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, 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

dependencies on the H-bond length RO···H. A

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.