Condensed-Phase Effects on the Structural Properties of Friedel-Crafts Intermediates: R-F’–BF 3

1. Condensed-Phase Effects on the Structural Properties of Friedel-Crafts Intermediates: R-F’–BF3 James A. Phillips, Department of Chemistry, University of Wisconsin – Eau Claire Organofluoride – boron trifluoride complexes (RF’–BF3) are key intermediates in Friedel-Crafts reactions, an important class of carbon-carbon forming processes that facilitate the conversion of petroleum feedstocks to commercially-viable compounds. The methylation of benzene is a common example: The first step in these processes are the formation RF’–BF3 intermediates, which subsequently react with an aromatic ring in a carbocation-like manner. However, we have found that CH3F–BF3, (CH3)2HCF–BF3, and (CH3)3CF–BF3are rather weak complexes in the gas-phase, as evidenced by long equilibrium B–F’ distances, and binding energies of a few kcal/mol. Thus, they presumably undergo major structural changes in solution in order to accomplish these reactions. The gas-phase potential (B98/aug-cc-pVTZ) is slightly flat along the inner wall, such that a few kcal/mol of solvent stabilization could enable a significant contraction of the B-F’ distance. Surprisingly, the minimum energy distance does not shift inward to any great extent in dielectric media (via PCM), The calculated (X3LYP/aug-cc-pVTZ) gas-phase structure of (CH3)3CF–BF3. The B-F’ distance is fairly long (2.32 Å), and the binding energy is 4.1 kcal/mol. but on the curves for = 5.0 and 10.0, one can see a shelf-like region emerge (not a true minimum), which is reminiscent of a solvent-stabilized structure with a B-F’ distance much shorter than the gas-phase value.