Ethyl cation geometry

The experimental 3J(HH) spin-spin coupling constants for Ha, Hp, and Hp for the -isomer 22 were quite satisfactorily reproduced (A = 0.1 - 1 Hz) by calculations, using a finite perturbation method (FPT level (26), Perdew/IGLO-III at a MP2/6-31G(d)) geometry for the model structure (E)-1 -cyclopropyl-2-(trimethylsilyl)ethyl cation. The calculations confirm the /rexperimentally observed carbocation 22 (20, 27, 28). The (E)- 1-cyclopropy 1-2-... 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

Figure 4 Geometry of l-phenyl-2-(trimethylsilyl)ethyl cation (14) (B3LYP/6-31G(d)) (20)...

Fig. 5. Calculated geometry of model structure E-1 -cyclopropyl-2-(trimethylsilyl)ethyl cation (2a) with selected bond lengths in A (upper MP2/6-31G(d) lower B3LYP/6-31G(d) angle S1-Cb-C 100.6° (MP2), 107.4°...

Experimental H-NMR chemical shifts [ppm] of -l-cyclopropyl-2-(triisopropylsilyl)ethyl cation (1) and calculated H-NMR chemical shifts [ppm] of the model structures -l-cyclopropyl-2-(trimethylsilyl)ethyl cation (2a) and Z-l-cyclopropyl-2-(trimethylsilyl)ethyl cation (2b) (B3LYP/6-3110(d,p) both at MP2/6-31G(d) and B3LYP/6-31G(d) geometry). 

The ethyl cation presents a case in which two alternative geometries have been considered as representing the energy minimum. A number of MO calculations have... 

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