Semiquinone radical anions derived from 2,3-dimethylchrysazin, 7-deoxyaklavinone, and aclacinomycin T. Computational studies of the influence of aprotic and protic solvents on the electron spin resonance spectra

Abstract

The influence of the solvent on the molecular structures and the ESR (EPR) spectra of the three title semiquinones is investigated by theoretical calculations, using density functional theory (DFT) and polarizable continuum models (PCM). The reordering of the relative magnitudes of the isotropic hyperfine constants observed when passing from an aprotic to a protic solvent is well reproduced by calculations using the integral equation formalism PCM (IEFPCM), provided discrete solute-solvent hydrogen-bonding interactions in the protic solvent is considered explicitly. A mixed discrete-continuum procedure with a simplified super-molecular model leads to excellent prediction of all proton hyperfine constants measured in methanol solution, including methine as well as alkyl protons. The dependence of the alkyl proton hyperfine constants on the conformation of the A rings in the semiquinones of 7-deoxyaklavinone and aclacinomycin T is discussed.