Benzenium Ion Chemistry on Solid Metal Halide Superacids: In Situ13C NMR Experiments and Theoretical Calculations

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Abstract

The benzenium, toluenium, and ethylbenzenium ions were synthesized on aluminum bromide by coadsorption of the precursors with either HBr or alkyl bromide. Principal components of the 13C chemical shift tensors for the ring carbons of these species were measured from magic angle spinning spectra. The benzenium ion was static at 77 K but underwent both proton scrambling and anisotropic rotation at 298 K as well as oligomerization at higher loadings. The para form of the toluenium ion was the dominant isomer at 77 K, but a temperature-dependent equilibrium between the para and ortho isomers was observed at 273 K. Observations of 1H−13C self-decoupling and loading-dependent 13C shifts for small amounts of toluene on high-purity AlBr3 demonstrate the existence of trace Brønsted sites on an important Friedel−Crafts catalyst. Geometries of the benzenium ions were optimized with both MP2/6-311+G* and density functional calculations at B3LYP/6-311+G*; these were in very good agreement with one another. Energy calculations at MP4(fc,sdq)/6-311+G*//MP2/6-311+G* with thermal corrections resulted in good agreement between calculated and measured proton affinities for benzene, toluene, and ethylbenzene. For the toluenium ion, the energies of the ortho and meta isomers were 1.2 and 5.4 kcal/mol, respectively, above the para isomer, consistent with the temperature-dependent 13C NMR spectra in the solid state. 13C chemical shift tensors calculated at the GIAO-MP2/tzp/dz//MP2/6-311+G* and GIAO-MP2/tzp/dz//B3LYP/6-311+G* levels of theory were in very close agreement with each other and generally in satisfactory agreement with experimental principal components. The calculated tensors of benzenium ion were modified to account for the two dynamical processes described above, and a combination of these reproduced the experimental observation of a single, axially symmetric tensor at 298 K.

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