Unpaired Electron Spin Research Articles

The millimeter and submillimeter spectrum of HO 2: The effects of unpaired electronic spin in a light asymmetric rotor

An extensive data set for the transient species HO 2 was acquired in the spectral region between 150 and 550 GHz. The complex spectrum of this light asymmetric rotor with unpaired electronic spin and nuclear hyperfine interactions was analyzed and fit to within experimental uncertainty (<0.1 MHz). This work provides, either by direct measurement or well-founded calculation, a map of the rotational transition frequencies of HO 2 over a wide range of states.

ESR Studies on Platinum Uracil Blue

Abstract ESR spectra of platinum uracil “blue” are discussed. Two main types o f paramagnetic species have been found, one of which can be explained by assuming an unpaired electron spin delocalized over dz²-orbitals of two equivalent Pt centers, the other one is attributed to a more localized d7 Pt(III) center. In both cases the hyperfine structure suggests an additional smaller probability density of the unpaired electron at one adjacent platinum center. The smallest possible oligomers to realize this facts are tetramers in the first case and dimers in the latter one.

ESR Study on Complexes Formed on Reaction of cis-Dichlorodiammineplatinum(II) with Cytosine and Cytidine

Abstract The ESR spectra of novel paramagnetic comple-xes between cis-dichlorodiammineplatinum(II) and cytosine or cytidine in aqueous solution are presented and discussed. The results imply a complex containing a binuclear metal-metal bonded platinum moiety with an unpaired electron spin delocalized over the dz2-orbitals.

G-Factors of pyrimidine free radicals

g-Factors of the short-lived radicals in irradiated aqueous solutions of uracil, thymine and 4,6-dihydroxypyrimidine were measured and correlated with Hückel's energetic parameter λ; a regression parabola was obtained. The g-factor of the regression equation was used as a useful empirical parameter in calculating the unpaired electron spin density distribution in the planar π radicals.

Generation of Cd +1 in x-irradiated frozen aqueous solutions: Similar hydration shells for Cd +2 and Cd +1 species

Cd +1 has been generated in frozen aqueous solution by X-irradiation at 4.2 and 77 K. No difference in the g-factors or isotropic hyperfine constant has been detected for the two irradiation temperatures in contrast to Ag° species. Although the isotropic coupling constant indicates that 30% of the unpaired electron spin density of Cd +1 is delocalized onto first solvation shell waters it appears that the equilibrium solvation environments for Cd +2 and Cd +1 are essentially the same.

Impurity‐Associated Colour Centres in Mg‐ and Ca‐Doped Al2O3 Single Crystals

AbstractIn Mg‐ and Ca‐doped α‐Al2O3 crystals irradiated by X‐rays at 80 K new paramagnetic colour centres are observed having an absorption band at about 255 nm and an emission band at 310 nm. The hyperfine structure of the EPR spectrum indicates that the unpaired electron spin interacts mainly with two Al27 nuclei. The optical and EPR spectra are consistent with the model of a centre involving one electron trapped at an anion vacancy adjoining the substitutional divalent ion (FMg or FCa). The luminescence band at 290 nm observed in the TSL peak at 225 K is suggested to result from the recombination of the holes with the FMg(FCa) centres.

A nonempirical SCF–MO study of the validity of the Solomon–Bloembergen equation for the hexa-aquonickel (II) ion

Nonempirical SCF–MO calculations of the effective distance between the ligand nuclei and the unpaired electron spins in the triplet ground state (3Ag) of the hexa-aquonickel (II) ion are presented and the validity of the point dipole approximation in the Solomon–Bloembergen equation is discussed. The calculations show that the effective distance between the oxygen atom and the unpaired electron is significantly shorter than the internuclear oxygen–nickel distance. For the case of the hydrogen atom, the point dipole approximation functions very well. Calculations of the isotropic hyperfine coupling constants are also reported.

Paramagnetic resonance, magnetic susceptibility, and antiferromagnetic exchange in a Cr5+ paramagnet: Potassium perchromate (K3CrO8)

EPR and magnetic susceptibility of potassium perchromate (K3CrO8) have been studied from liquid helium temperatures to 400 K. EPR spectra of dilute aqueous solutions of K3CrO8 yielded giso=1.9712±0.0005, and 53Aiso=(18.6±0.5)×10−4 cm−1. The (positive) sign for 53Aiso, deduced here from the dependence of the linewidths of hyperfine components on nuclear spin quantum numbers, constitutes the first direct experimental measurement of the (negative) sign of the unpaired electron spin density at a Cr5+ nucleus. EPR spectra of powdered K3CrO8 doped in (diamagnetic) K3NbO8 and of K3CrO8 crystals yielded g∥=1.9428±0.0005, g⊥=1.9851±0.0005, A∥(53Cr)=+(36.3±0.5)×10−4 cm−1 and A⊥(53Cr)=+(11±1)×10−4 cm−1, the ∥ direction coinciding with the crystal c axis. These parameters establish that the unpaired electron orbital is the metal dx2–y2 orbital, with considerable charge transfer from the peroxy ligands. In concentrated samples, the EPR line shape is Lorentzian, providing direct evidence for electron spin exchange between Cr5+ ions. Temperature dependence of magnetic susceptibility χ over 1.4 to 343 K follows the law χ(cm3/mol)=[2.7933/(T+2.7)]+χd, where χd=−1.5×10−4(cm3/mol) is estimated to be the diamagnetic contribution. From the Curie–Weiss temperature of 2.7 K, an exchange coupling J=1.35 K between the Cr5+ ions is estimated. The calculated exchange-narrowed EPR linewidth is consistent with the experimental value of 15 G observed for K3CrO8. No magnetic or structural phase transition was detected in the range of 1.4 to 400 K; hence K3CrO8 crystals can serve as useful internal standard for spin concentration measurements over a wide temperature range, especially for free radicals.

The novel “g' = 1.74” EPR spectrum of pink and purple uteroferrin.

Low temperature (T less than or equal to 20 K) EPR measurements have revealed the presence of a heretofore undetected signal in uteroferrin, a purple protein bearing a single iron atom per molecule. All three of its principal g values (1.923, 1.738, and 1.583) lie well below the free electron value of 2.0023. Magnetic susceptibility data from 2-77 K confirm that the novel EPR spectrum arises from a paramagnetic center with a single unpaired electron spin. Quantitative correlation of the EPR, susceptibility, and optical data point to chromophoric iron as the source of the rhombic EPR spectrum. Furthermore, close agreement between the concentration of iron and the integrated intensity of the rhombic EPR signal show that the iron in the paramagnetic center is mononuclear. Reduction of the protein to its pink form leaves the rhombic signal essentially unaltered. The previously reported g' = 4.3 EPR signal accounts for only a small fraction of the total iron in the protein and undoubtedly arises from adventitious iron. Collectively, these results strongly suggest that uteroferrin represents a new class of low spin iron proteins.

Interspecies hydrogen bonding between a nitroxide spin label and perfluoro- t-butanol studied by DNP and 1H and 19F relaxation measurements

Relaxation and polarization of 1H and 19F nuclei on (CF 3) 3COH in the presence of 4-oxo-2,2,6,6-tetramethylpiperidinooxy free radical have been studied. Although 19F polarization indicates both scalar and dipolar coupling of nuclear and unpaired electron spins, relaxation measurements show that the -OH proton interaction is dominated by a stronger coupling arising from only a dipolar contribution.

1H-NMR studies of ferric soybean leghemoglobin Assignment of hyperfine shifted resonances of complexes with cyanide, nicotinate, pyridine and azide

The 1H-NMR spectra of complexes of soybean ferric leghemoglobin a with cyanide, nicotinate, pyridine and azide have been recorded. Assignments of many of the hyperfine shifted resonances to specific groups on the periphery of the heme have been made on the basis of their intensities and chemical shifts, pH dependence, nuclear Overhauser effects, spin decoupling and the use of Gd 3+ as a relaxation probe. The resonances of the protons at positions 3 and 5 of pyridine and nicotinate ligands have also been assigned. The iron(III) atom in the cyanide, nicotinate and azide complexes is almost completely low spin. In the pyridine complex, which is predominantly low spin, a high-spin state is thermally populated at room temperature. Information on the conformation of the heme propionate and vinyl side-chains is obtained. The average rotational position of one of the heme vinyl groups appears to differ between the nicotinate and cyanide complexes. In both of these complexes conformational rearrangement of a heme propionic acid side-chain occurs upon deprotonation of its carboxylic acid group (p K approx. 5.0). A further change in the conformation of this group occurs in leghemoglobin nicotinate upon deprotonation of the distal histidine. The p K of the heme propionic acid side-chain in leghemoglobin pyridine is 5.6. Its conformation and environment appears to differ from that in the nicotinate and cyanide complexes. In leghemoglobin cyanide, evidence for an interaction between the protonated distal histidine and the cyanide ligand is obtained. In each of the complexes studied the unpaired electron spin distribution over the porphyrin ring is highly anisotropic. Considerable differences exist between the unpaired spin distribution in the cyanide complexes of leghemoglobin and myoglobin. The axial ligand field is stronger in leghemoglobin and this may be of significance in the reaction with oxygen.

The (CO 3Li) 2−[sbnd]Li + defect in synthetic calcite: Wave function and model

EPR spectra of the (CO 3Li) 2− Li + defect in X-irradiated single-crystal calcite were observed at RT immediately after irradiation. From the resolved hyperfine structure due to the coupling of the unpaired electron spin and the nuclear spin I =½ of 13C we were able to evaluate the O C O bond angle. Using the LCAO-MO method we have established the wave function of the single electron. This permitted us to propose a model for the (CO 3Li) 2− Li + center.

NMR studies of the interaction between Mn 2+ and ClO 4− in aqueous solution

Longitudinal and transverse relaxation rates of 1H, 35Cl, and 23Na in aqueous solution containing Mn(ClO 4) 2 and NaClO 4 have been studied as a function of temperature. The water proton relaxation rates are shown not to be affected by high concentrations of NaClO 4. It is shown that the chlorine relaxation of ClO 4 − is dominated by the interaction with the unpaired electron spin of Mn 2+ and that ClO 4 − forms an inner sphere complex with Mn 2+. The rotational correlation time ( τ r ) of ClO 4 − in this complex is 4.6 × 10 −11 sec and the lifetime τ M is less than 7 × 10 −9 sec (300 K). The 23Na + relaxation is dominated by the Mn 2+− 23Na electron-nucleus dipolar interaction and is not dependent on magnetic field. It is shown that Na + is probably separated from Mn 2+ by one water molecule and a rotational correlation time τ r ≥ 7 × 10 −11 sec (300 K) is obtained.

E′center in glassy SiO2: Microwave saturation properties and confirmation of the primarySi29hyperfine structure

Electron-spin-resonance studies of a series of air-annealed samples of glassy Si${\mathrm{O}}_{2}$ having various degrees of enrichment (or depletion) in the $^{29}\mathrm{Si}$ isotope have confirmed that a $\ensuremath{\gamma}$-ray-induced doublet of 420-G splitting is the $^{29}\mathrm{Si}$ hyperfine structure of the well-known ${E}^{\ensuremath{'}}$ center. This finding validates the widely accepted model of the ${E}^{\ensuremath{'}}$ center as an unpaired electron spin in a dangling $s{p}^{3}$ hybrid orbital of a silicon bonded to three oxygens in the glass structure and eliminates a recently proposed alternative model. Continuous-wave microwave saturation measurements at \ensuremath{\sim}9.2 GHz were carried out in order to establish spin-lattice relaxation behavior and to determine absolute line intensities in the low-power limit. The spin-lattice relaxation process for the $^{29}\mathrm{Si}$ ${E}^{\ensuremath{'}}$ center is shown to be dominated by a hyperfine mechanism. Spin-lattice relaxation times ${T}_{1}$ could be extracted from the cw saturation data only by means of a semiempirical formulation differing from the usual approaches found in the literature.

Superhyperfine Interaction in {Cu(NH3)4}2+Ion Complex

Study of superhyperfine interaction in {Cu(NH 3 ) 4 } 2+ ion complex was done by using the ESR-data-accumulation system. The main purposes of this study are to ascertain the existence of magnetic electrons on nitrogen atoms and to find a reasonable explanation of two-dimensional magnetic properties of CTS (copper tetra-amine sulfate, Cu(NH 3 ) 4 SO 4 H 2 O). The experiment was done by doping {Cu(NH 3 ) 4 } 2+ in Pd(NH 3 ) 4 Cl 2 H 2 O and the results show that about 14% of the unpaired electron spin are found on nitrogen atoms.

Recombination of radical pairs in high magnetic fields: A path integral–Monte Carlo treatment

The magnetic field dependence of photoinduced geminate hydrogen (electron) transfer processes originates from a Brownian motion of radical pairs, a hyperfine coupling, and Zeeman-induced precession of the unpaired electron spins and a spin-selective recombination. In this paper we introduce a Wiener path integral description for these processes. This approach entails an integration over all possible diffusion trajectories of two functionals, one that weights the diffusion paths according to the laws of Brownian motion, and a second that accounts for the spin motion and recombination of the radical pairs. This formulation maintains a close connection with the actual physical processes and is suitable for microscopic radical reaction processes under quite general circumstances. For the path integration we employ a Monte Carlo method, and for the electron spin precession we adopt a semiclassical approximation. The theory is then applied to the magnetic field dependence of a photoinduced hydrogen transfer process involving a ketyl–hexyl radical pair recently observed with nanosecond time resolution at fields of a few kG. It is argued that the magnetic field dependence observed cannot be reconciled with a simple hyperfine-coupled Zeeman-induced spin motion. Finally, a simple analytical expression for the magnetic field dependence of geminate radical pair recombination at high magnetic fields is presented.

Rotational energy transfer in collisions between orbitally non-degenerate open-shell systems

The theory is developed for the rotational energy transfer induced in an open-shell molecule by collision with an open-shell atom. The consequences of the exchange interaction between such systems are investigated and related to the behaviour of unpaired electron spins during inelastic collisions. The coupled equations for the closed shell case are first developed, and generalized equations derived for the matrix elements of a spin-independent potential. The spin-exchange interaction potential of open-shell species is then expressed in a form suitable for scattering theory and this leads to coupling matrix elements in a basis which includes electron spin. Nuclear spin is also included by a simple extension of the basis set. It is then argued that many open-shell collisions leading to rotational energy transfer will be sufficiently weak that the coupled equations can be treated by use of the restricted distorted-wave Born approximation. This method leads to the derivation of expressions for scattering cross-sections in both the spin-independent and spin-correlated cases. The influence of exchange forces is manifest in branching fractions for spin multiplets, which relate the cross-sections for the different ΔJassociated with each change in rotational quantum numbers. A general expression for the branching fractions in hyperfine multiplets is also derived. The discussion deals with the extent to which exchange forces will influence rotational energy transfer in practice. It shows how the results of experimental investigations such as those in the following paper might be interpreted. The way in which rotational propensity rules may be affected by exchange interactions is illustrated by reference to the model systems H + CN and H + NH2. In conclusion it is noted that open-shell open-shell collisions take place naturally both in the upper atmosphere and in interstellar space.

Radical pair formation from excited states in doped aromatic crystals. I. Epr studies of the guest-host system acridine-fluorene

Following light absorption in acridine doped fluorene single crystals formation of a triplet state complex (heteroexcimer) has been established earlier. More detailed EPR investigations presented here permit the clarification of the nature of this complex. Hyperfine structure has been resolved for three nuclear spins which could be assigned by selective deuteration experiments: the nitrogen and meso-proton spin located in the guest-constituent of the complex and one proton spin originating from the CH 2-group of a fluorene host molecule. The complete hyperfine tensors could be evaluated. The anisotropy is found to be that of a planar aromatic CH-fragment for both observed proton spins. As a consequence of the CH-fragment assignment the central CH 2-group of the fluorene constituent of the complex has to change from a sp 3 to a planar sp 2 configuration indicating a photoinduced radical pair formation involving hydrogen abstraction from a fluorene and hydrogen addition to an acridine molecule. The model is able to explain: (a) the high spin density located at the observed CH-fragments; (b) the geometry of the molecular partners involved in the complex, and (c) the fine structure tensor on the basis of a simple point charge dipole-dipole coupling of the two unpaired electron spins.

The average structure of 5.10-Dihydro-5.10-Diethylphenazinium-Iodide (E 2P·I 1.6): A new linear chain Polyiodide compound

Upon oxidation of 5.10-dihydro-5.10-diethylphenazine (E 2P) with iodine golden-green lustrous crystals of a compound with stoichiometry E 2P.I 1.6 were isolated. The compound crystallizes in the tetragonal space group D 4 2 with a = 12.321(2) A ̊ and c = 5.330(2) A ̊ . The E 2P and I form interpenetrating incommensurate sublattices along c, with an iodine repeat distance of 9.7 Å. Static susceptibility measurements at room temperature give χ g = + 0.994 × 10 −6 g −1 × cm 3. This corresponds to one unpaired electron spin per two formular units. Single-crystal EPR indicates that the paramagnetism is associated with weakly interacting E 2P + cation radicals. The 300K-d.c. conductivity of 3×10 −2Ω −1cm −1 and activation energy of 0.17±0.02eV for single crystals is consequently associated with the polyiodide chains, and not with the E 2P + cation radicals.

Metalloprotein models, location of the magnetic axes in low-symmetry complexes. Single crystal electron paramagnetic resonance, magnetic susceptibility anisotropy, and angular overlap ligand field calculations on a complex containing the distorted tetrahedral Co IIN 2O 2 coordination unit, bis(N-isopropylsalicyclaldiminato)cobalt(II)

Single-crystal and powder electron paramagnetic resonance (epr) spectra of bis(N-isopropylsalicylaldiminato)cobalt(II), Co(iPr-sal) 2, doped into the isomorphous zinc complexes were obtained at liquid helium temperatures. Principal molecular g-values of g x, 7.09; g y 0.57; and g z, <0.3, were obtained. The orientation of the principal magnetic axes with respect to the distorted tetrahedral Co IIN 2O 2 coordination unit of approximate C 2 symmetry was determined from the angular variation of the single-crystal epr signals using the method of Schonland. The x-axis approximately bisects the NCoO angles of the individual bidentate ligands, the y-axis is the approximate bisector of the NCoO angles formed between different ligands, while the z-axis is taken as the approximate molecular C 2 axis. Ligand field calculations employing the angular overlap model (AOM) predict principal molecular magnetic axis directions in excellent agreement with experiment. Principal crystal magnetic susceptibility anisotropies were measured by the critical torque method. The derived molecular susceptibilities were used to evaluate dipolar nuclear magnetic resonance shifts of ligand nuclei. When dipolar contributions to the observed shifts are accounted for, the unpaired electron spin delocalization patterns of the Co(iPr-sal) 2 and Ni(iPr-sal) 2 are quite similar. The relevance of these results to the problem of achieving a detailed understanding of the properties of cobalt(II)-substituted zinc metalloproteins is stressed.