Theoretical study of the symmetry of the (OH•••O)- hydrogen bonds in vinyl alcohol-vinyl alcoholate systems

The interactions between substituted vinyl alcohols and vinyl alcoholates (X = NH2, H, F, Cl, CN) are studied at the B3LYP/6-311++G(d,p) level of theory. In a first step, the conformation of the monomers is investigated and the proton affinities (PA(A(-))) of the enolates are calculated. The enols and enolates are held together by strong ((OHO)-O-...)(-) hydrogen bonds, the hydrogen bond energies ranging from 19.1 to 34.6 kcal mol(-1). The optimized (OO)-O-... distances are between 2.414 and 2.549 Angstrom and the corresponding OH distances from 1.134 and 1.023 Angstrom. The other geometry parameters such as C=C or CO distances also indicate that, in the minimum energy configuration, the hydrogen bonds are characterized by a double well potential. The Mulliken charges on the different atoms of the proton donors and proton acceptors and the frequencies of the v(OH) stretching vibrations agree with this statement. All the data indicate that the hydrogen bonds are the strongest in the homomolecular complexes. The transition state for hydrogen transfer is located with the transition barrier estimated to be about zero. Upon addition of the zero-point vibration energies to the total potential energy, the barrier vanishes. This is a characteristic feature of low-barrier hydrogen bonds (LBHBs). The hydrogen bond energies are correlated to the difference 1.5 PA(AH) - PA(A(-)). The correlation predicts different energies for homomolecular hydrogen bonds, in agreement with the theoretical calculations. Our results suggest that a PA (or pK(a)) match is not a necessary condition for forming LBHBs in agreement with recent data on the intramolecular hydrogen bond in the enol form of benzoylacetone.