Evaluation of reactivity of pesticides with ozone in water using the energies of frontier molecular orbitals

The rate constants of ozone with four groups of pesticides (4 phenolic-, 8 organonitrogen-, 8 phenoxyalkylacitic-, and 4 heterocyclic N-pesticides) were determined under controlled conditions at a pH of 7.5, ionic strength of ca 10−3 M, and 100 μM NaHCO3. The rate constant for BPMC was first determined from a pseudo-first order reaction model, the rate constants for the other pesticides were then obtained by a competition kinetic method using BPMC as the first reference compound. The rate constants were found to vary widely according to the pesticides. The highest rate constant was 27,600 M−1 s−1 for PCP and the lowest was 61.8 M−1 s−1 for cyanazine. In order to quantify the effects of structure on the reactivity of pesticide with ozone, for each group of pesticides the rate constants were correlated with the energies of the highest occupied molecular orbital (εHOMO). The logarithm of the rate constants was found to linearly increase with the increase of the εHOMO values except for the phenoxyalkylacitic group. When all examined pesticides were included in the correlation analysis, the rate constants gave a statistically unsatisfactory correlation with a coefficient of 0.92, but showed a similar trend of increasing reactivity with increasing εHOMO values of pesticides. The results of the correlation analysis suggested that the reactivity of pesticides with ozone follows the frontier orbital theory and can be estimated by εHOMO. The rate constants for the phenoxyalkylacitic group could be estimated accurately by a two-parameter QSAR model based on absolute electronegativity and εHOMO.