The rotational spectrum and molecular geometry of the cyclopropane–HF dimer

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Abstract
Four isotopic species of a hydrogen-bonded dimer formed between cyclopropane and HF have been observed through assignment of their rotational spectra using a pulsed, Fourier-transform microwave spectrometer in which a gas mixture is pulsed into an evacuated Fabry–Perot cavity. The geometry of the dimer has unambiguously been shown to have HF perpendicular to the cyclopropane edge and in the plane of the ring. HF, on average, lies on the C2 symmetry axis of the dimer which is additionally the a principal axis. Hyperfine structure due to the spin–spin interaction in HF and DF was resolved as well as the hyperfine structure due to the nuclear quadrupole moment of deuterium in the cyclopropane–DF species. The following spectroscopic constants have been obtained for the cyclopropane–HF species: The rotational constants are A = 19 948(60), B = 2753.706(2), and C = 2539.115(2) MHz, the HF spin–spin constant is 121(43) kHz, and the centrifugal distortion constants are DJ = 5.08(27) and DJK = 43.5(36) kHz.