Subtle changes in vibrational averaging upon complexation: The microwave spectra and molecular structures of argon-cis-allyl fluoride and argon-gauche-allyl fluoride

The 2–18 GHz microwave spectrum of jet-expanded allyl fluoride in argon is obtained using both broadband, chirped-pulse and cavity-enhanced, Balle-Flygare Fourier transform microwave spectrometers. Rotational transitions for both the achiral, cis , and chiral, gauche , rotamers of allyl fluoride are observed and assigned in addition to the gas-phase heterodimers of each with the argon atom. The spectra of the argon complexes are analyzed to provide the position of the argon atom in the heterodimer, which is found to be driven by a preference for interaction with the electron rich fluorine atom while simultaneously interacting with other heavy atoms as geometrically possible. Tunneling between the two equivalent (and enantiomerically related) forms of the gauche rotamer is observed via splitting of c -type rotational transitions, which was previously not resolved in the ground torsional state. In addition to this large amplitude motion, atypically large values of the centrifugal distortion constants for both rotamers of this fluoropropene suggest the existence of fairly soft vibrational modes that are sufficiently altered by the presence of argon in the heterodimer so that the usual assumption of unchanged monomer geometry upon complex formation is not completely valid. • Rotational spectra analyzed for two rotamers of allyl fluoride. • Centrifugal distortion constants, previously unavailable are obtained. • Rotational spectra analyzed for argon complexes of two rotamers of allyl fluoride. • Molecular structures determined for allyl fluoride-argon complexes. • Average structure of allyl fluoride impacted by argon atom.