EPR Spectroscopy of Mercury-Organic Compounds with Nitroxide Radicals

Intramolecular electron spin exchange as a function of temperature, solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy for two nitroxide biradicals containing mercury-organic groups in the bridge connecting two 1-oxyl-2,2,6,6-tetramethylpiperidine-3,4-ene-nitroxide rings, R. The temperature dependence of the isotropic hyperfine splitting (hfs) constant a ^N and the exchange integral value |J/a| of the biradicals were measured from EPR spectra and subsequently analyzed comparing to a ^N and a ^Hg hfs constants of ClHgR radical dissolved in the same solvents. In all cases, the interaction of solvent molecules (SM) with >N–O fragments of nitroxide rings led to a slight decrease in a values with increasing temperature. The |J/a| value varied slightly with temperature T changes. The changes of |J/a| are much less comparing to those with variation of the solvent polarity. The interaction between SM and Hg atoms inside the bridge is observed and discussed.     

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.     

Features of Spin Exchange in Short-Chain Nitroxide Biradicals in Ionic Liquids

The intramolecular electron spin exchange has been studied by electron paramagnetic resonance (EPR) spectroscopy in the room temperature ionic liquids (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) and 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF₄) for three nitroxide biradicals with analogous structures of the connecting bridge between two >N–O· centers as a function of temperature. Temperature variations of the isotropic nitrogen hyperfine splitting constant a, and exchange integral values |J/a| were determined from EPR spectra and analyzed. Thermodynamic parameters of the conformational rearrangements were obtained. The spin exchange in rather rigid short-chain biradicals dissolved in omimBF₄ and bmimPF₆ was compared with that in toluene solutions. Interesting features of the spin exchange in biradicals in RTIL were observed and explained as a result of the specific intramolecular conformational transitions. Examples when rather rigid biradical molecules become flexible under an influence of RTIL are reported.     

The Solvent Effect on Spin Exchange in Long-Chain Nitroxide Biradicals

Intramolecular electron spin exchange, as a function of temperature and the solvent viscosity, polarity and relaxation properties of the solvent molecules, has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two long-chain flexible nitroxide biradicals existing in fluid solutions in three spectroscopically different spatial conformations. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. Spin exchange in two biradicals dissolved in five different alcohols was compared with that in a nonpolar solvent (toluene), polar protic (water) and aprotic (acetonitrile), and with thermodynamic characteristics of the solvents. Distinct correlations were found between macroscopic (solvent viscosity, polarity) and microscopic (solvent longitudinal relaxation time) characteristics of solvents, and thermodynamic parameters of the intramolecular conformational transitions. Authors' address: Van Anh Tran, Institute of Physical and Theoretical Chemistry, Graz University of Technology, Technikerstrasse 4, 8010 Graz, Austria     

Time-Resolved and Pulse EPR Study of Triplet States of Alkylketones in β-Cyclodextrin

The triplet states of deoxybenzoin (DOB) and benzophenone (BP) molecules in randomly methylated β-cyclodextrin (CD) cavity are studied by time-resolved (TR) and pulse electron paramagnetic resonance (EPR). The observed TR EPR spectrum of DOB in β-CD at 30 K is close to the spectrum measured in polar solvent trifluoroethanol, revealing strong hydration by water molecules. At the same time, TR EPR spectrum of BP in β-CD corresponds to nonpolar surrounding of the CO-group. The electron spin relaxation times T ₁ and T ₂ of triplet BP at 30 K measured by pulse EPR are found to be different in β-CD compared to nonpolar toluene glass. The observed increase of T ₂ by up to a factor of four in β-CD is caused by the lower vibration amplitude of CO-bond of BP due to the confinement in β-CD. The influence of β-CD with covalently attached nitroxide on the triplet states of DOB and BP is principally different: the excited triplet states could not be observed by TR EPR due to the efficient quenching of the excited states by nitroxide.     

Influence of Pressure on the Intramolecular Spin Exchange in a Short Imidazolium-Nitroxide Biradical

A short-chain imidazoline-type nitroxide biradical R ₅ ^NO –CH=N–N=C(CH₃)–R ₅ ^N , B2, with nitroxide rings R ₅ ^N and R ₅ ^NO as 1-oxyl-2,2,5,5-tetramethyl-3-imidazoline and 1-oxyl-2,2,5,5-tetramethyl-3-N-oxide imidazoline, respectively, has been studied using X-band electron paramagnetic resonance (EPR) spectroscopy in CH₃CN solutions at variable temperature T and pressure P. Changes of the solution viscosity on the intramolecular electron spin exchange in B2 is characterized by calculating the value of the exchange integral |J/a|, where a denotes the ¹⁴N hyperfine splitting (hfs) constant. It is revealed that the intramolecular dynamics in B2 do not follow the Debye–Stokes–Einstein law, while the Arrhenius dependence is fulfilled. Probable reasons of such behavior are discussed.     

Determination of the 14N quadrupole coupling constant of nitroxide spin probes by W-band ELDOR-detected NMR

Nitroxide spin probe electron paramagnetic resonance (EPR) has proven to be a very successful method to probe local polarity and solvent hydrogen bonding properties at the molecular level. The g(xx) and the (14)N hyperfine A(zz) principal values are the EPR parameters of the nitroxide spin probe that are sensitive to these properties and are therefore monitored experimentally. Recently, the (14)N quadrupole interaction of nitroxides has been shown to be also highly sensitive to polarity and H-bonding (A. Savitsky et al., J. Phys. Chem. B 112 (2008) 9079). High-field electron spin echo envelope modulation (ESEEM) was used successfully to determine the P(xx) and P(yy) principal components of the (14)N quadrupole tensor. The P(zz) value was calculated from the traceless character of the quadrupole tensor. We introduce here high-field (W-band, 95 GHz, 3.5 T) electron-electron double resonance (ELDOR)-detected NMR as a method to obtain the (14)N P(zz) value directly, together with A(zz). This is complemented by W-band hyperfine sublevel correlation (HYSCORE) measurements carried out along the g(xx) direction to determine the principal P(xx) and P(yy) components. Through measurements of TEMPOL dissolved in solvents of different polarities, we show that A(zz) increases, while |P(zz)| decreases with polarity, as predicted by Savitsky et al.     

几种氮氧自由基与β-环糊精及其衍生物作用的EPR研究

测定了三种氮氧自由基与环糊精相互作用的EPR波谱，计算了波谱参数. 结果表明：与环糊精作用后，自由基的超精细分裂常数(a_n) 值及旋转相关时间(τ_c)都有规律变化，由此探讨了氮氧自由基与环糊精相互作用位点及作用力大小.     

The Structure and EPR Behavior of Short Nitroxide Biradicals Containing Sulfur Atom in the Bridge

Two short nitroxide biradicals of similar composition: S(OR₆)₂ (1) and O=S(OR₆)₂ (2), where OR₆ is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, have been studied by electron paramagnetic resonance spectroscopy, and X-ray structural analysis. Variations of the intramolecular electron spin exchange in the biradicals, dissolved in toluene and ethanol, as a function of temperature were characterized by changes in the isotropic ¹⁴N hyperfine splitting constant a, values of the exchange integral $ \left| J \right|, $ and compared with the X-ray structural data. Thermodynamic parameters of the conformational rearrangements were calculated. Geometry optimization and spin density distribution calculations of biradicals 1 and 2 were carried out on the DFT/UB3LYP/cc-pVdz and DFT/ROPBE/N07D levels of theory. Structural rigidity and probable differences in biradicals behavior are discussed.     

EPR spectra of gable-type copper(II) porphyrin dimers in fluid solution: extraction of exchange interaction in weakly coupled doublet pairs

A comparison between EPR spectra of rigidly linked dicopper porphyrin dimers and those of the corresponding monocopper dimers (copper porphyrin-free base porphyrin dimers) in fluid solution reveals a very weak exchange interaction between the two copper spins. In these dimers, two porphyrin moieties are linked via an aromatic spacer such as benzene, naphthalene or phenanthrene in a gable-type geometry, with a distance of 10–13 Å. Although essentially all the spectra from the monocopper dimers are the same, exhibiting hyperfine (hf) structure due to the copper and nitrogen nuclei, the EPR spectral patterns of the dicopper dimers depend on the spacer molecule. Differences in hf patterns among the dicopper porphyrin dimers are ascribed to isotropic spin—spin coupling, i.e., exchange coupling between the two copper spins. This is because the anisotropic dipole—dipole interaction is averaged out due to random tumbling of the solute molecules in fluid solution. From the line shape analysis, the absolute value of the exchange interaction (|J|) is found to be 4 × 10^?4 cm^?1 ≦|J| < 3 × 10^?3 cm^?1 for the benzene linked dicopper dimer (Cu—Bz—Cu) whereas |J| ～ 1 × 10^?4cm^?1 for the other two dimers (Cu—Np—Cu and Cu—Pn—Cu). These values are comparable with or much smaller than the dipole—dipole coupling, which is estimated as about 1–3 × 10^?3 cm^?1 from the centre-to-centre distance. Since Cu—Bz—Cu shows a significantly larger |J| than Cu—Pn—Cu, despite a slightly longer centre-to-centre distance, and since no correlation could be obtained between |J| and the separation of the two copper atoms, it is likely that the interaction via spacer molecules is dominant between the two halves.     

Saturation recovery EPR and ELDOR at W-band for spin labels

A reference arm W-band (94 GHz) microwave bridge with two sample-irradiation arms for saturation recovery (SR) EPR and ELDOR experiments is described. Frequencies in each arm are derived from 2 GHz synthesizers that have a common time-base and are translated to 94 GHz in steps of 33 and 59 GHz. Intended applications are to nitroxide radical spin labels and spin probes in the liquid phase. An enabling technology is the use of a W-band loop-gap resonator (LGR) [J.W. Sidabras, R.R. Mett, W. Froncisz, T.G. Camenisch, J.R. Anderson, J.S. Hyde, Multipurpose EPR loop-gap resonator and cylindrical TE₀₁₁ cavity for aqueous samples at 94 GHz, Rev. Sci. Instrum. 78 (2007) 034701]. The high efficiency parameter (8.2 GW^−1/2 with sample) permits the saturating pump pulse level to be just 5 mW or less. Applications of SR EPR and ELDOR to the hydrophilic spin labels 3-carbamoyl-2,2,5,5-tetra-methyl-3-pyrroline-1-yloxyl (CTPO) and 2,2,6,6,-tetramethyl-4-piperidone-1-oxyl (TEMPONE) are described in detail. In the SR ELDOR experiment, nitrogen nuclear relaxation as well as Heisenberg exchange transfer saturation from pumped to observed hyperfine transitions. SR ELDOR was found to be an essential method for measurements of saturation transfer rates for small molecules such as TEMPONE. Free induction decay (FID) signals for small nitroxides at W-band are also reported. Results are compared with multifrequency measurements of T_1e previously reported for these molecules in the range of 2–35 GHz [J.S. Hyde, J.-J. Yin, W.K. Subczynski, T.G. Camenisch, J.J. Ratke, W. Froncisz, Spin label EPR T₁ values using saturation recovery from 2 to 35 GHz. J. Phys. Chem. B 108 (2004) 9524–9529]. The values of T_1e decrease at 94 GHz relative to values at 35 GHz.     

Behavior of Short Nitroxide Biradical in Room Temperature Ionic Liquids

The intramolecular electron spin exchange in short nitroxide biradical O=S(OR₆)₂ (I), where OR₆ is 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine, dissolved in the room temperature ionic liquids (RTILs) 1-octyl-3-methylimidazolium hexafluorophosphate (omimPF₆), 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF₄), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF₄), and 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF₄) has been studied by electron paramagnetic resonance (EPR) spectroscopy as a function of temperature. Temperature variations of the isotropic nitrogen hyperfine splitting constant a were measured from EPR spectra. Thermodynamic parameters of the conformational rearrangements were calculated and compared with literature data. These intramolecular movements in rather rigid short-chain biradical I dissolved in four different RTILs are described well by the Debye–Stokes–Einstein law. Unrestricted density-functional-theory calculations of the geometry and electronic structure of the biradical were carried out using the ORCA program package, and showed that the O=S< group is available for the interaction with anions and cations of RTIL. The possible mechanism of such conformational transitions in biradical I in RTIL is discussed.     

Librational dynamics of nitroxide molecules in a molecular glass studied by echo-detected EPR

Nitroxides 2,2,6,6-tetramethyl-4-piperidone N-oxide (tempone), 3-carboxy-proxyl and potassium peroxylamine disulfonate (Fremy salt) in glycerol solution were studied in a wide temperature range near the glass transition temperatureT _g. The echo-detected (ED) electron paramagnetic resonance (EPR) lineshape reveals strong dependence on the time interval τ between the echo-forming microwave pulses which is readily explained by anisotropic phase relaxation. Employing a librational model of molecular motion and the Redfield relaxation theory, spectra were simulated for the τ’s varying in a large interval. The anisotropic relaxation rate increases with temperature increase and it is larger for nitroxide with a larger molecular size. The mean-squared amplitude of motion, obtained from reduced hyperfine splitting in continuous-wave EPR, near T_g linearly depends on temperature which is characteristic of harmonic solids. For tempone in a host crystal 2,2,4,4-tetramethyl-cyclobutan-1, 3-dione the anisotropic spin relaxation rate decreases with temperature increase so the found feature solely belongs to a glassy state. A new approach is proposed for modeling slow wobbling motion in a restricted angular space.     

151,153Eu electron paramagnetic resonance in SrMoO4 and determination of signs of spin Hamiltonian parameters at different temperatures

The electron paramagnetic resonance (EPR) spectra of Eu²⁺ impurity centers in SrMoO₄ crystals have been studied in the temperature range of 1.8, 100–300 K. A hyperfine structure has been simulated for ^151,153Eu of different EPR transitions observed experimentally at different temperatures and external magnetic field orientations. A unique set of all parameters of the spin Hamiltonian for the known sign of the hyperfine interaction parameters A _i has been determined. It has been found that the diagonal parameters |b _n ⁰ | of the spin Hamiltonian decrease with increasing temperature; however, the parameter b ₄ ⁴ increases. The results of the study have demonstrated that |b ₂ ⁰ (T)/P ₂ ⁰ (T)| ～ const for ^151,153Eu in this single crystal.     

Hyperfine structure of EPR spectra of Gd3+ odd isotopes in PbMoO4, Pb5Ge3O11, YVO4, and quadrupole interaction (temperature dependence)

A hyperfine structure of EPR signals of odd isotopes Gd³⁺ in Pb₅Ge₃O₁₁, PbMoO₄, and YVO₄ single crystals has been investigated at different temperatures. The observation of forbidden (with the nuclear spin flip) transitions has made it possible to determine quadrupole interaction P ₂ ⁰ associated with the gradient of the electric field of ligands at the impurity. It has been shown for the first time that, under the condition |P ₂ ⁰ | ≥ |A _{x, y} | (A _i are the tensor components), not only the magnitudes of splitting but also the observed asymmetry in a hyperfine structure (in perpendicular orientations of the magnetic field) depends on mutual signs of parameters of initial splitting b ₂ ⁰ and P ₂ ⁰ . Results of studying the spectra have demonstrated that |b ₂ ⁰ (T)|/|P ₂ ⁰ (T)| ～ const for a concrete single crystal, which assumes the similarity of physical mechanisms determining these parameters.     

Conformational dynamics of some short-chain biradicals in solutions

Intramolecular electron spin exchange, as a function of temperature and the solvent nature, has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in five short-chain flexible nitroxide biradicals. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. The process of spin exchange in short flexible biradicals has some peculiarities in comparison with that in long-chain molecules.     

Anisotropic hyperfine structure in the EPR spectrum of impurity ions in single crystals

Expressions are obtained for the hyperfine splitting in EPR spectra of impurity ions with electron spin 1/2 and arbitrary nuclear spin for arbitrary anisotropy of the g-factor and hyperfine structure constants. These results generalize the previously obtained results of Weil for the case of weakly anisotropic g-factors and hyperfine structure constants. Fiz. Tverd. Tela (St. Petersburg) 41, 1026–1027 (June 1999)     

Electronic properties of kekulene

The fluorescence and phosphorescence of kekulene in a host matrix of polycrystalline tetrachlorobenzene are investigated, together with the triplet zero field splitting parameters |D| and |E| obtained by ODMR in zero field. The D value is also calculated within a semi-empirical π-theory and compared with experiment. It could be shown that the triplet state energies of a number of different sites of kekulene in the host matrix and the zero field splitting parameters are related, in first order, by spin orbit interaction.     

Time-resolved EPR studies on magnetic interactions between excited triplet (tetraphenylporphinato) zinc and doublet nitroxide radical

By time-resolved electron paramagnetic resonance (TREPR), four (tetraphenylporphinato) zinc (ZnTPP) complexes coordinated by an axial ligand containing a nitroxide radical (NRX; X=4, 5, 8, and 10, denotes the bond number from zinc to nitroxide nitrogen) have been studied in terms of magnetic interactions between the photoexcited triplet state of ZnTPP and NRX. The TREPR spectrum of ZnTPP coordinated by NR10 is almost the same as the one of ZnTPP coordinated by pyridine, indicating that the electron exchange interaction,J, between ZnTPP and the doublet nitroxide is negligibly small. On the other hand, TREPR spectra of the NR4 and NR5 complexes are assigned to the Q₁ state constituted by the ZnTPP and the nitroxide radical. In the case of the ZnTPP-NR8 complex, both T₁ and Q₁ TREPR signals are seen, which may originate from two conformations or degenerate T₁ states of ZnTPP. This EPR study is useful for understanding the photophysical and photochemical properties of chromophores.     

EPR and Quantum Chemical Studies of the pH-sensitive Imidazoline and Imidazolidine Nitroxides with Bulky Substituents

The X- and W-band electron paramagnetic resonance (EPR) spectroscopies were employed to investigate a series of imidazolidine nitroxide radicals with different number of ethyl and methyl substituents at positions 2 and 5 of a heterocycle in liquid and frozen solutions. The influence of the substituents on the line shape and width was studied experimentally and analyzed using quantum chemical calculations. Each pair of the geminal ethyl groups in the positions 2 or 5 of the imidazolidine ring was found to produce an additional hyperfine splitting (hfs) of about 0.2 mT in the EPR spectra of the nitroxides. The effect was attributed to the hfs constant of only one of four methylene hydrogen atoms of two geminal ethyl substituents not fully averaged by ethyl group rotation and ring puckering. In accordance with this assumption, the substitution of hydrogen atoms of CH₂ groups in 2,2,5,5-tetraethyl-substituted imidazolidine nitroxides by deuterium leads to the substantial narrowing of EPR lines which could be useful for many biochemical and biomedical applications, including pH-monitoring. W-band EPR spectra of 2,2,5,5-tetraethyl-substituted imidazolidine nitroxide and its 2,2,5,5-tetraethyl–d₈ deuterium-substituted analog measured at low temperatures demonstrated high sensitivity of their g-factors to pH, which indicates their applicability as spin labels possessing high stability.