O–H flipping vibrations of the Cage water hexamer: An ab initio study

Abstract

In the Cage[1] isomer of the water hexamer, the free O–H bonds of the two end-cap water molecules can flip between “up” (u) and “down” (d) orientations, giving four conformers denoted uu, ud, du, and dd. Using the Møller–Plesset second order perturbation method and large basis sets, we calculate fully relaxed potential energy curves as a function of both u↔d torsional angles, denoted φ1, φ2. These predict du as the lowest conformer, with uu nearly degenerate and ud and dd at 30–40 and 50–70 cm−1 higher energy, respectively. Along φ1 the torsional barriers are about 200 cm−1, along φ2 between 80 and 110 cm−1. The torsional zero-point energies are high, the vibrational ground states are strongly delocalized and averaging of the cluster properties is important along both φ1 and φ2. The dipole moment components vary strongly along both φ1 and φ2: μa changes from +0.8 to +2.2 D, μb from +0.5 to +1.2 D, and μc from +1.4 to −0.9 D. The φ2 torsional fundamental of (H2O)6 is predicted in the range 65–72 cm−1 with an intensity of ≈0.5 D2, the φ1 fundamental is in the 22–32 cm−1 range, with an intensity of ≈0.3 D2. Both excitations are b/c hybrid perpendicular bands with a dominant b component. The torsional overtones should be very weak.