Density Functional Calculations of Electronic g-Tensors for Semiquinone Radical Anions. The Role of Hydrogen Bonding and Substituent Effects

• Published in: Journal of the American Chemical Society Vol. 124; no. 11; pp. 2709 - 2722
• Main Authors: Kaupp, Martin, Remenyi, Christian, Vaara, Juha, Malkina, Olga L, Malkin, Vladimir G
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 20.03.2002
• Subjects: 1-Propanol - chemistry; Anisotropy; Atomic and molecular physics; Benzoquinones - chemistry; Electron resonance and relaxation; Electron Spin Resonance Spectroscopy; Exact sciences and technology; Hydrogen Bonding; Models, Molecular; Molecular Conformation; Molecular properties and interactions with photons; Physics; Structure-Activity Relationship; Ubiquinone - chemistry; Water - chemistry; Substituent effect; Mean field approximation; Molecular structure; Hyperfine coupling constant; Benzoquinone derivatives; Theoretical study; Organic radical anion; Semiquinone; g-factor; Hyperfine structure; Hydrogen bonds; Density functional method; Hyperfine splitting; Monocyclic compound; EPR spectroscopy; Organic compounds
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0162764

A recently developed density functional approach has been used to carry out a systematic computational study of electronic g-tensors for a series of 1,4-semiquinone radical anions. Good agreement with high-field EPR data in frozen 2-propanol is achieved only after taking into account the significant reduction of g-tensor anisotropy caused by hydrogen bonding to solvent molecules. The comparison of various model systems for the first solvation shell suggests two hydrogen bonds from 2-propanol molecules to each of the carbonyl groups of the radical anions, and one additional hydrogen bond to each of the methoxy groups in ubiquinone systems. 2-Propanol makes stronger hydrogen bonds than water and thus influences g-tensor anisotropy more strongly. Substituent effects at the semiquinone are reproduced quantitatively by the calculations. The g-tensor anisotropy is influenced significantly by the conformations of methyl and methoxy substituents, with opposite contributions. Analyses and interpretations of the interrelations between structure, bonding, and spectroscopic data are provided. The relevance of the computational results for the EPR spectroscopy of semiquinone radical anions in photosynthetic reaction centers is discussed.