The rotational spectrum of the hydrogen bonded complex formed between hydrogen fluoride and formaldehyde has been obtained from 3 to 55thinsp;000 MHz using molecular beam electric resonance spectroscopy. The molecule is found to be a prolate slightly asymmetric rotor with kgr;=minus;0.98 and an inertial defect of Dgr;=0.62 amuthinsp;Aring;2. The rotational constants determine that the average geometry of the three heavy atoms is bent with an Ondash;F distance of 2.66 Aring; and a Cndash;Ondash;F angle of 109.5deg;. The large value of mgr;ashows that HF is hydrogen bonded to the oxygen. The average orientation of the HF axis with respect to the inertial axes is obtained from the two components (SaandSb) of the direct nuclear spinndash;spin hyperfine coupling tensor. Both the hyperfine data and the value of mgr;bindicate a nonlinear hydrogen bond in this species. Finally, the observation of hyperfine splittings in the spectrum attributable to the presence of the triplet spin state of formaldehyde provides experimental information regarding the tunneling of the formaldehyde protons. The spectra is consistent with a rigid planar structure. Some of the spectroscopic and average structural constants are summarized:Aminus;Dgr;K=50thinsp;543.205(23) MHz;B=4832.232(33) MHz;C=4386.621(33) MHz; mgr;a=3.7535(2) D; mgr;b=1.399(1) D;SHFa=93.6(8) kHz;SHFb=minus;31.7(11) kHz;rOndash;H=1.794 Aring;;rOndash;F=2.659 Aring;; angupr;C=Osdot;sdot;sdot;F=109.5deg;; angupr;C=Osdot;sdot;sdot;H=115deg;.
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