= Transition state
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A <transition state> describes configurations at the barrier between <reactants> and <reaction products> in an <elementary reaction>. It is a crossing region, rather than a stable substance that accumulates in a bottle. The IUPAC definition characterizes these configurations by equal probabilities of proceeding toward <reactants> or <reaction products>: https://goldbook.iupac.org/terms/view/T06468[IUPAC Gold Book: transition state]{ref}. Lowering the barrier relative to the starting configuration can increase a <reaction rate>, even when the overall change in <Gibbs free energy> is unchanged.
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