A Investigation of Transient Atmospheric Inorganic Peroxides: a Theoretical and Experimental Study

A combined theoretical and experimental study has been performed to characterize the peroxy isomers of NO_3 and ClO_2. Inorganic peroxy radicals are known to play key roles in chemical cycles controlling ozone in both the urban and remote troposphere as well as in the midlatitude and polar stratosphere. They may also be important in heterogeneous processes in the atmosphere. Until recently, because of their inherent lability and/or rapid dissociation, structural, spectroscopic, and thermodynamic information on the peroxy isomers of free radicals and intermediates involved in the NO _{rm X} and ClO_ {rm X} cycles were virtually unknown. Despite the recent advances in obtaining detailed thermodynamic information for the symmetric forms of these free radicals (sym-NO_3 and OClO), there remain significant uncertainties regarding their structural isomers, bonding patterns, relative stabilities, electronic states, and spectroscopic constants. This thesis presents the results of ab-initio Self-Consistent Field (SCF) calculations performed at the Hartree-Fock and Mfller-Plesset levels of theory for the electronic structure, equilibrium geometries, harmonic frequencies, and relevant thermodynamic parameters for transient peroxy isomers of NO_3 and ClO_2. Harmonic frequency analysis and calculation of isotopic shifts for several isomers of sym-NO_3 suggest a C _{{it 2/}v} equilibrium geometry. Energy and thermochemical analysis of the C_{s} peroxy isomers of NO_3 indicate that its formation is favorable in the NO_2 + O _3 reaction. Frequency analysis of cis and trans-OONO identify intense absorptions for the NO _2 asymmetric stretching and rocking modes in addition to the N = O stretching. The isomerization of OClO to ClOO has also been investigated. The results indicate that the isomerization may be favorable as an adiabatic process rather than the Herzberg-Teller allowed mechanism previously suggested. Calculated thermochemical constants for ClOO are in good agreement with experiment but extensive frequency analysis indicates strong electron correlation in the bonding of this molecule. In addition to the theoretical analysis, experimental studies employing the matrix-isolation technique coupled with infrared spectroscopy have also been performed. Spectral analysis of the product spectra indicate that both symmetrical nitrate and peroxynitrite are formed in the NO _2 + O_3 reaction. The observed product distribution suggests that OONO reactivity is dominated by rapid dissociation to NO and O _2 and reaction with NO_2 .