[1] Obtaining direct and detailed chemical information about the initial stages of nucleation continues to be a challenging task, but it is necessary for improved understanding of the growth process of particles in the atmosphere. To elucidate the role of organic molecules, specifically formic acid, in nucleation and particle growth, the present work uses a flow tube reactor to study formic acid-water interactions using water cluster distributions of 2-30 water molecules. The reactions studied herein are compared to methanol-water interactions to provide evidence of a change to the stable clathrate-like structure of the H + (H 2 O) 21 "magic" cluster. Intensity ratios of the form] are used to identify prominent clusters in spectra containing pure water, methanol-water mixed clusters, and formic acid-water mixed clusters. Relative concentration flow rates of 12.5, 25, 50, and 125.5 standard cubic centimeters per minute (sccm) of formic acid were added to a pure water distribution, and the resultant spectra were compared to those from methanol relative concentrations of 25, 50, and 100 sccm. The formic acid-water mixed cluster distributions observed in these reactions do not contain a prominent peak in the intensity ratio graphs, indicating a disruption in the clathrate structure of the 21-molecule cluster while a prominent peak for 21-molecule cluster is observed in the methanol system. Also observed was an increase in intensity with increased cluster size, which indicates that formic acid-water mixed clusters could serve as a prenucleation embryo for the nucleation of water molecules and a possible particle growth pathway.

Computational chemistry simulations were performed to determine the effect that the addition of a single formic acid molecule has on the structure and stability of protonated water clusters. Previous experimental studies showed that addition of formic acid to protonated pure water results in higher intensities of large-sized clusters when compared to pure water and methanol-water mixed clusters. For larger, protonated clusters, molecular dynamics simulations were performed on H(+)(H(2)O)(n), H(+)(H(2)O)(n)CH(3)OH, and H(+)(H(2)O)(n)CHOOH clusters, 19-28 molecules in size, using a reactive force field (ReaxFF). Based on these computations, formic acid-water clusters were found to have significantly higher binding energies per molecule. Addition of formic acid to a water cluster was found to alter the structure of the hydrogen-bonding network, creating selective sites within the cluster, enabling the formation of new hydrogen bonds, and increasing both the stability of the cluster and its rate of growth.

The specific rates of solvolysis of trimethylsilylmethyl trifluoromethanesulfonate have been measured at 25.0 °C in ethanol, methanol, and 2,2,2-trifluoroethanol (TFE) and their mixtures with water. Determinations were also made in aqueous acetone and in TFE-ethanol mixtures. An extended (two-term) Grunwald-Winstein equation correlation gave sensitivities towards changes in solvent nucleophilicity and solvent ionising power as expected for an S N 2 pathway, consistent with a previous proposal. For four solvents specific rates were determined at three or four additional temperatures and appreciably negative entropies of activation were observed, consistent with the proposed mechanism. At -20 °C, the specific rate of methanolysis is almost identical to that for methyl trifluoromethanesulfonate, suggesting a fortuitous balance between steric hindrance effects and a favourable electronic effect upon the introduction of the trimethylsilyl group.