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Precise measurement of all tunneling splittings in the ground vibrational state of the water dimer is essential for a complete and rigorous determination of the intermolecular potential energy surface. Here, accurate experimentally determined rotational constants and tunneling splittings were combined with estimates of the ground state acceptor switching (AS) splittings for both (H2O)2 and (D2O)2 in order to exactly predict the fingerprints of the weakly allowed E 2 ↔ E 1 transitions and to approximately predict their absolute frequencies. While these transitions are predicted to be quite weak, current technology guided by the now fairly complete water dimer data should permit their observation. The measurement of these weak transitions would permit the direct determination of AS splittings, which has so far eluded experiments.

Extensive new spectroscopic measurements are combined with a global analysis of the ground state data in order to re-examine and to refine the description of the excited vibration rotation tunneling (VRT) states of the water dimer. Notably, six new ‘donor torsion’ subbands are analytically identified, current vibrational assignments of the K a = 1 stacks are reassessed, the previously reported (H2O)2 donor torsion overtone (DT)2 and hydrogen bond stretch (S) data sets are augmented, and four new (S) subbands have been measured. Unusually large Coriolis effects are predicted, excited state E2 ↔ E1 assignments are reinforced, and possibilities of experimentally determining ground state AS splitting in (H2O)2 from excited state data are discussed.