Pseudorotation in the D2O trimer

Far-infrared spectra of Ar/D2O supersonic expansions have been recorded between 39 and 44 cm-1, in which a c-type band centered at 41.1 cm-1 was observed. This new band, attributed to the cyclic water trimer, has the same ground state rotational constants as the (D20)3 98 cm-1 a-type band reported by Liu et al. J. Am. Chem. Soc. 116 (1994) 3507]. Fits of the observed far-infrared band positions to an effective one-dimensional potential require a substantially larger pseudorotation barrier or effective moment of inertia than predicted theoretically. Estimated frequencies of unobserved transitions in the pseudorotation manifolds of (D2O)3 and (H20)3 are presented as a guide to future experimental searches.