Most organic compounds are transparent to the relatively high-energy radiation that constitutes the ultraviolet (200–400 nm) and visible (400–700 nm) portion of the electromagnetic spectrum, and consequently they appear colourless in solution. This is because the electrons in the σ bonds of organic molecules require wavelengths of even higher energy (such as those of X-rays) to excite them to the next higher accessible energy level. Electrons in π bonds, however, can be promoted to higher energy levels by ultraviolet and visible light, and UV-visible spectroscopy consequently provides useful structural information for molecules that contain π bonds. When multiple π bonds are separated from each other by intervening single bonds, they are said to be conjugated.
The UV-visible spectrum of a molecule is dramatically affected by the presence of conjugation. As the number of conjugated π bonds increases, the UV-visible spectrum shows light absorption at a greater number of different wavelengths (i.e., the spectrum contains more absorption peaks), and light of longer wavelengths (and lower energy) is absorbed. The many individual peaks of UV-visible spectra normally coalesce to produce a continuous absorption spectrum, with some of the strongest individual absorption peaks appearing as sharp spikes. For example, the UV-visible spectrum of azulene, a molecule that contains five conjugated π bonds, shows a strong absorbance in the visible region of the electromagnetic spectrum, which correlates with its intense blue colour.
Naturally occurring organic compounds that are highly coloured contain an extensive system of conjugated π bonds. The compound largely responsible for the bright orange colour of carrots, β-carotene, contains 11 conjugated π bonds. UV-visible spectroscopy is especially informative for molecules that contain conjugated π bonds.
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