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

Abstract

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 ∼ 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 Å for all three species. The simulated difference in hydrogen bond nonlinearity also supports the inequivalency of the two homogeneous mono mers 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.