It is often difficult to extract a great deal of information from a UV spectrum used by itself. It should be clear
by now that a UV spectrum is most useful when at least a general idea of the structure is already known; in this way,
the various empirical rules can be applied. Nevertheless, several generalizations can serve to guide our use of UV data.
These generalizations are a good deal more meaningful when combined with infrared and NMR data - which can, for instance,
definitely identify carbonyl groups, double bonds, aromatic systems, nitro groups, nitriles, enones, and other important
chromophores. In the absence of infrared or NMR data, the following observations should be taken only as guidelines.
- A single band of low to medium intensity (e = 100 to 10,000) at wavelengths less than
220 nm usually indicates an h to s* transition.
Amines, alcohols, ethers, and thiols are possibilities, provided that the nonbonded electrons are not included in a
conjugated system. An exception to this generalization is that the h to
p* transition of cyano groups appears in this region. However, this is a weak
transition (e < 100), and the cyano group is easily identified in the infrared. Do not
neglect to look for N-H, O-H, C-O, and S-H bands in the infrared spectrum.
- A single band of low intensity (e = 10 to 100) in the region 250 to 360 nm, with no
major absorption at shorter wavelengths (200 to 250 nm), usually indicates an h to
p* transition. Since the absorption does not occur at long wavelength, a simple,
or unconjugated, chromophore is indicated, generally one which contains an O, N, or S atom. Examples of this may include
C=O, C=N, N=N, -NO2, -CO2R, -CO2H, or -CONH2. Once again, infrared and NMR spectra
should help a great deal.
- Two bands of medium intensity (e = 1,000 to 10,000), both with
lmax above 200 nm, generally indicate the presence of an aromatic system. If an
aromatic system is present, there may be a good deal of fine structure in the longer-wavelength band (in nonpolar
solvents only). Substitution on the aromatic rings increases the molar absorptivity above 10,000, particularly if the
substituent increases the length of the conjugated system.
In polynuclear aromatic substances, a third band appears near 200 nm, a band which in simpler aromatics occurs below
200 nm, where it cannot be observed. Most polynuclear aromatics (and heterocyclic compounds) have very characteristic
intensity and band-shape (fine-structure) patterns, and they may often be identified via comparison to spectra which are
available in the literature.
- Bands of high intensity (e = 10,000 to 20,000) which appear above 210 nm generally
represent either an a,b-unsaturated ketone (check the infrared
spectrum), a diene, or a polyene. The greater the length of the conjugated system, the longer the observed wavelength.
For dienes, the lmax may be calculated using the Woodward-Fieser Rules.
- Simple ketones, acids, esters, amides, and other compounds containing both p systems
and unshared electron pairs show two absorptions: an h to
p* transition at longer wavelengths (>300 nm, low intensity) and a
p to p* transition at shorter wavelengths (<250 nm,
high intensity). With conjugation (enones), the lmax of the
p to p* band moves to longer wavelengths and can be
predicted by Woodward's Rules. The e value usually rises above 10,000 with conjugation, and,
as it is very intense, it may obscure or bury the weaker h to
p* transition.
For a,b-unsaturated esters and acids, Nielsen's Rules may be
used to predict the position of lmax with increasing conjugation and substitution.
- Compounds which are highly colored (have absorption in the visible region) are likely to contain a long-chain
conjugated system or a polycyclic aromatic chromophore. Benzenoid compounds may be colored if they have enough conjugating
substituents. For nonaromatic systems, usually a minimum of four to five conjugated chromophores are required to produce
absorption in the visible region. However, some simple nitro, azo, nitroso, a-diketo,
polybromo, and polyiodo compounds may also exhibit color, as may many compounds with quinoid structures.
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