Intermolecular hydrogen bond in molecules with large amplitude motions: rotational spectrum of the complex...

The rotational spectrum of the complex 3,3-dimethyloxetane...HF generated in a supersonic jet has been analyzed using molecular beam Fourier transform microwave spectroscopy in the frequency range 6-18 GHz. Only the axial conformer of the complex has been detected. The parent and 13C and 18O monosubstituted isotopic species were observed both for isolated 3,3-dimethyloxetane and the complex in their natural abundances. For 3,3-dimethyloxetane the methyl group carbon atoms have been observed to be equivalent despite the fact that the ring-puckering potential energy function is of the double minimum type with the ground vibrational state lying below the barrier. An effective (r0) structure of the adduct has been derived from the spectroscopic constants. A C(s) symmetry has been established with HF forming an axial hydrogen bond to the O atom of the ring molecule. No significant structural changes have been observed in 3,3-dimethyloxetane upon complexation. The non-observation of a plausible equatorial conformer is discussed with the help of ab initio computations.