Reactive and Dissociative Mechanisms of Propyl Radicals in the Troposphere

Luis Alejandro Del Valle and Charlie Jaramillo, California State University, Los Angeles

Mentor: Dr. Scott L. Nickolaisen, Department of Chemistry & Biochemistry, California State University, Los Angeles


Ozone in the polluted urban troposphere—associated with high smog episodes—is a primary contributor to the health issues seen in those susceptible to respiratory distress.  Alkyl peroxy radicals play a significant role as intermediates in the production of ozone in the troposphere.  Our research focuses on alkyl peroxy radical formation and understanding its role in the chemical mechanisms governing ozone formation.  Specifically, we have studied the reaction dynamics of the alkyl radical precursor using pyrolysis techniques at low to moderate pressures (1 – 50 Torr).  The mid-infrared spectra following pyrolysis of 1- and 2-iodopropane were collected using a long path length absorption/FTIR apparatus.  Pyrolysis was performed as a function of temperature and added oxygen.  Spectral results indicate that 1-iodopropane forms propene, ethene, and methane.  Pyrolysis of 2-iodopropane forms only propene.  Even in the presence of added oxygen, the reactions products were identical, and we observed no evidence for formation of propyl peroxy radical.  We used ab initio techniques to determine stationary points on the potential energy surface including transition states for each the reaction.  Molecular dynamic trajectory calculations were performed at various temperatures to determine the probability of dissociation of the propyl radical to products before addition of oxygen to form propyl peroxy radical.  At 300K, more than half of trajectories were dissociatve.  At 900K, all trajectories lead to dissociation.  If correct, our results indicate that the mechanism for ozone production from C­3 and larger alkanes an alkene intermediate, and not formation of an alkyl peroxy radical intermediate.