Solvent Effect on Absorption Spectra
In UV-Visible spectroscopy, solvent polarity significantly influences the absorption maximum (λmax) and absorption intensity of a chromophore. A change in solvent polarity alters the energy levels of ground and excited states differently, causing shifts in absorption bands (solvatochromism).
1. Effect of Solvent on n → π* Transitions
Increasing solvent polarity causes a blue shift (hypsochromic shift) to shorter wavelengths for n → π* transitions (e.g., carbonyl groups). Polar or protic solvents form strong hydrogen bonds or dipole-dipole interactions with non-bonding (n) electrons in the ground state, lowering its energy level significantly. This increases the energy gap (ΔE) required for excitation.
Example: The absorption maximum of acetone shifts from 279 nm in n-hexane to 264 nm in water. Increased solvent polarity similarly shifts n → σ* transitions to shorter wavelengths.
2. Effect of Solvent on π → π* Transitions
Increasing solvent polarity causes a red shift (bathochromic shift) to longer wavelengths for π → π* transitions. The excited π* state is typically more polar than the ground π state. Polar solvent molecules stabilize the excited state through dipole-dipole interactions more effectively than the ground state, thereby decreasing the energy transition gap (ΔE) and shifting the absorption peak toward longer wavelengths.
General Summary:
- If the ground state is more polar (or more capable of hydrogen bonding) than the excited state (n → π*), solvent polarity causes a blue shift.
- If the excited state is more polar than the ground state (π → π*), solvent polarity causes a red shift.
Choice of Solvent
An ideal solvent for UV spectroscopy must meet the following criteria:
- Transparency: It must be transparent and not absorb in the wavelength region where the analyte absorbs.
- Solvency & Interaction: It should dissolve the solute effectively while exerting minimal chemical interaction with the analyte molecules (non-polar solvents like n-hexane are preferred when fine spectral detail is needed).
The most widely used solvent is 95% ethanol due to its low cost, high solvating power, and optical transparency down to 210 nm. Non-polar solvents like n-hexane and cyclohexane are preferred for fine structure resolution, whereas solvents like benzene, chloroform, and carbon tetrachloride have higher cutoff wavelengths and obscure the far-UV region.
| Solvent | UV Cutoff Wavelength (nm) |
|---|---|
| Water | 190 - 205 |
| Methanol | 205 - 210 |
| Ethanol (95%) | 210 |
| Diethyl Ether | 215 |
| Cyclohexane | 210 |
| 1,2-Dichloroethane | 220 |
| Chloroform | 245 |
| Carbon Tetrachloride | 265 |