3. Franck-Condon principle. Vibrational fine structure
UV absorption bands generally appear as regular Gaussian curves, several thousand cm wide
–1
. In some cases, however, a fine structure can be discerned, especially in low-polarity solvents, when interactions with the medium are weaker. This fine structure becomes more pronounced in the gas phase, especially at low pressure.
Example
For example, benzene in solution in cyclohexane, figure
2
a, has a characteristic weak band at around 260 nm, known as the benzenoid band, with a fine structure of 6 or 7 equidistant components of regular contour. The spacing of these lines, of the order...
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Franck-Condon principle. Vibrational fine structure