Susan E. Barrows and Christopher J. Cramer, Department of Chemistry and Supercomputer Institute, University of Minnesota, Minneapolis, MN, 55455-0431
and
Michael S. Elovitz and Eric J. Weber, United States Environmental Protection Agency, 960 College Station Road, Athens, GA 30605-2700
Polynitroaromatics readily enter into the environment, since they are common in dyes, dye intermediates, and high-energy materials. Reduction of these compounds is observed to occur under biotic and abiotic conditions to yield aromatic amines. When multiple nitro groups are present, the rate of reduction of the first is generally much more rapid than the rate for any additional reduction of remaining nitro groups; understanding the specificity with which they reduce allows prediction of what longer-lived anilines should be expected to contaminate soils and groundwaters. Presumably, product partitioning occurs at the first protonation step following the initial electron transfer to create a polynitroaromatic radical anion. We calculate semiempirical Austin Model 1 (AM1) electrostatic potential maps on the van der Waals surfaces of a number of polynitroaromatic radical anions. When experimental reduction occurs exlusively at a single nitro position, in every case it is at the nitro group of the radical anion over which the most negative charge is localized. A key aspect of this work is that the gas-phase electrostatic potential does not permit much distinction to be made between nitro groups. However, when calculations are done that include the effect of solvation using the aqueous Solvation Model 2 (SM2), and either Class IV Charge Model 1 (CM1) or simple Mulliken atomic partial charges are employed to construct the potential surface, the selectivity becomes apparent. Finally, in order to evaluate certain issues of conformational analysis, AM1 is compared to ab initio levels of theory for the radical anions of nitrobenzene and 2-nitrotoluene.
Table of Contents
III. Discussion A. Geometries i. Gas phase calculations ii. Effects of solvation B. Solvation polarizarion of the electronic structures C. Point charge comparison