A simple and practical method has been developed for treating the non-adiabatic proton tunneling in malonaldehyde. A twodimensionalpotential energy surface function, which couples OH stretching and in-plane bending modes, has been constructed for the motion of hydrogen by the aid of quantum mechanical calculations at MP2/6-31Gpp and B3LYP/6-311þþGpp levels for a fixed skeleton geometry. This potential is used for calculation of energy levels from which a tunneling splitting of 20.8 and 22.8 cm21 at the MP2 and B3LYP levels, respectively, was obtained in an excellent agreement with the experimental value of 21.58 cm21. This simple two-dimensional potential function have been employed to study the interaction between the OH in plane bending and the OH stretching motions in bent hydrogen bonded systems. The coupling between OH stretching and bending modes explains the anomalous vibrational behavior of bent hydrogen bonded systems such as the cis enol form of b-diketones.

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