Structural and energetic properties of alkylfluoride-BF3 complexes in the gas phase and condensed-phase media: computations and matrix infrared spectroscopy

• Published in: Journal of physical organic chemistry 2012-06, Vol.25 (6), p.493-501
• Main Authors: Knauf, Robin R., Helminiak, Heather M., Wrass, John P., Gallert, Timothy M., Phillips, James A.
• Format: Article
• Language: English
• Subjects: BF3 complexes; Friedel-Crafts intermediates
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.1944

We have undertaken an experimental and computational study of the structural properties of a few alkylfluoride–BF3 complexes (RF′–BF3), which are proposed intermediates in a certain class of Friedel–Crafts reactions. Using density functional theory and second‐order Møller–Plesset calculations, we have obtained gas‐phase structures, frequencies, and B–F′ bond potentials for CH3F–BF3, (CH3)2CHF–BF3, and (CH3)3CF–BF3. All the complexes are weakly‐bonded in the gas phase, with B–F′ distances (X3LYP/aug‐cc‐pVTZ) of about 2.4 Å 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 Å), we observe no global or local minima at short B–F′ distances. For the (CH3)2CHF–BF3 and (CH3)3CF–BF3 potentials in dielectric media, we do observe a distinct flattening along the inner wall, which results in shelf‐like region near 1.7 Å, but this feature is not a true local minimum. We have also obtained low‐temperature infrared spectra of the (CH3)2CHF–BF3 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. Copyright © 2011 John Wiley & Sons, Ltd. The structural and energetic properties of a few Freidel‐Crafts intermediates (alkyl‐fluoride ‐ BF3 complexes) have been characterized via computations and infrared spectroscopy. These complexes are weakly‐bonded, with long intermolecular B‐F' distances, and the alkyl substituents exhibit little or no carbocation character. The structures that correspond to points along the inner wall of the B‐F' bond potential (1.4 to 1.6 Å) show some additional carbocation character, but these are predicted to lie at significantly higher energies, even in bulk, dielectric media.