Different Catalytic Effects of a Single Water Molecule: The Gas‐Phase Reaction of Formic Acid with Hydroxyl Radical in Water Vapor

Abstract

The effect of a single water molecule on the reaction mechanism of the gas-phase reaction between formic acid and the hydroxyl radical was investigated with high-level quantum mechanical calculations using DFT-B3LYP, MP2 and CCSD(T) theoretical approaches in concert with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. The reaction between HCOOH and HO has a very complex mechanism involving a proton-coupled electron transfer process (pcet), two hydrogen-atom transfer reactions (hat) and a double proton transfer process (dpt). The hydroxyl radical predominantly abstracts the acidic hydrogen of formic acid through a pcet mechanism. A single water molecule affects each one of these reaction mechanisms in different ways, depending on the way the water interacts. Very interesting is also the fact that our calculations predict that the participation of a single water molecule results in the abstraction of the formyl hydrogen of formic acid through a hydrogen atom transfer process (hat).

Perspective: Water cluster mediated atmospheric chemistry

“…Water, even one water molecule in a binary complex, has been shown by quantum chemistry to be able to stabilize a transition state. 4,5,7,44,[106][107][108] However, new results underscore the need to go beyond energetic arguments and evaluate the relative competing rates between reaction and dissipation to elucidate the role of water in chemistry. 55,68 Water clusters have been used successfully as models for reactions in gas-phase, in aqueous condensed phases and at aqueous surfaces.…”

“…106 Recent examples of catalysis by a single water molecule involve OH reactions with formic acid, 107 acetaldehyde, 108 propionaldehyde, 109 methane, 35 glyoxal, 110 acetone 111 as well as the self-reaction of HO 2 99 and OH with HO 2. 112 The water mediated acetaldehyde reaction with OH is an interesting and controversial example.…”

Formic Acid Catalyzed Gas‐Phase Reaction of H2O with SO3 and the Reverse Reaction: A Theoretical Study

“…The process is similar to the reactions of OH with compounds in the presence of water. [28,30,34] Based on previous theoretical and experimental studies, [34,[71][72][73] the major features of the hydrogen-bonded complexes between formic acid and water are discussed here. Although there are three distinguishable complexes formed between formic acid and water, the most stable complex (M1) is shown in Figure 1 because of its contribution to the reaction mechanism.…”

“…[34] From the geometrical viewpoint, the main change is that the interaction distance between H a and O is 2.002 at the MP2/aug-cc-pvTz level, which is shorter than the corresponding value [34] at the MP2/6-311 + G(2df,2p) level by 0.030 .…”

“…[34] In atmospheric chemistry, it is important to determine whether the reaction studied here can compete with the reaction of SO 3 with H 2 O. From Table 2, we note that the rate constant obtained by using the pseudo-second-order method is erroneous, because the estimated rate constant extends the limit rate constant of hard-sphere collision in the atmosphere.…”