The rotational and hyperfine spectrum and structure of H2CO–HF2

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Abstract
The rotational spectrum of the hydrogen bonded complex formed between hydrogen fluoride and formaldehyde has been obtained from 3 to 55 000 MHz using molecular beam electric resonance spectroscopy. The molecule is found to be a prolate slightly asymmetric rotor with κ=−0.98 and an inertial defect of Δ=0.62 amu Å2. The rotational constants determine that the average geometry of the three heavy atoms is bent with an O–F distance of 2.66 Å and a C–O–F angle of 109.5°. The large value of μa shows 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 (Sa and Sb) of the direct nuclear spin–spin hyperfine coupling tensor. Both the hyperfine data and the value of μb indicate 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: A−ΔK=50 543.205(23) MHz; B=4832.232(33) MHz; C=4386.621(33) MHz; μa=3.7535(2) D; μb=1.399(1) D; SHFa=93.6(8) kHz; SHFb=−31.7(11) kHz; rO–H=1.794 Å; rO–F=2.659 Å; ∠C=O⋅⋅⋅F=109.5°; ∠C=O⋅⋅⋅H=115°.

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