Hydrogen bond geometries from electron paramagnetic resonance and electron-nuclear double resonance parameters: Density functional study of quinone radical anion-solvent interactions

Density functional theory was used to study the impact of hydrogen bonding on the p-benzosemiquinone radical anion BQ(.-) in coordination with water or alcohol molecules. After complete geometry optimizations, H-1, C-13, and O-17 hyperfine as well as H-2 nuclear quadrupole coupling constants and the g-tensor were computed. The suitability of different model systems with one, two, four, and 20 water molecules was tested; best agreement between theory and experiment could be obtained for the largest model system. Q-band pulse H-2 electron-nuclear double resonance (ENDOR) experiments were performed on BQ(.-) in D2O. They compare very well with the spectra simulated by use of the theoretical values from density functional theory. For BQ(.-) in coordination with four water or alcohol molecules, rather similar hydrogen-bond lengths between 1.75 and 1.78 Angstrom were calculated. Thus, the computed electron paramagnetic resonance (EPR) parameters are hardly distinguishable for the different solvents, in agreement with experimental findings. Furthermore, the distance dependence of the EPR parameters on the hydrogenbond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected 1/R-O...H(3) dependencies on the H-bond length R-O...H. A 1/R-O...H(2) correlation was obtained for the g-tensor. It is shown that the point-dipole model is suitable for the estimation of hydrogen-bond lengths from anisotropic hyperfine coupling constants of the bridging H-1 nuclei for H-bond lengths larger than approximately 1.7 Angstrom. Furthermore, the estimation of H-bond lengths from 2 H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.