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The microwave and submillimeter spectra of the a-type K=0←0 and K=1←1, c-type K=1←0, and isotopically allowed b-type K=1←0 bands of the O–D bonded HDO–DOH water dimer isotopomer and the higher energy O–H bonded HDO–HOD isotopomer have been measured using molecular-beam electric resonance optothermal and pulsed-nozzle Fourier-transform microwave spectrometers. The present results obtained in He and He/Ne seeded molecular beams give the first evidence for the presence of the higher energy O–H bonded mixed protonated-deuterated water dimers. These species were not reported previously in studies using seeded Ar molecular beams. The donor–acceptor interchange tunneling splittings are found to be 1322.1019(43) and 5004.059(20) MHz for the HDO–DOH and the metastable HDO–HOD dimers, respectively. For both isotopomers, the donor-accepter interchange tunneling-state selections rules for the b- and c-type bands are consistent with tunneling pathways corresponding to geared partial internal rotation of the two subunits in double-minima potentials. The larger tunneling splitting in HDO–HOD is primarily the consequence of the smaller effective reduced mass for tunneling in this system compared to that in HDO–DOH. The presence of both b- and c-type K=1←0 bands allows the direct measurement of the largest tunneling splitting, that associated with the internal rotation about the O–H–O or O–D–O bond of the nonbonded proton/deuteron on the proton donating unit. For the K=0 state of HDO–DOH this splitting is 214 208.38(23) MHz, while for the K=0 state of HDO–HOD it is 117 440.97(17) MHz. A strong b-type Coriolis interaction is observed between the upper K=0 and lower K=1 states in HDO–DOH, similar to that observed previously in (H2O)2.

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