitted to such a theoretical analysis. In the second place, even for the case of polyethylene the answers are not in all instances unambiguous. Different assumptions as to the nature of the force field, and lack of knowledge of some of the force constants, has led to varying predictions of band positions in the observed spectrum. For the identification of certain modes, viz., those which retain the characteristics of separable group frequencies, such an analysis is not of primary importance, but for knowledge of skeletal frequencies and of interactions... 

A third absorption is usually observed at approximately 185-195 nm. For acyl silanes with aromatic substituents, this may be ascribed to a primary arene band or could arise from a second jr-jr transition, but for other acyl silanes, this transition (at 195 nm, e = 4200 for acetyl trimethylsilane) is presumably the latter. In general, however, the nature of the groups attached to the silicon atom (other than the acyl group) has little effect on the energies of the n-n and tt-tt transitions5. 

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