Raman DFT study of dimethylnaphthalenes: isomer identification and prediction of biodegradation rate coefficients

Abstract

Raman spectra of the dimethylnaphthalene (DMN) isomers (1,2-DMN, 1,3-DMN, 1,4-DMN, 1,5-DMN, 1,6-DMN, 1,7-DMN, 1,8-DMN, 2,3-DMN, 2,6-DMN, 2,7-DMN) were calculated in the gas-phase under the harmonic approximation at the DFT-B3LYP level. The effects of the position of the methyl substituents on the Raman spectra were explored, identifying vibrational markers potentially helpful to discriminate the DMN isomers. The results show that the summation of the Raman activity (ΣΑ Raman) over the CH3 stretching vibrations and over all the 3N-6 vibrational modes increase in the order α,α-DMN < α,β-DMN < β,β-DMN. Linear correlations between ΣΑ Raman values and the experimental first-order biomass-normalized biodegradation rate coefficients were established, revealing the crucial role of inductive and dispersive forces on the biodegradation pathways. ΣΑ Raman could be a valuable molecular descriptor for QSAR applications.