Isopropyl cation chemical shift tensors

Isopropyl alcohol, photoxidation, 41 238-239 Isopropyl cation, 42 154, 155 chemical shift tensors, 42 124 geometries, 42 135, 136 Isopropylcyclopentane, aromatization, 30 54 Isopropylcyclopropane, oxidation, 41 324 Isoteric enthalpy of adsorption, 38 154, 156 Isothermal heat of adsorption, 38 154, 158 Isotopes... 

Fig. 3. Chemical shift tensor orientations for the isopropyl cation, propionylium ion, benzoylium ion, and benzenium ion obtained from MP2 chemical shift calculations.

For our final example of theoretical NMR in catalysis, we again turn to carbenium ion chemistry. Here we study the formation of the isopropyl cation on frozen SbF5, a strong Lewis acid (27). In contrast to the studies presented earlier, with this system we were able to experimentally measure the chemical shift tensor. Because the full tensor is naturally obtained from NMR calculations, a comparison can readily be made. In addition, for the isopropyl cation we also studied the effect the medium (in this case, the charge balancing anion) had on the chemical shift tensor. 

Experimentally, the isopropyl cation was prepared by the low temperature reaction of 2-bromopropane-2-13C with frozen SbF5. The 13C spectrum was measured at 83 K using slow speed magic angle spinning. Analysis of the spectrum using the method of Herzfeld and Berger yielded tensor values of 8n = 497 ppm, 822 = 385 ppm and 833 = 77 ppm for the central carbon, which results in an isotropic chemical shift of 320 ppm. 

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