Calculated and experimental rotational constants of (D_2O)_3: Effects of intermolecular torsional and symmetric stretching excitations

摘要

Calculations of the torsional state dependence of A = B and C rotational constants of (D_2O)_3 are reported, for torsional energies up to 100 cm~(-1) above the ground state, extending our work on the rotational constants of all isotopomers of water trimer using ab initio four-dimensional torsional-stretching intermolecular potential-energy surface [D. Sabo et al., J. Phys. Chem. 110, 5745 (1999)]. Direct composition was made to the rotational constants measured by Viant et al. [J. Chem. Phys. 110, 4369 (1999)] for the same nine torsional levels of (D_2O)_3. In order to consistently reproduce the pronounced variations of the A = B and C rotational constants from one torsional eigenstate to the other, theory must take into account both the changes of rms torsional angles of the "free" O-D bonds and the changes in the oxygen-oxygen separation which accompany torsional excitation. The changes of C with torsional state. #DELTA#C (n), are distinctly non-monotonic; they depend mainly on the rms (root-mean-square) torsional angle and to a lesser degree on the interoxygen distance. The changes of A = B with torsional state, #DELTA#A (n) = #DELTA#B(n), depend on both rms torsional angles and interoxygen distance, but with opposite signs, giving rise to an apparent monotonic decrease with n which is smaller than the changes due to either mechanism alone. The #DELTA#A(n) = #DELTA#B(n) changes from the (3 + 1)D torsional-stretching calculations are in good agreement with experiment up to n = 5.