The rotational spectrum and molecular structure of the ethylene–HF complex

The microwave spectrum of the ethylene–HF complex in the ground vibrational state has been assigned using a Fourier transform microwave spectrometer, employing a Fabry–Perot cavity and a pulsed supersonic nozzle as a molecular source. Ethylene–HF is nonplanar, with the axis of the HF subunit oriented perpendicular to the plane of the ethylene molecule, and bisecting the carbon–carbon double bond. A hydrogen bond is formed between the HF proton and the π‐electron density of ethylene. The spectroscopic constants of ethylene–HF and ethylene–DF are: