NMR Study of Conformational Exchange and Double-Well Proton Potential in Intramolecular Hydrogen Bonds in Monoanions of Succinic Acid and Derivatives

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• 作者：Jing Guo ; Peter M. Tolstoy ; B. Koeppe ; Gleb S. Denisov ; Hans-Heinrich Limbach
• 刊名：Journal of Physical Chemistry A
• 出版年：2011
• 出版时间：September 8, 2011
• 年：2011
• 卷：115
• 期：35
• 页码：9828-9836
• 全文大小：977K
• 年卷期：v.115,no.35(September 8, 2011)
• ISSN：1520-5215

文摘

We present a ¹H, ²H, and ¹³C NMR study of the monoanions of succinic (1), meso- and rac-dimethylsuccinic (2, 3), and methylsuccinic (4) acids (with tetraalkylammonium as the counterion) dissolved in CDF₃/CDF₂Cl at 300鈥?20 K. In all four monoanions, the carboxylic groups are linked by a short intramolecular OHO hydrogen bond revealed by the bridging-proton chemical shift of about 20 ppm. We show that the flexibility of the carbon skeleton allows for two gauche isomers in monoanions 1, 2, and 4, interconverting through experimental energy barriers of 10鈥?5 kcal/mol (the process itself and the energy barrier are also reproduced in MP2/6-311++G** calculations). In 3, one of the gauche forms is absent because of the steric repulsion of the methyl groups. In all four monoanions, the bridging proton is located in a double-well potential and subject, at least to some extent, to proton tautomerism, for which we estimate the two proton positions to be separated by ca. 0.2 脜. In 1 and 3, the proton potential is symmetric. In 2, slowing the conformational interconversion introduces an asymmetry to the proton potential, an effect that might be strong enough even to synchronize the proton tautomerism with the interconversion of the two gauche forms. In 4, the asymmetry of the proton potential is due to the asymmetric substitution. The intramolecular H-bond is likely to remain intact during the interconversion of the gauche forms in 1, 3, and 4, whereas the situation in 2 is less clear.