Electronic polarizability as a predictor of biodegradation rates of dimethylnaphthalenes. an ab initio and density functional theory study.

Geometries, relative stabilities, electronic excited states, atomic charges, and electronic dipole polarizabilities of dimethylnaphthalene (DMN) isomers have been calculated in gas and aqueous phases by ab initio and DFT methods. At the highest levels of calculation, alpha,alpha-DMN (2,6-DMN, 2,7-DMN, and 2,3-DMN) are the lowest energy isomers, while 1,8-DMN is the less stable by 7-8 kcal mol(-1). The averaged electronic polarizability, <alpha>, is dependent on the position of the methyl substituents, increasing in the order alpha, alpha-DMN < (a, beta-DMN < beta, beta-DMN, with the largest values being obtained for 2,6-DMN and 2,7-DMN, while the lowest value is calculated for 1,8-DMN isomer. Polarizability differences among the isomers have been related to their spectroscopic properties. The computed <a> value of DMN isomers, with the notable exception of 2,7-DMN, is in excellent linear relationship with the observed first-order biomass-normalized rate coefficient, a parameter related to the rate of biodegradation of polycyclic aromatic hydrocarbons (PAHs). This result suggests that electronic polarizability may be a useful tool for prediction of biodegradation trends of series of compounds, and inductive and dispersive interactions play a fundamental role in the biodegradation process of DMNs. The present approach is potentially suitable for applications on PAHs with higher molecular weight.