Carbonic acid (CA) is a molecule of broad profile. Despite the long-held notion about its kinetic instability, there is now growing experimental and theoretical evidence suggesting that CA is quite stable in the gas-phase. However, the kinetic stability of CAis reduced in the presence of water vapors. Though the presence of water assists in the CAdecomposition, the activation barrier for the catalyzed reaction still remains very high (i.e., ~22.0 kcal/mol). In fact, Recent FT-IR and matrix isolation experiments reveal that tropospheric CA is stable even in highly humid conditions. Clearly, the precise mechanism of gas-phase decomposition of CA is far from being fully understood.Based on our high-level quantum chemical calculations, herein, we propose a new mechanism for the tropospheric decomposition of CA, which is catalyzed by an acid such as formic acid, HCOOH (FA) or CA. The involvement of an acid causes an enormous reduction in the barrier height of the reaction and thus provides a facile bimolecular route for the decomposition of gaseous CA. The calculated barrier height for the FA-catalyzed reaction is 8.7 kcal/mol upon inclusion of entropy effects, which is significantly lower as compared to the uncatalyzed (?G� = 41.0 kcal/mol) as well as H2O-catalyzed (?G� = 23.9 kcal/mol) reaction. The enormous catalytic activity of FA is attributed to the fact that the 1,3 H-shift reaction in the presence of FA is converted into two effective 1,2 H-shifts. As a result, an expanded eight-membered ring-like transition state in the formic acid-mediated decomposition is formed, which is sterically more relieved than the four-membered or six-membered cyclic transition state involved in the uncatalyzed or H2O-catalyzed reaction. The analysis of our results within transition state theory framework indicates that the presence of a single acid molecule produces an additional enhancement of 6-8 orders of magnitude in the rate of CA decomposition compared to the H2O-catalyzed one that further supports our mechanism.

Molecular modeling of carbonic acid decomposition

248th National Meeting (2014)

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