C–H⋯O Hydrogen Bonded Complexes Between Chloroform and Cyclic Ketones: Correlation of Spectral Shifts and Complex Stability with Ring Size

Summary

Stable C–H⋯O hydrogen bonded complexes between choloroform and three small cyclic ketones (cyclohexanone, cyclopentanone, cyclobutanone) are identified by use of FTIR spectroscopy in CCl₄ solution at room temperature. The C–H stretching fundamental of chloroform (ν_C-H) in the said three complexes exhibits blue shifting with enhancement in ν_C-H transition intensity. However, the red shifts of the ν_C=O bands of the cyclic ketones in the complexes show no apparent correlation with the corresponding blue shifts of ν_C-H. The spectral analysis reveals that the stability of the complexes decreases with the ring size of the cyclic ketones. Electronic structure calculation at DFT/B3LYP/6-311++G(d,p) and MP2/6-31+G(d) levels predict that the complex binding energies are correlated with the dipole moment, proton affinity, and n(O)→σ^∗(C–H) hyperconjugative charge transfer ability of the cyclic ketones.