High level ab initio investigation of the catalytic effect of water on formic acid decomposition and isomerization

Abstract

Formic acid (FA) is a ubiquitous molecule found in the atmosphere, and is relevant to many important processes. The FA molecule generally exists as the trans isomer, which can decompose into H₂O and CO (dehydration). It can also exist in the less favorable cis isomer which can decompose into H₂ and CO₂ (decarboxylation). Our work examines the complexes formed between each isomer of FA with water. We present geometries and vibrational frequencies obtained at the reliable CCSD(T)/aug-cc-pVTZ level of theory for seven FA⋯water complexes. We utilize the focal point method to determine CCSDT(Q)/CBS plus corrections binding energies of 7.37, 3.36, and 2.02 kcal mol⁻¹ plus 6.07, 3.79, 2.60, and 2.55 kcal mol⁻¹ for the trans-FA⋯water and cis-FA⋯water complexes, respectively. Natural bond orbital analysis is used to further decompose the interactions in each complex and gain insight into their relative strengths. Furthermore, we examine the effect that a single water molecule has on the barrier heights to each decomposition pathway by optimizing the transition states and verifying their connectivity with intrinsic reaction coordinate computations as well as utilizing a kinetic model. Water lowers the barrier to dehydration by at most 15.78 kcal mol⁻¹ and the barrier to decarboxylation by up to 15.90 kcal mol⁻¹. Our research also examines for the first time the effect of one water molecule on the interconversion barrier and we find that the barrier from trans to cis is not catalyzed by water due to the strong FA and water interactions. Our results highlight some instances where different binary complexes result in different decomposition pathways and even a case where one binary complex can form the same decomposition products via two distinct mechanisms. Our results provide a reliable benchmark of the FA⋯H₂O system as well as provide insight into future studies of similar atmospheric systems.