Anisotropic Hyperfine Coupling Constants Research Articles

A comparative analysis of powder ENDOR spectra of aromatic and aliphatic radicals by exact and 1st order simulation

Exact and perturbation methods were employed in simulations of powder ENDOR spectra to obtain the anisotropic hyperfine ( hfc ) and nuclear quadrupole ( nqc ) coupling constants of certain organic radicals of interest in fundamental research and in applications in radiation research, surface chemistry and biophysics. The principal hfc values of the ring protons and methyl substituents for several aromatic cations trapped in disordered matrices might be more accurate than those previously reported using regular EPR. Only one of the earlier assignments of the naphthalene cation spectrum was in acceptable agreement with the simulations. The proton couplings at the β-position of alkyl radicals were deduced by simulations while the spectrum due to α-couplings with appreciable anisotropy was weak. Accurate simulation of the 14 N (I = 1) spectra in bio-radicals was obtained by adjustment of the relative orientation of the principal hfc and nuclear quadrupole coupling ( nqc ) tensors as well as the principal values. Adjustment of the excitation width parameter employed in the software was also required in a few cases to improve the agreement with the experimental spectra. The hfc patterns due to matrix nuclei ( 1 H and 7 Li) around the radicals of X-irradiated samples were simulated to elucidate the nature of the trapping sites in materials used for EPR dosimetry. ENDOR simulation programs known to us are presented in an Appendix. The performance of the ENDORF2 program used in previous works was examined by comparison with exact treatment. Input and output examples, source and executable codes used in this work can be downloaded at https://github.com/EndorF2/Simulation .

Performance of the DLPNO-CCSD and recent DFT methods in the calculation of isotropic and dipolar contributions to 14N hyperfine coupling constants of nitroxide radicals.

In the present study, the performance of a set of density functionals: BP86, PBE, OLYP, BEEF, PBEpow, TPSS, SCAN, PBEGXPBE, M06L, MN15L, B3LYP, PBE0, mPW1PW, B97, BHandHLYP, mPW1PW, B98, TPSS0, PBE1KCIS, SCAN0, M06, M06-2X, MN15, CAM-B3LYP, ωB97x, B2PLYP, and the B3LYP/N07D and PBE/N07D schemes in the calculation of the 14N anisotropic hyperfine coupling (HFC) constants of a set of 23 nitroxide radicals is evaluated. The results are compared with those obtained with the DLPNO-CCSD method and experimental HFC values. Harmonic contribution to the 14N HFC vibrational correction was calculated at the revPBE0/def2-TZVPP level and included in the evaluation. With the vibrational correction, the DLPNO-CCSD method yielded HFC values in good agreement with the experiment (mean absolute deviation (MAD) = 0.3G for the dipole-dipole contribution and MAD = 0.8G for the contact coupling contribution). The best DFT results are obtained using the M06 functional with MAD = 0.2G for the dipole-dipole contribution and MAD = 0.7G for the contact coupling contribution. In general, vibrational correction significantly improved most DFT functionals' performance but did not change its overall ranking.

Self-trapped holes (small polarons) in ferroelectric KH2PO4 crystals

Density functional theory is used to establish the ground-state structure of the self-trapped hole (STH) in KH2PO4 crystals. The STHs in this nonlinear optical material are free small polarons, a fundamental intrinsic point defect. They are produced with ionizing radiation in the low-temperature orthorhombic structure of KH2PO4 and are only stable (i.e. long-lived) below approximately 70 K. A large 129-atom cluster, K19H40P14O56, is constructed to model the STH. The ωB97XD functional with the 6−31+G* basis set is used and geometry optimization is performed. Our results show that two of the oxygen ions in a PO4 unit relax toward each other and equally share the hole. These two oxygen ions do not initially have close hydrogen neighbors. This equal sharing of the hole is related to the presence of isolated, slightly distorted, PO4 units and is significantly different from the small-polaron behavior often observed in other oxide crystals where the hole is localized on only one oxygen ion. The computational results provide a detailed description of the lattice relaxation occurring during formation of the STH. Characteristic spectral features of this defect are a larger hyperfine interaction with one 31P nucleus and equal, but smaller, hyperfine interactions with two 1H nuclei. The computed values for these isotropic and anisotropic hyperfine coupling constants are in excellent agreement with results obtained from electron paramagnetic resonance experiments.

Performance of DFT methods in the calculation of isotropic and dipolar contributions to 14N hyperfine coupling constants of nitroxide radicals.

In the present study, we tested the widely used density functionals BP86, PBE, OLYP, TPSS, M06-L, B3LYP, PBE0, mPW1PW, B97, BHandHLYP, TPSS0, M06, M06-2X, CAM-B3LYP, ωB97x, and B2PLYP with the cc-pCVQZ basis set in calculations on a set of 23 nitroxide radicals with well-resolved 14N anisotropic hyperfine coupling (HFC) constants. The results were compared with those obtained using the B3LYP/N07D and PBE/N07D methods. The convergence of the HFC values to the complete basis set limit is briefly discussed. The best results were obtained using the M06/COSMO method, with a mean absolute deviation (MAD) of 0.4G for the dipole-dipole contribution and MAD = 0.6G for the contact coupling contribution (as compared to 1.1G and 1.0G, respectively, for the B3LYP/N07D/COSMO method and 1.7G and 0.5G, respectively, for the B3LYP/N07D method). The majority of the functionals yielded satisfactory results for the dipole-dipole contribution, but only the M06 functional yielded similar errors for both the dipole-dipole and isotropic contributions. The RIJCOSX and RI approximations introduced errors equal to or smaller than 0.01G.

Electron paramagnetic resonance study of gamma (γ)-irradiated methyl 4-methyl benzoate (C9H10O2)

ABSTRACTIn this study, gamma (γ)-irradiated methyl 4-methyl benzoate (C9H10O2) single crystals were investigated and the structure of the free radical occurring by γ-irradiation was determined by using Electron paramagnetic resonance (EPR) technique. C9H10O2 single crystals were produced by slow evaporation with distilled water solution at room temperature. These single crystals were irradiated by 60Co-gamma-ray source with a dose speed of 0.981 kGy/h at room temperature for 72 h. The EPR spectra of irradiated C9H10O2 single crystals were recorded between 125 and 400 K temperatures in steps of 10° intervals at different orientations in magnetic field with an X-band EPR spectrometer. The spectra were found to be slightly anisotropic and temperature independent; taking into consideration the chemical structure with experimental data we found only one free radical structure . The radical structure was simulated using the Bruker software WinEPR-Simfonia and the experimental g-value with anisotropic hyperfine coupling constants of radical were calculated as follows: g = 2.0124, = 26.51G, = 5.45G.

A [3Fe-3S](3+) cluster with exclusively μ-sulfide donors.

A [3Fe-3(μ-S)](3+) cluster is reported in which each ferric center has a distorted trigonal pyramidal geometry, with an S = 1/2 ground state for the cluster and unusually anisotropic hyperfine coupling constants as determined by variable temperature magnetometry and Mössbauer spectroscopy.

Methoxy Dihedral Angles of Ubiquinone Contribute More than 160 mV to the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of the Photosynthetic Reaction Center

Ubiquinone is an almost universal, membrane-associated redox mediator. Its redox properties are substantially determined by its environment in the binding sites of proteins and by the dihedral angles of the methoxy groups relative to the ring plane. In this work, we use the photosynthetic reaction center as a model system for understanding the role of methoxy conformations in determining the redox potential of the ubiquinone/semiquinone couple. Despite the abundance of X-ray crystal structures for the reaction center, the quinone site resolution has thus far been too low to provide a reliable measure of the methoxy dihedral angles of the primary and secondary quinones, QA and QB. We have performed HYSCORE on isolated reaction centers with ubiquinones 13C-labeled at the headgroup methyl and methoxy groups, and the isotropic and anisotropic hyperfine coupling constants were estimated. Comparison of our data to quantum mechanically derived models and available crystal structures gives a best fit for the 2-methoxy dihedral angle of QB as 50° more out of plane than in QA. This assignment corresponds to a redox potential gap (ΔEm) between QA and QB of ≈180 mV. This is consistent with the failure of a ubiquinone analog lacking the 2-methoxy group to function as QB in mutant reaction centers with ΔEm ≈160-195 mV. We conclude the 2-methoxy group of ubiquinone provides an essential mechanism by which the reaction center tunes the redox potential difference necessary for electron transfer. The influence of the methoxy groups on the second electron transfer is under investigation through similar EPR studies of the biradical, QA-QB-, generated in mutant reaction centers and at high pH in wild type reaction centers.

Electron Paramagnetic Resonance Studies of Nitrosyl and Thionitrosyl and Density Functional Theory Studies of Nitrido, Nitrosyl, Thionitrosyl, and Selenonitrosyl Complexes of Chromium

The novel S = 1/2 thionitrosyl complexes Cr(NS)(CN)(5)(3-), Cr(NS)(dmso)(5)(2+), and Cr(NS)(nmf)(5)(2+) (dmso = dimethyl sulfoxide, nmf = N-methylformamide) have been prepared, and their optical and electron paramagnetic resonance (EPR) spectra were studied. The values of the isotropic and anisotropic hyperfine and superhyperfine coupling constants A((53)Cr), A((14)N), and A((13)C) and of g were determined from the EPR spectra at room temperature and at 66 K. The values of A(⊥) and A(iso) in the thionitrosyl complexes were slightly higher than in the analogous nitrosyl complexes. A common feature in the optical absorption spectra of the thionitrosyl complexes in solution at 298 K is an absorption band around 600 nm with a vibronic structure whereas such a band is located around 450 nm in the analogous nitrosyl complexes. Density functional theory (DFT) studies of the series of complexes Cr(N)(H(2)O)(5)(2+), Cr(NO)(H(2)O)(5)(2+), Cr(NS)(H(2)O)(5)(2+), and Cr(NSe)(H(2)O)(5)(2+) show that the unpaired electron resides in a metal-based d(xy) orbital and that the electronic structure in the equatorial plane is similar in all four complexes and similar to Cr(3+). The σ donating ability was found in the order N(3-) ≫ NO < NS ≈ NSe and the π accepting ability in the order NO > NS ≈ NSe. Time dependent DFT calculations gave in all four complexes a d(x(2)-y(2)) ← d(xy) transition energy around 17 500 cm(-1).

The Equilibrium Between the Octahedral and Square Pyramidal Form and the Influence of an Axial Ligand on the Molecular Properties of VIVO Complexes: A Spectroscopic and DFT Study

The previously unreported equilibrium in aqueous solution between the V(IV)O square pyramidal and trans octahedral form with an axial water molecule for a number of bidentate ligands with (N,O) and (O,O) donor sets (6-methylpicolinic (6-mepicH) and 6-methyl-2,3-pyridinedicarboxylic (6-me-2,3-pdcH(2)) acids, dipyridin-2-ylmethanol (Hdpmo) and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (Hdhp)) has been demonstrated by the combined application of EPR spectroscopy and DFT methods. The EPR spectra suggest that, with increasing ionic strength, the equilibrium is shifted towards the formation of the pentacoordinated species and values of K approximately 4.0 and 7.0 for the systems containing 6-methyl-2,3-pyridinedicarboxylic acid and dipyridin-2-ylmethanol were measured. DFT calculations performed with Gaussian 03 and ORCA software predict the (51)V anisotropic hyperfine coupling constant along the z axis (A(z)), which can be used to demonstrate the presence of an axially bound ligand trans to the V=O bond. The results suggest that an axial donor (charged or not) can lower |A(z)|, in contrast to what was previously believed on the basis of the "additivity rule", and this explains the anomalous behaviour of the V(IV)O complexes formed by N-{2-[(2-pyridylmethylene)amino]phenyl}pyridine-2-carboxamide (Hcapca) and several amidrazone derivatives. The decrease in |A(z)| for the axial binding of a solvent molecule is mainly a result of the reduction of |A(iso)| and this was also observed when the solid [VO(6-methylpicolinato)(2)] was dissolved in DMSO or DMF. The variations in the structural (V=O, V-O and V-N distances, O-V-O and N-V-N angles, and the trigonality index tau) and spectroscopic (|A(z)|, |A(iso)| and nu(V=O)) properties as a function of the axial V-OH(2) distance (R) are also presented. Finally, the electronic structures of the penta- and hexacoordinated complexes are discussed.

Muon Spin Spectroscopy of the Nematic Liquid Crystal 4-n-Pentyl-4′-cyanobiphenyl (5CB)

Avoided level crossing muon spin resonance (ALC-microSR) spectroscopy has been used to study the four cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the rodlike liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). ALC-microSR spectra have been obtained over a wide temperature range in the isotropic, nematic, and crystalline phases. Four Delta0 resonances were observed in the ALC-microSR spectra, from which the methylene proton hyperfine coupling constants (hfcs) of the Mu adducts of 5CB were determined as a function of temperature. The methylene proton hfcs of two of the radicals have unusual temperature dependence in the nematic phase and have smaller values than would be predicted from extrapolating the data in the isotropic phase. We have used the Maier-Saupe theory for rodlike liquid crystals to explain the temperature dependence of the methylene proton hfcs, which results from the ordering of the 5CB molecules, the alignment of the molecules with the external magnetic field, and fluctuations that average the anisotropic hyperfine coupling constants. There are no Delta1 resonances in the ALC-microSR spectra of the nematic phase due to the radicals rotating rapidly around the long molecular axis and fluctuations about the local director. The Delta0 resonances broaden substantially as the temperature is lowered due to the slowing down of the fluctuations, which have an average activation energy of approximately 15.9 kJ mol(-1). Cooling the sample below 275 K stopped the rotation around the long molecular axis and led to the appearance of Delta1 resonances.

Theoretical study of the electronic and hyperfine structures of the HSO and SOH radicals

The electronic structures, the spin density distributions, and the isotropic and anisotropic hyperfine coupling constants of the HSO and SOH radicals have been studied by applying the natural bond orbital (NBO) method to the B3LYP/6-311++G(3df,3pd) densities. Within the NBO framework, the S O unit is properly described as intermediate between a single and a double bond and the HSO isomer is more stable due to the favorable p π(SO) overlap and the high strength of the H S bond. The computed spin densities indicate that both compounds are π-radicals, although the unpaired spin in HSO is shared by the sulfur and oxygen atoms and it is mostly located on the sulfur for SOH. The different spin density distributions and the values of the isotropic and anisotropic hyperfine coupling constants are properly explained on the basis of the proposed NBO structures.

Theoretical study of the electronic structure of HXY/XYH radicals (XC,Si;YO,S)

The electronic structures of the HXY/XYH compounds (X[Double Bond]C,Si;Y[Double Bond]O,S) on the (2)A(') electronic ground state were investigated by applying the natural bond orbital (NBO) method to the computed B3LYP6-311G(**) wave functions. Different localized structures are proposed for the HXY and XYH isomers and the central XY unit is described as intermediate between a double and a triple bond in HCO, HCS, HSiO, and HSiS, similar to a double bond in COH, CSH, and SiSH, and clearly a single bond in SiOH. Through the comparison between the NBO results for the diatomic and hydrogenated compounds, the energy preferences on each pair of isomers and the computed geometrical parameters are explained. According to the structures proposed, the HXY compounds are sigma radicals with the spin density distributed along the molecular framework, while the XYH compounds are pi radicals with most of the unpaired spin located on an almost pure p orbital of the X atom. Finally, the amounts of spin density on natural atomic orbitals provided by the NBO method are used to explain the computed values of the isotropic and anisotropic hyperfine coupling constants.

VIVO Complexes of Bis(imidazol‐2‐yl) Derivatives: A Potentiometric, Spectroscopic and DFT Study

AbstractThe complexation of the VIVO ion with four amino acid derivatives of bis(imidazol‐2‐yl)methylamine [N‐glycyl‐bis(imidazol‐2‐yl)methylamine = Gly‐BIMA, N‐α‐aspartyl‐bis(imidazol‐2‐yl)methylamine = α‐Asp‐BIMA, N‐α‐glutamyl‐bis(imidazol‐2‐yl)methylamine = α‐Glu‐BIMA and N‐histidyl‐bis(imidazol‐2‐yl)methylamine = His‐BIMA] was studied in aqueous solution through the combined application of potentiometric and spectroscopic (UV/Vis and EPR) techniques. For comparison, the complexing capability of three simple bis(imidazol‐2‐yl) derivatives [bis(imidazol‐2‐yl)methane = BIM, bis(imidazol‐2‐yl)methylamine = BIMA and bis(imidazol‐2‐yl)nitromethane = BINM] and two benzyloxycarbonyl (Z) derivatives (Z‐Gly‐BIMA and Z‐Ala‐BIMA) was reported. Mono‐ and bis‐chelated species with the (Nim, Nim) donor set were formed in both acid and neutral pH conditions, with the bis‐chelated complexes being characterised by a cis‐trans isomerism. In the basic pH range the complexation process continues with the formation of a mono‐hydroxo cis‐VOL2H–1 complex in systems with BIM, BIMA and BINM, and with the deprotonation and coordination of the amide nitrogen to give VOLH–1 and VOLH–2 in those with Gly‐BIMA, α‐Asp‐BIMA, α‐Glu‐BIMA and His‐BIMA. The results demonstrate that the bis(imidazol‐2‐yl)methyl residue is an anchoring group of intermediate strength, capable of avoiding extensive hydrolysis of the VIVO ion in the presence of a slight excess of ligand (L/M from 3:1 to 5:1). DFT calculations with progressively more complex basis sets were performed in order to obtain information on the structure of the VOLH–1 and VOLH–2 complexes. Finally, a discussion on the 51V anisotropic parallel hyperfine coupling constant (A∥) of VOLH–1 and VOLH–2 and on the EPR properties connected to the V–N–(amide) bond in VIVO complexes is presented. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)

The Structure of the Hydrated Electron. Part 1. Magnetic Resonance of Internally Trapping Water Anions: A Density Functional Theory Study

Density functional theory is used to rationalize magnetic parameters of hydrated electron trapped in alkaline glasses as observed using electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) spectroscopies. To this end, model water cluster anions (n=4-8 and n=20, 24) that localize the electron internally are examined. It is shown that hyperfine coupling tensors of H/D nuclei in the water molecules are defined mainly by the cavity size and the coordination number of the electron; the water molecules in the second solvation shell play a relatively minor role. An idealized model of the hydrated electron (that is usually attributed to L. Kevan) in which six hydroxyl groups arranged in an octahedral pattern point toward the common center is shown to provide the closest match to the experimental parameters, such as isotropic and anisotropic hyperfine coupling constants for the protons (estimated from ESEEM), the second moment of the EPR spectra, and the radius of gyration. The salient feature is the significant transfer (10-20%) of spin density into the frontal O 2p orbitals of water molecules. Spin bond polarization involving these oxygen orbitals accounts for small, negative hyperfine coupling constants for protons in hydroxyl groups that form the electron-trapping cavity. In Part 2, these results are generalized for more realistic geometries of core anions obtained using a dynamic one-electron mixed quantum/classical molecular dynamics model.

Density Functional Theory Study of the β-Carotene Radical Cation and Deprotonated Radicals

The beta-carotene radical cation and deprotonated neutral radicals were studied at the density functional theory (DFT) level using different density functionals and basis sets: B3LYP/3-21G, SVWN5/6-31G*, BPW91/DGDZVP2, and B3LYP/6-31G**. The geometries, total energies, spin distributions, and isotropic and anisotropic hyperfine coupling constants of these species were calculated. Deprotonation of the methyl group at the double bond of the cyclohexene ring of the carotenoid radical cation at 5 or 5' produces the most stable neutral radical because of retention of the pi-conjugated system while less stable deprotonation at 9 or 9' and 13 or 13' of the chain methyl groups causes significant distortion of the conjugation. The predicted methyl hyperfine coupling constants of 13-16 MHz of the neutral radicals are in good agreement with the previous electron nuclear double resonance (ENDOR) spectrum of photolyzed beta-carotene on a solid support. DFT calculations on the beta-carotene radical cation in a polar water environment showed that the polar environment does not cause significant changes in the proton hyperfine constants from those in the isolated gas-phase molecule. DFT calculated methyl proton hyperfine coupling constants of less than 7.2 MHz are in agreement with those reported for the radical cation in photosystem II (PS II) and those found in the absence of UV light for the radical cation on a silica alumina matrix.

A DFT study of the hyperfine coupling constants of triplet carbenes and biradicals

Motivated by our recent investigation of spin–orbit mixing in the simplest singlet carbene, HCF, we report Density Functional Theory (DFT) calculations of the isotropic and anisotropic hyperfine coupling constants (HFCCs) of HCF ( a ˜ A ″ 3 ) and other important triplet carbenes and biradicals. We conducted extensive DFT calculations on the prototypical triplet carbene, CH 2, and compare our results with experimental constants determined from Electron Spin Resonance (ESR) and microwave spectroscopy. We subsequently report DFT calculations of the HFCCs of other prototypical triplet carbenes and biradicals including HCF, HCCN, and C 3H 2, and the arylcarbenes diphenylmethylene (DPM; C 13H 10) and fluorenylidene (C 13H 8). The calculated HFCCs are compared in each case with experimental ESR and microwave data. This study provides an example of the utility of DFT for the calculation of HFCCs of open-shell species.

Erratum: “An ab initio calculation of the anisotropic hyperfine coupling constants in the low-lying vibronic levels of the XΠ2 electronic state of CCCH” [J. Chem. Phys. 121, 12361 (2004)

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Theoretical study of the nuclear degrees of freedom in the isotropic and anisotropic hyperfine coupling constants of the C2H5 radical: a Feynman path integral–density functional approach

The isotropic and anisotropic hyperfine coupling (hfs) constants of the C2H5 radical have been theoretically studied under the conditions of thermal equilibrium, i.e. under the explicit consideration of the nuclear degrees of freedom. For this purpose the Feynman path integral quantum Monte Carlo (PIMC) formalism has been combined with an electronic Hamiltonian of the B3LYP–EPRIII type. The density functional operator has been used to derive both the distribution functions for the isotropic and anisotropic hfs constants of the ethyl radical as well as the thermal mean values. The electron paramagnetic resonance (EPR) timescale enables only the measurement of the thermal averages. The underlying distribution functions of these mean values, however, offer insight into the nature and strength of the nuclear degrees of freedom contributing to the observable thermal averages. The EPR parameters of C2H5 have been studied between 25 and 1000 K. This temperature (T ) window is large enough to consider nuclear fluctuations beyond zero-point effects. The deviations between the thermally averaged hfs constants and the values at the minimum of the potential energy surface (PES) are caused by (i) enlargements of the bond lengths in thermal equilibrium under the influence of anharmonicities in the internuclear potential, and (ii) by the intramolecular methylene rotation. The latter degree of freedom leads to a planar CH2 unit in thermal equilibrium. At the minimum of the PES the methylene fragment exhibits a certain pyramidalization. The ensemble corrections as well as the T dependence of the isotropic hfs constants are larger than the ensemble shifts and T dependence of the anisotropic parameters. The non-validity of the crude Born–Oppenheimer approximation for the theoretical evaluation of physically meaningful isotropic hfs constants of the ethyl radical has been explained on the basis of specific nuclear degrees of freedom. Theoretical results of the ensemble averaged Monte Carlo type as well as single-nuclear configuration data are compared with experiment whenever available.

An ab initio calculation of the anisotropic hyperfine coupling constants in the low-lying vibronic levels of the X 2Π electronic state of CCCH

The results of ab initio calculations of the vibronically averaged components of the anisotropic magnetic hyperfine tensor in the low-lying vibronic species of the X (2)Pi electronic state of CCCH and CCCD are reported. The electronically averaged hyperfine coupling constants for hydrogen and (13)C in (12)C (12)C (12)CH, (13)C (12)C (12)CH, (12)C (13)C (12)CH, (12)C (12)C (13)CH, and (12)C (12)C (12)CD are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The results of the present study help to reliably interpret the experimental data previously published and predict the yet unobserved hyperfine structure in excited vibronic states of CCCH and CCCD.

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, (1)H, (13)C, and (17)O hyperfine as well as (2)H 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 (2)H electron-nuclear double resonance (ENDOR) experiments were performed on BQ(*-) in D(2)O. 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 A 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 R(O.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 (1)H nuclei for H-bond lengths larger than approximately 1.7 A. Furthermore, the estimation of H-bond lengths from (2)H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.