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

Calculations of the torsional state dependence of A = B and C rotational co nstants of (D2O)(3) are reported, for torsional energies up to 100 cm(-1) a bove the ground state, extending our work on the rotational constants of al l isotopomers of water trimer using ab initio four-dimensional torsional-st retching intermolecular potential-energy surface [D. Sabo , J. Phys. Chem. 110, 5745 (1999)]. Direct composition was made to the rotational constants measured by Viant [J. Chem. Phys. 110, 4369 (1999)] for the same nine torsi onal levels of (D2O)(3). In order to consistently reproduce the pronounced variations of the A = B and C rotational constants from one torsional eigen state to the other, theory must take into account both the changes of rms t orsional angles of the "free" O-D bonds and the changes in the oxygen-oxyge n separation which accompany torsional excitation. The changes of C with to rsional 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 th e interoxygen distance. The changes of A = B with torsional state, Delta A( n) = Delta B(n), depend on both rms torsional angles and interoxygen distan ce, 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) and Delta C(n) changes from the (3 + 1)D torsional -stretching calculations are in good agreement with experiment up to n = 5. (C) 1999 American Institute of Physics. [S0021-9606(99)01136-8].