SOLVENT-INDUCED CARBONYL FREQUENCY-SHIFTS - CYCLOPENTANONE AND CYCLOHEXANONE

The carbonyl stretching vibration, νC=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 νC=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 νC=O frequency for cyclohexanone decreases in frequency by approximately 22 cm-1, progressing through the series of solvents hexane through methyl alcohol. Moreover, the νC=O mode for cyclohexanone occurs at a lower frequency than the νC=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 νC=O of cyclohexanone compared to νC=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.