Solvent-Induced Carbonyl Frequency Shifts: Cyclopentanone and Cyclohexanone

The carbonyl stretching vibration, vC=O, for cyclopentanone is in Fermi resonance with a combination tone, and the amount of Fermi resonance interaction between these two modes is dependent upon the amount of solute/solvent interaction. Correction for Fermi resonance shows that vC=O for cyclopentanone decreases in frequency by approximately 17.1 cm⁻¹, progressing in the series of solvents from hexane through methyl alcohol. In the same series of solvents the vC=O frequency for cyclohexanone decreases in frequency by approximately 22 cm⁻¹, progressing through the series of solvents hexane through methyl alcohol. Moreover, the vC=O mode for cyclohexanone occurs at a lower frequency than the vC=O mode for cyclopentanone (after correction for Fermi resonance) by between 26.5 and 33.2 cm⁻¹, depending upon which particular solvent was used to dissolve cyclopentanone and cyclohexanone. The larger decrease in frequency for vC=O of cyclohexanone compared to vC=O for cyclopentanone in the same series of solvents is attributed to the fact that the (CH₂)₅ group contributes more electrons to the C=O group than does the (CH₂)₄ group. Consequently, the carbonyl group for cyclohexanone is more basic than the carbonyl group for cyclopentanone. Thus, there is a larger solute/solvent interaction in the case of cyclohexanone than in the case of cyclopentanone. The solvent acceptor numbers do not correlate well with the vC=O frequencies for either cyclohexanone or cyclopentanone (ACFFR).