Calculated and experimental rotational constants of $(H2O)3:$ Effects of intermolecular torsional and symmetric stretching excitations

Calculations of the torsional state dependence of the $A=B$ and C rotational constants of $(H2O)3$ are reported, for torsional excitations up to 144 cm⁻¹ above the ground state, extending our work on the rotational constants of all isotopomers of water trimer using an ab initio four-dimensional torsional-stretching intermolecular potential energy surface [D. Sabo et al., J. Chem. Phys. 110, 5745 (1999)]. Direct comparison was made to the rotational constants of $(H2O)3$ recently measured and analyzed by Brown et al. [J. Chem. Phys. 111, in press (1999)], for the lowest five torsional levels $n=0–5,$ with torsional energies up to 80 cm⁻¹. The results of the (3+1)-dimensional torsional-stretching calculations are in excellent agreement with experiment. The rotational constants of the next higher torsional level, $n=6,$ were calculated by the (3+1)-dimensional method. A comparison is made to the rotational constants calculated by a three-dimensional purely torsional model, which yields inferior results. 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–H bonds and the changes in the oxygen–oxygen separation which accompany torsional excitation.