The Shape of UV Absorption Curves

In some cases, the UV /VIS spectra will show the different energies associated with the vibrational sublevels. For example, simple molecules in the gas phase often show the vibrational levels superimposed on the electronic transitions, as seen in Fig. 5.11, the gas phase spectrum for benzene. The sharp peaks on top of the broad bands are called vibrational fine structure . This fine structure is usually lost at high temperatures in the gas phase due to population of higher vibrational energy levels in the ground state electronic level, with the result that many more lines are seen. Molecules in solution (such as the spectrum shown in Fig. 5.1) usually do not exhibit vibrational structure due to interactions between the solvent and the solute molecules. Compare the gas phase spectrum of benzene (Fig. 5.11) to the solution spectrum for benzene (Fig. 5.12) and note the loss of much of the fine structure in solution. Fine structure due to rotational sublevels is never observed in routine UV/VIS spectra the resolution of commercial instrumentation is not high enough to separate these lines. 

The solvent may affect the appearance of the spectmm, sometimes dramatically. Polar solvents generally wipe out the vibrational fine structure in a spectmm. Solvents may also shift the position of the absorption band, as will be discussed. For the visible region of the spectmm any colorless solvent can be used in which the sample is soluble. 

The common solvents used in UV/VIS spectroscopy are listed in Table 5.5, along with their low wavelength cutoff. At wavelengths shorter than the cutoff wavelength, the solvent absorbs too strongly to be used in a standard 1 cm sample cell. The cutoff is affected by the purity of the solvent. For spectroscopy, the solvents should be of spectral or spectro-chemical grade, conforming to purity requirements set by the American Chemical Society. 

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