By Kaylee Knox, Sophia Redmond, Grayson Camp

Faculty Mentor: Leanna Giancarlo

Abstract

The energetics of transition states govern chemical reactivity across atmospheric, industrial, and biological systems. Although Transition State Theory provides a framework for relating molecular structure to macroscopic rate constants, direct experimental characterization of transition states remains limited due to their transient nature. Computational chemistry therefore provides an essential approach for evaluating thermodynamic properties. In this study, transition states involved in the reaction of nitric oxide with ozone were computationally examined using the GAMESS program with the 6-31G* and 6-311G** basis sets to assess their ability to reproduce reaction energetics. The two methods produced distinct energetic trends. Calculated reaction enthalpies were 51.017 kJ/mol for 6-31G* (-11.58% error) and 54.218 kJ/mol for 6-311G** (-6.03% error). Forward rate constants differed by nearly two orders of magnitude, yielding 3.8573×10^7 and 1.8077×10^9, respectively. Both approaches predicted low activation energies, with 6-31G* (1918.6 J/mol, -86.20%) and 6-311G** (2421.8 J/mol, -82.58%) underestimating literature values. Examination of the reaction-coordinate demonstrated that only the 6-31G* method reproduced the correct energetic ordering of reactants, transition state, and products. These results illustrate the sensitivity of computed reaction energetics to basis-set selection in computational studies of atmospheric reaction mechanisms.