Calculations of the tunneling splittings in water dimer and trimer using diffusion Monte Carlo

Far-infrared VRT spectroscopy of two water trimer isotopomers: vibrationally averaged structures and rearrangement dynamics

We report the measurement of far-infrared vibration-rotation tunnelling parallel bands of two partially deuterated water trimer isotopomers: (D2O)2DOH and (H2O)2DOH at 97.2607 cm-1 and approximately 86 cm-1, respectively. The hydrogen bond rearrangement dynamics of the two mixed trimers can be described by the simplified molecular symmetry G8, which accounts for both the flipping and bifurcation tunnelling motions previously established for (H2O)3 and (D2O)3. The observed donor tunnelling quartet, rather than triplet, splitting indicates that the two homogeneous monomers (D2O or H2O) in each mixed trimer experience slightly different environments. Vibrationally averaged structures of (H2O)3, (D2O)3, and (D2O)2DOH were examined in a Monte Carlo simulation of the out-of-plane flipping motions of the free atoms. The simulation addresses both the symmetric top behaviour and the negative zero-point inertial defect for (H2O)3 and (D2O)3, which were insufficiently counted in all previous structure models. The average ground state O--O separations, which are correlated to other angular coordinates, were determined to be 2.84 +/- 0.01 angstroms for all three species. The simulated difference in hydrogen bond nonlinearity also supports the inequivalency of the two homogeneous monomers. The structural simulation shows that the unique H in (D2O)2DOH is free, while a torsional analysis suggests the unique D in (H2O)2DOH is bound within the cyclic ring. Both bands can be assigned to the pseudorotational transitions which correlate to those found in the pure trimers.

Quantifying hydrogen bond cooperativity in water: VRT spectroscopy of the water tetramer

Measurement of the far-infrared vibration-rotation tunneling spectrum of the perdeuterated water tetramer is described. Precisely determined rotational constants and relative intensity measurements indicate a cyclic quasi-planar minimum energy structure, which is in agreement with recent ab initio calculations. The O-O separation deduced from the data indicates a rapid exponential convergence to the ordered bulk value with increasing cluster size. Observed quantum tunneling splittings are interpreted in terms of hydrogen bond rearrangements connecting two degenerate structures.