Combined experimental and theoretical study on hydrogen-bonded complexes between cyclic ketones, lactones, and...

The interaction of 3,4 dinitrophenol (DNP) with cyclic ketones, lactones, and lactams was investigated by UV-visible spectroscopy and density functional theory (DFT) methods. Equilibrium constants K(HB) for 1:1 hydrogen bonded complexes were determined in solution in CCl(4) and C(6)H(12). For the entire range of studied bases, the pK(HB) scale, varying between 2.92 for beta-propiolactone to 5.53 for 1-methyl-epsilon-caprolactam, indicates that the basicity increases with the ring size. Geometries, energies, and vibrational characteristics of complexes were obtained by means of DFT calculations. For lactones and lactams, the energy difference between the two most stable conformers, cis and trans, with respect to the ring oxygen (nitrogen) atom, is relatively small, suggesting that the complex observed in solution is probably an equilibrium mixture of both forms. The good correlation between Gibbs free energies in solution and in the gas phase, computed at the B3LYP/6-311++G(3df,2p) level of theory, confirms the reliability of our results. The electron density of the complexes has been analyzed by means of the atoms in molecules (AIM) theory and the natural bond orbital (NBO) method have been used to characterize the orbital interactions. Our theoretical survey shows that the 1:1 complexes are stabilized by a network of conventional and/or nonconventional intermolecular hydrogen bonds.