To evaluate the abatement of organic micropollutants during ozonation processes, it is necessary to know the corresponding second order rate constants. Lee et al. are developing a software package (presented separately), which automates rate constant prediction, along with possible products, for cases where these data are not known experimentally. Instead of Hammet or Taft constants, this software uses one-electron wavefunction energies of highest occupied molecular orbitals (HOMOs) and natural bond orbitals (NBOs) to predict second order rate constants.[1] These HF- or B3LYP-computed HOMO energies require separate regression models for different classes of aromatics (e.g., anilines, phenols…), Also, HOMOs are often found delocalized over several reactive centers of complex molecules. This makes the assignment of site-specific rate constants, needed for product predictions, difficult.<br/>We evaluated differing quantum chemical descriptors, which address the above-mentioned shortcomings. For a general model of aromatics (n=120), explicitly computed vertical ionization energies (mean unsigned error (MUE) = 1.04 log(k)) provide an improvement over HOMOs (MUE = 1.33). This can be further improved by using a more capable density functional such as M11 (MUE = 0.93). M11 also yielded more suitable HOMO energies (MUE = 1.08). Alternatively, quantum chemistry allows the calculation of reaction barriers to evaluate rate constants. This requires detailed knowledge of the reaction mechanism and is, for theoretical reasons, difficult to perform for ozone and aromatics. We show results for the well-understood ozone-olefin reactions, which show a performance (n = 35, MUE = 0.40) similar to the best orbital-based model (natural localized molecular orbitals, MUE = 0.43). These alternative approaches will help to predict ozone reaction rates more accurately, and may be used in automated systems such presented by Lee et al.

QSAR prediction of rate constants for the reaction of ozone with organic compounds using quantum chemical descriptors beyond HOMO

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.