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.     

Multifrequency TREPR Investigation of Excited-State ZnTPP/Nitroxide Radical Complexes

Multiple-frequency (X-band and W-band) time-resolved electron paramagnetic resonance spectra of Zn-tetraphenylporphyrin (ZnTPP) triplet states coordinated to two different stable nitroxide moieties are presented and discussed. The position of the pyrimidyl nitroxide relative to the ZnTPP plane is the only structural change made between the two complexes, from ortho to para. This changes the angle between the interacting orbitals of the three-spin system with only a minor change in the distance between the unpaired electron of the nitroxide and the TPP ring system. Changes in the electron spin polarization patterns at the two different frequencies of observation are discussed in terms of a radical?Ctriplet pair spin-state mixing model, including the intersystem crossing processes. It is determined that the inclusion of an additional spin-selective relaxation process to the computational model for the spectral shapes gives adequate reproduction of the experimental results using the same parameters at each frequency.     

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.     

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     

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.     

NMR and EPR Study of the Nitroxide Radical Tempo Interaction with Phenols

The 2,2,6,6-tetramethyl-I-piperidinyloxy free radical (TEMPO) was used as a probe to study the changes in hydrogen bonding between the phenolic OH group and the ON group of the radical by means of NMR and EPR. ¹³C NMR contact shifts induced by TEMPO were measured for five phenols. Formation of intermolecular hydrogen bond between a phenol and TEMPO molecule causes noticeable increase of ¹⁴N hyperfme coupling constant in the radical and appearance of negative spin density on carbon nuclei of C-OH fragment in the phenol.     

Spin densities in some odd alternant radicals including linear polyene radicals

An approximate unrestricted Hartree-Fock method is used to calculate spin densities in the odd alternant radicals cyclohexadienyl, benzyl, perinaphthenyl and triphenylmethyl and the linear polyene radicals from allyl to C₁₉H₂₁. The results agree quite well with the exact calculations and with empirical estimates as well as with experimental data on hyperfine splitting constants and E.S.R. second moments.     

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.     

Formation of muonic radicals

Intensities of muonic radical spectra are compared with theoretical predictions to determine whether the radicals form epithermally or via thermal muonium.     

Muonium substituted hydrazyl radicals

Hyperfine coupling constants for muonium substitited hydrazyl radicals are reported. They are compared with the existing data for the hydrogen analogs. The influence of the temperature and the solvent are given.     

Rotational and Translational Mobility of Nitroxide Spin Probes in Ionic Liquids and Molecular Solvents

Rotational and translational movements of 1-oxyl-2,2,6,6-tetramethyl-4-oxypiperidine (TEMPOL) spin probe in the room temperature ionic liquid (RTIL) 1-octyl-3-methylimidazolium tetrafluoroborate (omimBF₄) and in two molecular solvents, 1-propanol and isopropyl benzene (cumene), have been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times τ _c of spin probes and the intermolecular spin exchange rate constants k _e were measured from EPR spectra at different temperatures and TEMPOL concentrations, and compared with the published data. The τ _c values were calculated both by known equations and from the EPR spectra simulation. Rotation movements of TEMPOL in omimBF₄ cannot be described by the model of the isotropic Brownian diffusion, which is valid for conventional solvents. The correct modeling of EPR spectra in RTIL can be achieved with the assumption of different rotational mobility of the spin probe around different molecular axes. The rotational, D _rot, and translational, D _tr, diffusion coefficients were calculated from τ _c and k _e values. The Debye–Stokes–Einstein law is valid in all three solvents while the dependence of D _tr on T/η is not linear in Stokes–Einstein coordinates. The effective activation energy E _rot^a of the rotational movements in omimBF₄ is noticeably higher than the corresponding values for conventional solvents, while the effective activation energies E _tr^a of the translational movements are comparable in all solvents studied.     

EPR Spectra of hexafluoride radicals

The preparation of various hexafluoride radicals is described, and their spectra are compared. The central-atom hyperfine interactions are large, indicative of an ²A _1g ground state in O_h symmetry. This is confirmed by the anisotropic ¹⁹F hyperfine structure. The semioccupied orbital consists of an antibonding combination of the central-atom ns atomic orbital, and the six fluorine 2p atomic orbitals. Changes in the central-atom contribution are correlated with electronegativity changes in the central atom.     

Muonium radicals in benzene-styrene mixtures

Muonium radicals were observed through theirSR precession frequencies in high transverse magnetic fields in pure benzene, pure styrene and their mixtures, all as liquids at room temperature. In benzene-styrene mixtures, the radicals obtained in each pure liquid are both present, so no slow (10^–9–10^–5 s) intermolecular exchange occurs; but strong selectivity was found with the formation of the radical from styrene being about eight-times more probable than the radical from benzene.