Overtone-Induced Decarboxylation:  A Potential Sink for Atmospheric Diacids

Abstract

Atmospheric photochemistry induced by solar excitation of vibrational overtone transitions has recently been demonstrated to be of importance in cleaving weak bonds (in HO(2)NO(2)) and inducing intramolecular rearrangement followed by reaction (in H(2)SO(4)). Here, we propose another potentially important process: the decarboxylation of organic acids. To demonstrate this possibility, we have calculated the decarboxylation pathways for malonic acid and its monohydrate. The barrier to the gas-phase decarboxylation was calculated to be in the range 26-28 kcal/mol at the B3LYP/6-311++G(3df,3pd) level of theory, in good agreement with previous results. The transition state is a six-membered ring structure which is accessed via concerted O-H and C-C stretches; excitation of v(OH) > or = 3 of either one of the OH stretching modes is sufficient to supply the energy needed for the decarboxylation. A low-energy isomer of the malonic acid-water complex forms an eight-membered, multiply hydrogen bonded structure, bound by 3-6 kcal/mol, somewhat less stable than the lowest energy, six-membered ring isomer. Decarboxylation of such complexes uses water as a catalyst; the water accepts an acidic proton from one malonic acid group and transfers a proton to the carbonyl of the other acid group. The barrier for this process is 20-22 kcal/mol, suggesting that complexes excited to v(OH) > or = 2 possess sufficient energy to react. Using estimated absorption cross sections for the OH overtone transitions, we suggest that the overtone-induced decarboxylation of malonic acid and its water complex is competitive with wet deposition of the acid and with gas-phase reaction with OH for removal of the acid.

Catalytic Role for Water in the Atmospheric Production of ClNO

“…This reaction is very fast even at low temperatures, making this relevant for chemistry in the tropopause, 100 the midlatitude and polar troposphere, 8,30,33 in volcanic plumes, [101][102][103] and indoors. 8 Catalytic effects of incorporating water into the transition state have been identified in other systems of atmospheric interest, [26][27][28][29] including the reaction of HCl with ClONO 2 to form Cl 2 , which occurs on polar stratospheric clouds. 25 The calculations show that the competing hydrolysis of ON-ONO 2 is not fast enough to significantly suppress the direct channel leading to ClNO formation.…”

Perspective: Water cluster mediated atmospheric chemistry

“…complexes at the transition state to decarboxylation. 79 In this example the cyclic transition state complex with water lowers the barrier energy by about 10 kcal mol −1 . Cluster formation opens the possibility of additional low barrier processes, which can be effective in energy dissipation from the initially excited state as illustrated in Figure 3 with the example of the water catalyzed reaction of SO 3 with H 2 O 50, 115-117 and the reverse reaction, dehydration of H 2 SO 4.…”

“…This possibility was suggested based on quantum computations of barrier heights by Staikova and Donaldson for vibrational overtone initiated reactions. 5,79 Nitric and sulfuric acid both form stable complexes with water, which could affect photochemical reaction. [80][81][82][83][84] The transition state energies are lowered in both HNO 3 and H 2 SO 4 by hydrogen bonding to water yet in these clusters the photon energy is dissipated by evaporation.…”

A theoretical study on ethylenediamine catalyzed decarboxylation of oxaloacetic acid

“…OAA is a b-keto acid, and keto carboxylic acid, such as malonic acid, decarboxylate thermally, with activation energies in the range of 20-40 kcal/mol [31,32]. This range of activation energy suggests that relatively low levels of vibrational excitation in the OH stretch mode might induce decarboxylation.…”

“…Later the direct decarboxylation of OAA was investigated experimentally by Itob et al [33], and the activation energy was found to be 23.6 kcal/ mol. According to the previous study [32], we have proposed a hydrogen migration mechanism for the direct decarboxylation of OAA. The optimized geometries with selected key geometry parameters are shown in Fig.…”