| RESEARCH |
Dynamics drive rearrangement pathway Transition-state structures do not necessarily determine the mechanisms of organic reactions, a new computational study suggests. When thermal effects are taken into consideration, the reaction course can deviate significantly from the minimum energy pathway, according to sophisticated calculations by chemist Michel Dupuis at Pacific Northwest National Laboratory and coworkers in Japan [Science, 299, 1555 (2003)]. The researchers studied the heterolysis of protonated pinacolyl alcohol, (CH3)3CCH(CH3)OH2+. Molecular orbital and density functional theory calculations show a single transition state corresponding to a concerted reaction with simultaneous loss of a water molecule and migration of a methyl group to give the tertiary carbocation final product (CH3)2C+CH(CH3)2. But molecular dynamics simulations at 400 K reveal a stepwise route: C2O bond cleavage gives a secondary cation that later rearranges to the tertiary cation product. "These results imply that a transition state of a given character may have only limited importance with respect to the actual mechanism," the chemists conclude. |
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