Esr-untersuchengen des L-adrenalin-semichinons

L-Adrenalin-semiquinone radicals generated by autoxidation of L-adrenalin have been studied in several organic solvents with different counterions by ESR-spectroscopy. A complete assignment of the hyperfine splitting constants has been made. The proton coupling constants are temperature independent and therefore a rigid conformation of the radical anion is proposed.     

Spin trapping of propagating radicals derived from dialkyl fumarates

Propagating radicals of dialkyl fumarates (DRFs) and deuterated fumarate were trapped by admitting a 2-methyl-2-introsopropane (BNO) solution to the polymerization mixture containing the active radicals or by the polymerizations initiated with di-t-butyl hyponitrite in the presence of BNO. Although ESR spectra of the propagating radicals were appreciably changed with the size of the ester alkyl groups, all the nitroxyl radicals resulting from the spin trapping exhibited similar six-line spectra. The hyperfine splitting constant for the α-hydrogen of the radical moiety showed a slight dependence on the chain length, and the bulkiness of the ester alkyl group did not affect splitting of the spectra. These findings indicate that a substituted methylene radical is produced by addition of the primary radical to DRF followed by propagation throughout the polymerization and that poly(DRF) radical does not encounter severe hindrance in the reaction with BNO.     

Simplification of complex EPR spectra by cepstral analysis

As the Fourier transform of time-series data is known as the spectrum, the Fourier transform of the logarithm of the time-series data is called the cepstrum of the data. When cepstral analysis is applied to free induction decay signals of free radicals showing first-order EPR spectra, the identification of nuclear hyperfine coupling constants becomes simple. In a systematic manner, we have examined how the technique of cepstral analysis is affected by the presence of aliasing, noise, uncertainty in the time origin of the free induction decay, the presence of second-order hyperfine couplings, and the applications of various apodization methods. This technique was then applied to analyze the EPR spectrum of anthraquinone anion radical, and anion radicals of porphycene and tetrapropyl-porphycene, and the hyperfine coupling constants thus obtained were compared with published data. A good agreement was always found. We make a case for the usefulness of cepstral analysis in determining the hyperfine coupling constants of complex EPR spectra of organic free radicals.     

Aromatic Radical Anions in Neat Aromatic Hydrocarbons as Solvents. Direct evidence of through space spin-density transfer to the ligand of the counter ion

Contact ion pairs of aromatic radical anions, with a crown ether complex of potassium as cation in a neat aromatic hydrocarbon, can be obtained by reducing the aromatic hydrocarbon in which a small amount of crown ether is dissolved. The unpaired electron stays attached to one aromatic molecule during a time interval which is long on the ESR. time scale. The radicals are stabilized by ion-pair formation in the low polarity solvent. As a consequence of this stabilization, radicals of compounds with low electron affinities, e.g. mesitylene, can be prepared. Mesitylene, m-xylene, and toluene show additional hyperfine splitting in the ESR. spectra of their anion radical pairs of the order of 18 μT. The proton ENDOR. spectra have signals at the corresponding frequencies, indicating a hyperfine coupling with protons of the crown ether ligand. Using mixtures of two aromatic compounds, their relative electron affinities can be determined by studying the temperature dependence of the radical concentrations.     

DFT and direct ab-initio MD study on hyperfine coupling constants of methyl radicals adsorbed on model surface of silica gel

Methyl radicals interacting with silica gel surfaces have been investigated by means of DFT and direct ab-initio molecular dynamics (MD) methods using cluster models. Four typical binding sites of CH₃ on the cluster models were found in the geometry optimization from several initial geometries of CH₃ around the silica gel clusters. These were two silanol Si–OH sites and two siloxane Si–O–Si sites. In both sites, magnitude of hyperfine coupling constants of the methyl radical (a_H) was smaller than that of free CH₃ (a_H = 23.04 G). Temperature effects on a_H of the methyl radical were investigated by means of the direct ab-initio MD method. The hyperfine coupling constant of CH₃ interacting with the SiOH group decreased with increasing temperature. The methyl radical interacting with alkali metal supported silica gel was also investigated for comparison. The electronic states of methyl radicals on silica gel were discussed on the basis of theoretical results.     

An analysis of the through-bond interaction using the localized molecular orbitals—II : Long-range proton hyperfine coupling in bridgehead alkyl radicals: bicyclo[1.1.1]pent-1-yl,bicyclo[2.1.1]hex-1-yl and bicyclo[2.2.1]hept-1-yl radicals

The INDO calculations were performed on three bridgehead alkyl radicals; bicyclo[1.1.1]pent-1-yl, bicyclo[2.1.1]hex-l-yl and bicyclo[2.2.1]hept-1-yl radicals. We have transformed the canonical molecular orbitals obtained by the INDO method into the localized molecular orbitals. With the use of the obtained localized molecular orbitals, the variation in the hyperfine coupling constant at the bridgehead proton in these radicals was pursued in terms of the through-bond (and/or the through-space) interaction according to the method by which we selectively can pick up a particular interaction between the specified localized molecular orbitals in a radical. As a result of this analysis, it was found that the hyperfine coupling constants in these radicals can be expressed by the summation of several terms; through-virtuals, through-space, through-bond, and some other coupling terms.     

Hyperconjugation and homoconjugation in alkyl radicals bearing β-phosphorus substituents

The isotropic ³¹P hyperfine splitting constants derived from the ESR spectra of a series of β-phosphorus-substituted alkyl radicals are in the order expected for hyperconjugative spin transmission.     

L-tryptophan radical cation electron spin resonance studies: connecting solution-derived hyperfine coupling constants with protein spectral interpretations

Fast-flow electron spin resonance (ESR) spectroscopy has been used to detect a free radical formed from the reaction of l-tryptophan with Ce (4+) in an acidic aqueous environment. Computer simulations of the ESR spectra from l-tryptophan and several isotopically modified forms strongly support the conclusion that the l-tryptophan radical cation has been detected by ESR for the first time. The hyperfine coupling constants (HFCs) determined from the well-resolved isotropic ESR spectra support experimental and computational efforts to understand l-tryptophan's role in protein catalysis of oxidation-reduction processes. l-Tryptophan HFCs facilitated the simulation of fast-flow ESR spectra of free radicals from two related compounds, tryptamine and 3-methylindole. Analysis of these three compounds' beta-methylene hydrogen HFC data along with equivalent l-tyrosine data has led to a new computational method that can distinguish between these two amino acid free radicals in proteins without dependence on isotope labeling, electron-nuclear double resonance, or high-field ESR. This approach also produces geometric parameters (dihedral angles for the beta-methylene hydrogens) that should facilitate protein site assignment of observed l-tryptophan radicals as has been done for l-tyrosine radicals.     

Electron spin resonance analysis of the redox indicator variamine blue II

The oxidation of the redox indicator Variamine Blue in an aqueous medium has been studied by the ESR method. The optimum conditions for the formation of the radical have been determined. The spectra have in some cases shown hyperfine splitting, from which conclusions could be drawn for the nature of the oxidation reaction and structure of the oxidized product. The coupling constants have also been determined for the radicals. The correctness of the assumptions for the structure of the radicals has been supported by the simulation of spectra and the investigation of deuterated derivatives.     

Hyperfine structure observed in EPR spectra of polyphenylacetylene solutions

The high-temperature (>120C) electron paramagnetic resonance (EPR) spectrum of solutions of polyphenylacetylene have been deconvoluted into the spectra of two separate radicals, a delocalized π radical, whose EPR spectrum consists of a single 15-G wide Gaussian line comprising about 90% of the total signal and a second, more localized π radical exhibiting complex hyperfine structure in its EPR spectrum. Some possible structures for the minor component radical are suggested and their hyperfine splitting constants calculated using molecular orbital theory.     

Q-band EPR and ENDOR of low temperature X-irradiated beta-D-fructose single crystals

Beta-D-fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2-C3 bond and a double C2-O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.     

Combined electron magnetic resonance and density functional theory study of 10 K X-irradiated beta-D-fructose single crystals

Primary free radical formations in fructose single crystals X-irradiated at 10 K were investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR induced EPR (EIE) techniques. ENDOR angular variations in the three principal crystallographic planes and a fourth skewed plane allowed the unambiguous determination of five proton hyperfine coupling tensors. From the EIE studies, these hyperfine interactions were assigned to three different radicals, labeled T1, T1* and T2. For the T1 and T1* radicals, the close similarity in hyperfine coupling tensors suggests that they are due to the same type of radical stabilized in two slightly different geometrical conformations. Periodic density functional theory calculations were used to aid the identification of the structure of the radiation-induced radicals. For the T1/T1* radicals a C3 centered hydroxyalkyl radical model formed by a net H abstraction is proposed. The T2 radical is proposed to be a C5 centered hydroxyalkyl radical, formed by a net hydrogen abstraction. For both radicals, a very good agreement between calculated and experimental hyperfine coupling tensors was obtained.     

Positional and angular dependence of hyperfine interactions in cyclic and bicyclic iminoxy radicals with CO or CH2 group – DFT and EPR studies

UB1LYP method was used to study the geometries together with hyperfine coupling constants (hfccs) and natural atomic occupancies (NAO) for the cyclic (cyclohexanone-type) and bicyclic (camphor-type) iminoxy radicals. The positional and angular dependence of the hyperfine interactions, affected by radical substituents and conformations, was analyzed in terms of different mechanisms of spin density transmission. The calculations predicted a significant distortion of regular conformations and change of hyperfine couplings upon introduction of CO into cyclohexane iminoxyl and the CNO spin label into a boat cyclohexane. Hyperfine splittings of the EPR spectra were compared with the computed hfccs to verify their assignment.     

ESR studies on radical polymerization of vinyl ethers

ESR studies on the radical polymerization of vinyl ethers were performed from ?50°C to room temperature using di-tert-butylperoxide as a photoinitiator. Well resolved ESR spectra were assigned to propagating radicals of vinyl ethers. Their hyperfine splitting constants due to α-proton were about 16 G, being smaller than those of ethyl acrylate and vinyl acetate. The smaller constants is ascribed to a deviation of the propagating radicals from sp² hybrid structure. The reason why high polymers are not obtained from vinyl ethers by radical polymerization is discussed on the basis of information from the ESR studies.     

Advances in solid alanine radiolysis understanding

To better understand the composite character of irradiated alanine ESR spectra, a comparative study of few simple amino acids is performed in order to identify the different radio-induced radicals and their proportions. A dedicated spin-trapping method coupled with High Performance Liquid Chromatography (HPLC) is developed and carried out on irradiated alanine, glycine and valine; labeled or not. This study leads us to obtain different isolated trapped radical spectra where hyperfine coupling constants could be evaluated. For alanine, only two species are identified with relative proportions around 97 and 3% in contradiction with recent published articles. The main species has a particularity on its hyperfine coupling constants when labeled carbons are used. Very high hyperfine coupling constants are observed with the carboxylic acid function carbon for the three studied amino acid.     

Enolization of acetone in superheated water detected via radical formation

Muoniated free radicals have been detected in muon-irradiated aqueous solutions of acetone at high temperatures and pressures. At temperatures below 250 degrees C, the radical product is consistent with muonium addition to the keto form of acetone. However, at higher temperatures, a different radical was detected, which is attributed to muonium addition to the enol form. Muon hyperfine coupling constants have been determined for both radicals over a wide range of temperatures, significantly extending the range of conditions under which these radicals and the keto-enol equilibrium have been studied.     

Density functional theory study of the beta-carotene radical cation and deprotonated radicals

The beta-carotene radical cation and deprotonated neutral radicals were studied at the density functional theory (DFT) level using different density functionals and basis sets: B3LYP/3-21G, SVWN5/6-31G*, BPW91/DGDZVP2, and B3LYP/6-31G**. The geometries, total energies, spin distributions, and isotropic and anisotropic hyperfine coupling constants of these species were calculated. Deprotonation of the methyl group at the double bond of the cyclohexene ring of the carotenoid radical cation at 5 or 5' produces the most stable neutral radical because of retention of the pi-conjugated system while less stable deprotonation at 9 or 9' and 13 or 13' of the chain methyl groups causes significant distortion of the conjugation. The predicted methyl hyperfine coupling constants of 13-16 MHz of the neutral radicals are in good agreement with the previous electron nuclear double resonance (ENDOR) spectrum of photolyzed beta-carotene on a solid support. DFT calculations on the beta-carotene radical cation in a polar water environment showed that the polar environment does not cause significant changes in the proton hyperfine constants from those in the isolated gas-phase molecule. DFT calculated methyl proton hyperfine coupling constants of less than 7.2 MHz are in agreement with those reported for the radical cation in photosystem II (PS II) and those found in the absence of UV light for the radical cation on a silica alumina matrix.     

HYSCORE and Davies ENDOR study of irradiated ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been studied with different magnetic resonance techniques to elicit information on the nature and the location of radicals generated during high energy irradiation. Field swept electron paramagnetic resonance, pulsed Davies electron nuclear double resonance and hyperfine sublevel correlation spectroscopic measurements allowed extracting for the first time the full ¹H hyperfine coupling tensors of the most abundant radical, i.e. a secondary alkyl radical and to ascertain the formation of allyl radicals in the first stages of the irradiation process. The ¹H hyperfine coupling tensors are analogous to those reported for single crystal irradiated polyethylene, suggesting that radicals generated in UHMWPE are located in the crystalline region of the polymer. Copyright © 2012 John Wiley & Sons, Ltd.     

All electron calculations of open-shell polyatomic molecules. III. Perturbation treatment of the spin polarization in vinyl and methyl radicals

A first order perturbation treatment starting with SCF-MO 'S in canonical or equivalent quasi-localized form is presented for the hyperfine coupling constants of vinyl and methyl radicals. The spin-polarisation contribution to hyperfine splittings is found to be large, negative for the proton of the radical center in both radicals and positive for the β protons of vinyl.     

An NMR,ESR, ENDOR and TRIPLE resonance investigation of a cation radical formed from 1,2,4,5-tetramethoxybenzene

Several methods have been established for preparing cation radicals from 1,2,4,5-tetramethoxybenzene that allow highly resolved ESR spectra to be recorded. Precise values of the hyperfine coupling constants for the aromatic and methoxy protons have been obtained; the values are 0.2268±0.0004 and 0.0863±0.0002 mT, respectively, with dichloromethane as solvent. No temperature dependence is evident. TRIPLE resonance experiments showed that both coupling constants have the same sign. NMR experiments provided contact shift and line broadening measurements; these proved that both the above constants are positive and led to a value of 3.1 (±0.3)×10⁸M^?1 s^?1 at 23°C for the rate constant for electron exchange between the cation radical and the parent compound.