FLUXIONALITY AND LOW-LYING TRANSITION STRUCTURES OF THE WATER TRIMER

The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies < 1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree-Fock and second-order Moller-Plesset levels, using the 6-311 ++ G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O-H bonds on one and the third O-H bond on the other side of an approximately equilateral hydrogen-bonded O ... O ... O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O-H bonds which are on the same side of the plane across this plane via a transition structure with this O-H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational-tunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C6 symmetry and a very low barrier V6 = 0.1 +/- 0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further low-energy saddle points on the surface are of second and third order; they correspond to crown-type and planar geometries with C3 and C3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetry-adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O-H bonds. Implications of these results for the interpretation of spectroscopic data are discussed.