LOW-LYING STATIONARY PAINTS AND TORSIONAL INTERCONVERSIONS OF CYCLIC (H2O)(4) - AN AB-INITIO STUDY

The global and local minima, stationary points, and torsional rearrang ement processes of cyclic homodromic (H2O)(4) were studied on its four -dimensional torsional intermolecular potential energy surface. Eight different energetically low-lying torsional stationary point structure s were found by ab initio theory, and fully structure-optimized at the second-order Moller-Plesset level, using large basis sets. Second-ord er energies close to the one-particle basis set limit were obtained at these geometries using the explicitly correlated Moller-Plesset metho d. The effects of higher-order correlation energy terms were investiga ted by coupled cluster theory, and terms beyond second order were foun d to cancel in good approximation. The S-4 symmetric global minimum ha s a square and almost planar O...O...O...O arrangement with free O-H b onds alternating ''up'' and ''down'' relative to this plane, with two isometric versions of this structure. Another torsional conformer with two neighboring up O-H bonds followed by two neighboring down O-H bon ds is a local minimum, 0.93 kcal/mol above the global minimum. The fou r versions of this structure are connected to the global minima via tw o distinct but almost degenerate first-order torsional saddle points, which occur as two sets of eight isometric versions each, both about 1 .24 kcal/mol above the global minimum. Yet another set of eight second -order saddle points lies at 1.38 kcal/mol. The structure with three O -H bonds up and one down is not a stationary point, while the structur e with all four O-H bonds on the same side of the plane is a first-ord er saddle point. The fully planar C-4h symmetric structure is a fourth -order stationary point 2.8 kcal/mol above the minimum. The torsional interconversion paths between this multitude of points are complex, an d are discussed in three-dimensional spaces of symmetry-adapted torsio nal coordinates, and also in a network representation. The torsional n ormal-mode eigenvectors point fairly directly along the torsional inte rconversion pathways, but the harmonic frequencies are well below the corresponding barriers. Tunneling interconversion between torsional co nformers is, hence, less important than for the water trimer. (C) 1995 American Institute of Physics.