Pair exchange interaction in a crystalline nitroxide radical ESR study of dimers in the triplet state

The dimerization of the 9-aza-bicyclo (3,3,1) nonan-3-one-9-oxyl in the solid state is investigated by use of ESR spectroscopy. The ESR spectrum of a single crystal is characteristic of symmetric pairs of exchange-coupled radicals in a thermally accessible triplet state. The presence of well-resolved hyperfine structure is evidence for strongly localized excitations with a jumping rate lower than 10⁷ Hz.

The ESR spectrum is well described by the spin hamiltonian

At 35 GHz the observed splitting of the m _s=+ 1?0 transition has been found to be slightly different from that of the m _s=0?-1 one; this anomaly is explained by the mixing of the m _s electronic states.

The parameters and the principal directions of the zero-field splitting, spectroscopic and hyperfine tensors are determined and discussed. The principal directions of the dipolar tensor indicate a nearly equal spin density on the nitrogen and oxygen atoms; from the fine structure parameters D and E, determined to be (-0·0723 ± 0·0005) cm^-1 and (-0·0044±0·0003) cm^-1 at T=293 K respectively, it is suggested that the unpaired electron is partly delocalized on the molecule.

The singlet-triplet energy gap (J) and the zero-field splitting parameters are shown to be linearly temperature-dependent. These variations with temperature are attributed to the thermal expansion of the crystal lattice.     

Nuclear spin induced collapse and revival shape of Rabi oscillations of a single electron spin in diamond

A collapse and revival shape of Rabi oscillations in an electron spin of a single nitrogen-vacancy centre has been observed in diamond at room temperature. Because of hyperfine interaction between the host ¹⁴N nuclear spin and the nitrogen-vacancy centre electron spin, different orientations of the ¹⁴N nuclear spins lead to a triplet splitting of the transition between ground state (m_s =0) and excited state (m_s =1). The manipulation of the single electron spin of nitrogen-vacancy centre is achieved by using a combination of selective microwave excitation and optical pumping at 532 nm. Microwaves can excite three transitions equally to induce three independent nutations and the shape of Rabi oscillations is a combination of the three nutations.     

Fine and hyperfine structure in three low-lying 3Σ+ states of molecular hydrogen

The fine structure constant (electron spin-spin coupling) and the hyperfine structure parameters (electron-nuclear spin coupling, including spin-rotation and electron-nuclear quadrupole coupling) in the low-lying triplet states b³Σ⁺ _u, a³Σ⁺ _g and e³Σ⁺ _u of molecular hydrogen and deuterium are calculated using a recently developed technique with full configuration interaction and multiconfiguration self-consistent field wave functions. The second-order spin-orbit coupling contribution to the ³Σ⁺ states splitting is negligible, and the calculations therefore provide a good estimate of the zero-field splitting based only on the electron spin-spin coupling values. For the bound a³Σ⁺ _g state a negligible zero-field splitting is found, in qualitative agreement with the e-a spectrum. The zero-field splitting parameter is considerable for the repulsive b³Σ⁺ _u state (?1 cm^?1) and of intermediate size for the bound e³Σ⁺ _u state. The isotropic hyperfine coupling constant is very large not only for the valence b³Σ⁺ _u state (1580 MHz) but also for the Rydberg a and e triplet states (?1400 MHz). The quadrupole coupling constants for the deuterium isotopes are negligible (0.04–0.07 MHz) for all studied triplet states. The electric dipole activity of the spin sublevels in the triplet-singlet transitions to the ground state is estimated by means of the quadratic response technique.     

ESR, ENDOR, and ESEEM investigations on UV-irradiated 2,4,6-tri-tert-butyl phenol crystals

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) measurements were carried out for UV-irradiated 2,4,6-tri-tert-butyl phenol in the polycrystalline state. The radical produced in the crystal was detected by ESR and identified to be the corresponding phenoxyl radical, which is well characterized in the chemical oxidations in solutions. ENDOR and ESEEM spectra were unambiguously analyzed in terms of the hyperfine coupling constants determined from well-resolved ESR in solutions. Radical pairs in the crystals were also ascertained, and together with the single-crystal study the analysis disclosed zero-field splitting parameters in the triplet states. ESEEM time decays gave relaxation timesT ₁ = 5.94 andT ₂ = 1.12 μs at room temperature. These appropriate values permit an easy detection of the spin echoes, and therefore this radical matrix can be used as a useful standard for pulsed ESR investigations.     

EPR of photo-excited triplet states: A personal account

A sketch is presented of the path that has led from Zavoisky’s pioneering experiments to modern investigations by electron paramagnetic resonance (EPR) of the phosphorescent (S = 1) triplet state of polyatomic molecules or ions. The group-theoretical method first introduced by Wigner in his analysis of the multiplets of atomic spectroscopy, likewise provides a key for understanding the zero-field splitting and selection rules for radiative decay of the phosphorescent triplet state. Examples to illustrate the progress made through EPR experiments are selected from three fields. (i) Conformational instability on excitation. Both the zero-field splitting and the electron spin density distribution provide unique fingerprints of a triplet state’s geometry — structural information of a kind that is nonexistent for singlet states! Illustrations are provided by benzene C₆H₆ and fullerene C₆₀. (ii) The optical pumping cycle. The spin selectivity of singlet-to-triplet intersystem crossing and radiative decay of the individual spin components of the triplet state is discussed. In practice this selectivity is put to advantage by performing EPR on triplet states in zero-field by means of optical detection. In turn, such experiments have led to a detailed insight into the spin-orbit coupling mechanisms responsible for the spin selectivity of the above processes. The high sensitivity attainable with optical detection has recently culminated in EPR experiments on single molecules. (iii) Quantum interference. In a triplet state of low symmetry two of the spin sublevels may decay to the ground state by the emission of photons of a common polarization (i.e., out of plane for an aromatic hydrocarbon). In such a situation quantum interference between the two decay channels can be induced by an appropriate preparation of the excited state. An example is shown where flash-excitation in the singlet manifold followed by rapid intersystem crossing causes theS = 1 spin angular momentum to be created in a spin state which is not an eigenstate of the zero-field splitting tensor. This nonstationary character of the initial triplet state, which reflects the spin-orbit coupling pathway, is observed through the detection of a spontaneous microwave signal following the 25 ps laser flash.     

ESR study of exchange coupled pairs in copper diethyldithiocarbamate—explanation of the low temperature hyperfine anomaly

A study of the hyperfine interaction in the ESR of Cu-Cu pairs in single crystals of copper diethyldithiocarbamate as a function of temperature has shown distinct differences in the hyperfine structure in the two fine structure transitions at 20 K, the spectrum not having the same hyperfine intensity pattern in the low field fine structure transition in contrast to that of the high field transition. The details of the structure of both the fine structure transitions in the 20 K spectrum have now been explained by recognizing the fact that the mixing of the nuclear spin states caused by the anisotropic hyperfine interaction affects the electron spin states | + 1 > and | −> differently. This has incidentally led to a determination of the sign ofD confirming the earlier model. The anomalous hyperfine structure is found to become symmetric at 77 K and 300 K. It is proposed that the reason for this lies in the dynamics of spin-lattice interaction which limits the lifetime of the spin states in each of the electronic levels | − 1 >, | 0 > and | + 1 > The estimate of spin-lattice relaxation time agrees with those indicated from other studies. The model proposed here for the hyperfine interaction of pairs in the electronic triplet state is of general validity.     

Rubidium and proton ENDOR on ion pairs in solution

The E.S.R. spectrum of the o-dimesitoylbenzene anion-alkali cation radical shows unusually large isotropic alkali hyperfine splitting constants. We report a solution ENDOR study of this radical in which both alkali (^85,87Rb) and proton ENDOR spectra were recorded. Both the alkali and proton intensities showed a strong dependence on the metal ion nuclear spin quantum number of the E.S.R. line being saturated. This dependence is attributed to strong flip-flop cross-relaxation induced by modulation of the isotropic alkali hyperfine splitting. The powder E.S.R. spectrum of the complex reveals a small anisotropy of the Rb hyperfine splitting tensor. This indicates a small metal non-s-contribution to the half-filled molecular orbital, which is consistent with the observed relaxation behaviour and the small g shift. The intensity variations in the alkali and proton ENDOR spectra were used to determine the relative signs of all hyperfine splitting constants, and the absolute signs of the hyperfine splitting constants are deduced from a model of the structure of the complex.     

Hyperfine patterns of infrared absorption lines of Ho3+ C4v centres in CaF2

Hyperfine structure is observed on several infrared transitions to the ⁵I₇ and ⁵I₆ multiplets of the Ho³⁺ C_4v centre in CaF₂. Particularly complex and detailed hyperfine structures are observed for transitions to the Y₃ level of the ⁵I₇ multiplet, which include the appearance of ΔI_z ≠ 0 transitions. The hyperfine lines of this level are satisfactorily accounted for by strong mixing of close-lying levels of this multiplet by the perpendicular hyperfine interaction, which leads to relaxation of the ΔI_z = 0 selection rule and a redistribution of the hyperfine intensities over many transitions. An excellent correspondence is found between the measured and simulated spectra.     

Evidence for a localized magnetic moment in paramagnetic α-Mn from multiplet splitting

High resolution X-ray photoemission spectra of paramagnetic α-Mn are reported. The 3s and 2s levels show considerable multiplet splitting. Analysis of the 3s splitting yields a spin S ～ 1.25, implying a magnetic moment of 2.5 μ_B, in excellent agreement with a recent susceptibility value, but larger than the neutron scattering result.     

Theoretical study of the intensity of chemically induce dynamic electron polarization of radical-triplet pairs

Considering the interaction between encited triplet molecule and cloublet radical,based on the second-order perturbation theroy and the motion equation of density matrix,the polarzation intersity of RTPM were theoretically calculated with the overpopulated doublet spin states and quartet spin states of radical0triplet paris as initial conditions the radical result from the zero-field-splitting(zfs)and the multiplet A/E and E/A polarization result from hyperfine (hf) interactions of the triplet molcule,The hyperfine ralated A A/E or E E/A CIDEP on the radical were the overpopulation of the net abscrptive or emissive polarization and multiple A/E or E/A polarization.     

Electron spin resonance in the photo-excited triplet state of free base porphin in a single crystal of n-octane

E.S.R. experiments have been performed on the lowest triplet state of free base porphin (H₂P) in a n-octane single crystal at 1·3 K. The results demonstrate that the large majority of guest molecules occur in two orientations. While the molecules in these two orientations are coplanar, they have their N-H H-N axes at right angles. The fine structure results show that the two molecular orientations have zero-field splittings that differ by a few per cent in magnitude. Further, the 65 cm^-1 doublet separation which appears in the fluorescence spectrum of H₂P is related to the occurrence of these two orientations.

Resolved hyperfine structure is obtained for the two in-plane canonical orientations of the magnetic field and also when the field bisects the angle between these two directions. From an analysis of the fine structure and hyperfine structure results it is established that the zero-field splitting pattern is described by the parameters (average over the two orientations) the x axis is taken along the N-H H-N direction and z is the out-of-plane axis. From a computer simulation of the hyperfine structure it further follows that this structure is dominated by a high spin density at the methine carbons; with the coupling constants of the C-H fragment proposed by Hirota et al. the methine density is found to be ρ_m = 0·163.

In order to interpret the experimental results the zero-field splitting parameters and spin density distribution have been calculated for the lower triplet states of H₂P on the basis of a set of PPP-SCF-MO-CI calculations. From these calculations it follows that the lowest triplet state must correspond to the excitation e_g ←a _2u in Gouterman's four-orbital model. In terms of the D _2h symmetry of the H₂P molecule the assignment is ³ B _2u (b _3g ←b _1u ). For this assignment the calculations yield .     

EPR investigation of endofullerenes in solution

82 and Lu@C₈₂ were prepared by arc burning and subsequent HPLC purification. EPR spectra of Lu@C₈₂ could be interpreted as arising from unresolved hyperfine interaction with the I=7/2 nuclear spin of ¹⁷⁵Lu. At temperatures above 250 K, a thermally activated process, which is tentatively attributed to a hopping process of the encapsulated ion or to time-dependent population of close-lying electronic states, leads to pronounced line broadening. For the Ho@C₈₂ sample, no EPR signals could be detected, indicating a high spin state of this molecule. Spin relaxation data of N@C₆₀, which was prepared by ion bombardment, could be interpreted by assuming that collision-induced deformation of the carbon shell leads to a fluctuating zero-field splitting, sensed by the quartet spin state of the central encapsulated nitrogen atom. Received: 25 August 1997/Accepted: 6 October 1997     

Electron spin resonance of symmetrical three-spin systems in nematic liquid crystals

An E.S.R. line-shape model is developed for fast tumbling three-spin systems in nematic liquid crystals. The line positions are calculated from a spin hamiltonian, which considers Zeeman, exchange, dipolar and hyperfine interactions of the three unpaired electrons. The dominant spin relaxation process, determining the line-widths, is assumed to result from the anisotropy of the zero-field splitting coupled to the molecular motion. The predictions of the theory are tested by comparison with the temperature-dependent E.S.R. spectra of trisverdazyl radicals in 4,4′-azoxydianisole. Good agreement is found between experimental and simulated spectra. A detailed analysis provides values for the solute order parameters [Pbar] ₂ and [Pbar] ₄. They correspond surprisingly well to predictions of the molecular-field theories of nematic liquid crystals.     

Applications of automatic fittings to powder EPR spectra of free radicals,<Emphasis Type="Italic">S</Emphasis>>1/2, and coupled systems

An automatic fitting procedure has been employed to analyze experimentally studied paramagnetic complexes in powder form by electron paramagnetic resonance (EPR). A least-squares fitting procedure utilizing analytical derivatives of the theoretically calculated spectrum with respect to theg-, zero-field, nuclear quadrupole, and hyperfine tensors was used to refine those parameters. An anisotropic line width could also be fitted. The theoretically calculated spectra were obtained by matrix diagonalization of a general spin Hamiltonian allowing also magnetically coupled systems to be analyzed. A VO²⁺ S=1/2 complex showingg and hyperfine anisotropic interactions and free radical systems featuring Δm ₁≠0 transitions due to the direct field effect or the presence of quadrupolar nuclei have been analyzed as well as NO _x species on surfaces and radiation defects employed for EPR dosimetry. Analysis of systems withS>1/2, and magnetically coupled species has also been attempted.     

Hyperfine splittings of infrared lines of the cubic centre in CaF2:Ho3+

Hyperfine structure is observed on three transitions of the ⁵I₈?→?⁵I₇ absorption spectrum of the cubic-symmetry centre in CaF₂:Ho³⁺. Hyperfine mixing between the two lowest levels of the ⁵I₈ ground multiplet produces a complex and rich spectrum, which is only partially resolved. Simulated spectra are generated using one parameter to determine electronic crystal-field wavefunctions and one for the crystal-field splitting of the near-degenerate ground state. The simulated spectra correspond well with the measured spectra and demonstrate that the complex structure is well accounted for by dipole-hyperfine mixing between the lowest triplet and doublet states in the ⁵I₈ ground multiplet.     

Hyperfine structure and absolute frequency measurements of <Superscript>127</Superscript><Emphasis Type="Italic">I</Emphasis><Subscript>2</Subscript> transitions with monolithic Nd:YAG 561-nm lasers

The precision hyperfine structures of the ¹²⁷ I ₂ transitions at 561.4 nm are measured by the heterodyne beat between two home-made ¹²⁷ I ₂-stabilized Nd:YAG lasers. The theoretical distributions of the observed transitions’ hyperfine sublevels are used to identify the two transitions. High-accuracy hyperfine constants are obtained by fitting the measured hyperfine splittings to the four-term Hamiltonian, which includes the electric quadruple, spin-rotation, tensor spin–spin and scalar spin–spin interactions. The absolute frequencies of the observed four transitions are measured by an optical frequency comb based on a mode-locked erbium-fiber laser.     

Features of spin exchange in nitroxide biradicals in the ionic liquid bmimPF<Subscript>6</Subscript>

The intramolecular electron spin exchange has been studied by electron paramagnetic resonance (EPR) spectroscopy in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) for various nitroxide biradicals as a function of temperature and the nature of the connecting bridge between two >NO^· centers. Temperature variations of the isotropic nitrogen hyperfine splitting constant a and exchange integral values |J/a| were measured from EPR spectra and analyzed. Thermodynamic parameters of the conformational rearrangements were obtained. The spin exchange in rigid and flexible biradicals dissolved in the ionic liquid bmimPF₆ was compared with that in toluene solutions. Interesting features of the spin exchange in biradicals in ionic liquid were observed and explained as a result of the specific intramolecular conformational transitions. The first example of a rather rigid biradical molecule becoming flexible under the influence of an ionic liquid is reported.     

A hole-burning study onRhodobacter sphaeroides with absorbance-detected magnetic resonance (ADMR)

The triplet state of the primary donor in mutant reaction centers ofRhodobacter sphaeroides, in which amino acids near the primary donor were substituted, was investigated by absorbance-detected magnetic resonance. (ADMR). The mutations are associated with the substitution of leucines at site L131 and M160 near the bacteriochlorophyll halves of the primary donor by histidines, resulting in the formation of a hydrogen bond with the carbonyl group at ring V of either bacteriochlorophyll halves of the primary donor. For both mutant reaction centers the zero-field splitting parameters were slightly changed compared to native reaction centers, indicating that the dimer-halves of the primary donor in the latter are coupled in the triplet state: In addition, results from hole burning ADMR experiments on the mutants were compared with those forRhodobacter sphaeroides R26. The linewidths of the holes burnt in the zero-field transitions were similar for the mutant reaction centers involving the mutation at site L131 and for reaction centers ofRhodobacter sphaeroides R26; a somewhat larger linewidth was experiments on¹⁴N- and¹⁵N-containing reaction centers ofRhodobacter sphaeroides R26 showed that at high microwave power the holewidths of¹⁴N-reaction centers are determined by the quadrupole lines, but that at low microwave power the holewidths are mainly determined by unresolved hyperfine interactions with the protons. From the similarity in the holewidths for all reaction centers, we therefore conclude that the hyperfine interactions between the protons and the triplet spin, and thus the electronic composition of the triplet state, are similar for all reaction centers studied. The slight differences in the holewidths of the zero-field transitions and in the zero-field splitting parameters of the triplet state of the primary donor, and the differences previously observed for the interaction between the primary donor and neighboring bacteriochlorophylls (Vrieze J., Williams J.C., Allen J.P., Hoff A.J.: Biochim. Biophys. Acta1276, 221–228 (1996)), are attributed to small changes in charge-transfer contributions to the triplet state.     

Hyperfine interaction in a quantum spin chain

From an electron spin resonance measurement on a single crystal sample of theS=1 linear chain Heisenberg antiferromagnet Ni(C₃H₁₀N₂)₂NO₂ClO₄ (NINO) containing a small amount of Cu impurity atoms, we have observed two sets of four hyperfine lines, one of which has almost three times larger field splitting than the other. The hyperfine lines are well explained as arising from the hyperfine interaction between the Cu nuclear spin andthe Cu electron spin which interact with theS=1/2 degrees of freedom induced at the Ni sites by the quantum effect. A large anisotropy in the hyperfine constant is observed andanalyzed using a ligand field theory with covalency effects.     

The Zeeman effect in spin-allowed and forbidden doublet-doublet transitions in orthorhombic rotating molecules

Theoretical predictions of the Zeeman effect on spin-allowed and forbidden doublet-doublet transitions in near-symmetric top molecules are reported, with the assistance of computer simulations of band profiles. Bandwidths in the high-field limit have been determined. Marked differences have been found between the behavior of spin-allowed and forbidden transitions. The intermediate-field effect on spin-allowed transitions is sensitive to the relative energy order of the zero-field spin components of the rotational sublevels in the combining vibronic states. In the high-field limit the rotational components of spin-allowed transitions give rise to a single band, corresponding to ΔM_S = 0, whereas the bands with ΔM_S = ±1 are also expected in spin-forbidden transitions. The applicability to actual cases is finally discussed.