Effect of solvent nature on spin exchange in rigid nitroxide biradicals

Intramolecular electron spin exchange as a function of temperature and solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two rigid nitroxide biradicals existing in one spatial conformation only. Temperature variations of the isotropic hyperfine splitting constanta and exchange integral value |J/a| were measured from EPR spectra and subsequently analyzed. The interaction of polar solvent molecules with >N-O fragments of nitroxide groups led to a slight decrease of the |J/a| value with the increase of temperatureT. In contrast, the interaction of polar solvent molecules with functional groups inside the bridge resulted in a noticeable increase of |J/a| vs.T. In the last case, a coverse relationship between the values of |J/a| and the hyperfine splitting constanta has been observed for solvents with different polarity.     

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.     

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.     

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.     

Tailored Nitroxide Radicals and Biradical Containing <Superscript>13</Superscript>C Enriched Acetylene Groups: ENDOR and DFT Investigation

A specially synthesized nitroxide biradical R₅-C≡¹³C-(p-C₆H₄)₂-¹³C≡C-R₅ (B4) and two radicals, R₅-C≡¹³CH (RCC) and R₅-C≡¹³C-C₆H₅ (RCCPh), where R₅ is 1-oxyl-2,2,5,5-tetramethyl-pyrroline group, have been studied by X- and W-band electron paramagnetic resonance (EPR) spectroscopy, and by W-band electron-nuclear double resonance (ENDOR). Spin density distribution and hyperfine splitting (hfs) constants on ¹³C atoms were experimentally determined and also calculated using ORCA 3.0.3 program package. The biradical and radicals geometries were optimized on UKS/B3LYP/cc-pVDZ level. Hfs constants were calculated using density functional theory (DFT) with PBE0 functional and N07D, and were compared with the experimental value of the hfs constant on ¹³C atoms, measured from ENDOR spectra. It is concluded that at small values of the exchange integral as J ≤ a/2 ≈ 7–8 G, the current quantum chemical approaches do not allow determining precise values of the hfs constants on the ¹³C atoms in the bridge connecting two paramagnetic nitroxide rings of the biradical.     

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     

Unexpected Features of the Intramolecular Spin Exchange in Imidazoline Nitroxide Biradicals Dissolved in Ionic Liquids

Three imidazoline-type nitroxide biradicals of the similar composition R ₅ ^NO –CH=N–N=CH–R ₅ ^N , B1, R ₅ ^NO –CH=N–N=C(CH₃)–R ₅ ^N , B2, and R ₅ ^N –C(CH₃)=N–N=C(CH₃)–R ₅ ^N , B3, with R ₅ ^N and R ₅ ^NO denoting, respectively, the nitroxide rings 1-oxyl-2,2,5,5-tetramethyl-3-imidazoline and 1-oxyl-2,2,5,5-tetramethyl-3-N–oxide imidazoline, have been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Variations of the intramolecular electron spin exchange in these biradicals dissolved in ethanol and the room temperature ionic liquid bmimBF₄ were characterized as a function of temperature by means of the analysis of the EPR lines shape. Thermodynamic parameters of the conformational rearrangements in ethanol were calculated. Analyzing the EPR spectra of these biradicals in bmimBF₄, it was revealed that the two-conformational model does not describe their conformational transitions. Moreover, the observed EPR spectra are not central symmetric especially at low temperatures that cannot be described and explained in the framework of the current theory of the intramolecular spin exchange. Probable reasons of this “strange” behavior are discussed.     

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.     

Effect of single-ion uniaxial anisotropy on the phase diagrams of magnetic system in the presence of internal spin fluctuation

Basing on the two-spin-per-site Heisenberg model, the effect of single-ion uniaxial anisotropy on the phase diagrams of magnetic system in the presence of internal spin fluctuation has been investigated by use of the mean field theory. It was found that single-ion uniaxial anisotropy has important effect on the phase digrams. In the ferromagnetic case (J₃>0) the positive single-ion uniaxial anisotropies (D) suppress the internal spin fluctuation and raise the phase trasition temperature, and negative single-ion uniaxial anisotropies (D) increase the internal spin fluctuation and reduce the phase trasition temperature. In the antiferromagnetic case (J₃<0), there exist two critical values J_c1 and J_c2 (|J_c2|<|J_c1|) in the positive D values. In the |J₃|<|J_c2| range intra-spin exchange coupling prevails inter-spin exchange coupling, the positive D values suppress the internal spin fluctuation and raise the phase transition temperature. In the |J₃|>|J_c1| range the two sub-spins behave as a rigid spin and the positive D values make the reduction of the phase transition temperature. We also observe that the larger D values make the range of internal spin fluctuation to move towards the larger |J₃| range.     

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.     

Intramolecular spin exchange in flexible short-chain biradicals in solutions with various viscosity

Intramolecular electron spin exchange as a function of temperature and solvent viscosity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two short-chain flexible nitroxide biradicals. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. The process of spin exchange in these biradicals dissolved in six different alcohols is compared with that in a nonpolar solvent (toluene) and with the thermodynamic characteristics of the solvents. No correlation is found on a macroscopic level (solvent viscosity) but on a microscopic level (solvent longitudinal relaxation times) a distinct correlation is seen. The original results are compared with literature data.     

Imidazoline biradical spin labels with EPR spectral sensitivity to the local concentration of protons

EPR spectral sensitivity of the three biradicals of imidazoline and imidazolidine types to the local concentration of protons (pH) in aqueous solutions has been studied. Significant increase ing-factor (Δg ≈ 0.0002) and decrease in hyperfine interaction constant (Δa_N = 0.8?1.2 G) upon the protonation of atom N-3 of the radical heterocycle were observed. The pH dependence of a_N andg- factor is described by conventional titration curve with one or two pK values. The latter case was explained by nonindependent protonation of two monoradical fragments closely located in the biradical structure. EPR spectra of protonated and deprotonated forms of the biradicals have significantly different intensities of “biradical” components, I_b, reflecting changes in spin exchange interaction. It has been found that activation energy of spin exchange, E_a, estimated from temperature dependence of I_b in glycerol/water solution is lower for the biradical conformation with larger I_b. This is in agreement with main contribution of direct collisions of monoradical fragments in spin exchange interaction so larger value of I_b corresponds to the conformation with less spatial restrictions for the motion of monoradical fragments (lower E_a). The potential use of the pH-sensitive biradicals as spin labels is discussed.     

Hyperfine structure in optical spectra of LiYF4-Ho

We report high resolution infrared absorption spectra of LiYF₄-Ho exhibiting well resolved nuclear hyperfine structure (hfs). The halfwidths of some hfs components do not exceed 0.03 cm^?1. Magnetic field at the nucleus is evaluated. Non-radiative relaxation rates due to phonon emission are estimated. With the help of experimentally found 〈Γ₃|J_z|Γ₃〉 matrix elements g-factors of the excited crystal field levels are calculated. Some hfs peculiarities are discussed.     

EPR measurements of weak exchange interactions coupling unpaired spins in model compounds

I review electron paramagnetic resonance (EPR) measurements performed to evaluate very weak exchange interactions (defined as ?_ex(i, j) = ?J _ij S _i S _j , with 10^?3 cm^?1<|J _ij|<1 cm^?1) between unpaired spins, transmitted through long and weak chemical pathways typical of protein structures. They are performed in appropriate model compounds, mainly copper derivatives of amino acids and peptides, making use of the phenomenon of exchange narrowing and collapse of the resonances. I describe the theoretical basis and the implementations of the method to different situations, including selected experimental values of the exchange couplingsJ between metal centers, and briefly discuss correlations betweenJ and the structure of the paths. Results obtained in relatively simple EPR experiments performed at room temperature in single-crystal samples are compared with those obtained from thermodynamic magnetic measurements having higher experimental difficulties. The experimental information allows describing the role of molecular segments typical of biomolecules (H bonds, aromatic ring stacking, cation-π contacts, etc.) in the transmission of the exchange interaction. The values ofJ obtained in some model compounds are compared with those obtained in proteins to conclude that the magnitudes of the exchange interactions are useful to characterize long and weak biologically relevant chemical pathways. One observes that these exchange couplings are weakly dependent on the nature of the unpaired spins and strongly dependent on the chemical pathway. Thus, measurements of exchange couplings in model compounds may provide useful information about biological function, particularly about electron transfer in proteins.     

Collective quantum decoherence induced by qubit-electron interaction

We investigate a model of quantum register composed of N qubits coupling with itinerant electrons by adopting the Born-Markov master equation. Decoherence induced by this coupling is studied for various initial states. By solving the master equation for N=4 with the numerical integration, we obtain time evolution of fidelity and linear entropy of the register. The decoherence rate of this model is proportional to ²|J^′| with J^′ being the exchange coupling strength of electrons and qubits. We also investigate the decoherence free subspace which provides a possible routine of applications in quantum computation.     

Self-induced generation of an off-axis frequency shifted radiation from atoms

The transient resonant linear response at wavelength λ_a of an N two-level atom vapor driven by a strong pulse with wavelength λ_f = λ_a - |Δλ| is shown to promote an emission of radiation peaked at wavelength λ_c = λ_a + |Δλ| in a conical shell around the propagation axis of the incident beam. In the limit of weak excitation, i.e. for an incident Rabi frequency much smaller than the detuning, the cone angle is found to be equal to 2λμ(2N/ch|Δλ|)^{$\text{1}\text{2}$} where μ is the transition dipole moment.     

Triplet excitons in (TEA) (TCNQ)2

We report the results of EPR studies on the ionic-radical salt (TEA)⁺ (TCNQ)₂^- composed of an oganic free radical anion and a diamagnetic cation. Between about 40 and 80 K this crystal exhibits the triplet exciton EPR spectrum characteristic of an alternating chain of spins. The triplet spin Hamiltonian parameters are |D| = 44 ± 2 G and |E| = 5.5 ± 1 G. The directions of the zero field splitting principal axes are determined through single crystal rotation studies at 55 K and related to the crystal structure.     

Tunable diode laser measurements of N2- and air-broadened halfwidths: Lines in the (ν4 + ν5)0 band of 12C2H2 near 7.4 μm

Nitrogen- and air-broadened Lorentz halfwidths have been determined for 29 lines in the P and R branches of the (ν₄ + ν₅)⁰ combination band of ¹²C₂H₂ using a tunable diode laser spectrometer. Two tunable diode lasers operating in the region 1250–1380 cm^?1 were used in recording the data. For nitrogen broadening, the measured halfwidths at 296 K decrease from about 0.11 cm^?1 atm^?1 at |m| = 1 to about 0.05 cm^?1 atm^?1 at |m| = 30, where m = J″ + 1 for R-branch lines, m = ?J″ for P-branch lines, and J″ is the lower state rotational quantum number. On the average, the air-broadened halfwidths are 97% of the N₂-broadened halfwidths.     

Instanton approximation to the graded nonlinear sigma model for the integer quantum hall effect

We construct the multi-instanton solutions for the graded nonlinear σ model with symmetry U(1,1/2)/U(1/1) ⊗ U(1/1), and we calculate the quantum fluctuations around these solutions. The determinant of the fluctuation operator for a fixed multi-instanton solution turns out to be UV finite. However, the integration over instanton parameters contains an integral, ∫d|a| |a|⁻³, over the size, |a|, of each instanton, which is quadratically singular at |a|=0. It is shown that these quadratic divergences cancel exactly in the calculation of all Green functions. The applicability of the present results to the integer quantum Hall effect is discussed.