MICROWAVE AND SUBMILLIMETER-WAVE SPECTRA OF THE MIXED DEUTERATED-PROTONATED WATER-DIMER ISOTOPOMERS

The microwave and submillimeter-wave rotation-tunneling spectra of sev eral mixed deuterated-protonated isotopomers of the water dimer have b een measured up to 460 GHz using an electric-resonance optothermal spe ctrometer, a pulsed-nozzle Fourier-transform microwave spectrometer, a nd a far-infrared CO2-laser difference-frequency spectrometer. Spectra were recorded for the a-type K-a = 0-0 and 1-1 and b- and/or c-type K -a = 1-0 bands for H2O-DOD, D2O-HOH, D2O-DOH, H2O-HOD, D2O-HOD, H2O-DO H, DHO-HOH, and DHO-DOD, where O-D or O-H denotes the two atoms direct ly involved in the hydrogen bonding. Spectra of D2O-HOH, H2O-HOD, D2O- HOD, and HDO-HOH, which are higher-energy isomers of H2O-DOD, H2O-DOH, D2O-DOH, and H2O-DOH, respectively, have not been reported previously . These higher-energy isomers are not significantly populated in Ar mo lecular beams, such as used in many of the previous microwave studies. Here, we obtain information on two of the three major tunneling proce sses found in the water dimer, the tunneling interchange of the two pr otons/deuterons on the proton-acceptor subunit and the tunneling inter change of the two protons/deuterons on the proton-donor subunit. The o bserved rotation-tunneling selection rules are consistent with previou sly proposed pathways for these tunneling processes. For H2O-DOD, D2O- HOH, and DHO-HOH we find donor interchange tunneling splittings of 24. 4(11), 864.14(29), and 1809.39(9) MHz, respectively, averaged over K-a = 0 and 1, and for DHO-DOD we find a donor-interchange tunneling spli tting of 16.8(1) MHz for K-a = 0. For DHO-DOD we also determined the p roton/deuteron acceptor interchange tunneling splitting for K-a = 0 as 107 723.7(1) MHz, similar to the 117 441.0(2) MHz valued determined p reviously for DHO-HOD. Because the position of the D/H involved in the hydrogen bonding is nearly coincident with the center of mass of the dimer, isotopomers which only differ by isotopic nuclei at this positi on have strikingly similar spectra. The present measurements furnish r otational constant and tunneling splitting data to use in the testing and refinement of proposed intermolecular potentials describing water dimer. (C) 1997 Academic Press.