Kumar Manoj, Busch Daryle H, Subramaniam Bala, Thompson Ward H
Department of Chemistry and §Department of Chemical and Petroleum Engineering, University of Kansas , Lawrence, Kansas 66045, United States.
J Phys Chem A. 2014 Jul 10;118(27):5020-8. doi: 10.1021/jp5037469. Epub 2014 Jun 27.
Density functional theory calculations predict that the gas-phase decomposition of carbonic acid, a high-energy, 1,3-hydrogen atom transfer reaction, can be catalyzed by a monocarboxylic acid or a dicarboxylic acid, including carbonic acid itself. Carboxylic acids are found to be more effective catalysts than water. Among the carboxylic acids, the monocarboxylic acids outperform the dicarboxylic ones wherein the presence of an intramolecular hydrogen bond hampers the hydrogen transfer. Further, the calculations reveal a direct correlation between the catalytic activity of a monocarboxylic acid and its pKa, in contrast to prior assumptions about carboxylic-acid-catalyzed hydrogen-transfer reactions. The catalytic efficacy of a dicarboxylic acid, on the other hand, is significantly affected by the strength of an intramolecular hydrogen bond. Transition-state theory estimates indicate that effective rate constants for the acid-catalyzed decomposition are four orders-of-magnitude larger than those for the water-catalyzed reaction. These results offer new insights into the determinants of general acid catalysis with potentially broad implications.
密度泛函理论计算预测，碳酸的气相分解反应，一种高能的1,3-氢原子转移反应，可由一元羧酸或二元羧酸催化，包括碳酸本身。研究发现羧酸比水是更有效的催化剂。在羧酸中，一元羧酸比二元羧酸表现更优，因为二元羧酸中分子内氢键的存在阻碍了氢转移。此外，计算结果揭示了一元羧酸的催化活性与其pKa之间存在直接关联，这与之前关于羧酸催化氢转移反应的假设相反。另一方面，二元羧酸的催化效能受分子内氢键强度的显著影响。过渡态理论估计表明，酸催化分解的有效速率常数比水催化反应的有效速率常数大四个数量级。这些结果为一般酸催化的决定因素提供了新的见解，可能具有广泛的意义。
