2.4 CLASSIFICATION OF UV ABSORPTION BANDS

UV absorption bands have fine structure due to the presence of vibrational sub-levels, but this is rarely observed in solution due to collisional broadening. As the transitions are associated with changes of electron orbitals, they are often described in terms of the orbitals involved, e.g.

Another method of classification uses the symbols: B (for benzenoid) E (for ethylenic) R (for radical-like) K (for conjugated - from the German "konjugierte")

For example that an n → π. (transition requires less energy than π → π For example that an n → π * (transition requires less energy than π → π * or (σ → σ*) transition . n → π * less energy < π → π * < σ → σ*

It seems simplest to use electronic transitions or the letter designation assigned by Burawoy as shown in Table I and described below

The n→ tt* transitions (also called R -bands) of single chromophoric groups, such as the carbonyl or nitro group are forbidden and the corresponding bands are character­ized by low molar absorptivities, ɛmax generally less than 100. They are further characterized by the hypsochromic or blue shift observed with an increase in solvent polarity. They frequently remain in the spectrum when modifica­tions in molecular structure introduce additional bands at shorter wavelengths. When additional bands make their appearance, the n → tt* transition is shifted to a longer wavelength but may be submerged by more intense bands.

Bands attributed to tt → tt Bands attributed to tt → tt* transitions (K-Bands) appear in the spectra of molecules that have conjugated tt -systems such as butadiene or mesityl oxide. Such absorptions also appear in the spectra of aromatic molecules possessing chromophoric substitution—styrene, benzaldehyde, or aceto-phenone. These tt → tt* transitions are usually characterized by high molar absorptivity, ɛmax > 10,000.

The tt → tt* transitions (k-bands) of conjugated di- or poly-ene systems can be distinguished from those of enone systems by observing the effect of changing solvent polarity. The tt → tt* transitions of diene or polyene systems are essentially unresponsive to solvent polarity; the hydrocar­bon double bonds are nonpolar.The corresponding absorp­tions of enones, however, undergo a bathochromic shift, frequently accompanied by increasing intensity, as the polarity of the solvent is increased. The red shift presum­ably results from a reduction in the energy level of the ex­cited state accompanying dipole-dipole interaction and hydrogen bonding.

The effect of solvent has been measured for the n → tt The effect of solvent has been measured for the n → tt* transition of acetone. This maximum is at 279 nm in hexane, and decreases to 272 and 264.5 nm for the solvents ethanol and water, respectively

B-Bands (benzenoid bands) are characteristic of the spectra of aromatic or heteroaromatic molecules. Benzene shows a broad absorption band, containing multiple peaks or fine structure, in the near ultraviolet region between 230 and 270 nm (ɛ of most intense peak ca. 255 nm). The fine structure arises from vibrational sublevels affecting the electronic transitions. When a chromophoric group is attached to an aromatic ring, the B-bands are observed longer wavelengths than the more intense tt → tt* tions. For example, styrene has a tt → tt* transition at 244 nm (ɛmax 12,000), and a B-band at ɛmax 282 nm (ɛmax 450).

When an n → tt* transition appears in the spectra an aromatic compound that contains tt → tt* transitions (including B-bands), the n →tt* transition is longer wavelengths. The characteristic fine structure of the B-bands may be absent in spectra of substituted arc The fine structure is often destroyed by the use of solvents.

E-Bands (ethylenic bands), like the B-bands, are characteristic of aromatic structures. The E1 and E2 of benzene are observed near 180 nm and 200 nm. resptively. Auxochromic substitution brings the E2 near ultraviolet region, although in many cases it may not appear at wavelengths much over 210 nm.

In auxochromic substitution, the heteroatom with the lone pair of electrons shares these electrons with the tt –electron system of the ring, facilitating the tt → tt* transition causing a red shift of the E-bands. The molar offE-bands generally varies between 2000 and 14,000.

A bathochromically displaced E2 -band is probably resposible for the intense, fine-structured bands of polynuclear aromatics. With the appearance of the E-bands as a results of auxochromic substitution, the B band shifts to longer wavelengths and frequently increases in intensity.

Molecules such as benzylideneacetone in which more complex conjugated chromophoric occur, produce spectra with both E- and K-bands are obscured by the displaced K-bands.

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