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On the g Factor of Hydrogen Atoms Trapped in Phosphates
The Journal of Chemical Physics
Abstract
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... As far as we know there are no publications about the phosphate centres in the biominerals. Therewith, previous investigators have registered and described EPR signals that they have connected with atomic hydrogen [14,16]. But, according to our opinion, these centres are phosphate ones and cannot be attributed to the atomic hydrogen. ...
... So, P 2 centre can be an indicator of the tricalcium phosphate phase in biominerals and annealed hydroxylapatite. EPR signals of P 2 centres in the biominerals were interpreted as monatomic hydrogen in works [13,14,16]. In our opinion, it is unlikely because usually monatomic hydrogen is not stable at the room temperature. ...
Electron paramagnetic resonance (EPR) method was applied to study phosphate centres in different types of biominerals, synthetic hydroxylapatite and tricalcium phosphate. The values of the g-factor and the hyperfine splitting were measured for all the substances. The effect of presence of defects on characteristics of the EPR signals and their decay was investigated. Possibilities of using the obtained experimental data to study assimilation dynamics of implanted substance by organism, regeneration of bone tissue and to detect small amount of tricalcium phosphate phase in biominerals and their synthetic analogues are discussed.
... Formation of atomic hydrogen with ionizing irradiation has been observed in a number of crystalline matrices including CaF2 [1,2], alkali halides [3,4], quartz [5][6][7], beryl [8] and some phosphates [9,10]. In m any systems they can only be observed at low temperatures, but in CaF2 and in the phosphates they are stable up to temperatures in the range of 100 °C. ...
EPR parameters for atomic hydrogen in three nonequivalent special positions at the twofold axis of the monoclinic structure of brazilianite, formed by X-ray irradiation at room temperature, are reported. The g-factors are practically isotropic and close to the free electron value, but the hyperfine splittings exhibit considerable anisotropy. One of the centers shows an additional doublet splitting, most likely due to hyperfine interaction with the proton of one of the OH groups. These hydrogen atoms have the highest thermal stability yet reported. With repeated thermal destruction and formation by ionizing radiation one of the centers is rapidly depleted. Thus they must be formed at defect sites, not from the undisturbed OH groups, and trapped near the sites of formation and their thermal destruction must at least in part be irreversible. Possible defects for their formation are discussed.
Summary This document is part of Subvolume A ‘Atoms, Inorganic Radicals, and Radicals in Metal Complexes’ of Volume 9 ‘Magnetic Properties of Free Radicals’ of Landolt-Börnstein - Group II Molecules and Radicals.
Summary This document is part of Subvolume A ‘Atoms, Inorganic Radicals, and Radicals in Metal Complexes’ of Volume 9 ‘Magnetic Properties of Free Radicals’ of Landolt-Börnstein - Group II Molecules and Radicals.
The influence of synchrotron radiation (SR) in vacuum-deposited layers of copper phthalocyanine (CuPc) and the influence of the following vacuum thermal development in the positive and negative exposed CuPc layers were investigated by EPR, Raman spectroscopy, UV-visible spectroscopy, X-ray diffraction, mass spectrometry using Knudsen cell. It was established that the layers of α-CuPc possessed EPR anisotropy. The anisotropy decreased with an increase of SR exposure and changed for negative exposed layers and for layers heated at development temperature. For positive exposed and unexposed layers of CuPc, the α → β-phase transition occurred at development temperature, but this process was not observed at development of negative exposed layers. Mass spectra of evaporated CuPc molecules from unexposed and positive exposed layers were similar. Only for negative exposed layers the additional protonated molecules of CuPc were recorded in mass spectra. Unexposed, positive exposed and negative exposed CuPc layers had different temperature dependencies of pressure of evaporated molecules. All measured spectra and dependencies were analysed and SR photochemistry of CuPc layers was discussed.
Two types of trapped hydrogen atoms (Ht) were found in a natural kaolinite, Kanpaku, irradiated with an ionizing radiation at 77 K. One (Ht(o)) showed a hyperfine coupling (hfc) of 1435 MHz, which is larger than
1420 MHz for the free hydrogen atom. The line-width of the EPR signal is very wide (ca. 0.6 mT) and its half-life is as long as several minutes even at 250 K. From these characteristics we deduced a small atomic cage consisting of six oxygen atoms (and six aluminium atoms in the next nearest shell) found in the crystalline structure of kaolinite as the trapping site for this Ht. The other (Ht(i)) showed an EPR spectrum with a hfc a little smaller than that of the free hydrogen atom. The yield of this component was saturated at a low level with a dose of above 10 kGy, and it decreased further by repeated irradiation after annealing. Thus, the trapping site of Ht(i) may be a crystalline defect, whose density is usually small and consumed by radiolysis. In the case of synthetic kaolinite, both types of Ht's were also observed as the main components, which were interpreted with the same model as above. In addition,
another Ht showing a sharp signal with
a short lifetime was found in the non-crystalline part.
Frozen solutions of silver salts exposed to 60Co γ-irradiation form silver atoms by reaction of radiation-produced electrons with the silver ion. At 4K the silver atoms are initially produced in a nonequilibrium or presolvated state and upon brief thermal excitation to 77K the first solvation shell geometry changes towards an equilibrium or solvated silver atom. This is most pronounced in water but also occurs in methanol, ethanol and n-propanol matrices. The changes in the electron spin resonance magnetic parameters upon silver atom solvation have been determined. In alcohol-water mixtures Ag0 is preferentially solvated by polycrystalline water at low alcohol concentration. Above a particular alcohol mole percent Ag0suddenly changes its environment to a glassy alcohol one. This sudden change occurs at 17, 13 and 6 mol % methanol, ethanol and n-propanol, respectively. These mole percents correlate with the minimum of the excess enthalpy of mixing and with the hydrogen atom trapping ability of these alcohol-water mixtures. The results also suggest that the local environmental disorder around Ag0 increases with alcohol chain length in alcohol-water frozen solutions.
Cd+1 has been generated in frozen aqueous solution by X-irradiation at 4.2 and 77 K. No difference in the g-factors or isotropic hyperfine constant has been detected for the two irradiation temperatures in contrast to Ag° species. Although the isotropic coupling constant indicates that 30% of the unpaired electron spin density of Cd+1 is delocalized onto first solvation shell waters it appears that the equilibrium solvation environments for Cd+2 and Cd+1 are essentially the same.
SO2Cl2·- is the only observable product of electron attachment in γ-irradiated crystalline sulfuryl chloride at -196 °. By contrast, SO2Cl2·-, Cl2·-, and SO2·- have been identified by their esr spectra in γ-irradiated glassy solutions of sulfuryl chloride. The three species are readily distinguished by selective photobleaching, and this has been utilized in assigning an absorption band at 432 nm (λmax) to SO2Cl2·-. A similar absorption band is observed at 415 nm in crystalline sulfuryl chloride. The esr parameters of SO2Cl2·- in the glasses, A∥(35Cl) = 62.4-62.9 G and g∥ = 2.0058-2.0060, are close to the corresponding values (64.1 G and 2.0055) determined in polycrystalline sulfuryl chloride. Although the perpendicular features are not well defined in the powder spectra, the large anisotropy of the chlorine hyperfine splitting has been demonstrated by use of an aligned crystalline sample. The calculated spin density in the chlorine p orbitals is incompatible with the structures proposed for other 33-valence electron radicals and a novel structure has been proposed for SO2Cl2·-. The unpaired electron is visualized as occupying a supramolecular orbital comprising the lowest unoccupied orbitals of essentially separate SO2 and Cl2 molecules. Iterative extended Hückel calculations support this model and yield, in addition, electron spin density distributions for the radical similar to that found experimentally.
Setzt man gefrorene AgClO4-L?sungen bei 4 K "Covy-Strahlung aus, werden durch Reaktion Strahlen-generierter Elektronen mit Ag+ Ag-Atome gebildet.
Procedures for producing 2-D images of the distribution of paramagnetic centers in solids using cw EPR procedures are described. The method makes use of static magnetic field gradients. An on-line microcomputer is used for data capture and for image generation in a gray-scale or color pixel representation. Applications of the method to a number of systems including natural and electron-irradiated diamond, irradiated silica gloss, and irradiated calcium orthophosphate are described. Representative images of paramagnetic defect distributions are presented. The resolution achieved is limited by the natural linewidth; the gradient magnitude, and the pixel size used. These factors are discussed in relation to methods for improving resolution.
It is suggested that relatively sharp spectral lines at millimeter and centimeter wavelengths may arise from neutral or ionized atomic species in interstellar dust. Frequencies at which the F = 0 - F = 1 ground-state hyperfine transition of atomic hydrogen occurs in various solids are listed, and the column density required to yield measurable absorption is estimated. Zero-field splitting of the ground state of Fe(3+) and Cr(3+) in different crystals is examined, and the frequency range at which magnetic dipole transitions between the zero-field levels occur is shown to be 6 to 80 GHz. It is concluded that transitions between zero-field levels of Fe(3+) and other transition-metal ions in interstellar dust may be observable in absorption in typical interstellar dust clouds. It is noted that nonthermal emission at the frequencies of the absorption features considered is also a distinct possibility.
Epr studies were carried out on trapped H atoms in 60Co -γ-irradiated calcium phosphate powder at room temperature. The radiation yield of H atoms was relatively small [G(HT) ≅ 0.002] and inversely related to the : extent of hydration of the phosphate. The H atoms decayed slowly at room temperature with an initial half-time of 4 days. The epr characteristics of the H-atom doublet were very similar to those found for the same species in acidic ices except that the epr line width in phosphate was only 0.35 G compared with approximately 3.4 gauss in H2SO4 glassy ice at 77° K. Paramagnetic relaxation measurements vs. radiation dose were made on the H-atom epr lines. These data taken together with the thermal decay and dose saturation data indicated that the H atoms are produced initially in a nonuniform spatial distribution which gradually changes with dose and irradiation time to a spatially more uniform distribution.
Epr studies were carried out on trapped H atoms (Ht) in 60Co γ-irradiated lithium ortho-, meta-, and pyrophosphate polycrystals at room temperature. The radiation yields of Ht varied from G(Ht) = 0.08 for lithium orthophosphate to G(Ht) = 0.01 for lithium meta- and pyrophosphate. The Ht decayed slowly at room temperature with initial halftimes which varied from 14 hr for lithium orthophosphate to 48 hr for lithium pyrophosphate. The widths of the epr lines of Ht varied from 0.74 G for lithium orthophosphate to 3.2 G for lithium metaphosphate. Two sets of satellite lines were observed in the epr spectra of the Ht doublet in lithium orthophosphate and lithium pyrophosphate. These satellites were attributed to forbidden spin flips of adjacent H and P nuclei which populate the H atom traps. Paramagnetic relaxation time measurements vs. radiation dose were made on the Ht epr lines in lithium orthophosphate. These data when correlated with thermal decay and dose saturation data supported the hypothesis that the Ht are distributed uniformly in the polycrystalline matrix.
Rat femora, fetal calf cancellous and compact bone, demineralized bovine cortical bone, human enamel, and various synthetic calcium phosphates were x-irradiated at 105 °K and examined by electron spin resonance spectroscopy at various temperatures. All mineral-containing specimens exhibited a complex main signal and two satellite resonance lines attributed to trapped hydrogen atoms. Satellite signals from mineral samples with large crystal sizes showed a fine detail probably due to proton interaction, possibly from hydroxide ions. Noncrystalline calcium phosphate yielded spectra different from those of crystalline apatite, and percentage crystallinity measurements based on these differences indicated that bone tissue contains both of these phases. Low-temperature spectra of demineralized bone protein and young rat femora exhibited poor resolution attributed to interference by collagen degradation products in the former and noncollagenous proteins in the latter case. Resonance signals from mechanical mixtures of organic bone matrix and synthetic calcium phosphates differed qualitatively and quantitatively from those of bone tissue itself. These data indicate that a transfer of free radical electron spin energy may take place between bone mineral and bone protein. Thus it is suggested that there is an intimate interaction, perhaps through chemical bonding, between the organic and mineral phases of skeletal tissue.
Hydrogen atoms have been stabilized in nonequivalent lattice sites in matrices of the rare gases at liquid helium temperature. Electron spin resonance spectra of H atoms in argon, krypton, and xenon show that at least two trapping sites are involved in each case. In a neon matrix, H atoms have been stabilized in only one site. Attainability of the various trapping sites apparently depends on the initial energy of the H atom, a simple doublet spectrum being obtained when the atoms are deposited from the gas phase, while multiple trapping spectra are obtained when the atoms are produced by photolysis in the solid.
The hyperfine coupling contants and the electronic g factors for H atoms trapped in the various matrix sites have been determined. The deviation of the hyperfine coupling constant from the free-state value is positive in some cases and negative in others. The experimental results are in good agreement with theoretical predictions. A complex multicomponent H atom spectrum was obtained by photolysis of HI in xenon. The subsplitting in the spectrum is attributed to magnetic hyperfine interactions with matrix nuclei (Xe129 and Xe131). The nature of the trapping sites in rare gas matrices is discussed. Evidence for trapping in substitutional sites and octahedral sites is presented.
The electron-spin resonance spectra of various γ-irradiated phosphates show that hydrogen atoms are trapped, the centres being stable at room temperature in some cases. A model of the trapping site is suggested which accounts qualitatively for the observed hyperfine interaction and g-tensors, and for the observed increase in stability with increase in basicity of the anion.
Free radicals produced in various substances by Co60 gamma rays have been observed by the paramagnetic resonance method. A sufficient concentration has been obtained by performing the irradiation and observation at liquid nitrogen temperature, where the radicals are stably trapped. One pair of lines observed in irradiated H2SO4, HClO4 and H3PO4 has been identified as arising from atomic hydrogen as shown by deuterium substitution experiments and a consideration of the strength of the hypexfine interaction. Atomic hydrogen is also formed from water adsorbed on glass surfaces. Atomic hydrogen was not found in irradiated ice. The presence of additional, weak paramagnetic resonance lines gives information on the environment of the atomic hydrogen, while warming experiments give rate data that indicate second order kinetics for the disappearance of atomic hydrogen.
Electron spin resonance spectra of H, D, N, and C trapped in solid matrices at liquid helium temperature have been observed and interpreted. The effect of the matrix field on the resonance properties of the radicals has been investigated by depositing the radicals in matrices with different binding energies. The effect of the matrix on the g factor is extremely small in all cases. The deviation of the hyperfine coupling constant from the free-state value increases in a systematic way with increase in binding energy of the matrix, the percentage deviations being small for H, D, and C but rather large for the case of N. The widths and shapes of the spectral lines are discussed in terms of dipolar broadening, spin-lattice relaxation, anisotropic broadening, rate of passage and the modulation parameters used for observation.
Electron spin resonance (ESR) spectra of the formyl radical (HCO) and
deuterated radical (DCO) were observed in solid carbon monoxide over the
temperature range 4.2 to 30 deg K. The observed line shapes were temperature
dependent, but the cause of the temperature dependence was not definitely
determined. The radical was produced by the reaction of CO with hydrogen atoms
produced by the photolytic decomposition of HI. The radical was also produced by
the photolytic decomposition of formaldehyde in solid argon, and it was iound to
be one of the products of the photolysis of methyl alcohol in solid argon. The
most striking feature of the ESR spectrum of HCO is the very large proton
hyperfine splitting (137 gauss). Also the lines were broad and unsymmetrical
with the linewidth varying from one hyperfine component to the next. It was
shown that the line shapes were due to the combination of a rather pronounced
anisotropy in the electronic g factor with a smaller anisotropy in the proton
hyperfine interaction. These characteristics of the radical gave a good deal of
information about its structure which supported the conclusion of others that the
formyl radical is not a pi -electron radical. (auth)
More lines than can be explained by first‐order theory have been observed in the ESR spectra of several alkyl radicals in solution. The additional structure is predicted if the usual treatment is extended to second order in the coupling constants. This extension has been made in such a way as to eliminate the necessity of explicitly considering the 2n nuclear‐spin wave functions which occur with spin‐☒ nuclei. The spectrum of the ethyl radical is used to demonstrate the agreement of actual spectra with the treatment presented here. A table is included which facilitates calculation of the second‐order structure in the spectra of radicals containing spin‐☒ nuclei.
Electron spin resonance methods were used to observe alkyl radicals in
liquid hydrocarbon systems during irradiation with 2.8-Mev electrons. The
investigations provided detailed structural, radiation chemical, and kinetic
information about a large number of radicals. In general, the ESR lines were
found to be narrow; considerable fine structure was observable, permitting
positive assignment of the radical species. Accurate hyperfine constants are
reported for 21 alkyl and cycloalkyl radicals (including several deuterated
species), vinyl, 1 -methylvinyl, 3-butenyl, allyl, and cyclohexadienyl radicals,
and hydrogen and deuterium atoms. Except for cyclopropyl radical, all the alkyl
and cycloalkyl radicals have alpha coupling constants in the range 21 --23 G.
The BETA coupling constants in cases where they have been rotationally averaged
isotropically were found to decrease with increasing substitution of alkyl groups
on the alpha carbon atom. In general the values for primary, secondary, and
tertiary radicals appear to be represented by the splittings observed for the
methyl protons in ethyl (26.87 G), isopropyl (24.68 G), and tert-butyl (22.72 G)
radicals. A possible explanation for this trend is discussed. A number of
examples showing a significant departure from the above isotropically averaged
values were found. In several cases this departure, and the resultant strong
temperature dependence of the BETA coupling constants, was taken as evidence
for a barrier hindering rotation about the alpha -carbon -- BETA -carbon bond.
Splittings caused by gamma protons and range from 0.4 to 1.1 G were resolved in
five cases. A pronounced angular dependence of the coupling constant was
demonstrated in the case of propyl radical. The angular dependence is important
in considerations of the mechanism of the gamma hyperfine interaction. Three
radicals that do not have the usual pi -electron configuration were observed:
vinyl, 1-methylvinyl, and cyclopropyl. The splittings by the alpha protons of
vinyl and cyclopropyl radicals are 13.39 and 6.51 G. The small values indicate
that as the orbital associated with the unpaired electron acquires s character
the coupling constant increases from approximates --23 G. The coupling
constants for the two conjugated radicals, allyl and cyclohexadienyl, support the
theoretical prediction of negative spin density at the unstarred positions of the
odd-alternant radicals. Spectra with relatively narrow lines are reported for
transient species in a number of solid hydrocarbons. The radicals observed in
various hydrocarbons are discussed in terms of the radiation chemical reactions
expected in the systems. In most cases the radicals represent fragments that
result from rupture of a single bond. In certain cases secondary reactions,
such as the addition of hydrogen atoms to unsaturates, were also found to be
important. The use of ESR methods in obtaining information concerning the rate
of radical reactions is illustrated by two studies. In the first an activation
energy of 3.3 kcal/mole was estimated for the addition of vinyl radicals to
ethylene from measurements on the temperature dependence of the relative vinyl
and 3-b tenyl radical concentrations in liquid ethylene. In the second the
kinetics of the disappearance of ethyl radicals liquid ethane were examined in
detail. The absolute second-order rate constunt for the disappearance as
obtained from absolute concentration and dose rate measurements was found to be 3
x 10/sup 8/ liters mole/sup -1/ sec/sup -1/ at --175 deg C. The activation energy
for the reaction of ethyl radicals in liquid ethane is 780 cal/mole, or
essentially that for the diffusion-controlled process. (auth)