MICROWAVE AND AB-INITIO INVESTIGATION OF HF-BF3

The complex HF-BF3 has been examined by ab initio methods and the rota tional spectra of four isotopic species have been observed via pulsed- nozzle Fourier transform microwave spectroscopy. The experimental stru cture places the fluorine of the HF near the C-3 axis of the BF3, with an intermolecular B-F separation of 2.544(2)Angstrom. The proton is o ff axis, but rapid vibrational averaging produces a complex which is e ffectively a symmetric top in the ground vibrational state. The averag e B- -F--H angle determined from the moments of inertia is 104.1(1)deg rees. These results are in excellent agreement with the ab initio calc ulations, which give a binding energy of 3-4 kcal/mol for the complex and a 2 degrees out-of-plane distortion of the BF3 moiety. Taken toget her, the results indicate a complex which is essentially weakly bound in nature, though interestingly, the intermolecular B-F distance is sl ightly shorter than the sum of reasonable van der Waals radii. The com plex resembles an incipient donor-acceptor adduct of the kind frequent ly observed or invoked in solution-phase Friedel-Crafts reactions, and its relationship to such chemistry is discussed. The experiments were performed on a newly constructed instrument featuring direct software -driven control of the pulse timing sequence without the need for home -built digital electronics. The essential characteristics of the appar atus are described.