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Abstract

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, ¹H, ¹³C, and ¹⁷O hyperfine as well as ²H 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 ²H 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 Å 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 hydrogen-bond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected 1/R³_O···H dependencies on the H-bond length R_O···H. A 1/R²_O···H 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 ¹H nuclei for H-bond lengths larger than approximately 1.7 Å. Furthermore, the estimation of H-bond lengths from ²H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.