Proton ENDOR Research Articles

Assignment of Proton Endor Resonances of Nitroxyl Spin-Labels in Frozen Solution

Spin-label nitroxyl derivatives of tetramethylpyrroline and tetramethylpyrrolidine in frozen solutions of perdeuterated methanol have been characterized by electron nucleus double resonance (ENDOR spectroscopy). With use of selectively deuterated derivatives of 2,2,5,5-tetramethylpyrroline-1-oxyl-3-carboxamide, proton ENDOR resonance features have been assigned to the vinylic proton in the five membered pyrrolinyl ring and to the methyl groups. The ENDOR resonance features were analyzed on the basis of their dependence on H0. Two pairs of resonance features were assigned to the vinylic proton and were shown to correspond to parallel and perpendicular hyperfine coupling (hfc) components. Six pairs of resonance features were ascribed to the methyl groups. The proton ENDOR spectra of the 3-carboxylic acid spin-label derivatives of tetramethylpyrroline and of tetramethylpyrrolidine compounds exhibited comparable features with nearly identical line splittings. From the observed ENDOR splittings, we have estimated the isotropic hfc component of the vinylic proton in 2,2,5,5-tetramethylpyrroline-1-oxyl-3-carboxamide to be -1.81 +/- 0.04 MHz in frozen methanol. On the basis of the anisotropic dipolar hfc components, the electron-to-vinylic proton distance is estimated as 3.78 +/- 0.01 A, in excellent agreement with that of 3.79 A calculated from X-ray defined coordinates.

Electron spin resonance and proton ENDOR studies in potassium hydrogen oxalate single crystals

Radicals trapped in a γ-irradiated single crystal of potassium hydrogen oxalate have been investigated using ESR and ENDOR techniques. The CO–2 radical produced exhibited hyperfine interaction with nearby protons, and detailed ENDOR study revealed that one of these protons was partially transferred onto the radical site along the hydrogen bridge. Theoretical interpretation of the dipolar couplings indicate that the shift form the crystallographic position of this proton is small. A second species was observed and tentatively assigned to a protonated anion radical. These findings are discussed in relation to the anomalous couplings observed in sodium hydrogen oxalate monohydrate.

Radical anions of nitrobenzene on MgO and Al2O3, as investigated by electron spin resonance and electron nuclear double resonance spectroscopies

Radical anions formed upon adsorption of nitrobenzene at room temperature on the surface of MgO and γ-Al2O3, previously activated by heating under vacuum, have been characterized by means of proton ENDOR and ESR spectroscopies. Fully deuterated nitrobenzene and oxides whose surface protons were exchanged with deuterons have been used in addition to non-deuterated products. On MgO, ESR experiments with deuterated nitrobenzene showed the presence of two radical species whose relative concentration depends strongly on the activation temperature of the oxide (673–1173 K). Hyperfine interactions with ring protons (ortho, meta and para) of the radical anion were resolved in the ENDOR spectra. Calculated spectra fitted the experimental ones well. Weak interactions of the radical with distant surface protons were observed, and surface protons of hydroxyl groups of γ-Al2O3, but not those of MgO, were observed to exchange with aromatic protons of the radical.

The Hydration of Vanadyl Doped into Magnesium Acetate: Single Crystal Proton ENDOR at 20 K

AbstractThe single crystal ESR and proton ENDOR of VO2+ doped into magnesium acetate tetrahydrate have been studied at 20 K. The VO2+ ion occupies Mg2+ sites, replacing Mg2+ together with one water molecule. Hyperfine tensors for the six protons in the three coordinating water molecules have been determined and their dipolar components used to construct a model of the proton geometry. The relation of the structure to that of {VO(H2O)5}2+ in a Tutton salt and in frozen aqueous solutions of vanadyl sulphate is discussed.

EPR, proton and carbon-13 ENDOR studies of a quintet state 13C-labeled galvinoxyl-type tetraradical

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEPR, proton and carbon-13 ENDOR studies of a quintet state 13C-labeled galvinoxyl-type tetraradicalBurkhard Kirste, Michael Grimm, and Harry KurreckCite this: J. Am. Chem. Soc. 1989, 111, 1, 108–114Publication Date (Print):January 1, 1989Publication History Published online1 May 2002Published inissue 1 January 1989https://doi.org/10.1021/ja00183a019RIGHTS & PERMISSIONSArticle Views952Altmetric-Citations23LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (835 KB) Get e-Alerts Get e-Alerts

Proton ENDOR at a microwave frequency of 97 GHz

Most commercial EPR spectrometers work at microwave frequencies below 40 GHz; practical considerations such as sample handling, sensitivity, and equipment cost favor X-band (910 GHz) spectrometers over S (2-4 GHz)and Q-band (3436 GHz) extensions, which are normally used only for special purposes. There are, however, many unsolved problems in molecular spectroscopy, such as lack of resolution in disordered samples, which require taking advantage of higher microwave frequencies (hmf) and, correspondingly, higher magnetic fields, &. The first advantage of hmf EPR is the improvement of spectral resolution due to the larger Zeeman interaction. It brings about a more precise determination of g tensors of frozen solutions or polycrystalline samples (1-4). In spectra of yor X-ray irradiated single crystals with different sites and small g-factor anisotropy a better separation of EPR lines becomes possible at hmf (5). An increasing field of investigations is mixed radicals in solution with small g-factor differences and anisotropic rotation of radicals in viscous solvents (3,4). In the slow-motion regime hmf EPR is distinguished by its increased spectral sensitivity to motional dynamics and important applications in this field are in progress (6). The second advantage of hmf EPR is the higher sensitivity. The two important factors that determine the sensitivity of an EPR spectrometer are the filling factor and the loaded Q of the resonator. Although sensitivity does not increase with a high power of B,, (the power ranging between f and g depending on experimental conditions (7)), there is a significant sensitivity improvement compared with X-band EPR in the investigation of small single crystals. Another advantage of hmf EPR spectrometers becomes apparent when investigating high-spin transition metal complexes with large zero-field splittings. Xor Q-band frequencies are often too low for measuring the zero-field splitting parameters D and E. Examples for such a situation occur for many compounds of biological interest (2). With these advantages in mind hmf EPR spectrometers have been constructed in a few laboratories, at 70 GHz (X = 4.3 mm) (8), at 94 GHz (X = 3.2 mm) (3, 9), at 150 GHz (X = 2 mm) (I, 4, 5) and, most recently, even at 250 GHz (X = 1.2 mm) (6). Additional advantages are envisaged when extending hmf EPR to hmf ENDQR spectroscopy. Due to the larger Zeeman interaction, selective saturation of EPR lines

Frozen solution proton endor spectroscopy of solvated Mo(V) oxochloro complexes

Proton ENDOR spectra of randomly oriented [MoOCl 4(S)] − complexes, with S = H 2O, CH 3OH, C 2H 5OH, and CH 3CHO, have been measured at ≈15 K. Complete hyperfine coupling parameters have been obtained for the hydroxyl and aldehydic protons, as well as for some of the alkyl hydrogens. Structural information concerning coordination of the solvent molecules has been obtained in the limit of the point dipole approximation. Resonance lines ascribable to the presence of an ordered secondshell solvation sphere have been detected in the spectrum of the methanol solvate.

23Na hyperfine and quadrupolar interactions with CO 2− in sodium hydrogen oxalate. An ENDOR study

The results of proton and sodium ENDOR measurements on CO 2 − in X-irradiated sodium hydrogen oxalate have been used to deduce the trapping site of CO 2 −. The hyperfine and quadrupolar (for 23Na) tensors have been determined. A method of calculating the hyperfine and quadrupolar tensors from ENDOR data is briefly discussed. In the calculations, no restrictions regarding the relative magnitudes of the terms in the spin Hamiltonian are made, except that the electron Zeeman term is dominant.

Proton endor spectra of phenanthrene-TCNB triplet traps in single crystals of naphthalene-TCNB

The ENDOR spectra of triplet traps of phenanthrene-TCNB in doped single crystals of naphthalene-TCNB have been recorded and the complete hyperfine tensors of all the protons have been measured. The spin densities on the carbon atoms are obtained from the isotropic as well as from the anisotropic part of the tensors. The spin density distribution on phenanthrene inphenanthrene-TCNB differs from that of the pure phenanthrene in its first excited triplet state, due to a polarizing effect of the acceptor. The CT degree is obtained from the reduction of the total spin density on the donor molecule and it results to be 40%. The structure of the trap is also discussed.

ChemInform Abstract: The Radical Anions of 1,2‐Diphenylcyclohexene and Structurally Related Compounds. Conformational ESR and ENDOR Studies

ESR, ENDOR, and TRIPLE resonance studies have been performed on the radical anions of 1,2-diphenylcyclohex-1-ene (4), 1,2-di(perdeuteriophenyl)cyclohex-1-ene ((D10)4) the trans-configurated 3,4-diphenyl-8-oxabicyclo[4.3.0]non-3-ene (5) and its 2,2,5,5-tetradeuterio derivative (D4)5, and 2,3-diphenyl-8,9,10-trinorborn-2-ene (6). The spectra of exhibit strong temperature dependence along with a specific broadening of ESR hyperfine lines and proton ENDOR signals. The coupling constant, which bears the main responsibility for these features, is that of the β-protons in the quasi-equatorial positions of the cyclohexene ring, and the experimental findings are readily rationlized in terms of relatively modest conformational changes without invoking the inversion of the half-chair form. The hyperfine data for the β-protons in closely resemble the corresponding low-temperature values for , However, the ‘unusual’ features observed for are absent in the ESR and ENDOR spectra of , because the half-chair conformation of the cyclohexene ring in is deprived of its flexibility. Although the boat form of this ring in is also rigid, the spectra of are temperature-dependent, due to an interconversion between two propeller-like conformations of the phenyl groups. The pertinent barrier is 30 ± 5 kJ·mol−1. An analogous interconversion presumably takes place in and as well, but, unlike , it is not amenable to experimental study.

The Radical Anions of 1,2‐diphenylcyclohexene and Structurally Related Compounds. Conformational ESR and ENDOR studies

AbstractESR, ENDOR, and TRIPLE resonance studies have been performed on the radical anions of 1,2‐diphenylcyclohex‐1‐ene (4), 1,2‐di(perdeuteriophenyl)cyclohex‐1‐ene ((D10)4) the trans‐configurated 3,4‐diphenyl‐8‐oxabicyclo[4.3.0]non‐3‐ene (5) and its 2,2,5,5‐tetradeuterio derivative (D4)5, and 2,3‐diphenyl‐8,9,10‐trinorborn‐2‐ene (6). The spectra of \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document} exhibit strong temperature dependence along with a specific broadening of ESR hyperfine lines and proton ENDOR signals. The coupling constant, which bears the main responsibility for these features, is that of the β‐protons in the quasi‐equatorial positions of the cyclohexene ring, and the experimental findings are readily rationlized in terms of relatively modest conformational changes without invoking the inversion of the half‐chair form. The hyperfine data for the β‐protons in \documentclass{article}\pagestyle{empty}\begin{document}$ 5^{- \atop \dot{}} $\end{document} closely resemble the corresponding low‐temperature values for \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document}, However, the ‘unusual’ features observed for \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document} are absent in the ESR and ENDOR spectra of \documentclass{article}\pagestyle{empty}\begin{document}$ 5^{- \atop \dot{}} $\end{document}, because the half‐chair conformation of the cyclohexene ring in \documentclass{article}\pagestyle{empty}\begin{document}$ 5^{- \atop \dot{}} $\end{document} is deprived of its flexibility. Although the boat form of this ring in \documentclass{article}\pagestyle{empty}\begin{document}$ 6^{- \atop \dot{}} $\end{document} is also rigid, the spectra of \documentclass{article}\pagestyle{empty}\begin{document}$ 6^{- \atop \dot{}} $\end{document} are temperature‐dependent, due to an interconversion between two propeller‐like conformations of the phenyl groups. The pertinent barrier is 30 ± 5 kJ·mol−1. An analogous interconversion presumably takes place in \documentclass{article}\pagestyle{empty}\begin{document}$ 4^{- \atop \dot{}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ 5^{- \atop \dot{}} $\end{document} as well, but, unlike \documentclass{article}\pagestyle{empty}\begin{document}$ 6^{- \atop \dot{}} $\end{document}, it is not amenable to experimental study.

Single crystal proton ENDOR of γ-irradiated ammonium paramolybdate at 15 K

Irradiation of ammonium paramolybdate yields a paramagnetic centre which is trapped uniquely on one of the molybdenum atoms in the {Mo7O24}6- ion. The centre has low symmetry so that its location cannot be deduced from the E.S.R. parameters. Hyperfine coupling tensors for six protons have been obtained from ENDOR measurements at 15 K and interpreted in the point-dipole approximation to construct the proton geometry in the vicinity of the paramagnetic centre. Comparison of this model with the heavy atom positions, known from X-ray diffraction, has enabled the location of the paramagnetic centre to be identified.

ChemInform Abstract: Proton ENDOR Spectroscopy of Bis(2,2,6,6‐tetramethylheptane‐3,5‐dionato)copper(II) and Crystal Structure of Bis(2,2,6,6‐tetramethylheptane‐3,5‐dionato)palladium(II)

ChemInformVolume 18, Issue 17 Physical Organic Chemistry ChemInform Abstract: Proton ENDOR Spectroscopy of Bis(2,2,6,6-tetramethylheptane-3,5-dionato)copper(II) and Crystal Structure of Bis(2,2,6,6-tetramethylheptane-3,5-dionato)palladium(II) G. J. BAKER, G. J. BAKER Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorJ. B. RAYNOR, J. B. RAYNOR Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorJ. M. M. SMITS, J. M. M. SMITS Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorP. T. BEURSKENS, P. T. BEURSKENS Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorH. VERGOOSSEN, H. VERGOOSSEN Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorC. P. KEIJZERS, C. P. KEIJZERS Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this author G. J. BAKER, G. J. BAKER Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorJ. B. RAYNOR, J. B. RAYNOR Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorJ. M. M. SMITS, J. M. M. SMITS Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorP. T. BEURSKENS, P. T. BEURSKENS Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorH. VERGOOSSEN, H. VERGOOSSEN Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this authorC. P. KEIJZERS, C. P. KEIJZERS Dep. Chem., Univ. Leicester, Leicester LE1 7RH, UKSearch for more papers by this author First published: April 28, 1987 https://doi.org/10.1002/chin.198717068Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume18, Issue17April 28, 1987 RelatedInformation

Single-crystal proton ENDOR of the SO–3 centre in γ-irradiated sulphamic acid

The proton ENDOR spectra of the SO–3 centre in irradiated crystals of sulphamic acid are rich but poorly resolved. Coupling tensors for three protons have been determined; for two of them it being necessary to assume that the tensors were uniaxial. The relation of the proton coordinates estimated from the tensors to those of hydrogen atoms in the undamaged crystal, which are known from neutron diffraction, is discussed.

ESR and ENDOR study of poly (phenylene vinylene)

ESR and ENDOR measurements were performed on undoped poly (p-phenylene vinylene) films obtained by stretching the precursor films under inert gas atmosphere at temperatures between 100°C and 400°C. Weak ESR signals of the spin concentration being roughly 1 spin / 10 6 C 8H 6 unit appeared as the temperature of preparation increased. g value was about 2.003. Spectra showed a clear anisotropy both in g value and line width, where both quantities are larger for the stretch direction. Proton ENDOR signals were also detected and showed larger coupling values for the stretch direction. Observed anisotropy of ESR and ENDOR is qualitatively similar as those observed in stretch oriented polyacetylene, suggesting that the unpaired electrons are π-electron defects on the conjugated molecular chain.

A single-crystal proton ENDOR study of the ClO3centre in γ-irradiated barium perchlorate at 120 K

Hyperfine coupling tensors for 24 protons have been determined from ENDOR measurements of the ClO3 centre trapped in γ-irradiated single crystals of barium perchlorate trihydrate at 120 K. The tensors have small isotropic components and their dipolar components are close to axial so that their interpretation in the point-dipole approximation to reconstruct the proton geometry in the vicinity of the trapped radical is justifiable. The model thus obtained indicates that there is no severe distortion of the crystal structure in the immediate vicinity of the trapped radicals. There are two chemically identical but ENDOR-distinguishable sites related by a reflection in the a1a2 plane of the hexagonal crystal. This observation supports the assignment of the P63/m space group to the crystal: X-ray diffraction measurements had indicated that the P63 space group was also a possibility.

ESR and ENDOR studies of irradiated potassium dihydrogen phosphate

Irradiated single crystals of potassium dihydrogen phosphate were studied via ESR and ENDOR techniques in order to define phosphate group structures. A precursor cation species was produced at 4.2 K which converted upon warming to a radical previously reported for 77 K irradiation. The g values and proton ENDOR results allowed detailed models for both species to be constructed. Phosphorous ENDOR resonances were also detected and provided complete hyperfine splitting tensors for both species which reflected small internal phosphate group polarization densities.

ENDOR study of Gd 3+ complexes in frozen solutions

The coordination environment of Gd 3+ in frozen solutions is investigated by application of electron nuclear double-resonance spectroscopy. Proton ENDOR spectra of the Gd 3+ ion in frozen methanol-water cosolvent mixtures obtained with the static laboratory magnetic field (Ha) at the turning point of the electron paramagnetic resonance absorption exhibit single-crystal-type line pairs. With use of selectively deuterated materials, the ligand origin of each pair of ENDOR lines has been assigned. For GdCl 3 there are two distinguishable types of protons due to the HO groups of metal-coordinated solvent molecules, and there is one set belonging to the methyl group of metal-coordinated methanol. Similarly, for Gd(CH 3000) 3 and Gd(CH 3CH 2000) 3, the set of ENDOR lines belonging to the methyl group of acetate and to the methylene group of propionate ligands have been identified. By analysis of the dependence of the ENDOR spectra on Ho, we have determined the values of the principal hyperfine coupling (hfc) components of each of the metal-bound ligands. The hfc components of methyl protons of Gd 3+-bound acetate and of the methylene protons of Gd 3+-bound propionate exhibit axial symmetry. Under the point-dipole approximation, they yield correspondingly calculated metal-proton distances of 4.53 and 4.42 Å in good agreement with the value of 4.73 A deduced from crystallographic data for inner sphere coordinated ligands. The hfc components of the HO protons of metal-bound solvent molecules do not exhibit axial symmetry. One set is assigned to inner sphere coordinated H2O while the other is assigned to outer sphere bound CH 30H. The metal-proton distances, calculated on the basis of the largest anisotropic hfc components as lower limit estimates, support these structural assignments. Application of ENDOR spectroscopy is made to identify the primary lanthanide binding site in a-chymotrypsin and to demonstrate the accuracy with which this method of analyzing ENDOR can be employed for structural characterization of metal complexes in frozen solutions. Comparison of the proton ENDOR spectrum of Gd 3+ bound to a-chymotrypsin in frozen D20 solution to those of Gd(CH 3000) 3 and Gd(CH 3CH 2000) 3 indicates that the lanthanide binding site in the protein includes a glutamate residue rather than an asparlate residue. Identification of this protein residue by ENDOR spectroscopy resolves discussions in the literature about the primary lanthanide binding site of a-chymotrypsin.

Harmonics of 13C nuclear frequency observed by endor in 13C-enriched cis-rich (CH) x and reinterpretation of the spectra

ENDOR measurements were performed on 13C-enriched undoped cis-rich (CH) x. Precise measurements of the spectra as a function of the external magnetic field, microwave power and temperature revealed that some of the structures of the line shape, which were formerly assigned to 13C or proton hyperfine tensor components by Thomann et al., are due to higher harmonics of 13C nuclear frequency. For example, the structure around 7.3 MHz, which was assigned to a 13C tensor component, is the second harmonic. Comparison between pristine and 13C-enriched system at given temperatures clearly showed that the intrinsic spin density of the soliton, confirmed by our previous proton ENDOR analysis in pristine system, is not observable in the latter system because of the line shape distortion due to the overlapping of those harmonics.

Proton endor study of the photoexcited triplet state P T in Rps. sphaeroides R-26 photosynthetic reaction centres

The photoexcited triplet state P T of Rhodopseudomonas sphaeroides R-26 has been investigated by ENDOR measurements performed on frozen photosynthetic reaction centre solutions. For the first time hyperfine data could be obtained for P T. These data indicate a delocalisation of the triplet state over two bacteriochlorophyll a molecules.