Blue shifting C−H···O hydrogen bonded complexes between chloroform and cyclic ketones: ring-size effects on stability and spectral shifts

Abstract

Blue-shifting C−H···O hydrogen bonded complexes between chloroform and three small cyclic ketones (cyclohexanone, cyclopentanone, and cyclobutanone) have been identified by use of FTIR spectroscopy in CCl₄ solution at room temperature. The shifts of the C−H stretching fundamental of chloroform (ν_C−H) in the said three complexes are +1, +2, and +5 cm^-1, respectively, and the complexation results in enhancement of the ν_C−H transition intensity in all three cases. The 1:1 stoichiometry of the complexes is suggested by identifying distinct isosbestic points between the carbonyl stretching (ν_C═O) fundamentals of the monomers and corresponding complexes for spectra measured with different chloroform to ketone concentrations. The ν_C═O bands in the three complexes are red-shifted by 8, 19, and 6 cm^-1, and apparently have no correlation with the respective blue shifts of the ν_C−H bands. Spectral analysis reveals that the complex with cyclohexanone is most stable, and the stability decreases with the ring size of the cyclic ketones. A qualitative explanation of the relative stabilities of the complexes is presented by correlating the hydrogen bond acceptor abilities of the carbonyl groups with the ring size of the cyclic ketones. Quantum mechanical calculations at the DFT/B3LYP/6-311++G(d,p) and MP2/6-31+G(d) levels were performed for predictions of the shapes of the complexes, electronic structure parameters of C−H (donor) and C═O (acceptor) groups, intermolecular interaction energies, spectral shifts, and evolution of those properties when the hydrogen bond distance between the donor−acceptor moieties is scanned. The results show that the binding energies of the complexes are correlated with the dipole moments, proton affinity, and n(O) → σ*(C−H) hyperconjugative charge transfer abilities of the three ketones. NBO analysis reveals that the blue shifting of the ν_C−H transition in a complex is the net effect of hyperconjugation and repolarization/rehybridization of the bond under the influence of the electric field of carbonyl oxygen.