C-13 CHEMICAL-SHIFT TENSORS FOR ACYLIUM IONS - A COMBINED SOLID-STATENMR AND AB-INITIO MOLECULAR-ORBITAL STUDY

We report the principal components of the C-13 chemical shift tensors for seven acylium ions, determined by both slow speed magic angle spin ning (MAS) nuclear magnetic resonance (NMR) and theoretical methods. E xperimentally, the acylium ions were prepared either by direct reactio n of the parent acyl halides with metal halide powders, including froz en antimony pentafluoride, or by the reaction of alkyl halides with ca rbon monoxide on aluminum chloride (AlCl3). The generalization of our recent observation of the acetylium ion on AlCl3 to other cations is d irect proof of free acylium ion intermediates in Friedel-Crafts acylat ion reactions. C-13 CP MAS NMR spectra of the acylium ions were acquir ed at temperatures ranging from 83 to 298 K, and the principal compone nts of the C-13 chemical shift tensors were extracted by fitting the s ide band intensities of the MAS spectra. With the exception of the chl oroacetylium ion, the acylium ions studied have isotropic C-13(1) chem ical shifts of 154 +/- 1 ppm, but clear variations in the principal co mponents of the shift tensors were measured. The carbenium carbons of the acetylium and 2,2-dimethylpropionylium ions have axially symmetric C-13 chemical shift tensors, consistent with the molecular symmetry ( C-3 upsilon), while the chemical shift tensors of the other cations we re characterized by non-zero asymmetry parameters. The observation of appreciably smaller chemical shift anisotropies for C-1 in the benzoyl ium ions versus the values for the corresponding carbon in the alkanoy l cations is consistent with charge delocalization into the ring subst ituent. Additional information on the acylium cations is provided by t heoretical calculations. We optimized the geometries of the acylium io ns using second-order Moller-Plesset perturbation theory (MP2) and the 6-311G basis set. We then calculated the NMR data at the MP2 level u sing the gauge-including atomic orbital (GIAO) method and the double-z eta (dz) and triple-zeta polarized (tzp) basis sets of Horn and Ahlric hs. While the isotropic shifts calculated at the GIAO-RHF/tzp/dz level were in error by as much as 26 ppm, the GIAO-MP2 values were in excel lent agreement with the experimental measurements, as were those for m ost of the principal components. The calculations were also used to de termine the orientations of the principal components. The results of a nalysis of the MP2 wave functions help answer long standing questions regarding the structure and bonding of acylium cations.