Organic Acids Tunably Catalyze Carbonic Acid Decomposition Manoj Kumar, †,‡ Daryle H. Busch, †,‡ Bala Subramaniam, ‡,§ and Ward H. Thompson* ,†,‡ † Department of Chemistry and § Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, Kansas 66045, United States ‡ Center for Environmentally Beneficial Catalysis, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States * S Supporting Information ABSTRACT: 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 pK a , 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. 1. INTRODUCTION Carbonic acid (H 2 CO 3 ) is a molecule of wide importance. 1-5 Despite the long-held notion that it is kinetically unstable, there is now growing experimental and theoretical evidence suggesting that H 2 CO 3 is quite long-lived in the gas phase. 6-17 Pure H 2 CO 3 has been synthesized, isolated, and characterized using various techniques, 6-11 and the half-life of an isolated gaseous H 2 CO 3 molecule is estimated to be 0.18 million years at 300 K. 12 However, the kinetic stability of H 2 CO 3 is reduced in the presence of water (H 2 O) vapor, e.g., the half-life of H 2 O-bound H 2 CO 3 is only 10 h at 300 K, reflecting an enhancement of ∼10 8 in the decomposition rate constant (Scheme 1). 12,13 In addition, it has recently been reported that H 2 CO 3 , which forms dimers in the gas phase, 7,18-20 can catalyze its own decomposition. 17 Interest- ingly, recent FT-IR 21 and matrix isolation 11 experiments suggest that H 2 CO 3 in the troposphere is stable even in the presence of water vapor. Although H 2 CO 3 decomposition has been intensely studied experimentally and theoretically during the last 3 decades, key questions remain; for example, do other mechanistic pathways contribute to its gas-phase decomposi- tion? In this work, we propose a new mechanism for the catalyzed tropospheric decomposition of H 2 CO 3 by carboxylic acids. Monocarboxylic acids such as formic acid, HCOOH, and dicarboxylic acids such as oxalic acid are considered along with H 2 CO 3 itself. The involvement of an acid causes a dramatic reduction in the barrier height of the reaction and thus provides a facile bimolecular route for decomposition of gaseous H 2 CO 3 . The autocatalytic activity of H 2 CO 3 has already been reported, 17 including, more recently, 18 two additional, more effective, autocatalytic pathways based on different conforma- tions of H 2 CO 3 that result in multiple conformers for the H 2 CO 3 dimer. These reactions are considered in the general context of the catalytic role of organic acids, which is important for several reasons. Both HCOOH and H 2 CO 3 are produced in Received: April 16, 2014 Revised: June 13, 2014 Published: June 16, 2014 Scheme 1. General Mechanistic Pathways for the Unassisted and the Assisted Gas-Phase Decomposition of the syn-anti H 2 CO 3 Conformer Article pubs.acs.org/JPCA © 2014 American Chemical Society 5020 dx.doi.org/10.1021/jp5037469 | J. Phys. Chem. A 2014, 118, 5020-5028