Electron spin resonance studies of steric and solvent interactions in organic radical anions

Abstract

Steric effects of nitroaromatic radical anions have been studied utilizing esr spectroscopy. We have chosen to study the sym-trialkylnitrobenzene radical anions in solvents which offer additional steric requirements such as acetonitrile, propionitrile, isobutyronitrile, t-butyl cyanide and hexamethylphosphoramide. Steric decoupling of the nitro group from the benzenering increases as the steric hindrance in the nitroaromatic radical anion increases. The nitrogen coupling constant decreases as the steric requirement of the solvent increases. In hexamethylphosphoramide, the nitroaromatic series studied exhibit the lowest nitrogen coupling constants observed so far which indicates the small degree of solvent-ion and ion-ion interactions in this solvent. Solvent exchange between the nitrile solvents and hexamethylphosphoramide have been studied for the following processes: [equation on PDF]. The equilibrium constant for such a process decreases as the steric requirement of the solvent increases. We have investigated the dependence of β-proton coupling constant on solvent media for the sym-triethyl and sym-triisopropylnitrobenzene radical anions. Such studies indicate that the equilibrium conformation of the alkyl substituents is a function of the nature of the solvent and its ability to complex with the nitro group. The esr spectra of the thioxanthone series have been investigated and compared to the dibenzothiophene series. Thioxanthone sulfoxide reduced in dimethoxy-ethane and tetrahydrofuran leads to the corresponding radical anion with subsequent cleavage to the thioxanthone radical anion, when treated with excess alkali metal. Both radical anions are observed in the same alkali metal reduction.