Abstract
The carbonyl stretching vibration, <i>v</i>C=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 <i>v</i>C=O for cyclopentanone decreases in frequency by approximately 17.1 cm<sup>−1</sup>, progressing in the series of solvents from hexane through methyl alcohol. In the same series of solvents the <i>v</i>C=O frequency for cyclohexanone decreases in frequency by approximately 22 cm<sup>−1</sup>, progressing through the series of solvents hexane through methyl alcohol. Moreover, the <i>v</i>C=O mode for cyclohexanone occurs at a lower frequency than the <i>v</i>C=O mode for cyclopentanone (after correction for Fermi resonance) by between 26.5 and 33.2 cm<sup>−1</sup>, depending upon which particular solvent was used to dissolve cyclopentanone and cyclohexanone. The larger decrease in frequency for <i>v</i>C=O of cyclohexanone compared to <i>v</i>C=O for cyclopentanone in the same series of solvents is attributed to the fact that the (CH<sub>2</sub>)<sub>5</sub> group contributes more electrons to the C=O group than does the (CH<sub>2</sub>)<sub>4</sub> 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 <i>v</i>C=O frequencies for either cyclohexanone or cyclopentanone (ACFFR).
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