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Study Notes: How Organic Compounds Absorb UV-Vis Radiation

Fundamentally, absorption of ultraviolet and visible radiation comes about as the result of transitions of valence shell electrons between different energy levels.

What is the nature of such transitions in organic compounds? How do they occur? The answer is tied up in the bonding and non-bonding electrons within the molecules that comprise the compounds.

Consider electrons in covalent bonds and in lone pairs on atoms in saturated organic compounds (ie compounds containing single bonds only):

Electrons and bonds in Ethane. Ask your tutor for a model. Electrons and bonds in Ethyl Ether. Ask your tutor for a model.

These electrons can be excited by UV radiation however they are held so tightly that the lambdamax values fall well below 200 nm where it is technically very difficult to measure and, as a consequence, is uninformative for spectroscopic analysis.

There are two electronic transitions of principal importance in UV-Vis spectroscopy (note pi* is the excited state and is termed an ‘antibonding pi orbital’):

pi arrow right pi* and n arrow right pi*

These transitions involve compounds that have double or triple bonds (pi) or compounds with lone pair electrons (n) as part of a saturated structure such as in the carbonyl (C = O) functional group.

Electrons and bonds in Ethylene. Ask your tutor for a model. Electrons and bonds in Acetone. Ask your tutor for a model.

Because the electrons involved in pi arrow right pi* and n arrow right pi* transitions are bound less tightly within the molecular structure they require less energy to excite and therefore have lambdamax values at longer wavelengths.

Shifting to longer lambdamax values is even more pronounced when conjugation is present ie. where there is a sequence of unsaturated bonds separated by a single bond. An isolated carbonyl group or an isolated double bond do not have a strong maximum above 200 nm – this circumstance changes with conjugation:

Electrons and bonds in Butadiene. Ask your tutor for a model. Electrons and bonds in alpha betaunsat ketone. Ask your tutor for a model.

The longer a conjugated system, the longer will be the wavelength of the absorption. In effect, conjugation pushes lambdamax to longer wavelengths (absorption intensity, epsilonmax, normally increases as well).

The conjugation in carotene (a polyene with multiple double bonds), for instance, is of such a length that the compound absorbs in the visible region – this is consistent with carotene being coloured.

Spectrum of carotene showing absorption peaks at wavelengths in the visible region.

In practice, UV-Vis spectroscopy of organic compounds is limited in the most part to conjugated systems where absorption occurs above 200 nm. This is that part of the spectrum that is of most value in spectroscopy.

Note that:

  • the effective or useable part of the UV-Vis range therefore lies between 200 and 780 nm
  • wavelengths between 200 and 380 nm are referred to as the ‘near UV’ region.
That structural part of a compound that contains the electrons responsible for absorption is termed the chromophore and involves saturated bonds and functional groups such as the carbonyl and carboxyl (COOH) groups.

The most important chromophores are those where conjugation is present.

Tables are available that show lambdamax values associated with the absorption bands of common organic chromophores. Note that there may be more than one absorption band per chromophore (and therefore more than one lambdamax). The tables may also show the corresponding values for epsilonmax as well as the solvent in which the spectral data was obtained.

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