Calculated and experimental rotational constants of (D2O)3: Effects of intermolecular torsional and symmetric stretching excitations

Abstract

Calculations of the torsional state dependence of A=B and C rotational constants of (D2O)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 (D2O)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, ΔC(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, ΔA(n)=ΔB(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 ΔA(n)=ΔB(n) and ΔC(n) changes from the (3+1)D torsional-stretching calculations are in good agreement with experiment up to n=5.