Structural and energetic properties of alkylfluoride-BF _3 complexes in the gas phase and condensed-phase media: Computations and matrix infrared spectroscopy

摘要

We have undertaken an experimental and computational study of the structural properties of a few alkylfluoride-BF 3 complexes (RF'-BF _3), which are proposed intermediates in a certain class of Friedel-Crafts reactions. Using density functional theory and second-order M?ller-Plesset calculations, we have obtained gas-phase structures, frequencies, and B-F' bond potentials for CH _3F-BF _3, (CH _3) _2CHF-BF _3, and (CH _3) _3CF-BF _3. All the complexes are weakly-bonded in the gas phase, with B-F' distances (X3LYP/aug-cc-pVTZ) of about 2.4 ? and binding energies (MP2/aug-cc-pVTZ) ranging from 5.4 and 6.7 kcal/mol. Accordingly, gas-phase bond potentials are relatively shallow and flat for these complexes. However, even though the inner walls of the potentials are rather soft (the energies rise by only about 5 to 10 kcal/mol between 2.4 and 1.6 ?), we observe no global or local minima at short B-F' distances. For the (CH _3) _2CHF-BF _3 and (CH _3) _3CF-BF _3 potentials in dielectric media, we do observe a distinct flattening along the inner wall, which results in shelf-like region near 1.7 ?, but this feature is not a true local minimum. We have also obtained low-temperature infrared spectra of the (CH _3) _2CHF-BF _3 complex in solid neon, and the frequencies agree quite favorably with those obtained via computations, which validates the computational assessment of the gas-phase complexes.