Rotational constants of all H/D substituted water trimers: Coupling of intermolecular torsional and symmetric stretching modes

A four-dimensional (4D) intermolecular potential energy surface (IPES) for the water trimer was calculated in the subspace of the three torsional coor dinates and the symmetric intermolecular stretch coordinate, employing high -level ab initio theory. Torsionally adiabatic potential energy curves for the intermolecular symmetric stretching vibration were constructed based on this IPES. They were used to calculate the symmetric stretch fundamentals and stretching-averaged interoxygen R(O ... O) distances [R-H] and [R-D], f or the n = 0-6 torsional levels of (H2O)(3) and (D2O)(3). [R-H] and [R-D] i ncrease with n up to n = 5, and decrease for n = 6. Torsionally averaged ro tational constants A, B, and C of all 20 isotopomers of water trimer, for t he torsional levels n =0, 5 and 6, were obtained by averaging the inverse i nertia tensor over the 3D torsional wave functions. Two approaches were exa mined: (i) setting the interoxygen R(O ... O) distance to a fixed value, in dependent of torsional excitation; (ii) effectively incorporating the vibra tional averaging due to the intermolecular symmetric stretching mode by usi ng the appropriate [R-H] and [R-D] values to define the R(O ... O) distance s for the isotopomers in the torsional state n. Both approaches yielded n=0 , 5 rotational constants in good agreement with experiment. However, only a pproach (ii) reproduced the experimentally observed decrease in the rotatio nal constants A and B upon 5 <--0 torsional excitation. Fixing the R(O ... O) distances to R-H and R-D values obtained by fitting the 3D torsionally a veraged rotational constants to the experimental values for (H2O)(3) and (D 2O)(3), only marginally improved the agreement with experiment for other is otopomers. (C) 1999 American Institute of Physics. [S0021-9606(99)01012-0].