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

摘要

We report the measurement of far-infrared vibration-rotation tunnelling parallel bands of two partially deuterated water trimer isotopomers: (D_2O)_2DOH and (H_2O)_2DOH at 97.2607 cm~(-1) and ～86 cm~(-1), respectively. The hydrogen bond rearrangement dynamics of the two mixed trimers can be described by the simplified molecular symmetry G_8, which accounts for both the flipping and bifurcation tunnelling motions previously established for (H_2O)_3 and (D_2O)_3. The observed donor tunnelling quartet, rather than triplet, splitting indicates that the two homogeneous monomers (D_2O or H_2O) in each mixed trimer experience slightly different environments. Vibrationally averaged structures of (H_2O)_3, (D_2O)_3, and (D_2O)_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 (H_2O)_3 and (D_2O)_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 A 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 (D_2O)_2DOH is free, while a torsional analysis suggests the unique D in (H_2O)_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.