Primary and secondary electron spin polarization (CIDEP) in photoreductions of carbonyl compounds

It is shown that electron polarization can be transmitted along a reaction pathway and that the observed polarization of a radical may result from both primary and secondary processes. These secondary processes affect measurement of polarization ratios, and hence experimental tests of polarization theory. However they constitute a useful method for producing polarized species.     

Longitudinal electron spin relaxation induced by degenerate electron exchange as studied by time-resolved magnetic field effects

T(1) paramagnetic relaxation of radical ions induced by degenerate electron exchange (DEE) reactions is studied theoretically and experimentally. Our theoretical analysis shows that T(1) relaxation time is well described by the Redfield theory at arbitrary values of the characteristic DEE time tau. Longitudinal relaxation of norbornane (NB) radical cation is studied by means of the time-resolved magnetic field effects (TR-MFE) technique; the rate constant of DEE involving NB(*+) radical cation and NB neutral molecule is obtained. Advantages of the TR-MFE technique and its potential for measuring the short DEE times are discussed in detail.     

Chemically induced dynamic electron polarization (CIDEP) in nitrogen heterocyclic radicals: a stringent test of polarization theory

Transient ESR spectra are presented of polarized radicals formed following photolysis of quinoxaline, quinoline and pyrazine in solution. Triplet and geminate radical-pair mechanism polarizations are reported and discussed. The phase of the triplet polarization varies with the radical concerned in a manner which confirms the central assumption of the triplet theory.     

The electron spin polarized (CIDEP) spectrum of the propan-2-olyl radical

It is demonstrated that the origin of the absorptive contribution observed in the spin polarized esr spectrum of the propan-2-olyl radical when it is created by flash-photolysis of propan-2-one (acetone) in a hydrogen-donating solvent is not the triplet mechanism of CIDEP, as previously assumed. It is shown to arise not in the photochemistry or photophysics of the parent molecule, but rather to be a property of the radical pair itself. Arguments are presented to demonstrate that its origins may lie in an unidentified novel polarization process, rather than the possible redistribution of level populations via very fast relaxation processes.     

Solvent-induced textural changes of as-synthesized mesoporous alumina, as reported by spin probe electron spin resonance spectroscopy

The effect of solvent used during the synthesis and postsynthesis treatment on textural properties of organized mesoporous aluminas was investigated and related to the behavior of spin probes studied by electron spin resonance (ESR) spectroscopy. It was found that the structure of surfactant aggregates serving in the as-synthesized precipitates as templates could be easily modified by treatment with different solvents. This treatment induces corresponding variations in surface areas, mesopore volumes, and mesopore diameters of the final products. The ESR spectrum of 5-doxyl stearic acid spin probe properly reflects the changes in template structure based on changes of the solvent used and represents an early indicator of the corresponding textural modifications of the mesoporous alumina.     

The Kinetics of alkyl radical reactions with aliphatic alcohol glasses as studied by electron spin resonance

Methyl radical reactions with matrix molecules in glasses C₂H₅OH, (CH₂OH)₂, n- and i-C₃H₇OH, n- and i-C₄H₉OH, n- and i-C₅H₁₁OH, C₂D₅OH, and i-C₃D₇OD, and the reactions of ?₂H₅, ?₃H₇, ?₄H₉, ?₅H₁₁ with methanol glasses have been studied. Alkyl radicals were produced by photolysis of diphenylamine–alkylhalide–alcohol mixtures using ultraviolet light. In all cases the alkyl radical decay follows the law c = c₀ exp(-k √t). The √t law should not be associated with alkyl radical diffusion in a matrix. A method of processing the kinetics of those reactions in which one paramagnetic species changes into another with the total concentration being constant and the electron spin resonance spectra of both species overlapping, is described.     

The concentration dependence of the phase of radical-pair mechanism chemically induced dynamic electron polarization (CIDEP)

In flash-photolysis experiments the phase of RPM polarization in the radicals commonly changes in time after the flash, the patterns observed at longer times being apparently inconsistent with observations on the same radical systems made using alternative experimental techniques. It is shown here that the phase inversion is concentration dependent and that this removes the apparent inconsistencies. The results can be interpreted in two independent ways which at present cannot be distinguished.     

Time-resolved electron spin resonance of gallium and germanium porphyrins in the excited triplet state

Gallium and germanium porphyrin complexes in the lowest excited triplet (T1) state have been studied by time-resolved electron spin resonance (TRESR). It is found that for Ge(TPP)(OH)2 (TPP = dianion of tetraphenylporphyrin) intersystem crossing (ISC) from the lowest excited singlet (S1) state to the T1x and T1y sublevels is faster than that to the T1z sublevel (T1x, T1y, and T1z are sublevels of the T1 state), while the ISC of ZnTPP and Ga(TPP)(OH) is selective to the T1z sublevel. This is interpreted by a weak interaction between the dpi orbital of germanium and LUMO (eg) of the porphyrin ligand, resulting in small spin-orbit coupling (SOC). The interpretation is supported by molecular orbital calculations. The ISC of Ge(OEP)(OH)2 (OEP = dianion of octaethylporphyrin) and Ge(Pc)(OH)2 (Pc = dianion of tetra-tert-butylphthalocyanine) is found to be selective to the T1z sublevel in contrast to Ge(TPP)(OH)2. This dependence on the porphyrin ligand is reasonably explained by a difference between the 3(a(1u)eg) (the OEP and Pc complexes) and 3(a(2u)eg) (the TPP complex) configurations. This is the first observation of a difference in selective ISC between the 3(a(1u)eg) and 3(a(2u)eg) configurations. The TRESR spectrum of Ge(TPP)Br2 is different from those of Ge(TPP)Cl2 and Ge(TPP)(OH)2, and is interpreted by SOC between the T1 and T2 states. From ESR parameters the square of the coefficient of the eg orbital on bromine is evaluated as 0.018 in the T1 state.     

High frequency electron paramagnetic resonance (HFEPR) study of a high spin Co(II) complex

Variable high-frequency electron paramagnetic resonance data were collected for a single crystal of [Zn(hmp)(dmb)Cl]₄ (1) doped with a small quantity of high spin Co(II), where dmb is 3,3-dimethyl-1-butanol and hmp- is the monoanion of 2-hydroxy-methylpyridine. The lack of solvent in the lattice of complex 1 results in very little disorder. Consequently, the EPR spectra are extremely sharp, enabling precise comparisons with theoretical simulations. We find the ground state of the Co(II) ions to be an effective spin S′ = 1/2 Kramers’ doublet with a highly anisotropic g-tensor. The anisotropy is found to be of the easy-axis type, with the single-ion easy axis directions tilted away from the crystallographic c direction by 58°.     

Kinetics of O2 + TEOS gas-phase chemical reactions in a remote RF plasma reactor with electron spin resonance

Kinetics of the reaction O+TEOS (tetraethoxysilane) was studied by means of the ESR method. Molecular oxygen was partially atomized by a 13.56-MHz discharge in the quartz tube that passed inside an ESR cavity. TEOS vapors were injected between the discharge and ESR cavity. O-atom loss occurring due to the reaction of atomic oxygen with TEOS molecules was measured and the rate coefficient k, of the reaction of O atoms with TEOS molecules was determined. The obtained results could be used for deposition process simulations.     

Unusual observation of chemically induced dynamic electron polarization (CIDEP) in systems containing the propan-2-olyl radical: Involvement of a metastable product

A large group of polarized radicals derived pom polyatomic alcohols heretofore unexplored by CIDEP techniques are characterized in viscous media. The TR ESR technique is shown to be capable of unravelling the mechanisms of the complicated dehydration processes of the primary olyl radicals in neutral, acidic and basic media.     

Comparison of electron spin resonance probe and dielectric relaxation in physically and chemically crosslinked polymers

Dielectric relaxation spectroscopy in the frequency range from 10⁻² Hz to 10⁶ Hz and electron spin resonance (ESR) experiments are employed to study the dynamics in chemically and physically crosslinked networks. As examples for physically crosslinked networks ortho- and para-cresol novolacs were investigated. Dielectrically these materials show low-temperature β- and high-temperature α-relaxation. Both relaxation regions differ for both types of novolacs. This is also reflected by the ESR measurements and is discussed in terms of different hydrogen bonds found to be stronger in para-cresol novolac. For the chemically crosslinked poly(triallyl isocyanurate) only a β-peak is found by the dielectric measurements. Also in the ESR experiment the slow motion regime is characterized up to high temperatures. This means that the segmental motion is strongly suppressed by chemical crosslinking. Nevertheless the obtained change in the formal T_50G value can be used to characterize the glass transition in highly crosslinked systems by the ESR method.     

Bimolecular homolytic substitution (S(H)2) reactions with hydrogen atoms. time-resolved electron spin resonance detection in the pulse radiolysis of alpha-(methylthio)acetamide

Pulse radiolysis of aqueous solutions of alpha-(methylthio)acetamide produced unexpectedly large quantities of acetamide radicals that were identified by time-resolved electron spin resonance (TRESR) spectroscopy. The pH dependence of the TRESR-measured radical yields, results from selective scavenging reactions, and density functional theory predictions of the reaction thermochemistry prove that bimolecular homolytic substitution, S(H)2, of the acetamide radical fragment by a H atom is the most likely formation pathway.     

Detection of prior irradiation of dried fruits by electron spin resonance (ESR)

Dried almonds, raisins, dates and pistachio were irradiated using either gamma radiation or electron beam, at an average absorbed dose of 5 kGy. To detect the previous irradiation different parts of the dried fruits were analyzed by ESR spectroscopy: almonds: skin; raisins: dried pulp; dates: dried pulp and stone; pistachio: nutshell. Analyses were carried out 2–3 months and 6 months after irradiation. A series of signals tentatively described as “cellulose-like”, “sugar-like” and “complex” radical were observed, and some slight differences between spectra from samples irradiated with gamma rays and electrons were evident.     

Time-resolved determination of Fe(II) ions using cysteine-bridged Mn-doped ZnS quantum dots as a phosphorimetric probe

A time-resolved phosphorescence (TRP) is applied to the highly sensitive determination of Fe(II) ions. The method is based on the use of a phosphorescent probe consisting of cysteine-bridged Mn-doped ZnS quantum dots (Mn/ZnS QDs). The presence of cysteine enhances the phosphorescence of the QDs and also increases the efficiency of quenching caused by Fe(II) ions. This results in strongly improved selectivity for Fe(II). The linear response is obtained in the concentration range of 50–1000 nM with a 19 nM detection limit. Phosphorescence is recorded at excitation/emission peaks of 301/602 nm. The interference of short-lived fluorescent and scattering background from the biological fluids is eliminated by using the TRP mode with a delay time of 200 μs. The determination of Fe(II) in human serum samples spiked at a 150 nM level gave a 92.4% recovery when using the TRP mode, but only 52.4% when using steady-state phosphorescence. This demonstrates that this probe along with TRP detection enables highly sensitive and accurate determination of Fe(II) in serum.

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Graphical abstract Schematic of a novel phosphorescent method for the detection of Fe²⁺ ions based on cysteine-bridged Mn-doped ZnS quantum dots. The sensitivity of this assay greatly increases due to the addition of cysteine. Interferences by short-lived auto-fluorescence and the scattering light from the biological fluids is eliminated by using time-resolved phosphorescence mode.

     

SURF_ER--surface electron spin resonance (ESR) of the surface domain of large objects

SURF_ER is a method for spectral and spatial electron spin resonance measurements on the surface of large objects which extension is only restricted by the width of the pole gap of the magnet and the homogeneity of the magnetic field and not by the cavity dimensions. The application of several techniques like SURF_ER for spectroscopic measurements, SURF_ERM for spatial scanning and SURF_ERI for spatial measurements of the depth of the surface region are discussed and represented for the skin of a human being as an example.     

Rotational vibration of poly(tetrafluoroethylene) chains by the electron spin resonance method

Temperature‐dependent electron spin resonance spectra of main‐chain free radicals, ? CF₂(β)? C · F(α)? CF₂(β)? , in poly(tetrafluoroethylene) (PTFE) were analyzed by the change in the hyperfine splitting due to β‐fluorines, which was a decreasing function of the observation temperature. The results were interpreted in terms of the rotational vibration around the C_α? C_β bond. The amplitude of the vibration was estimated on the assumption of its harmonic oscillation. The vibration of the PTFE chain was found to have a large amplitude in comparison with that of a polyethylene chain in single crystals. The vibration of the large amplitude was caused by a weak interchain interaction in the PTFE matrices. The amplitude of the vibration in crosslinked PTFE was much larger than that in noncrosslinked PTFE. This result indicated that the free radicals in crosslinked PTFE were trapped in the amorphous region, which had the disordered sites of crosslinking, whereas the free radicals in noncrosslinked PTFE were mainly trapped in the paracrystalline region. The decay reaction of the free radicals in the PTFE matrices was also related to the heterogeneity in the structure and the rotational vibration. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1539–1547, 2004