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

Stable C-H center dot center dot center dot 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 (nu(C-H)) in the said three complexes exhibits blue shifting with enhancement in nu(C-H) transition intensity. However, the red shifts of the nu(C=O) bands of the cyclic ketones in the complexes show no apparent correlation with the corresponding blue shifts of nu(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) -> sigma*(C-H) hyperconjugative charge transfer ability of the cyclic ketones.