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Home » Chemistry Homework Help » Physical Chemistry » Conjugated Systems Spectra
Conjugated Systems Spectra
The absorption of visible-ultraviolet radiation by molecules with conjugated double bonds can be attributed to changes in the ∏=electron system.

For larger and generally shaped molecule, detailed descriptions of the states involved in an electronic transition cannot be achieved. Spectroscopic techniques still provide a considerable insight into electronic properties.

The electronic absorptions of organic compounds, usually found in the ultraviolet region, can often be identified with a group within the molecule. Electrons in single covalent bonds such as C-C and C-H require very large energies to produce electronic excitation. Saturated hydrocarbons absorb only very high energy radiation, usually beyond 160 nm, far in the ultraviolet region. A simple olefin, however, has an absorption band at around 170 nm, and this can be attributed to the excitation of the ∏ electrons from the electron paired bonding configuration to a high energy, or antibonding state. Such a transition is referred to as a ∏ ∏* transition, the asterisk implying an antibonding orbital.

Some molecules have electronic configurations which can be altered in different ways to lead to an excited or high energy, electronic state. This situation arises, for example, with compounds containing a carbonyl group C=O: for such a group the possibility of exciting the ∏ electrons to the excited ∏* state bonding electrons of the oxygen might be excited to the higher energy ∏* electron state, and the absorption would then be characterized as an n ∏* transition, where the n signifies a nonbonding electron. 

Not all electronic transitions of organic compounds occur in the ultraviolet region. The occurrence of colored compounds indicates absorptions of radiation in the visible spectrum. Such absorption requires the electronic energy levels to be more closely spaced than in most molecules. The most common type of organic molecule that absorbs in the visible region, i.e. is colored, consists of a conjugated system, frequency involving aromatic rings. The qualitative explanation for the closer spacing that results from the delocalization of the conjugated electrons is most easily given by regarding such electrons as being free particles within the potential box of the molecules. For sufficiently long “boxes”, the electronic energy spacing is small enough to bring the absorption of radiation into the visible part of the spectrum.

Consider a fairly long conjugated system such as β-carotene:

If resonance structures are drawn, each carbon-carbon bond along the chain has appreciable double bond character. The ∏ electrons are therefore not localized but are relatively free to move throughout the entire carbon skeleton. This suggests that the skeleton be considered as a roughly uniform region of low potential bounded at the ends of the molecule by the regions of infinitely high potential. The resulting square potential well is to be the receptacle of the 22 ∏ electrons. An expression for the allowed energies of these electrons, electron-electron repulsions being ignored, was obtained. The energies are given by

ε = n2h2/8ma2, n = 1, 2, 3 ….., where a is the effective length of the molecule. Two electrons, one with each spin direction, can be placed in each square well orbital. The electron energies described by this molecular-orbital approach.

The chief merit of this treatment of conjugated systems is that it offers an easy way t related the wavelength of light absorbed to the length of the conjugated system. The visible region absorption of carotene consists of a band centered at 451 nm. Absorption at this wavelength implies the absorption of quanta of energy 4.41 × 10-19 J. according to free-electron model for the electron system; this result can be related to the energy difference

ε12 – ε11 = (122 – 112)h2/8ma  

Substitution of numerical values leads to a value of a, the length of the region in which the electrons are free to move, of 1.77 nm or 1770 pm. This value, as we shall see when experimental methods for determining molecular dimensions are developed in the following aspects, is very close to that expected for the extended conjugated carbon chain of the β-carbonate molecule.
   
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