The pa R-branch rotational spectra of Lhe ground and first exnlcd nateS of tie Wee lowcsr vibrauonal modes of tie H,O._.HF hekrodiier have been observed in the frequency range 40-80 GHz Canolis perturbations between Ihe ground vibrational stik (u = 0) and the first excited smte oE the OUI-of-plane bending vibration (uacO, -1) show ~h.at for a pven f the K_1 -2 levels of uBcO, -1 lie approxkmxely 3 cm-' above the K_1 = 3 levels 01 D -O_ The vibrational separation behveen these LWO slates is esUmaled 10 be 7033 cm-l. This val ue is co114~te.n~ with drose determined by other melhods and reinrorces Lhe conclusion that Y,.,(,,, is governed by a double-mmirn urn potential energy function wilh Lhe quantitative Zorro previously publlsbbed. A pctiurbarion is also observed io the frost excited state or tie hydrogen-bond slrelching vibration U, = 1. llus manifests Itself as a large, negative centrilugal distortion constant D,, = -8-5 MHz compared wxlh 2 MHz in the other vlbrarional spates l_ Jiltroduction The microwave spectrum of the hydrogen-bonded heterodimer formed between water and hydrogen fluoride was first observed by Bevan et al. [l] _ In two later papers [2,3] the molecular structure was determined and the spectra of vibrationally excited states involving the three lowest modes were assigned. One of the most interesting conclusions from this work iSthat the molecule has a non-planar equilibrium geometry with an out-of-plane angle C$ = 45O and a barrier to inversion of _ -1.5 kJ mol-l. The three lowestenergy vibrational modes are shown sche-0 009-2614/85/s 03.30 0 Elsevier Science Publishers B.V.