Proton ENDOR Research Articles

Methyl group tunneling rotation in the lowest nπ* state of toluquinone. An optically detected ENDOR, LAC and CR study

In this paper we report and discuss the effects of methyl group tunneling rotation on the methyl proton ENDOR, LAC and CR spectra in the lowest triplet state of toluquinone at 1.8 K. From a detailed analysis of the ENDOR spectra in the lowest rotational state (A) we obtain for the methyl protons the following isotropic hyperfine interaction constants: A 1 = A 2 = 34.4 MHz and A 3 = −53.7 MHz. The tunneling frequency (3 J) is calculated to be 2.9 GHz. The most likely equilibrium configuration of the torsional oscillator is found to be the one whereby the molecular plane is a mirror plane and the out-of-plane methyl protons point towards the closest oxygen atom. We also show that cross-relaxation between |A> and |E> states of translational equivalent toluquinone molecules is responsible for the observed sidebands in the level-anti-crossing spectra of toluquinone.

Electron nuclear double resonance of cytochrome oxidase: Nitrogen and proton endor from the ‘copper’ EPR signal

Proton ENDOR resonances have been found from at least two different protons with fairly large and isotropic couplings of about 12 and 19 MHz. It is possible that such protons are attached to carbons that are one bond removed from the point of ligation to copper. A number of weakly coupled protons with anisotropic couplings have also been seen. None of the protons, either weakly or strongly coupled, appears to exchange with 2H 2O. We have obtained nitrogen ENDOR from at least one nitrogen with a hyperfine coupling large enough for the nitrogen to be a ligand of copper. We have not yet demonstrated experimentally ENDOR characteristic of the copper nucleus itself.

Strucrure and dynamics of the lowest triplet state in p-benzoquinone IV. The effect of mild substitution on the proton endor spectra

The effect of mild (deuterium and methyl) substitution on the proton ENDOR spectra in the lowest nπ* triplet state of p-benzoquinone is reported and analyzed. The conclusion is that, as recently suggested by Merienne-Lafore and Trommsdorff, p-benzoquinone in its lowest triplet state is best described as a dimer.

Linear electric field effect on the ground state splitting of Mn2+in La2Mg3(NO3)12.24H2O

The effect of an applied electric field on the EPR spectrum of Mn2+ (3d5:6S) in La2Mg3(NO3)12.24H2O has been measured by means of electron spin echo combined with a pulsed electric field. The third rank tensor describing the linear electric field effect on the ground state splitting has the following non zero elements: mod R1E mod =7.5(2), mod R2E mod =6.0(4), mod R14 mod =11.4(4), mod R15 mod =14.6(4) and mod R3D mod =2.8(1) kHz per kV cm-1 (systematic error +or-10%). To increase the accuracy of the measurements the orientation of the static magnetic field with respect to the crystallographic reference frame was done with the aid of spin echo proton ENDOR.

Sodium and proton ENDOR and triple-resonance experiments on biphenyl and fluorenone ion pairs in solution

ENDOR and triple-resonance spectra of fluorenone ketyl and biphenylide sodium ion pairs in ethereal solutions are reported. For the first time alkali metal cation resonance lines could be observed at frequencies below 4 MHz belonging to counterion hyperfine constants as small as 100 kHz. The temperature dependence of the hyperfine constants and their signs were determined by triple resonance and were compared with previous ESR and NMR results. This comparison shows that, for fluorenone, different ion pair structures occur in concentrated and diluted solutions.

Photophysics of small α-dicarbonyls

The triplet kinetics of glyoxal-h 2 and -d 2 neat crystals have been studied by the method of microwave-saturation recovery and microwave-induced delayed phosphorescence. In all cases studied the spin level closely associated with the carbonyl direction is the most radiative. The populating rates, however, show a rich diversity among the various x-traps. This is interpreted by having minute distortions of the trap-molecules out of planarity. A large spin-dependence on the deuterium effect of non-radiative decay is reported and an explanation is suggested. A resolved proton hyperfine structure is observed for glyoxal-h 2 at zero field. Preliminary results on zero-field proton ENDOR are presented.

Endor investigationon Cu 2+-doped TGS and DTGS single crystals

Proton ENDOR spectra of the Cu(II)-glycinate chelate in ferroelectric triglycine sulphate single crystals are reported. Magnetic hyperfine coupling constants of the ligand protons of the amino and methylene groups are given and discussed. The ENDOR investigations confirm the correctness of the assumed structure of the copper chelate and show that its point group is only C 1 in the ferroelectric phase of TGS.

Electron—proton distance changes during solvation of electrons in 3-methylhexane and other organic glasses by matrix ENDOR and second moment EPR lineshape analyses

Matrix proton ENDOR signals have been measured for stabilized electrons produced by irradiation at 4 K and at 77 K in 3-methylhexane glass. The matrix proton ENDOR linewidths are smaller when irradiation is carried out at 77 K. This difference is analyzed to show a 0.15 Å change in the average electron—proton distance as the electron produced at 4 K is solvated by warming to 77 K. Similar distance changes have been deduced from a second moment EPR lineshape analysis for electrons in several organic glasses for which the proton hyperfine interaction is mainly dipolar.

Optical detection of proton endor of short-lived phosphorescent triplet states of aromatic molecules

Electron nuclear double resonance of protons (proton ENDOR) of the short-lived triplet states of benzil in a benzophenone crystal and 2,3-dichloroquinoxaline (DCQ) in a 1,2,4,5-tetrabromobenzene crystal which are not detectable by the normal ENDOR was observed by monitoring phosphorescence intensity at ENDOR transitions are interpreted by a simple model. The spin densities on the carbons bonded to hydrogens in the triplet state of DCQ were found to be ϱ5 = ϱ8 = 0.239 and ϱ6 = ϱ7 = 0.072.

ENDOR Relaxation Study on Phenyl Substituents

Abstract The proton ENDOR spectra of various phenyl substituted hydrocarbon radicals in solution show a marked temperature dependence of the positions, widths, and amplitudes of particular lines. These effects are analyzed in terms of three relaxation mechanisms: (i) internal hindered rotation and (ii) torsional oscillations of the phenyl rings, and (iii) molecular tumbling. From the tempera-ture dependence of relaxation effects caused by mechanism (i) hindrance potentials for internal rotation of phenyl rings could be determined. The results are in good agreement with the pre-dictions of simple π-MO theory. Relaxation effects caused by (ii) turned out to be negligible. Mechanism (iii) produces strong variations in linewidths at low temperatures due to the different hyperfine anisotropics depending on nuclear position.

Structure and dynamics of the lowest triplet state in p-Benzoquinone: II. An optically detected EPR and proton ENDOR study

Optically detected high field EPR and proton ENDOR experiments on the zero-point ( g-inversion) level of the lowest triplet state in p-benzoquinone- h 4 as guest in p-benzoquinone- d 4 are reported. The results are used to obtain the parameters of the molecular spin hamiltonian and the following results are obtained at 1.8 K: The fine-structure principal values are X = −798.2 ± 0.6 MHz, Y = −569.1 ± 0.6 MHz, and Z = 1367.3 ± 0.6 MHz; while the molecular g-values are found to be g xx = 2.0045 ± 0.0005, g yy = 2.0035 ± 0.0005, and g zz = 2.00994 ± 0.00008. The principal z axes of both the fine-structure and g-tensor are found to coincide with the carbonyl bond direction of the p-benzoquinone ground state structure. The orientation of the x and y axes of the fine-structure and proton hyperfine-structure tensor however indicates that the molecule in the excited state is distorted into a centrosymmetric chair-form, which also implies that the symmetry of this state is only A g. The 1 3 trace of the proton hyperfine interaction tensor is found to be 9.6 MHz and from this we calculate that 1.8% of the unpaired n-electron density is at each proton. We further report the effect of mono deuteration on the fine-structure and g-tensor principal values and these data are used to calculate the fine-structure parameter Z of the u-inversion level (20 cm −1 above the zero-point level) in the triplet state of p-benzoquinone- h 4. This method yields Z = 5.8 GHz which is to be compared with the value 6 ± 1.3 GHz obtained from the temperature dependence of the corresponding fine-structure parameter of the zero-point level. Finally, a new effect - the observation of radio-frequency induced changes in the phosphorescence cross relaxation signal — is reported and used to identify the electron spin state, in which the (ENDOR) proton spin flips take place.

Endor spectra of Cu(sal) 2:Ni(sal) 2

Proton, nitrogen and 63Cu ENDOR spectra of the bis(salicylaldoximato)Cu(II) complex substituted in bis(salicylaldoximato)Ni(II) single crystals are reported. Magnetic hyperfine coupling constants of ligand proton and nitrogen nuclei derived by first order analysis are given and discussed. Dipolar parts of some proton hyperfine interactions are shown to correlate with the Cu—proton distance.

Study of solvation of excess electrons in ethylene glycol/water and methyltetrahydrofuran matrices by matrix ENDOR

Matrix proton ENDOR signals have been measured for stabilized excess electrons produced by x irradiation at 4 K in ethylene glycol/water and in 2−methyltetrahydrofuran glasses. The matrix proton ENDOR linewidths are smaller when irradiation is carried out at 77 K. The contribution to the matrix ENDOR linewidth comes principally from predominantly dipolar interactions between the unpaired electron and the surrounding matrix protons. The change in ENDOR linewidth can be interpreted as due to a slight increase in the average distance from the trapped electron to the matrix protons contributing to the matrix ENDOR line as the temperature is raised to 77 K. This is a reflection of the solvation process for the excess electron in these organic glasses. Based on a simple model, the CH bond and molecular dipoles reorient such that the negative H end of the average CH dipole moves 0.2−0.4 Å further away from the unpaired electron as solvation becomes more complete at higher temperature. These results are consistent with the change in electron solvation implied by irreversible changes upon warming of the optical absorption spectra and the EPR linewidths of excess electrons generated at 4 K in these matrices.

ENDOR detection of proton configuration at an HAsO 3− radical in antiferroelectric ADA

An HAsO 3 − radical has been identifìed by proton ENDOR in the antiferro-electric phase of the ADA crystal after room temperature gamma irradiation. The observation of hyperfine interactions for protons surrounding the radical provides a way for studying the hydrogen bonds linked with the radical. The proton positions at the radical sites are found to correspond to ‘lateral’ Slater confìgurations in agreement with the antiferroelectric ordering of protons in the bulk crystal proposed by Nagamiya.

Matrix proton ENDOR studies of the overlap of trapped electron wavefunctions with first solvation shell molecules in methanol glass at 77 K

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Rubidium and proton ENDOR on ion pairs in solution

The E.S.R. spectrum of the o-dimesitoylbenzene anion-alkali cation radical shows unusually large isotropic alkali hyperfine splitting constants. We report a solution ENDOR study of this radical in which both alkali (85,87Rb) and proton ENDOR spectra were recorded. Both the alkali and proton intensities showed a strong dependence on the metal ion nuclear spin quantum number of the E.S.R. line being saturated. This dependence is attributed to strong flip-flop cross-relaxation induced by modulation of the isotropic alkali hyperfine splitting. The powder E.S.R. spectrum of the complex reveals a small anisotropy of the Rb hyperfine splitting tensor. This indicates a small metal non-s-contribution to the half-filled molecular orbital, which is consistent with the observed relaxation behaviour and the small g shift. The intensity variations in the alkali and proton ENDOR spectra were used to determine the relative signs of all hyperfine splitting constants, and the absolute signs of the hyperfine splitting constants are deduced from a model of the structure of the complex.

ENDOR of trapped hydrogen atoms in gamma-irradiated glassy acid matrices and in polycrystalline oxyanion matrices

Trapped H atoms are produced in H2SO4/H2O and H3PO4/H2O glasses and in KHSO4/H2O and KH2PO4/H2O polycrystalline matrices by γ-irradiation at 77°K. High power proton ENDOR spectra of the H atoms have been obtained and analyzed. Prominent matrix ENDOR lines at the free proton frequency together with weak sidebands are observed. Two methods have been developed for increasing the resolution of weak sidebands due to isotropic hyperfine interaction. The two methods involve high irradiation dose and the addition of paramagnetic transition metal ions. The ENDOR line shapes are analyzed in terms of modulation of the electron-nuclear dipolar interaction by the relaxing electron. The isotropic hyperfine constant is about 0.75 G in all matrices and is assigned to the nearest neighbor matrix protons 1.8 Å distant. The center matrix ENDOR lines have been analyzed by computer simulation of the line shape to arrive at the spatial extent of the H atom wavefunction. In glassy matrices the wavefunction is nearly unperturbed from that of the free atom, while in polycrystalline matrices the extreme tail of the wavefunction shows significant distortion.

Proton ENDOR [electron nuclear double resonance] of .gamma.-irradiated Y-type zeolites

Proton ENDOR [electron nuclear double resonance] of .gamma.-irradiated Y-type zeolites

ENDOR spectra of some semiquinone radical-anions

The proton ENDOR spectra of the radical-anions of several semiquinones in alkaline ethanol solution are reported. p-Benzosemiquinone and 2,5-dimethyl-p-benzosemiquinone are converted to di-alkoxy substituted derivatives through reaction with the solvent. When this happens the e.s.r. spectra become complicated because the spectra of the primary and secondary radicals overlap: the power of ENDOR for analyzing overlapping e.s.r. spectra is illustrated.

Endor in solutions of mononegative ions of aromatic hydrocarbons

In this work it is shown that proton ENDOR spectra of the mononegative ions of naphthalene, anthracene, tetracene, p-terphenyl and m-terphenyl dissolved in DME can be detected just above the freezing point of the solvent. ENDOR due to hyperfine interaction with the counterion nuclei could not be observed.