Time-resolved spectroscopy and chemical reactivity of energetic transient species of nitroaromatics

Abstract

Electronic excitation of symmetric trinitrobenzene (TNB) in aerated protic solvents generates a transient species the yield of which is a function of the oxygen concentration. Spectroscopic and kinetic data suggest the formation of a complex between TNB and oxygen. Electronic excitation of symmetric 2,4,6-trinitrotoluene (TNT) indicates a photochemically induced isomerization leading to an aci-quinoid transient in non-polar solvents or the 2,4,6-trinitrobenzyl anion in polar solvents. In the gas phase, electronic excitation of TNT generates the 2,4,6-trinitrobenzyl radical. This species is also formed from electronically excited TNT, 2,4,6-trinitrobenzyl chloride and hexanitrobibenzyl in 1,4-dioxane at room temperature and in ether—isopentane—ethanol at 77 K. Nanosecond laser time-resolved spectroscopy studies of mononitronaphthalenes and dinitronaphthalenes in non-polar and polar solvents indicate that the lowest triplet excited states of these compounds behave like n,π * states in non-polar media, while in polar solvents the n,π * character of these states is reduced with a simultaneous increase in their charge transfer character. In the case of 1,4-dinitronaphthalene, owing to the symmetry of nitro substitution, the triplet state retains its n,π * character even in polar solvents.