Polyene spectrum

The UV/vis spectrum of the chlorophylls is a combination of the spectrum of an aromatic system with a relatively intense (e = 1 x 10 ) Soret band and a polyene spectrum represented by an almost equally intense long-wavelength band (y = 660 nm, = 8 x lO ) (Fig. 6.2.9). The spectrum is understandable be-... 

The spectrum shown in Fig. 7.5 shows the appropriate portion of the spectrum for a copolymer prepared from a feedstock for which fj = 0.153 It turns out that each polyene produces a set of three bands The dyad is identified with the peaks at X = 298, 312, and 327 nm the triad, with X = 347 367, and 388 nm and the tetrad with X = 412 and 437 nm. Apparently one of the tetrad bands overlaps that of the triad and is not resolved. Likewise only one band (at 473 nm) is observed for the pentad. The identification ol these features can be confirmed with model compounds and the location and relative intensities of the peaks has been shown to be independent of copolymer composition. 

The polyene Amph B (intravenous formulation) has the broadest spectrum, is fungicidal and shows its superiority in immunosuppressed patients. Its only drawback is its infusion-related toxicity and its negative influence on renal function. Acute reactions to Amph B - usually fever chills, rigor and nausea - can be... 

According to the Woodward-Hoffmann rule [6, 7], conjugate polyenes with 4n and 4n+2 n electrons undergo cychzations in conrotatory and disrotatory fashions under the thermal conditions, respectively. Recently, novel cycloisomerizations were found to be catalyzed by Lewis acid and to afford bicychc products [39] as photochemical reactions do [40]. The new finding supports the mechanistic spectrum of chemical reactions. 

Allenic groups — Neoxanthin, a xanthophyll found in many foods, has an allenic group at the C-6,7,8 position where the two double bonds are perpendicular to each other, and the C-7,8 double bond coplanar with the polyene chain contributing effectively to the chromophore since the C-6,7 bond is in a different plane, it makes no contribution. Therefore, neoxanthin, despite its 10 conjugated double bonds, has a UV-Vis spectrum similar to that of a conjugated nonaene such as violaxanthin. 

When dichloromethane (DCM) solutions of the polyenes which had been prepared as described (39) were added to DCM solutions of TFA new species were formed which had strong absorptions in the region 500-850 nm. Figure 6 shows seme typical spectra for such a solution, (a) immediately after mixing, (b) after a further 20 minutes and (c) after 200 minutes. In spectrum (a) clearly defined maxima are visible at 590, 660, 730 and 790 nm the intensities of which change with time in a way which indicates that they are inter-related (figure 7). As Arg0 decreases,... 

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

The ultraviolet/visible absorption spectrum of a polyene shows an intense absorption band and an extremely weak absorption band which is located below the strong absorption band, as described in the following section. This spectral pattern is a general property of linear polyenes of all chain lengths independent of local symmetry and/or the presence of cis bonds. This is the reason why in the literature on polyenes the labels 1 kg for So, 2 kg for Si and 1 feu for Si are used even in cases where Cih symmetry is not realized. The ordering that the 2 kg excited state is located below the 1 feu excited state is peculiar to linear polyenes. 

As a real example we show in Figure 2 the PE spectrum of 1,1-divinylcyclopropane (46 in Table 1), taken from the considerable number of diene and polyene PE spectra published by R. Gleiter and his coworkers. In the second column of the insert (5) are listed the / values in eV corresponding to the first bands of 46. 

We mentioned in Section III.A that one of the unique features of radical ion optical spectroscopy is that it allows one to measure excited-state energies of a molecule at two different geometries, namely that of the neutral species (If in PE spectra) and that of the relaxed radical cation (Xmax of the EA bands). In many cases this feature is of little relevance because either the geometry changes upon ionization are too small to lead to noticeable effects (e.g. in aromatic hydrocarbons), or because such effects are obscured, due to the invisibility of the states in one or other of the two experiments (i.e. strong cr-ionizations in the PE spectrum) or because of the near-cancellation of opposing effects (as in the case of linear conjugated polyene radical cations). 

The use of UV-VIS spectra to analyse dienes and polyenes was historically the first method of choice. The spectra of isolated non-conjugated polyenes is actually the superposition of the spectrum of each one of the double bonds. For each double bond the spectrum depends on the various substituents and also on its location in the molecule. It also depends on the stereochemistry, since conjugated double bonds have either E or Z configuration around each jr-bond but also a cisoid and transoid conformer3 around the single bond marked as s-cis and s-trans4. 

Very powerful tools for the study of dienes and, to some extent, polyenes (in particular annular polyenes) are both H and 13 C NMR spectroscopies, which will be discussed in a separate section. As previously mentioned 1,3-butadiene is more stable in the s-trans conformation and in the H NMR spectrum both butadiene (1) and 2,3,6,7-tetramethyl-2,4,6-octatriene (3) display the vinyl proton at a low chemical shift value. In these simple examples the S value can be predicted theoretically. The 111 NMR spectrum of a C25-branched isoprenoid was examined as part of the structural determination for biomarkers and is shown in Figure l6. The other spectral and structure assignments are described later in this review. 

In contrast, if the same polyene were to be open , no ring current would exist and the NMR spectrum would be very different (see discussion on carotenoids). 

The 13C NMR spectrum of 64, an amide of 63, showed sixty-two carbon signals of which partial assignments, shown in Table 16, were made based upon distortionless enhancement by polarization transfer(DEPT), H-13C correlation experiments and literature data describing 13C NMR analysis of polyene macrolides. 

In the 1H noise-decoupled 75.5 MHz 13C NMR spectrum of 72, the signals of the sp-hybridized carbon atoms C15 and C. 15 are found at 98.3 and 97.3 ppm. This is in the expected region for substituted alkynes and the chemical shifts agree very well with those of other didehydrocarotenoids. As can be seen in Table 21, the 15,15 -triple bond leads to an upheld shift of ca 22 ppm for the directly connected C14 and C. 14. The chemical shifts of the other carbon atoms of the polyene chain are also affected a downheld shift is observed for the odd carbon atoms and a (shght) upheld shift for the even carbon atoms, both decreasing with increasing distance from the central part. 

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