Role of Entropy in Increased Rates of Intramolecular Reactions
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- 作者：Anthony A. Armstrong and L. Mario Amzel
- 刊名：Journal of the American Chemical Society
- 出版年：2003
- 出版时间：November 26, 2003
- 年：2003
- 卷：125
- 期：47
- 页码：14596 - 14602
- 全文大小：325K
- 年卷期：v.125,no.47(November 26, 2003)
- ISSN：1520-5126
文摘
The rates of intramolecular condensation of a series of monoesters of dicarboxylic acids havebeen shown to be highly dependent on the nature of the intervening groups. To understand the origin ofthis effect, we estimated 
SNAC,S, the entropy difference between the ensemble of accessible ground stateconformers and a single ground state conformer having transition-state-like geometry. SNAC,S differs fromthe activation entropy for the reaction by STS,NAC, the difference in vibrational entropy between the selectedground state conformer and the transition state. The estimated values of SNAC,S correlate well (R2 = 0.96and 0.73 using dielectric constant values of 80 and 1, respectively) with experimentally determined reactionrate constants. Normal-mode analysis performed on minimized ground state conformations of each moleculesuggests that the change in vibrational entropy makes only a small contribution to the total activation entropy.These results indicate that the conformational entropy difference between the transition and the groundstates contributes significantly to the free energy of activation.
SNAC,S, the entropy difference between the ensemble of accessible ground stateconformers and a single ground state conformer having transition-state-like geometry. SNAC,S differs fromthe activation entropy for the reaction by STS,NAC, the difference in vibrational entropy between the selectedground state conformer and the transition state. The estimated values of SNAC,S correlate well (R2 = 0.96and 0.73 using dielectric constant values of 80 and 1, respectively) with experimentally determined reactionrate constants. Normal-mode analysis performed on minimized ground state conformations of each moleculesuggests that the change in vibrational entropy makes only a small contribution to the total activation entropy.These results indicate that the conformational entropy difference between the transition and the groundstates contributes significantly to the free energy of activation.