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

The intermolecular potential energy surface and the torsional-puckering (TP ) motion of the cyclic homodromic water pentamer were theoretically investi gated at the level of second-order Moller-Plesset perturbation theory using double-and triple-zeta correlation-consistent basis sets including diffuse functions. Extensive searches yielded three cyclic homodromic stationary p oints: (i) the puckered global minimum structure GS, (ii) a torsional-pucke ring saddle point, 62 cm(-1) higher, denoted TS1, and (iii) a planar C-5h s ymmetric structure (Hessian index 7), 800 cm(-1) higher in energy. Harmonic vibrational frequencies and normal modes were calculated for all stationar y points. Based on the energies and second derivatives of the GS and TS1 st ructures a cyclic minimum energy path for the large-amplitude TP motion wit h ten permutationally equivalent minima was constructed. The ground and all excited states are delocalized by TP pseudorotation, giving vibrationally averaged C-5h symmetry. The first pseudorotational TP level lies at approxi mate to 1 cm(-1) (approximate to 0.6 cm(-1)) and the ten lowest TP states w ithin a band of approximate to 20 cm(-1) (8 cm(-1)) for (H2O)(5) [(D2O)(5)] . The corresponding TP dipole moment functions were also calculated, and in tensities for the far-infrared torsional-puckering transitions of (H2O)(5) and (D2O)(5) evaluated up to 300 cm(-1). (C) 1999 American Institute of Phy sics. [S0021-9606(99)30615-2].