No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
An EPR study of LiOH2+ in lithium sulfate monohydrate
Abstract
Crystals of Li2SO4 · H2O irradiated with X-rays for 6 hours at room temperature contain a paramagnetic center identified as LiOH+2. The observed hyperfine structure comes from two types of nuclei, one with I = and the other with I = 1. The first is assigned to the lithium and the second to two equivalent hydrogens. From the tensor directions the location and orientation of the center is determined. A positive g-shift indicates a hole-type center.
Electron paramagnetic resonance spectra of 7-irradiated Li:2SO4-K2SO4 mixed system are measured at X-band and 300 K. Two types of centers, A and B, identified as LiOH+2 and SO-3 are studied. The EPR spectrum of LiOH+2 has a well-resolved hyperfine structure. The effective g-value and the hyperfine structure constant of it are calculated. On the other hand, the SO-3 center is characterized by an intense isotropic signal at a g-value ≈ 2. 0035E Mechanisms of center formation are proposed. Infrared transmittance spectra were recorded for unirradiated and gamma-ray irradiated samples. A decrease in the transmittance spectra was observed after irradiation. The ionic conductivity, sigma, has been measured for the Li2SO4-K2SO4 system before and after irradiation in the temperature range 30-400°C. The activation energy was found to be 1. 1 eV. A considerable decrease in the conductivity value accompanied by an increase in the activation energy was observed after gamma-irradiation.
ESR of LiNH4SO4 (LAS) X-irradiated at ambient temperature and investigated from -40 to 200°C but focussing on ferro-para-electric phase transition (186.5°C) is reported. The observed ESR spectrum is attributed to an unusual type of NH+3 radical, not hither to detected in γ-irradiated LAS or (NH4)2SO4, and characterized by a nearly isotropic g = 2.0045, a very low 14N hyperfine coupling constant, and a normal, isotropic 1H hyperfine coupling constant AH = 21.5 ± 0.1 G. The species is apparently trapped at two physically inequivalent sites in the LAS lattice. The AN value is found to drop, gradually, from ≌ 8 G at RT to ≌ 5.6 G at 186.5°C, but to rise abruptly thereafter, in the manner of a first order phase transition. The intensity of one of the resolved hyperfine lines also shows a similar behaviour through both the low-temperature (10°C, antiferro-ferro electric) and high temperature phase transitions.
Fine structure has been observed in the nuclear paramagnetic resonance absorption line for protons in crystalline hydrates. The magnetic field of 6820 gauss was provided by a permanent magnet, the inherent stability of which facilitated detailed study of line shape. Measurements on a single crystal of CaSO4·2H2O show a splitting into four component lines with maximum separation varying from zero to 22 gauss, depending upon the direction of the externally applied magnetic field in the crystal. Both the number of component lines and the dependence of their spacing on field direction are calculated by treating the magnetic dipole-dipole interaction as a perturbation of the proton two-spin system within the water molecule; the effect of the more distant protons, neglected in this calculation, gives a finite width to the component lines. Variation of the splitting with field direction determines the orientation of the line joining protons in the water molecule, which is found to be consistent with positions ascribed to hydrogen nuclei in the lattice through simple considerations of chemical bonding. The distance between protons in the water molecule is measured by the splitting to be 1.58A for CaSO4·2H2O; if one assumes an H&sngbnd;O&sngbnd;H bond angle of 108°, the O&sngbnd;H distance is 0.98A. Powdered hydrates show a characteristic fine structure arising from isotropic distribution in solid angle of single crystal granules. This type of fine structure determines the proton-proton distance somewhat less accurately than does the single crystal experiment.
The crystal structure of lithium sulfate monohydrate has been reinvestigated in order to determine the position of the hydrogen bonds and to check on the asymmetry of the sulfate group in this substance suggested by the earlier work of Zeigler.
A disposition of the hydrogen bonds in agreement with the proton magnetic resonance results has been found and the sulfate group has the configuration of a regular tetrahedron within the limits of error of our measurements. Our results are in agreement with Zeigler in most essentials.
A description is given of a paramagnetic center produced in single crystals of iodic acid, HIO3, when irradiated at room temperature with either x or γ rays. This center is interpreted as neutral iodine atoms, I0, trapped in the crystal. ESR studies indicate four different trapping sites that are related by the crystal symmetry. The g values, the hfs tensor, and the quadrupole interaction tensor are presented and discussed.
EPR studies of x-ray damaged crystals of (NH4)2SO4 show that long lived defects, NH3+ ions, are produced in two inequivalent sites with each site having two magnetically inequivalent orientations. All sites have approximately the same isotropic hyperfine splittings which are found to be 19.0 G for nitrogen and -24.6 G for hydrogen. The anisotropic components are different for the two inequivalent sites indicating different amounts of restricted rotation for the two sites. A second transient defect is identified as SO3-.
Proton magnetic resonance absorption of water molecules in the single crystal of Li2SO4\cdotH2O was observed, and the length and directions of the proton-proton line (p-p) of the water of crystallization were determined. The results obtained are as follows: there are two kinds of orientations of the p-p line in the unit cell, which contains two molecules of Li2SO4\cdotH2O, and they are symmetrically disposed with respect to the ac-plane, the plane of mirror, making the angles ± 51° to that plane. The angle between the projection of each p-p line on the ac-plane and the positive direction of a-axis is 94°, and the length of p-p line is 1.59 Å. From the results obtained and the crystal structure by X-ray analysis, the bond angle of H-O-H is expected to lie between 106°50' and 112°40', and presumably some value nearer to 112°40' may be taken to be favorable from the consideration of the bonding character. The single crystals were prepared by a slow evaporation method from an aqueous solution in a water bath at 80°C.
An extended proton magnetic resonance study of lithium sulfate monohydrate is presented. An oddity in the resulting Pake curves appears. Some of the curves show a shift so that they are described by equations of the form, ΔH=2α[3 cos2(φ+φ0) cos2δ−1] +constant shift.
There are two orientations of the hydrated water molecule. For these the direction angles of the proton-proton axes with respect to the crystallographic a, b, and c axes are, respectively, α0=97°, β0=45°, γ0=48°, and α0=74°, β0=38°, γ0=127°. Apparently the hydrogens bond O5— —H–O5–H— —O1, where O5 are neighboring water oxygens and O1 is a sulfate oxygen.
Several possible origins of the shifts in the Pake curves are discussed with limited success. An adequate theoretical explanation is lacking.
Single crystals of Li2SO4 · H2O have been grown with labeled Li- and H-isotopes near room temperature, and irradiated in 1 MeV electron beams near 77°K. EPR studies of the irradiated samples have also been conducted near 77°K, without warming the samples. The anisotropic spectra with resolved proton (or deuteron) hyperfine structure are satisfactorily described in terms of a spin Hamiltonian consisting of a Zeeman term and a proton (or deuteron) hyperfine interaction term where both the g-tensor and the hyperfine tensor are axially symmetric. The Li-hyperfine interaction is not needed to describe the observed anisotropy, and its only noticeable effect is on the width of individual hyperfine lines. The defect of axial symmetry responsible for the spectra is probably an OH radical with insignificant bonding to its neighbor. If Ziegler's crystallographic data are taken to be correct, the defect is oriented very closely to the direction of O5-O4.
Analytic formulas derived in an earlier paper for the frequency of tunneling through high periodic barriers are here used to derive from nuclear magnetic resonance data in solids the heights of the potential barriers hindering the rotational motion of molecules. The rotations of CH3 groups, NH3+ groups, benzene, NH2 groups and water molecules of crystallization are considered. The use of the barrier heights in correcting for the effect of torsional vibrations of molecules on molecular dimensions obtained from nuclear magnetic resonance data is illustrated. A possible explanation of an isotropic part of the splitting of the proton resonance in Li2SO4⋅H2O, reported earlier by Pake, is presented. Comments are made on the relative magnitudes of the barrier heights in the different molecules considered.
Paramagnetic resonance in a series of hydrated and deuterated molecular crystals irradiated with one MeV electrons at liquid air temperature has been associated with a hole localized on the hydrogen bonded oxygen of the oxyion group. In each case hyperfine structure due to the hydrogen bonding proton has been analyzed with the aid of a Hamiltonian containing an anisotropic term and an isotropic term and the results interpreted to give the relative position of the nucleus and the paramagnetic center.
Single crystals of lithium sulfate have been grown with labeled Li and H isotopes and irradiated in a 1 MeV electron beam at 77°K. After warming to room temperature and again returning to 77°K, the previously reported OH radical spectrum disappears and two new types (B and C) of spectra are observed.The type B spectrum can be explained by an axially symmetric Hamiltonian containing a Zeeman term and an isotropic proton hyperfine interaction term. This spin Hamiltonian is assigned to a modification of the original OH radical, forming an O2H radical. The type C spectrum can be explained by an axially symmetric Hamiltonian containing only a Zeeman term, and is attributed to an O2− radical produced simultaneously with the type B center.