Intramolecular Electron Spin Exchange Research Articles

Intramolecular Mobility in Nitroxide Biradicals with Flexible Linkers

Intramolecular electron spin exchange has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two long-chain flexible nitroxide biradicals existing in fluid solutions in three spectroscopy-different spatial conformations as a function of temperature, solvent viscosity and polarity. Certain thermodynamic parameters of the conformational transitions were calculated from the EPR spectra. The process of spin-exchange in these biradicals dissolved in five different alcohols was compared with that in the non-polar solvents (toluene) and aprotic (acetonitrile), as well as with two other biradicals studied earlier, and with thermodynamic characteristics of the solvents. A distinct correlation was found between macroscopic (solvent viscosity) characteristics of solvents and thermodynamic parameters of the intramolecular conformational transitions.

Specific Features of the Intramolecular Spin Exchange in a Novel Nitroxide Triradical

An X-band EPR spectroscopic study was performed of the temperature dependence of the dynamics of the intramolecular electron spin exchange in a triradical where the radical rings are linked by acetylene groups –C≡C–. A thermodynamic characteristic was determined for the conformational transition in the triradical between conformers where the spin exchange is of the triradical or biradical type. The geometric structure of the novel stable nitroxide triradical was determined using density functional theory calculations.

The Structure and Internal Dynamics of R6-p-C6H4-R6 Biradical: EPR, X-ray Crystallography and DFT Calculations

A purposefully synthesized nitroxide biradical R6-p-C6H4-R6 (B1), where R6 is the 1-oxyl-2,2,6,6-tetramethyl-1,2,5,6-tetrahydropyridine group with a relatively short distance between the two radical sites, has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Hyperfine splitting (hfs) constants on the 14N atoms, electron spin exchange integral |J|, and the distance between the two N–O fragments rNO–NO were experimentally measured. Density functional theory, DFT, calculations were performed using the ORCA 4.0.1.2 program package. The optimized geometry was compared with X-ray crystallographic data and theoretical hfs constants were compared with the respective experimental EPR values. It is concluded that the current quantum chemical approaches provide good results in calculating hfs constants as well as some other EPR parameters. It is confirmed that the intramolecular electron spin exchange in biradicals analogous to B1 is realized by the indirect mechanism rather than direct collision of the N–O· groups. It is also shown that one can calculate and predict values of |J| in other similar biradicals based on the principle of “attenuation coefficients.

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.

Peculiarities of Spin Exchange in Nitroxide Biradicals Containing Two para-Phenylene Groups in the Bridge: EPR Investigation and DFT Calculations

Two nitroxide biradicals of similar composition: R5-C≡C-Ph-Ph-C≡C-R5 (B3a) and R6-C≡C-Ph-Ph-C≡C-R6 (B3b), where Ph = p-C6H4, and R6 is 2,6,6-tetramethyl-5,6-dihydropyridin-1(2H)-yloxyl, and R5 is 2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-yloxyl nitroxide rings, have been studied by electron paramagnetic resonance (EPR) spectroscopy. Variations of the intramolecular electron spin exchange in the biradicals, dissolved in toluene, as a function of temperature were characterized by changes in the isotropic 14N hyperfine splitting (hfs) constant a, values of the exchange integral |J|, and compared with the data obtained by density functional theory calculations (DFT). Thermodynamic parameters of the conformational rearrangements were calculated. Geometries of nitroxide biradicals in PES local minima and transition states in the triplet state were calculated at UDFT/B3LYP level with split-valence basis set cc-PVTZ. Probable differences in biradicals behavior are discussed.

Spin Density Distribution in a Nitroxide Biradical Containing 13C-Enriched Acetylene Groups in the Bridge: DFT Calculations and EPR Investigation

A specially synthesized nitroxide biradical R6-13C≡C-p-C6H4–C≡13C-R6, B3, where R6 = 1-oxyl-2,2,6,6-tetramethyl-1,2,3,6-tetrahydropyridine nitroxide, has been studied by electron paramagnetic resonance spectroscopy, and electron-nuclear double resonance (ENDOR). Spin density distribution and hyperfine splitting (hfs) constant on 13C atoms calculations for biradical B3 were carried out using B3LYP and PBE0 functionals and several different basis sets including N07 family and were compared with the experimental value of the hfs constant on 13C atoms, measured from ENDOR spectra of B3. The mechanism of the intramolecular electron spin exchange in B3 biradical is discussed.

Influence of Pressure on Intramolecular Dynamics in a Long-Chain Flexible Nitroxide Biradical

A flexible long-chain nitroxide biradical of the structure R6–(CH2)2OOC–(CH2)8–COO–(CH2)2–R6, 1, where –R6 is the 1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl residue, has been investigated by X-band electron paramagnetic resonance spectroscopy in CH3CN solutions at variable temperature and pressure. The effect of changes of the solution viscosity on the intramolecular electron spin exchange in 1 has been characterized in terms of dynamic and thermodynamic parameters such as the equilibrium constant describing the transition of the elongated and coiled effective configuration, τc/τ, and the characteristic spin-exchange time τeff and its associated Arrhenius activation energy. It was revealed that the intramolecular dynamics in 1 do not follow the Debye-Stokes–Einstein law.

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 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.

Spin exchange in piperidineoxyl polyradicals with bridges containing methylene groups

The temperature-dependent dynamics of the intramolecular electron spin exchange in tri- and tetraradicals in which piperidineoxyl rings are connected via methylene groups are studied using EPR technique. The thermodynamic characteristics of conformational transitions in these polyradicals are obtained. The effect of the substitution of an −OH group for the >C=O group at the fourth position of the piperidineoxyl rings of the tetraradicals on the dynamics of spin-exchange interaction in them is examined. The structure of a new asymmetrical nitroxide triradical is determined from X-ray structural analysis.

Behavior of Nitroxide Biradicals with Acetylene Bridges in Organic Solvents and Ionic Liquids

Intramolecular electron spin exchange (IESE) in two nitroxide biradicals, R6–C≡C–C≡C–R6 (1) and R6–C≡C–p-C6H4–C≡C–R6 (2), is studied as a function of temperature and solvent properties. The effect of molecular solvents and ionic liquids (ILs, [1-methyl-3-butylimidazolium]+[PF6]−, bmimPF6, and [1-methyl-3-octylimidazolium]+[BF4]−, omimBF4) on the IESE in magnetically diluted solutions is investigated. Changes in electron paramagnetic resonance spectra are analyzed and the thermodynamic parameters of these changes are calculated. Geometry optimization and D-tensor calculations of biradicals 1 and 2 were carried out on the DFT/UB3LYP/cc-pVdz and DFT/ROPBE/N07D levels of theory. The probable differences in biradical behavior are discussed.

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.

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 bmimPF6

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.

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.

Solvent effects on the intramolecular spin exchange in biradicals at room temperature

Solvent effects on intramolecular electron spin exchange in biradicals have been investigated in various solvents by electron spin resonance (ESR) spectroscopy at room temperature. Biradicals containing different radical fragments and bridges of different length and composition were used. Interactions between solvent molecules and biradicals were found to take place at the radical fragment as well as at the functional group in the connecting bridge and have been proved to have an influence on the intramolecular spin exchange. The experimentally measured parameters, the exchange integral (J) and the characteristic time of an intramolecular movement (τ eff), have been compared with solvent viscosity and polarity parameters (macroscopic level) and the longitudinal solvent relaxation time (microscopic level). The dependencies on viscosity and polarity were in good agreement with reported results. In addition, strong evidence of hydrogen bonding between the solvent and different sites of the biradicals was found, in some cases even surpassing the other effects.

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.

Regularities of the Spin Exchange Coupling Through a Bridge in Nitroxide Biradicals

AbstractFor Abstract see ChemInform Abstract in Full Text.

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.

Regularities of the spin exchange coupling through a bridge in nitroxide biradicals

The current state, achievements, problems and prospects of the intramolecular electron spin exchange interaction as a function of the nitroxide biradical composition and structure, type of the radical ring, temperature and the solvent nature are considered on the basis of the literature data and the results of our own experiments.