Do Hydroxyl Radical−Water Clusters, OH(H₂O)_n, n = 1−5, Exist in the Atmosphere?

Abstract

It has been speculated that the presence of OH(H2O)n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of the hydroxyl radical. We have used the G3 and G3B3 model chemistries to model the structures and predict the frequencies of hydroxyl radical/water clusters containing one to five water molecules. The reaction between hydroxyl radical clusters and methane was examined as a function of water cluster size to gain an understanding of how cluster size affects the hydroxyl radical reactivity.

Perspective: Water cluster mediated atmospheric chemistry

“…40 The influence of water on the gas phase reactivity of OH and HO 2 with other molecules such as SO 3 , CH 4 and DMS has been investigated with varying conclusions about the potential for water catalysis. 35,101,102 The unstable radical HOOO can form water complexes with longer atmospheric lifetimes than the HOOO radical, allowing for the possibility of reaction with other species. However, the atmospheric importance of this radical water complex depends on its stability, which is currently under investigation.…”

“…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.…”

“…A related study on propionaldehyde 109 with OH finds a less pronounced effect explained by the formation of stable complexes of this aldehyde with water which compete with the positive rate enhancing effect of barrier lowering by water. An interesting study by Allodi et al 35 of the free energy of activation for abstraction of a hydrogen from methane predicts that H 2 O suppresses the oxidation of CH 4 by OH. Careful studies of the stability of water complexes are unquestionably needed to understand the effect of water on radical reactions.…”

“…In addition to conflicting reports on the rate enhancement of radical reactions by water, the importance of single water catalysis of bimolecular reactions has to consider the atmospheric abundance of radical-molecule complexes even when catalysis is observed in the lab. Several studies point to the low tropospheric abundance of radical-water complexes and conclude that an atmospheric effect is unlikely, 35,106,110,113 in contrast with documented important examples of single molecule catalysis such as the hydration of SO 3 50, 114-117 and autocatalytic decomposition of sulfurous acid or carbonic acid. 118 In any discussion of gas-phase water cluster chemistry, a distinction should be made between thermally activated reactions where the excitation energy is deposited in the molecule according to the Boltzmann distribution and photochemical reactions where the energy is deposited selectively, in a non-statistical fashion in a specific mode of the molecule.…”

“…4,9,25 Water can form open and closed shell molecular complexes with low binding energies (van der Waals interactions) and more strongly bound complexes (dipolar interactions and hydrogen bonding) with atmospheric species (e.g., O 2 , N 2 , Ar, OH, HO 2 , RO 2 , O 3 , OCS, SO 2 , SO 3 , NO, SH, ClO, NH 3 , HNO 3 , HCl, H 2 SO 4 , organic acids, aldehydes, and ketones, etc.). 19,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] We illustrate results of calculations of the abundance of water complexes with altitude in Figure 1, pointing out that while the temperature minimum at the tropopause enhances the equilibrium constant, it also reduces the partial pressure of water and other monomeric condensable species leading to an overall decrease of the complex abundance with altitude. 4,8,10,12,29 Nevertheless, even low concentrations of weakly bound complexes can have a significant effect on atmospheric chemistry.…”