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

Abstract

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 Møller–Plesset 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−1 higher, denoted TS1, and (iii) a planar C5h symmetric structure (Hessian index 7), 800 cm−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 TS1 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 C5h symmetry. The first pseudorotational TP level lies at ≈1 cm−1 (≈0.6 cm−1) and the ten lowest TP states within a band of ≈20 cm−1 (8 cm−1) for (H2O)5 [(D2O)5]. The corresponding TP dipole moment functions were also calculated, and intensities for the far-infrared torsional-puckering transitions of (H2O)5 and (D2O)5 evaluated up to 300 cm−1.