Resolving Discrepancy between Theory and Experiment in 4-Nitrotoluene Oxidation

Abstract

We have performed calculations of possible oxidation pathways of 4-nitrotoluene (4NT) by permanganate anion and evaluated relative contributions of oxidation of the methyl group and aromatic ring. Although a few theory levels matched the experimental results obtained by compound specific isotope analysis (CSIA) for 4NT, they failed in reproducing results for other nitrotoluene derivatives studied previously [Wijker, R.S.; Adamczyk, P.; Bolotin, J.; Paneth, P.; Hofstetter, T.B. Environ. Sci. Technol., 2013, 47, 13459-13468]. This discrepancy prompted us to reevaluate the experimental isotopic fractionation of carbon and hydrogen for 4NT on which the relative contributions of the oxidation channels has been based. Using position specific isotope analysis (PSIA) for hydrogen isotopic fractionation we have found that the previously determined value was incorrect. Reexamination of theory levels that are in agreement with these new findings indicated that while a better agreement for this particular case can be reached, overall, the previously used B3LYP functional expressed in the 6-31+G(d,p) basis set with inclusion of the polarized continuum model (PCM) of aqueous solution remains the theoretical level of choice in modeling oxidation of nitrotoluene derivatives by permanganate.