engleski

Quantum Chemical Study of Degenerate Hydride Shifts in Acyclic Tertiary Carbocations

Quantum chemical calculations were carried out to study the mechanism of degenerate 1, 2-, 1, 3-, and 1, 4-hydride shifts in acyclic tertiary carbocations 2, 3-dimethyl-2-butyl, 2, 4-dimethyl-2-pentyl, and 2, 5-dimethyl-2-hexyl. Stable structures and transition structures were calculated at the B3LYP and MP2 levels using the 6-31G(d) and 6-311G(d, p) basis sets. The potential energy profile for these degenerate hydride shifts has global minima potential wells that correspond to the two interchanging open-chain carbocation structures and a high lying local minimum corresponding to the symmetrically hydrido-bridged intermediates. Unsymmetrically hydrido-bridged carbocations were located as transition structures. The calculated energy barriers (at the MP4/6-311G(d, p)//MP2/6-311G(d, p) level) for 1, 2-, 1, 3-, and 1, 4-hydride shift are 3.9, 4.2, and 7.5 kcal/mol, respectively. The trend of the calculated energy barriers agrees with the experimentally observed values. Electron correlation using MP2/6-311G(d, p) is superior to B3LYP DFT hybrid methods for structures with hypercoordinated hydrogens involved in three-center two electron C-H-C bonds.

quantum-chemical study; carbocation; rearrangement

nije evidentirano