Fourier Transform Electron Paramagnetic Resonance Spectra Research Articles

Fourier Transform EPR Spectroscopy of Trityl Radicals for Multifunctional Assessment of Chemical Microenvironment

Pulse techniques in electron paramagnetic resonance (EPR) allow for a reduction in measurement times and increase in sensitivity but require the synthesis of paramagnetic probes with long relaxation times. Here it is shown that the recently synthesized phosphonated trityl radical possesses long relaxation times that are sensitive to probe the microenvironment, such as oxygenation and acidity of an aqueous solution. In principle, application of Fourier transform EPR (FT-EPR) spectroscopy makes it possible to acquire the entire EPR spectrum of the trityl probe and assess these microenvironmental parameters within a few microseconds. The performed analysis of the FT-EPR spectra takes into consideration oxygen-, proton-, buffer-, and concentration-induced contributions to the spectral shape, therefore enabling quantitative and discriminative assessment of pH, pO2, and concentrations of the probe and inorganic phosphate.

FT-EPR study on the sign of exchange interaction in triplet naphthalene-galvinoxyl encounter pair

Chemically induced dynamic electron polarization (CIDEP) created in the quenching of triplet naphthalene by galvinoxyl were investigated by time-resolved Fourier-transform electron paramagnetic resonance (FT-EPR) measurements with monitoring a free induction decay signal of a pulsed microwave irradiation. Transient FT-EPR spectra of galvinoxyl with CIDEP were observed in various nonpolar solvents with different viscosity. A transient FT-EPR signal phase shows remarkable dependence on the viscosity: FT-EPR signal phases were absorption and emission in the solvents with low and high viscosity, respectively. Time evolutions of the FT-EPR signal of galvinoxyl were well simulated by a model of the radical-triplet pair mechanism (RTPM) for CIDEP. A sign of theJ value in the triplet naphthalene-galvinoxyl system in various solvents were discussed on the basis of the sign rule in the RTPM and the transient FT-EPR signal phase. One of possible explanation for the solvent viscosity dependence of the transient FT-EPR signal phase was pressented on the basis of hypothetical model of theJ value.

FT-EPR Study of the Wavelength Dependence of the Photochemistry of Phenols

The excitation wavelength dependence of the photochemistry of phenol, p-cresol, and tyrosine in neutral and acidic aqueous solution was examined using Fourier Transform Electron Paramagnetic Resonance (FT-EPR). Photoexcitation gives rise to formation of phenoxyl, p-cresyl, or tyrosyl radicals (PhO•), hydrated electrons ( ), and hydrogen atoms (H•). From Chemically Induced Dynamic Electron Polarization (CIDEP) effects and the effect of addition of N2O on the spectra it can be deduced that the mechanism of hydrogen atom formation depends on the wavelength of excitation. Excitation into the S2 absorption band (λex = 193 nm) gives rise to H-atom formation by O−H bond homolysis and by capture by protons. The electron capture reaction produces a net absorption signal contribution in the FT-EPR spectrum of H• due to transfer of spin polarization from . This demonstrates that the reaction occurs with H+ in the bulk of the solution. The spectrum of H• produced by excitation into the S1 band (λex = 266 nm) can be completely quenched by addition of N2O. In this case H-atom formation is due entirely to the reaction of with H+. The excitation energy apparently is insufficient to cause O−H bond cleavage. The spectra show no evidence of an in-cage H-atom formation mechanism such as 1PhOH* → [PhOH+··· ] → [PhO•···H•] → PhO• + H•. Excitation to S1 also leads to triplet-state formation. FT-EPR spectra show that triplet-state phenol (p-cresol) reacts with ground-state phenol (p-cresol) to give a cyclohexadienyl-type radical and PhO•.

Energy- and Electron-Transfer Quenching of Porphyrin Triplets by C60

Energy- and electron-transfer from porphyrins in the triplet excited state to C60 in toluene and benzonitrile has been studied with Fourier transform EPR (FT-EPR). Pulsed-laser excitation of the systems magnesium tetraphenylporphyrin (MgTPP)/C60 and free-base octaethylporphyrin (OEP)/C60 in toluene gives rise to a resonance peak from C60 triplets (3C60) generated primarily by the energy-transfer process 3MgTPP (3OEP) + C60 → MgTPP (OEP) + 3C60 as evident from the emissive spin polarization of the 3C60 signal observed at early times. No FT-EPR signals from redox products could be detected. In benzonitrile, triplet energy transfer from 3MgTPP or 3OEP to C60 is a minor process. In this polar solvent photoexcitation of MgTPP/C60 produces FT-EPR spectra with signal contributions from MgTPP+ and C60- in addition to the 3C60 resonance. From the spin polarization and time profile of the signal from the anion radical, it can be deduced that the primary route of electron transfer is oxidative quenching of the porphyrin triplets, 3MgTPP (3OEP) + C60 → MgTPP+ (OEP+) + C60-. The reduction in 3C60 lifetime indicates that C60- is generated as well by the reductive triplet quenching reaction 3C60 + MgTPP (OEP) → C60- + MgTPP+ (OEP+).

FT-EPR Study of Methyl Radicals Photogenerated from [Ru(Me)(SnPh3)(CO)2(iPr-DAB)] and [Pt(Me)4(iPr-DAB)]: An Example of a Strong Excitation Wavelength Dependent CIDEP Effect

The photoinduced methyl radical formation from the title complexes [Ru(R)(SnPh3)(CO)2(iPr-DAB)] (R = CH3, CD3; iPr-DAB = N,N‘-diisopropyl-1,4-diaza-1,3-butadiene) and [Pt(Me)4(iPr-DAB)] was the subject of a detailed time-resolved Fourier transform EPR (FT-EPR) study. The FT-EPR spectra of the radicals show pronounced chemically induced dynamic electron polarization (CIDEP) effects due to the ST0 and ST-1 radical pair mechanisms (RPM). The relative contributions of the two CIDEP mechanisms depend on solvent polarity and viscosity. In the case of the [Ru(R)(SnPh3)(CO)2(iPr-DAB)] complexes, the polarization pattern is also strongly excitation wavelength dependent. This effect is attributed to extremely fast reactions from different thermally nonequilibrated σ-bond-to-ligand charge transfer (SBLCT) excited states.

FT-EPR study of the pH dependence of the photochemistry of sesamol in aqueous solution

A Fourier transform electron paramagnetic resonance (FT-EPR) study was made of the photochemistry of 3,4-methylenedioxyphenol (sesamol, SEOH)) in aqueous solution. FT-EPR measurements show that in alkaline (pH 11) solution, pulsed-laser excitation of SECT leads to photoionization giving the hydrated electron and SEO⋅ free radical. Resonance signals from these paramagnetic species develop with instrument-controlled rise time. They exhibit a low-field emission/ high-field absorption (E/A) CIDEP pattern with the transition from emission to absorption occurring at the resonance of the hydrated electron. It is shown that the spin polarization stems from contributions from the ST0 radical pair mechanism (E/A) and triplet mechanism (A). From this it is concluded that photoionization of sesamol occurs via the triplet excited state. In neutral and acidic (pH 4–7) aqueous solution, photoexcitation generates SEO⋅ and cyclohexadienyl-type radicals. In this case, radicals grow in over a period of 1–2 μs and FT-EPR spectra display an E/A pattern with the inversion point in the center. The lowering of the pH of the solution apparently is accompanied by a strong reduction in the relative importance of photoionization. From the FT-EPR data it can be deduced that in neutral and acidic solutions the dominant reaction channel is H-atom transfer. In this respect, the photochemistry of sesamol differs from that of phenol andp-cresol. For these phenols the change in pH does not affect the appearance of the FT-EPR spectra. Apparently, the change in electronic structure caused by the methylenedioxy substituent strongly affects the excited state reactivity of sesamol.

FT-EPR study of alkyl radicals formed in the photochemical reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) complexes

A Fourier transform EPR (FT-EPR) study was made of the photochemistry of [Re(R)(CO)3 (α-diimine)] and [Ru(E)(R)(CO)2(α-diimine)] complexes, where R = alkyl or benzyl, E = I or SnPh3, and α-diimine = 4,4′-dimethyl-2,2′-bipyridine (DMB) orN,N′-diisopropyl-1,4-diazabutadiene (iPr-DAB). Photoexcitation of these complexes leads to homolysis of the metal-alkyl (benzyl) bonds as evident from the detection of the spectra of the alkyl (benzyl) radicals. FT-EPR spectra display strong spin polarization effects attributed to Triplet Mechanism (TM) and Radical Pair Mechanism (RPM) Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP patterns point to bond dissociation via a triplet state precursor. For a number of complexes, spin polarization was found to exhibit unusually large solvent effects, whereas for one complex the CIDEP pattern proved to be sensitive to the wavelength of laser light used to initiate bond dissociation. These effects reflect the strong dependence of CIDEP on the character of the excited states involved in the photochemical reactions and contribute to the understanding of the reaction mechanism.

FT-EPR and HPLC study of the mechanism of 4-chlorophenol photolysis

The mechanism of 4-chlorophenol (4CP) photolysis was investigated with the aid of Fourier Transform Electron Paramagnetic Resonance (FT-EPR) and pulsed-laser photolysis combined with High Performance Liquid Chromatography (HPLC) detection-of stable (diamagnetic) products. With FT-EPR transient free radicals produced by pulsed-laser excitation of solutions of 4CP in alcohols could be identified. Time profiles of the FT-EPR spectra provided information on reaction kinetics and Chemically Induced Dynamic, Electron Polarization (CIDEP) effects. It was found that 4CP photolysis in alcohols leads to the simultaneous formation of the phenoxyl radical and radicals produced by hydrogen abstraction from the solvent. CIDEP patterns establish that these radicals are formed in a reaction sequence involving a triplet state precursor and radical pair intermediate. Results of earlier transient optical absorption measurements indicate that the triplet precursor must be the carbene 4-oxocyclohexa-2,5-dienylidene. This assignment is supported by the finding that photolysis of quinone diazide in a hydrogen-donating solvent gives the same free radical products as those obtained from 4CP. The formation of the phenoxyl radical intermediate accounts for the finding that photolysis of deoxygenated solutions of 4CP in alcohols gives phenol as stable diamagnetic product. By contrast, photolysis of aerated and deoxygenated aqueous solutions of 4CP produces benzoquinone and hydroquinone as primary products, respectively.

Nonlinear-Least-Squares Analysis of Slow-Motion EPR Spectra in One and Two Dimensions Using a Modified Levenberg–Marquardt Algorithm

Abstract The application of the “model trust region” modification of the Levenberg–Marquardt minimization algorithm to the analysis of one-dimensional CW EPR and multidimensional Fourier-transform (FT) EPR spectra especially in the slow-motion regime is described. The dynamic parameters describing the slow motion are obtained from least-squares fitting of model calculations based on the stochastic Liouville equation (SLE) to experimental spectra. The trust-region approach is inherently more efficient than the standard Levenberg–Marquardt algorithm, and the efficiency of the procedure may be further increased by a separation-of-variables method in which a subset of fitting parameters is independently minimized at each iteration, thus reducing the number of parameters to be fitted by nonlinear least squares. A particularly useful application of this method occurs in the fitting of multicomponent spectra, for which it is possible to obtain the relative population of each component by the separation-of-variables method. These advantages, combined with recent improvements in the computational methods used to solve the SLE, have led to an order-of-magnitude reduction in computing time, and have made it possible to carry out interactive, real-time fitting on a laboratory workstation with a graphical interface. Examples of fits to experimental data will be given, including multicomponent CW EPR spectra as well as two- and three-dimensional FT EPR spectra. Emphasis is placed on the analytic information available from the partial derivatives utilized in the algorithm, and how it may be used to estimate the condition and uniqueness of the fit, as well as to estimate confidence limits for the parameters in certain cases.

Photooxidation of zinc tetraphenylporphyrin by benzoquinone: a Fourier transform electron paramagnetic resonance investigation

Fourier transform electron paramagnetic resonance (FT-EPR) has been used to study the photooxidation of zinc tetraphenylporphyrin (Zn-TPP) by benzoquinone (BQ). A method was developed for the analysis of the time dependence of the FT-EPR spectra, following pulsed laser excitation of ZnTPP, that gives rate constants for the electron-transfer process as well as for the spin-polarization evolution in paramagnetic species involved in the reaction. It is found that the rate constant of electron transfer from photoexcited triplet ZnTPP to BQ in ethanol is concentration dependent. The data yield a value for the electron-transfer-reaction radius of triplet ZnTPP, the limiting rate constant that would obtain for [BQ]{yields}0, and the radical pair lifetime. In addition, values are found for the spin-lattice relaxation times of the BQ{sup {minus}} anion radical and triplet porphyrin precursor and for electron transfer from BQ{sup {minus}} to BQ. 31 refs., 8 figs., 1 tab.

FT-EPR study of electron spin polarization in the acetone ketyl radical formed by photolysis of tert-butylperoxide in 2-propanol

The kinetics of formation and decay of acetone ketyl radicals produced by photocleavage of tert-butylperoxide in 2-propanol and the generation of electron spin polarization has been investigated with Fourier-transform electron paramagnetic resonance (FT-EPR). It is found that the evolution of the FT-EPR spectra over a time domain ranging from nanoseconds to 500 μs can be completely explained in terms of: (1) kinetics of radical formation and decay, (2) relaxation of tert-butoxy radical precursors — born with zero spin polarization via the singlet excited state of the peroxide — to thermal equilibrium, (3) spin polarization created by the radical pair mechanism involving random encounters of ketyl radicals (F-pair CIDEP), and (4) spin-lattice relaxation of the ketyl radicals. A detailed numerical analysis of the time dependence of the amplitudes of the central three hyperfine components in the spectrum of the ketyl radical gives a value of 85±30 ns for the spin—lattice relaxation time of the tert-butoxy radical. The analysis also gives the value of the pseudo-first-order hydrogen abstraction rate constant (2.5 × 10 7 s −1) and the magnitude of the F-pair spin polarization.

Fourier transform-electron paramagnetic resonance study of the photochemical reaction of acetone with 2-propanol

Fourier transform-electron paramagnetic resonance (FT-EPR) was used to study the pulsed-laser induced reduction of acetone with 2-propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin-correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2-propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2-propanol(D8) at room temperature corresponds to what would be expected for diffusion controlled cage escape. The rate constant for ketyl(D7) radical formation is found to be 5.8±0.5×106 s−1. The decay rate constant kdN(0), where N(0) is the maximum radical concentration, is found to be 1.0×105 s−1. The rate constant for radical formation increases to 1.1×107 s−1 for the acetone(H6)/2-propanol(H8) system. The pronounced isotope effect on radical formation is consistent with a transition state involving C–H bond stretching. The FT-EPR spectra show the effects of radical pair CIDEP. However, a detailed analysis of signal growth and decay kinetics shows that there is no triplet mechanism CIDEP contribution. This finding is in agreement with the relative magnitudes of acetone triplet spin lattice relaxation and radical formation rate constants. The results also demonstrate that the rate constant for hydrogen abstraction is triplet sublevel independent.