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Quantum Chemical NMR Shift Calculations of Carbocation Hydride Shift Intermediates and Transition States

Carbocation are important reactive intermediates in the course of chemical and biochemical transformations. Due to the shallow potential energy surface hydride shifts reactions are common in carbocations. Using quantum chemical methods we have investigated a series of degenerate hydride shifts in model carbocations of type 1a/1b. Degenerate hydrogen shifts in 1a/1b proceed via cyclic mi-hydrido-bridged structures of type 2 with a hypercoordinated hydrogen involved in a two-electron three-center (2e-3c) C-H-C bond. Depending on the ring size, stationary points with Cs-symmetric cyclic structure were characterised by frequency calculations either as local minima or transition state structures. For the mi-hydrogen highly shielded chemical shifts of delta(1H) up to -8 ppm were calculated. These shifts are diagnostic for the 2e-3c C-H-C bond and are very sensitive to variations of the geometrical parameters. The chemical shift data calculated for the model structures are in accord with experimental and calculated chemical shifts for transannular *-hydrido-bridged carbocations. Experimental NMR results obtained for fast degenerate hydride shifts in acyclic carbocations are rationalised by comparing the relative energies of open chain 1a/1b and hydrido-bridged structures 2. The results demonstrate that the calculation of magnetic parameters using contemporary quantum chemical methods at DFT and ab initio level of theory are in good agreement with experimental data and are a useful tool for the reliable prediction of unusual bonding situation and the investigation of the interplay between dynamic and structure of reactive intermediates.

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