Transition structures for the interchange of hydrogen atoms within the water dimer

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Abstract
High levels of a b i n i t i o molecular orbital theory were used to examine rearrangement processes in the water dimer corresponding to the interchange of various hydrogen atoms. Our most reliable calculations involve MP4/6‐311+G(2d f,2p) energy evaluations at MP2/6‐311+G(d,p) optimized structures. The lowest energy rearrangement pathway corresponds to the interchange of hydrogen atoms of the acceptor molecule within the C s water dimer structure (1). This proceeds via a transition structure of C 1 symmetry (2) and requires an energy of 0.59 kcal mol− 1. The interchange of donor and acceptor molecules can be achieved via a transition structure with C i symmetry (4) and requires an energy of 0.87 kcal mol− 1. Finally, the interchange of hydrogen atoms of the donor molecule, via a C 2v transition structure (9), requires 1.88 kcal mol− 1. The rearrangements via 2 and 4 lead to complete scrambling of hydrogen atoms within the individual H2O moieties at a cost of 0.87 kcal mol− 1; the transition structure 9 is not necessary for this process. The implications of these results with regard to the interpretation of spectroscopic data on the water dimer will be of interest.