Solvent effects A most suitable solvent is one which does not itself absorb in the region under investigation. A dilute solution of the sample is always prepared for the spectral analysis. Most commonly used solvent is 95% ethanol. For ultraviolet spectroscopy, ethanol, water and cyclohexane serve the purpose best. The position and the intensity of absorption maximum is shifted for a particular chromophore by changing the polarity of the solvent. By increasing the polarity of the solvent, compounds like dienes and conjugated hydrocarbons do not experience any appreciable shift. Thus, in general, the absorption maximum for the non – polar compounds is the same in alcohol (polar) as well as in hexane ( non- polar ). The absorption maximum for the polar compounds is usually shifted with the change in polarity of the solvents. α, β – unsaturated carbonyl compounds show two different shifts. 1) n→ π* transition In such a case, the absorption band moves to shorter wavelength by increasing the polarity of the solvent. In n→ π* transition, the ground state is more polar as compared to the excited state. The hydrogen bonding with solvent molecules takes place to lesser extent with the carbonyl group in the excited state. For example, absorption maximum of acetone is at 279 nm in hexane as compared to 264 nm in water. 2) π → π* transition For such a case, the absorption band moves to longer wavelength by increasing the polarity of the solvent. The dipole interactions with the solvent molecules lower the energy of the excited state more than that of the ground state. Thus, the value of absorption maximum in ethanol will be greater than that observed in hexane. In general, we say that a) If the group (carbonyl) is more polar in ground state than in the excited state, then increasing polarity of the solvent stabilizes the non bonding electron in the ground state due to hydrogen bonding. Thus, absorption is shifted to lower wavelength. b) If the group is more polar in the excited state, then absorption is shifted to longer wavelength with increase in polarity of the solvent which helps in stabilizing the non bonding electrons in the excited state. It has been found that increase in polarity of the solvent generally shifts n → π* and n →σ* bands to shorter wavelength and π → π* bands to longer wavelengths.
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