An ab initio study of the torsional-puckering pseudorotation in the cyclic water pentamer.

The intermolecular potential energy surface and the torsional-puckering (TP) motion of the cyclic homodromic water pentamer were theoretically investigated at the level of second-order MoilerPlesset perturbation theory using double- and triple-zeta correlation-consistent basis sets including diffuse functions. Extensive searches yielded three cyclic homodromic stationary points: (i) the puckered global minimum structure GS, (ii) a torsional-puckering saddle point, 62 cm[SUP-1] higher, denoted TS1, and (iii) a planar C[SUB5h] symmetric structure (Hessian index 7), 800 cm[SUP-1] higher in energy. Harmonic vibrational frequencies and normal modes were calculated for all stationary points. Based on the energies and second derivatives of the GS and TS 1 structures a cyclic minimum energy path for the large-amplitude TP motion with ten permutationally equivalent minima was constructed. The ground and all excited states are delocalized by TP pseudorotation, giving vibrationally averaged C[SUB5h] symmetry. The first pseudorotational TP level lies at ≈1 cm[SUP-1] (≈0.6 cm[SUP-1]) and the ten lowest TP states within a band of ≈20 cm[SUP-1] (8 cm[SUP-1]) for (H[SUP2]O)[SUB5] [(D[SUB2]O)[SUB5]]. The corresponding TP dipole moment functions were also calculated, and intensities for the far-infrared torsional-puckering transitions of (H[SUB2]O)[SUB5] and (D[SUB2]O)[SUB5] evaluated up to 300 cm[SUP-1]. [ABSTRACT FROM AUTHOR]