Closed-Shell Organic Compounds Might Form Dimers at the Surface of Molecular Clusters

The role of covalently
bound dimer formation is studied using high-level
quantum chemical methods. Reaction free energy profiles for dimer
formation between common oxygen-containing functional groups are calculated,
and based on the Gibbs free energy differences between transition
states and reactants, we show that none of the studied two-component
gas-phase reactions are kinetically feasible at 298.15 K and 1 atm.
Therefore, the catalyzing effect of water, base, or acid molecules
is calculated, and sulfuric acid is identified to lower the activation
free energies significantly. We find that the reactions yielding hemiacetal,
peroxyhemiacetal, α-hydroxyester, and geminal diol products
occur with activation free energies of less than 10 kcal/mol with
sulfuric acid as a catalyst, indicating that these reactions could
potentially take place on the surface of sulfuric acid clusters. Additionally,
the formed dimer products bind stronger onto the pre-existing cluster
than the corresponding reagent monomers do. This implies that covalent
dimerization reactions stabilize the existing cluster thermodynamically
and make it less likely to evaporate. However, the studied small organic
compounds, which contain only one functional group, are not able to
form dimer products that are stable against evaporation at atmospheric
conditions. Calculations of dimer formation onto a cluster surface
and the clustering ability of dimer products should be extended to
large terpene oxidation products in order to estimate the real atmospheric
significance.