Proton Hyperfine Interactions Research Articles

Making triplets from photo-generated charges: observations, mechanisms and theory

Triplet formation by charge recombination is a phenomenon that is encountered in many fields of the photo-sciences and can be a detrimental unwanted side effect, but can also be exploited as a useful triplet generation method, for instance in photodynamic therapy. In this Perspective we describe the various aspects that play a role in the decay of charge separated states into local triplet states. The observations and structures of a selection of (pre-2015) molecular electron donor-acceptor systems in which triplet formation by charge recombination occurs are reported. An overview is given of some more recent systems consisting of BODIPY dimers, and BODIPYs attached to various electron-donor units displaying this same triplet formation process. A selection of polymer–fullerene blends in which triplet formation by (non-geminate) charge recombination has been observed, is presented. Furthermore, in-depth information regarding the mechanistic aspects of triplet formation by charge recombination is given on spin dephasing, through hyperfine interactions, as well as on spin–orbit coupling occurring simultaneously with charge recombination. The limits and constraints of these factors and their role in intersystem crossing are discussed. A pictorial view of the two mechanisms is given and this is correlated to aspects of the selection rules for triplet formation, the so-called El-Sayed rules. It is shown that the timescale of triplet formation by charge recombination is indicative for the mechanism that is responsible for the process. The relatively slow rates (CRkT ~ 1 × 108 s−1 or slower) can be correlated to proton hyperfine interactions (also called the radical pair mechanism), but substantially faster rates (CRkT ~ 1 × 109 up to 2.5 × 1010 s−1 or faster) have to be correlated to spin-orbit coupling effects. Several examples of molecular systems showing such fast rates are available and their electron donor and acceptor orbitals display an orthogonal relationship with respect to each other. This orientation of (the nodal planes of) the π-orbitals of the donor and acceptor units is correlated to the mechanisms in photodynamic agents and photovoltaic blends

The Current State of Measuring Bimolecular Spin Exchange Rates by the EPR Spectral Manifestations of the Exchange and Dipole–Dipole Interactions in Dilute Solutions of Nitroxide Free Radicals with Proton Hyperfine Structure

Experimental studies of 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (Tempol) in 60 wt% aqueous glycerol were carried out for temperatures from 273 to 340 K. Selective isotope substitution allowed comparisons between the experimental spectral manifestations of spin exchange and dipole–dipole interactions for protonated, deuterated, 15N, and 14N Tempol. Theoretical spectra were computed from a rigorous theory specifically formulated to include proton hyperfine interactions over a wide range of spin exchange and dipole–dipole interactions to compare with the experimental data. For spin exchange and dipole–dipole interactions small compared with the proton hyperfine coupling constant, spectra were calculated with perturbation theory to gain insight into the behavior of individual proton lines. The theoretical and experimental spectra were analyzed by least-squares fitting to Voigt shapes or by a new two-point method. For most accessible experimental designs, the comparisons are rather good; however, for an experiment constrained to low concentrations and high viscosities, the methods are less accurate.

The effect of Gd on trityl-based dynamic nuclear polarisation in solids.

In dynamic nuclear polarisation (DNP) experiments performed under static conditions at 1.4 K we show that the presence of 1 mM Gd(iii)-DOTAREM increases the (13)C polarisation and decreases the (13)C polarisation buildup time of (13)C-urea dissolved in samples containing water/DMSO mixtures with trityl radical (OX063) concentrations of 10 mM or higher. To account for these observations further measurements were carried out at 6.5 K, using a combined EPR and NMR spectrometer. At this temperature, frequency swept DNP spectra of samples with 5 or 10 mM OX063 were measured, with and without 1 mM Gd-DOTA, and again a (13)C enhancement gain was observed due to the presence of Gd-DOTA. These measurements were complemented by electron-electron double resonance (ELDOR) measurements to quantitate the effect of electron spectral diffusion (eSD) on the DNP enhancements and lineshapes. Simulations of the ELDOR spectra were done using the following parameters: (i) a parameter defining the rate of the eSD process, (ii) an "effective electron-proton anisotropic hyperfine interaction parameter", and (iii) the transverse electron spin relaxation time of OX063. These parameters, together with the longitudinal electron spin relaxation time, measured by EPR, were used to calculate the frequency profile of electron polarisation. This, in turn, was used to calculate two basic solid effect (SE) and indirect cross effect (iCE) DNP spectra. A properly weighted combination of these two normalized DNP spectra provided a very good fit of the experimental DNP spectra. The best fit simulation parameters reveal that the addition of Gd(iii)-DOTA causes an increase in both the SE and the iCE contributions by similar amounts, and that the increase in the overall DNP enhancements is a result of narrowing of the ELDOR spectra (increased electron polarisation gradient across the EPR line). These changes in the electron depolarisation profile are a combined result of shortening of the longitudinal and transverse electron spin relaxation times, as well as an increase in the eSD rate and in the effective electron-proton anisotropic hyperfine interaction parameter.

Terahertz Spectroscopy of 25MgH (X2Σ+) and 67ZnH (X2Σ+): Bonding in Simple Metal Hydrides

Pure rotational spectra of (25)MgH (X(2)Σ(+)) and (67)ZnH (X(2)Σ(+)) were recorded using direct absorption techniques. These free radicals were synthesized by the reaction of metal vapor, generated in a Broida-type oven, with H2 in a dc discharge. The N = 0 → 1 and N = 1 → 2 transitions were recorded for both species in the frequency range 342-789 GHz. Spin-rotation and metal and proton hyperfine interactions were resolved in the spectra. From these data, rotational, fine structure, and hyperfine constants were determined, including the Fermi contact, dipolar, and electric quadrupole parameters of the (25)Mg and (67)Zn nuclei. Comparison of the metal and proton hyperfine terms suggests that the unpaired electron resides in a σ molecular orbital that has significant s contributions from both the metal and the hydrogen atoms. The dipolar terms for both metals are relatively large, in contrast to those of the proton, and indicate spσ and possibly sdσ (zinc only) orbital hybridization. The quadrupole constants arise principally from the p/d orbital character of the unpaired electron, although there is a non-negligible polarization contribution. These results suggest significant covalent character in both MgH and ZnH, in contrast to their fluoride analogs.

Excitons and charges at organic semiconductor heterojunctions

All-organic heterojunction solar cells now provide very high quantum efficiencies for charge generation and rapidly-improving power conversion efficiencies. Charge generation and separation however, must overcome the strong Coulomb interactions between electrons and holes in these materials that is manifest also through the large exchange energies usually observed. We show for a polymer-polymer system with low charge generation efficiency that this arises through intersystem crossing from the photogenerated charge-transfer state to a lower lying triplet state, mediated by the proton hyperfine interaction, and that the activation barrier for full separation of electrons and holes is of the order of 250 meV. We observe, using transient optical spectroscopy, the processes of charge separation, recombination and sweep-out in efficient polymer-fullerene devices. We report also on the process of singlet exciton fission to form a pair of triplet excitons in pentacene that can later be dissociated against a heterojunction formed with C60.

Pulse EPR and ENDOR study of 1,2,3-benzodithiazolyl, 2,1,3-benzothiaselenazolyl and 1,2,3-benzodiselenazolyl radicals

1,2,3-Benzodithiazolyl, 2,1,3-benzothiaselenazolyl and 1,2,3-benzodiselenazolyl radicals were generated by the reduction of the corresponding cations and investigated by pulse EPR and ENDOR in frozen CHCl(3) solutions at 30 and 80 K. These methods, in combination with density functional theory calculations, were used to study the magnetic parameters of the radicals, namely the principal values of the nitrogen and proton hyperfine interactions and g-tensors. The spin density distribution was shown to be nearly the same for all investigated radicals and, therefore, replacement of sulfur by selenium leads to a limited perturbation of the radicals' electronic structure. A high anisotropy of the g-tensors was found for the selenium-containing radicals.

The effect of spin polarization on zero field splitting parameters in paramagnetic π-electron molecules

Spin polarization effects play an important role in the theory of isotropic hyperfine interactions for aromatic protons. The spin polarization gives rise to significant isotropic proton hyperfine interactions--spin-dependent one-electron properties--smaller than 0 MHz and the effect has been theoretically described [H. M. McConnell and D. B. J. Chesnut, Chem. Phys. 28, 107 (1958)]. The influence of spin polarization on the zero field splitting parameters, which are spin-dependent two-electron properties, has not been clearly identified yet. A phenomenological equation is proposed here for the contribution of spin polarization to the zero field splitting parameter D in analogy to McConnell's equation for hyperfine interactions. The presence of the effect is demonstrated in a series of calculations on polyacenes in the triplet state and turns out to be responsible for up to 50% of the D parameter in the case of naphthalene! It is found that spin-unrestricted single-determinant methods, including the widely used density functional theory methods, do not accurately reproduce the two-electron reduced electron density required for the evaluation of two-electron spin-dependent properties. For the accurate calculation of zero field splitting parameters by quantum chemical methods, it thus seems necessary to resort to correlated ab initio methods which do not give rise to spin contamination and which do provide an accurate description of the two-electron reduced electron density.

A Nickel Hydride Complex in the Active Site of Methyl-Coenzyme M Reductase: Implications for the Catalytic Cycle

Methanogenic archaea utilize a specific pathway in their metabolism, converting C1 substrates (i.e., CO2) or acetate to methane and thereby providing energy for the cell. Methyl-coenzyme M reductase (MCR) catalyzes the key step in the process, namely methyl-coenzyme M (CH3-S-CoM) plus coenzyme B (HS-CoB) to methane and CoM-S-S-CoB. The active site of MCR contains the nickel porphinoid F430. We report here on the coordinated ligands of the two paramagnetic MCR red2 states, induced when HS-CoM (a reversible competitive inhibitor) and the second substrate HS-CoB or its analogue CH3-S-CoB are added to the enzyme in the active MCR red1 state (Ni(I)F430). Continuous wave and pulse EPR spectroscopy are used to show that the MCR red2a state exhibits a very large proton hyperfine interaction with principal values A((1)H) = [-43,-42,-5] MHz and thus represents formally a Ni(III)F430 hydride complex formed by oxidative addition to Ni(I). In view of the known ability of nickel hydrides to activate methane, and the growing body of evidence for the involvement of MCR in "reverse" methanogenesis (anaerobic oxidation of methane), we believe that the nickel hydride complex reported here could play a key role in helping to understand both the mechanism of "reverse" and "forward" methanogenesis.

Multifrequency EPR analysis of the positive polaron in I2-doped poly(3-hexylthiophene) and in poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene

The W-band continuous-wave electron paramagnetic resonance (EPR) analysis of chemically induced polarons in drop-cast and spin-coated polyphenylenevinylene-type and polythiophene-type polymer films reveals rhombic g tensors in both cases. The dependence of the W-band EPR signals on the orientation of the spin-coated films with respect to the magnetic field indicates a high degree of backbone alignment with the substrate and allows a partial assignment of the g tensor orientation. The derived molecular orientations of the polymer chains in the spin-coated films show clear differences between the two types of polymers. The proton hyperfine interactions obtained from X-band HYSCORE (hyperfine sublevel correlation) and Q- and W-band pulsed ENDOR (electron-nuclear double resonance) experiments are interpreted in terms of earlier theoretical studies on the extension of the polarons.

Nuclear spin relaxation in free radicals as revealed in a stimulated electron spin echo experiment

Electron spin echo envelope modulation (ESEEM) in a three-pulse stimulated echo experiment, when the time interval between the first and second pulses τ is varied, is attributed to a spontaneous change of the electron spin Larmor frequency in the time intervalT between the second and third pulses, due to the longitudinal relaxation of nearby nuclei. It is observed for nitroxide radicals in glassy matrices in the temperature range of 130–240 K. Nuclear relaxation is assumed to arise from fluctuation of the proton hyperfine interaction, due to fast rotation of the methyl groups. This contribution to ESEEM and the conventional one that is induced by the simultaneous excitation of allowed and forbidden electron spin transitions were found to be multiplicative. As the latter does not depend on the timeT, both contributions can be easily separated. The rate of nuclear spin relaxation was determined, and correlation time of methyl group rotation was estimated by Redfield theory of spin relaxation. Arrhenius parameters of this motion were estimated on the basis of these data and those at 77 and 90 K, where the previously developed approach was used (L.V. Kulik, I.A. Grigor’ev, E.S. Salnikov, S.A. Dzuba, Yu.D. Tsvetkov, J. Phys. Chem. A 106, 12066–12071, 2003).

A Generalized Unpaired Electron Spin Density Equation and Organic Hyperconjugation Mechanism

A large class of stereochemcial and related interactions in organic chemistry are repulsive and others are attractive, but the relative orientation of two methyl groups and the amount of energy required to twist one relative to the other (the hindered rotation energy barriers), or the alignment of such a group with respect to a conjugated ring to which it is attached (widely attributed to a mechanism called “hyperconjugation”) are estimated to be small in compared with the total energy of the molecule. We used theories of both isotropic and anisotropic proton hyperfine interactions in the π-electron systems developed in the early sixties. They are approximated by the magnetic dipole nteractions between each proton and an electron spin magnetization that is distributed in 2s and 2p Slater atomic orbitals center on carbon atoms. We have extended these theories to the non-planar olefinic cation radicals, which are very important in biochemistry as well as in petroleum catalysis. A three dimensional electron spin density equation has been developed in this paper to handle some Jahn-Teller vibronic molecules. The new electron spin density equation related the observed proton hyperfine splittings to the non-planar structures of the open-chain alkene cation radicals generated by radiolysis and various chemical oxidation methods. The spin densities and the conformational calculations based on valence bond theory and symmetry principles are compared with some more elaborated molecular orbital calculations in the literature. The localized valence bond approaches are better in accord with our experimental results. The anomalous line-width effect of the four methyl groups observed in the 2,3-dimethyl-2-butene cation radicals also confirmed the positive sign of the electron-proton hyperfine constant of hyper-conjugation mechanism. A methyl substituent attached to a conjugated molecule often behaves as if it formed part of the region of conjugation; the charge appears to flow from the methyl group into the π electron system and it may also give rise to an appreciable dipole moment. Methylation also gives rise to an appreciable dipole moment, and the resultant red shift of electronic absorption bands is of some importance in the design of dye molecules.

EPR and ENDOR studies of single crystals of 2-oxazolidinone X-irradiated at 295 K

When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C3O2NH5) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable to analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one α-coupling, and three β-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carbon-centered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C1–H bond with respect to the N–C1–C2 plane is estimated to be 13–15° (bending angle θ ≈ 7°). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogen-bonded proton hyperfine interactions.

Perturbation-allowed rotational transitions and A1–A2 splitting transitions in the ground, v2=1 and v4=1 vibrational states of SbH3 observed by microwave Fourier transform spectroscopy: Extension of the effective hyperfine Hamiltonian

Using the technique of Fourier transform microwave spectroscopy in the range 1–14 GHz Q-branch rotational transitions have been observed for SbH3122 and SbH3123 in the ground, v2=1 and v4=1 vibrational states with an accuracy of 0.1–100 kHz. A1–A2 splitting transitions for k=±3 in the ground state, k=±3, ±9 in v2=1, and for kl=+1, −2, +4 in the v4=1 vibrational state have been observed. We also measured perturbation-allowed transitions with selection rule Δk=±3 in the ground vibrational state for k=±1↔∓2, with selection rules Δ(k–l)=3,6,9 in the v2=1 state for k=±1↔∓2, 0↔±3, ±2↔∓4, ±2↔∓7, and 0↔±9 and in the v4=1 state for kl=+2↔+3, +2↔+6, and +3↔+5. The transitions show hyperfine structures due to the quadrupole and spin-rotation coupling of the nuclear spin ISb and the rotational angular momentum J. Hyperfine structures in the dyad v2=1, v4=1 have been analyzed using an effective Hamiltonian extended to higher order spin-rotation coupling terms and including spin-vibration coupling. A total of 21 hyperfine parameters has been determined for each isotopomer including quadrupole and spin-rotation constants of the (Δl,Δk)=(0,3), (2,2), and (2,−1) interactions. A similar analysis has been performed for the ground vibrational state yielding 7 (6) hyperfine parameters for SbH3121 (123SbH3) including the (0,3) interaction constants. Splittings of transitions between E-states involving basis states with k=±1 have been observed in the ground, v2=1 and v4=1 vibrational states. This splitting has been unequivocally explained as lifting of parity degeneracy by proton hyperfine interactions. From the analysis of the ground state hyperfine doublets, tensorial constants of the H spin-rotation coupling and the Sb–H spin–spin interaction have been accurately determined.

Molybdopterin radical in bacterial aldehyde dehydrogenases.

The EPR spectra of three different molybdoprotein aldehyde dehydrogenases, one purified from Comamonas testosteroni and two purified from Amycolatopsis methanolica, showed in their oxidized state a novel type of signal. These three enzymes contain two different [2Fe-2S] centers, one flavin and one molybdopterin cytosine dinucleotide, as cofactors all of which are expected to be EPR silent in the oxidized state. The new EPR signal is isotropic with g = 2.004 both at X-band and Q-band frequencies, consists of six partially resolved lines, and shows Curie temperature behavior suggesting that the signal is due to an organic radical with S = 1/2. The EPR spectra of Comamonas testosteroni aldehyde dehydrogenase obtained after cultivation in media containing 15NH4Cl and/or after substitution of H2O for D2O show the presence of both nitrogen and proton hyperfine interactions. Simulations of the spectra of the four possible isotope combinations yield a single set of hyperfine coupling constants. The electron spin shows hyperfine interaction with a single I = 1 (0.9 mT) ascribed to a N nucleus, with a single I = 1/2 (1.5 mT) ascribed to one nonexchangeable H nucleus, and with two, exchangeable, identical I = 1/2 spins (0.6 mT) ascribed to two identical exchangeable protons. Taken together, the observations and simulations rule out amino acid residues or flavin as the origin of the radical. The values of the various hyperfine coupling constants are consistent with the properties expected for a molybdenum(VI)-trihydropterin radical in which the N5 atom is engaged in two hydrogen-bonding interactions with the protein. The majority of the electron (spin) density of the radical is located at and around the N5 atom and at the proton bound to the C6 atom of the pterin ring. The EPR spectrum of the molybdopterin radical broadens above 65 K and is no longer detectable above 168 K, indicating that it is not magnetically isolated. The line broadening is ascribed to cross-relaxation with a nearby, rapidly relaxing, oxidized [2Fe-2S] center involving its magnetic S = 1 excited state in this process. The amount of radical was apparently not changed by addition of aldehydes or oxidants, but it disappeared upon reduction by sodium dithionite. Therefore, whether the molybdenum(VI) trihydropterin radical as detected here is a functional intermediate in catalysis remains to be investigated further.

Electronic Structure of the Neutral Tyrosine Radical in Frozen Solution. Selective 2H-, 13C-, and 17O-Isotope Labeling and EPR Spectroscopy at 9 and 35 GHz

Selective 2H-, 13C-, and 17O-isotope labeling of the tyrosine amino acid has been used to map the unpaired π-electron spin-density distribution of the UV-generated neutral l-tyrosine phenoxy radical in alkaline frozen solution. The use of 13C and 17O labels allowed accurate determination of the full spin-density distribution and provided more insight in the geometrical structure of the neutral tyrosine radical in vitro. Simulations of the X-band (9.2 GHz) and Q-band (34.8 GHz) EPR powder spectra yielded the principal components of the 1H-, 13C-, and 17O-hyperfine tensors. For the two β-methylene hydrogens, a static conformational distribution of the dihedral angles (90° < θ1 < 60° and 60° < θ2 < 30°) was taken into account. The major proton hyperfine interactions and the principal g values for the neutral tyrosine radical, obtained from selectively deuterated samples, are consistent with literature values. The spin density at the specifically labeled postitions (C1‘, C2‘, C3‘, C4‘, C5‘, O4‘) was evaluated...

A Practical Strategy for Determination of Proton Hyperfine Interaction Parameters in Paramagnetic Transition Metal Ion Complexes by Two-Dimensional HYSCORE Electron Spin Resonance Spectroscopy in Disordered Systems

A practical strategy is outlined for the determination of proton hyperfine parameters in paramagnetic transition metal ion complexes in disordered systems from a single two-dimensional hyperfine sublevel correlation spectroscopy (2D HYSCORE) electron spin resonance experiment. Both dipolar and isotropic hyperfine interaction parameters can directly be determined from the cross peak ridges in the HYSCORE spectrum in the limit of the point dipole approximation. This approach is justified by spectral simulations for isotropic and axially symmetric g tensors. If the HYSCORE spectrum is measured at the g⊥ spectral region of the electron spin resonance powder spectrum, the orientation of the hyperfine interaction tensor with respect to the g tensor frame can also be deduced from the shape of the cross peak ridges in many cases. Two experimental examples are presented. Using this approach, the hyperfine interaction parameters for protons in [Cu(H2O)6]2+ and in [Cu(C5H5N)4]2+ complexes, both incorporated into mesoporous (L)Cu−MCM-41 silica tube material, are determined from a single 2D HYSCORE spectrum. The parameters are in good agreement with independent measurements by electron spin echo modulation spectroscopy.

Isotope effects and proton hyperfine interactions in the lowest 3nπ* state of substituted benzaldehydes

The zero-field splittings, principal spin axes, kinetic parameters, and nuclear hyperfine interactions of the 3nπ* state of p-chloro- and p-methylbenzaldehyde and several of their deuterated derivatives are investigated by zero- and low-field optically detected magnetic resonance (ODMR) at 1.4 K in a p-dimethoxybenzene host. The zero-field splittings show large isotope effects. These are interpreted in terms of spin–orbit interaction with the nearby but higher lying 3ππ* state, yielding the energy gap between the two states in both benzaldehyde derivatives. The locations of the spin axes are approximately along the local symmetry axes of the carbonyl group and are insensitive to isotope. But, the spin axis most nearly normal to the plane of a host molecule deviates from the normal by an angle of 7°–13°. The kinetic parameters of the 3nπ* state also are relatively insensitive to isotope. The dominant hyperfine interactions are associated with the aldehyde hydrogen and indicate that the 3nπ* state is largely localized on the aldehyde moiety. Various properties of the 3nπ* and 3ππ* states are compared.

Low temperature1H NMR study of electronic structure parameters (T 1e T) in zirconium dihydride

Measurements of the proton-spin-lattice relaxation times (T1) are reported for zirconium dihydrides: ZrH1.87, ZrH1.93 and ZrH2.00 at nominal frequency of 41 MHz, with emphasis on the low temperature (4.2–25 K) behaviour of theT1. The dominant contributions are originated from the hyperfine interactions of the protons with the conduction electrons and the paramagnetic impurities. The (T1e T)−1/2 values determined from the low temperature range (4.2–25 K) agree with those obtained in the extended range (4.2–300 K) as well as with the values reported by the other researches. The paramagnetic impurity contribution is found to be small and teperature independent.

Analysis of powder EPR and ENDOR spectra of the biphenyl radical cation on H-ZSM-5 zeolite, silica gel and in CFCl3 matrix

The biphenyl radical cation absorbed on H-ZSM-5 zeolite and silica gel and isolated in frozen CFCl3 has been studied by EPR and ENDOR spectroscopy. The recorded spectra can be explained by anisotropy of proton hyperfine interactions in para and ortho position and of the g-tensor. The experimental results indicate that the cation has a nearly planar structure, with a twist angle of less than 10°. The ENDOR spectra are analysed by computer simulations. A simple theory to calculate ENDOR transition moments to first order in disordered systems is presented. The simulations show that the observed ENDOR line positions correspond to principal hyperfine tensor values of the para and ortho ring protons.

ESR study of the tricyclohexylmethyl radical

AbstractThe ESR spectrum of the tricyclohexylmethyl radical was obtained from tricyclohexylmethyl chloride and was studied in solution in the range 275–455 K. It has a half‐life of about 30 min at room temperature. The complex hyperfine structure of the radical was analysed with the help of MNDO/UHF calculations. Hyperfine interactions with three beta protons, twelve gamma protons and six delta protons were resolved. The alpha and the three beta carbon atoms are coplanar. The temperature dependence of the beta and six of the gamma proton hyperfine interactions was ascribed to torsional motions of the cyclohexyl rings about the CαCβ bond. The radical has C3h symmetry, the Cα and Cβ atoms being coplanar. Evidence for the existence of hexacyclohexylethane at about 200 K was obtained for the first time.