| The first part of the thesis investigates the use of theoretical quantum chemical calculations in studying the pathways for the atmospheric aromatic-OH reaction. The computational model is comprised of a semi-empirical PM3 geometry optimization followed by a single point calculation using density functional theory (Becke3LYP/6-31G(d,p)). Full mechanisms for the OH-initiated photooxidation of toluene, m-xylene, p-xylene, 1,2,4-trimethylbenzene and m-ethyltoluene are developed. The lowest energy intermediates are determined and predicted products from these structures compared to available experimental product data. These studies serve to refine proposed mechanisms currently available for toluene, m-xylene and p-xylene, while providing new information on the 1,2,4-trimethylbenzene and m-ethyltoluene reaction pathways.;The second part reports the application of a technique for probing the dynamics of charged droplet evaporation and discharge in electrospray ionization. Charged droplets from an electrospray are injected into a cylindrical drift cell, in which a phase Doppler anemometer (PDA) measures the diameter and charge of individual droplets. After initial detection and characterization, the electric field within the cell is repeatedly reversed to pass the droplet through the measurement volume of the PDA. This "ping-pong" experiment yields the size and charge of a single droplet as a function of time. We applied this technique to solvents used commonly in electrospray, and report the following observations: (1) The discharge dynamics of droplets with the same initial diameter and charge are highly reproducible for all solvents and analyte/solvent combinations studied to date, in temporal patterns dictated by solvent evaporation and the approach to the Rayleigh limit of charge. (2) Droplet discharge events occur at, or slightly above, the Rayleigh limit of charge, and are characterized by loss of 15--30% of the charge from methanol, acetonitrile and water droplets, with little accompanying loss of solvent. (3) The addition of biomolecules or salt (up to 10--3 M) to the solution does not significantly alter discharge dynamics. (4) Droplet polarity does not influence discharge dynamics. (5) No significant displacement of the droplet arises from discharge events. (6) Droplet size-charge correlations show that, in certain instances, droplets from an electrospray fall into discrete groupings that can be attributed to fission events. |
