Pseudorotation in tetrahydrofuran center dot center dot center dot HF heterodimer

Seven isotopomers of the hydrogen-bonded heterodimer tetrahydrofuran ... HF have been investigated in the frequency range 6-18 GHz using molecular-bea m Fourier transform microwave spectroscopy. The rotational spectra of C4H8O ... HF and C4H8O ... DF exhibit small tunneling splittings, which were not observed for C4D8O ... HF and the four single C-13 isotopomers. These obse rvations analyzed in terms of symmetry considerations allow us to conclude that these splittings are due to pseudorotation within the tetrahydrofuran subunit of the complex and not to HF inversion. The spectroscopic parameter s of the complex have been interpreted in terms of a geometry in which tetr ahydrofuran has a conformation close to the twisted ring form, with HF lyin g on the plane bisector to the COC ring angle. (C) 2000 American Institute of Physics. [S0021-9606(00)00331-7].