Combination bands dienes

As in aromatic compounds, dienes demonstrate multiple bands associated with conjugated doublebond and C-H stretching. The mid-infrared spectra of conjugated polyenes exhibit a band in the 1000-to 900-cm" (10,000- to 11,111-nm) region, which does not have overtones or combination bands observed in the near-infrared. This band is indicative of cis and trans groups within the different polyenes. The near-infrared bands associated with v H-C=C are observed near 6110 cm (1637 nm), 5960 cm (1678 nm), 4710 cm (2123 nm), 4595 cwr (2176 nm), and 4470 cm i (2237 nm) and are very similar to those shown for 1-hexene in Figure 3.1. The 3v C=C band is observed near 4950 cm (2020 nm). Note that the Appendix material provides more examples of model spectra as well as tables showing precise band positions. 

Tables 6.3-6.5 record data developed to undertake structural analysis in systems possessing chromophores that are conjugated or otherwise interact with each other. Chromophores within a molecule interact when linked directly to each other or when they are forced into proximity owing to structural constraints. Certain combinations of functional groups comprise chromophoric systems that exhibit characteristic absorption bands. In the era when UV-VIS was one of the principal spectral methods available to the organic chemist, sets of empirical rules were developed to extract as much information as possible from the spectra. The correlations referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 6.3) and enones and dienones (Table 6.4) to be predicted. When this method is applied, wavelength increments correlated to structural features are added to the respective base values (absorption wavelength of parent compound). The data refer to spectra determined in methanol or ethanol. When other solvents are used, a numerical correction must be applied. These corrections are recorded in Table 6.5.

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