Ab initio study of the potential-energy well of malondialdehyde on varying the O center dot center dot center dot O distance

The evolution of the proton-transfer potential-energy curves of malondialdehyde on decreasing the O ... O distance until the barrier to C-s --> C-2v --> C-s interconversion disappeared, was studied at the ab initio level (6-31G** basis sets including and excluding correlation energy). The frequencies of the stretching and in-plane-bending modes were also calculated for conformations with different r(O ... O) values. The results obtained suggest that the energy cost when two oxygen atoms approach each other is not relevant and the remarkable shifts of the O-H stretching mode can be accounted for mainly by the modification of the potential-energy well accompanying the molecular geometry variations; only a moderate increase in the hydrogen-bridge strength is induced by the decrease in r(O ... O).