The Spectra of Aromatic Hydrocarbons

Bands of the first type ( L,) are of low intensity (e = 10 -10 ), may be hidden by the other bands, and often possess a complicated vibrational structure. Bands of the second type ( L or BJ are moderately intense (e 1(H) Lg bands usually show a regular vibrational structure ,  

Bands of the third type ( B,) are very strong (e 10 ) and have little vibrational structure. 

The Lh, Lj, Bb, B nomenclature originates in the perimeter model, discussed in detail in Section 2.2.2. 

In general the HOMO and LUMO of condensed aromatic hydrocarbons are not degenerate, so simple theory gives a HOMO- LUMO transition and a degenerate transition from the HOMO pi into the second LUMO or from the second HOMO to the LUMO 

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To determine the effect of the positive charge on the changes in chemical shifts in benzenium ions at sp -hybridized carbons these changes should be counted off from the position of protons and proton-containing groups in the spectra of aromatic hydrocarbons rather than from that of the signals in the appropriate unsaturated compounds. 

Ham, N. S., and Ruedenberg, K., J. Chem. Phys. 25, 1, 13, Electron interaction in the free-electron network model for conjugated systems. I. Theory. II. Spectra of aromatic hydrocarbons."... 

MO) with the protons in the nodal plane. The mechanism of coupling (discussed below) requires contact between the unpaired electron and the proton, an apparent impossibility for n electrons that have a nodal plane at the position of an attached proton. A third, pleasant, surprise was the ratio of the magnitudes of the two couplings, 5.01 G/1.79 G = 2.80. This ratio is remarkably close to the ratio of spin densities at the a and (3 positions, 2.62, predicted by simple Hiickel MO theory for an electron placed in the lowest unoccupied MO (LUMO) of naphthalene (see Table 2.1). This result led to Hiickel MO theory being used extensively in the semi-quantitative interpretation of ESR spectra of aromatic hydrocarbon anion and cation radicals. 

Fig. B3.1. Illustration of the vibrational bands in the absorption and fluorescence spectra of aromatic hydrocarbons. Broadening of the bands will be explained in Section 3.5.1.

The fluorescence spectrum of a compound may be used in some cases for the identification of species, especially when the spectrum exhibits vibronic bands (e.g. in the case of aromatic hydrocarbons), but the spectra of most fluorescent probes (in the condensed phase) exhibit broad bands. 

Jones, R. N. The Ultra-violet absorption Spectra of Aromatic Hydrocarbons. 

Marchetti, A. P., Kearns, D. R. Investigation of singlet-triplet transitions by the phosphorescence excitation method. IV. The singlet-triplet absorption spectra of aromatic hydrocarbons. J. Am. Chem. Soc. 89, 768 (1967). 

Absorption and fluorescence spectra of aromatic hydrocarbons are not greatly affected by change of solvent, except for small solvent shifts. At low temperatures the vibrational structure of the bands sharpens up, and some peculiar solvent effects have been noted. When frozen in solution of normal paraffins coronene shows doubling of some of its vibrational bands, and the separation of the components varies with the number of carbon atoms in the solvent molecule chain. The most probable cause is some size-relationship factor between solvent and solute molecules (7). 

All the properties that can be calculated by ab initio methods can in principle also be calculated semiempirically, bearing in mind that the more the molecule of interest differs from the training set used to parameterize the semiempirical program, the less reliable the results will be. For example, a program parameterized to predict the UV spectra of aromatic hydrocarbons may not give good predictions for the UV spectra of heterocyclic compounds. NMR spectra are usually calculated with ab initio (Section 5.5.5) or density functional (Chapter 7) methods. UV... 

UV spectra of aromatic hydrocarbons are characterized by three sets of bands (El, E2, and B bands) that originate from ti ti transitions. Generally the E2 and B bands are of most interest to chromatographers, since the solvent cutoff for most mobile phases is <200 nm. 

The azido group affects the spectrum of the hydrocarbon itself in two ways it causes a red shift of the L -band and a smaller red shift of the other bands (the size of the aromatic system is increased) it also reduces the symmetry of the molecule, thus enhancing the extinction of the symmetry forbidden transition at the expense of the associated B-transition (intensity borrowing ). From the size of the effect it can be inferred that the inductive interaction of the azido group with the ring is comparable with, if somewhat weaker than, that of the amino group For the spectra of aromatic diazides see reference 40. 

The electronic spectra of cyclic conjugated n systems depend inherently on the number of n electrons. Closed-ring systems with AN+l n electrons in the perimeter are aromatic compounds, of which benzene is the most important representative. Benzenoid hydrocarbons constitute a class of compounds whose UV spectra have been investigated most extensively both experimentally and theoretically. The fact that the spectra of aromatic compounds are so characteristic meant that formerly they were of considerable importance in the structure determination of organic compounds. However, these spectra cannot be explained in terms of the simple HMO model. If one seeks a theoretical basis for an understanding, one has the choice between the perimeter model and the Pariser-Parr-Pople or a more complicated numerical method. Before discussing these theoretical models, some empirical relations will be presented. Finally, cyclic systems derived from a perimeter of 4N Jt electrons will be considered. 

Figure 2.7. Absorption spectra of naphthalene, anthracene, and tetracene as typical examples of the UV spectra of aromatic hydrocarbons (by permission from DMS UV-Atlas, 1966-71).

This book is based on the reactions of thermal electrons with molecules. The ECD, negative-ion chemical ionization (NICI) mass spectrometry, and polaro-graphic reduction in aprotic solvents methods are used to determine the kinetic and thermodynamic parameters of these reactions. The chromatograph gives a small pure sample of the molecule. The temperature dependence of the response of the ECD and NIMS is measured to determine fundamental properties. The ECD measurements are verified and extended by correlations with half-wave reduction potentials in aprotic solvents, absorption spectra of aromatic hydrocarbons and donor acceptor complexes, electronegativities, and simple molecular orbital theory. 

In a series of papers published in 1953, Pariser, Fumi, and Parr16 w 54>55 put forward a set of simplifying assumptions which together constituted a semiempirical theory for the interpretation of aromatic hydrocarbon spectra. 

It may be said, therefore, that the electronic spectra of aromatic hydrocarbons are now well understood. The essential features of the present theory are... 

The establishment of a satisfactory theory of the electronic spectra of aromatic hydrocarbons has paved the way for a useful... 

Van Duuren, B.L. The fluorescence spectra of aromatic hydrocarbons and heterocyclic aromatic compounds ... 

With the advent of esr spectroscopy aromatic-antimony pentachloride precipitates were shown to contain the aromatic cation radical (Weissman et al., 1957), and this in turn accounted for the earlier discovery of paramagnetism in the salts obtained from reaction of aromatic amines with antimony pentachloride (Kainer and Hausser, 1933). Characterization of cation radicals in nitro-methane and nitrobenzene solutions of antimony pentachloride by visible spectroscopy soon followed. Eventually, by working with degassed solutions of antimony pentachloride in dichloromethane at —70° it was possible to obtain esr spectra of aromatic hydrocarbon cation radicals with extraordinarily well-resolved hyperfine patterns (Lewis and Singer, 1965, 1966). Similar success was obtained with alkyl aryl ether (Forbes and Sullivan, 1966), and organosulfur cation radicals in aluminum chloride-nitromethane solutions at —50° (Shine and Sullivan, 1968 Sullivan, 1968). In this work, resolution... 

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