Ultraviolet Spectra The Effect of Conjugation

The wavelength necessary to effect the it it transition in a conjugated molecule depends on the energy gap between HOMO and LUMO, which in turn depends on the nature of the conjugated system. Thus, by measuring the UV spectrum of an unknown, we can derive structiual information about the nature of any conjugated tt electron system present in a molecule. 

Other kinds of conjugated systems, such as conjugated enones and aromatic rings, also have characteristic UV absorptions that are useful in stmcture determination. The UV absorption maxima of some representative conjugated molecules are given in Table 14.2. 

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Ultraviolet and Infrared Spectra.—The effect of conjugation in the excited state of some silicon-substituted thiophens has been discussed. The u.v. and i.r. characteristic absorption spectra of some pairs of cis- and rra/75-isomers of 2 - and 3 -substituted 2-styrylthiophens have been determined. It is claimed that the spectra of the stereoisomers are sufficiently different to assign the configuration. The i.r. and Raman spectra of 4-chloro-, 4-bromo-, 4-iodo-, and 4-cyano-2-fluoro-thiophens have been studied in the region 500—540 cm . The fundamental frequencies have been assigned, and normal co-ordinate calculations have been performed. The force constants were adjusted to reproduce accurately the fundamental frequencies. Thermodynamic functions and mean amplitudes of vibration for these molecules were also calculated. Thiophen-2-aldehyde and 19 related aldehydes have been studied under high resolution in the i.r. C=0 region. 

The following are data in ethanol [4] which illustrate the effect of lengthening the conjugated system in aliphatic unsaturated compounds Table (13). Kochany and Piotrowska [10] examined the ultraviolet-spectra of a number of nitroalkenes. Their w — tt were observed between 360 and 420 run. The ultraviolet absorption spectra of nitroethylene, nitropropenes and nitro methane have been taken and interpreted with the Pariser-Parr-Pople self-consistent field — MO calculation. The absorption bands it it are in good agreement with calculated values [11]. 

Figure 7.19 shows the ultraviolet spectra of naphthalene and anthracene. Notice the characteristic shape and fine structure of each spectrum, as well as the effect of increased conjugation on the positions of the absorption maxima. 

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic ultraviolet absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylcarbinols and the corresponding carbocation. 

The ultraviolet spectrum of 247 (R = R = H) is similar to that of o-aminodiphenylamine and a comparison of the two spectra indicates that in the case of 247 the restricting effect of the —CH2NH— bridge actually allows better conjugation than in the unbridged analog.287... 

Fluoride-ion-induced reactions A similar polymer to that in Figure 7.87 is obtained upon anionic polymerisation of hexafluoro-2-butyne initiated by fluoride ion in a solvent [311-313] (Figure 7.89). This is a clear example of an anionic polymerisation of an unsaturated fluorocarbon, although the growing anion can be trapped by a sufficiently reactive system [291, 314], such as pentafluoropyridine [315] (Figure 7.90). There is little difference between the ultraviolet spectra of 7.90A and 7.90B, confirming that conjugation in the polyene system is inhibited by steric effects. 

The conformational dependence of the 7c-donor properties of the cyclopropyl group is of decisive importance in the behavior of this substituent, and measurements of ultraviolet spectra, particularly in conformationally fixed derivatives, have been especially useful in elucidating this effect " . Maximum conjugation occurs when the plane of the cyclopropyl ring is most nearly parallel to the adjacent 7c-system, and this is most commonly expressed as an angular dependence, with overlap at a maximum in the bisected conformation 7a, b and at a minimum in the perpendicular arrangement 7c, d (Figure 1). 

Moreover, all the complicating factors mentioned above in connection with the use of bo for determination of helix content may also affect the estimates by Eqs. (19) and (20). This includes overlapping effects of transitions of molecules conjugated to proteins however, at a distance from the active band center their influence would, in general, be smaller in CD spectra because of their steeper wavelength dependence as compared to ORD. Among possible contributions to the optical activity by other structures, -structures may be prominent in proteins [e.g., (15,61,69) and refs, cited therein]. Fig. 4 illustrates far ultraviolet CD spectra of polyamino acids in different conformations. 

An important feature of ultraviolet absorption spectra is that chromophores that are not conjugated give, not ir ir bands, but a summation of n w bands. A -CH2- group is sufficient to isolate two chromophores, but -0-, -S-, or -NH- is not. An example of this effect is seen for hexacene, a green compound, and 6,15-dihydrohexacene, a colorless compound whose absorption spectrum is essentially the sum of the spectra of anthracene and naphthalene ... 

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