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Gerd N La Mar - One of the best experts on this subject based on the ideXlab platform.
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the use of chemical shift temperature gradients to establish the Paramagnetic Susceptibility tensor orientation implication for structure determination refinement in Paramagnetic metalloproteins
Journal of Biomolecular NMR, 2000Co-Authors: Zhicheng Xia, Bao D Nguyen, Gerd N La MarAbstract:The use of dipolar shifts as important constraints in refining molecular structure of Paramagnetic metalloproteins by solution NMR is now well established. A crucial initial step in this procedure is the determination of the orientation of the anisotropic Paramagnetic Susceptibility tensor in the molecular frame which is generated interactively with the structure refinement. The use of dipolar shifts as constraints demands knowledge of the diamagnetic shift, which, however, is very often not directly and easily accessible. We demonstrate that temperature gradients of dipolar shifts can serve as alternative constraints for determining the orientation of the magnetic axes, thereby eliminating the need to estimate the diamagnetic shifts. This approach is tested on low-spin, ferric sperm whale cyanometmyoglobin by determining the orientation, anisotropies and anisotropy temperature gradients by the alternate routes of using dipolar shifts and dipolar shift gradients as constraints. The alternate routes ultimately lead to very similar orientation of the magnetic axes, magnetic anisotropies and magnetic anisotropy temperature gradients which, by inference, would lead to an equally valid description of the molecular structure. It is expected that the use of the dipolar shift temperature gradients, rather than the dipolar shifts directly, as constraints will provide an accurate shortcut in a solution structure determination of a Paramagnetic metalloprotein.
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solution nmr determination of the anisotropy and orientation of the Paramagnetic Susceptibility tensor as a function of temperature for metmyoglobin cyanide implications for the population of excited electronic states
Journal of the American Chemical Society, 1999Co-Authors: Bao D Nguyen, Zhicheng Xia, K Vyas, And Helen Deaguero, Deok Cheon Yeh, Gerd N La MarAbstract:Comprehensive 1H NMR assignments of the heme cavity proton resonances of sperm whale metmyoglobin cyanide have provided the dipolar shifts for nonligated residues which, together with the crystal c...
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solution 1h nmr structure of the heme cavity in the low affinity state for the allosteric monomeric cyano met hemoglobins from chironomus thummi thummi
FEBS Journal, 1996Co-Authors: Wei Zhang, Gerd N La Mar, Klaus GersondeAbstract:Solution 1H-NMR studies of the heme cavity were performed for the cyanomet complexes of monomeric hemoglobins III and IV from the insect Chironomus thummi thummi, each of which exhibit marked Bohr effects. The low pH 5, Paramagnetic (S= 1/2) derivatives were selected for study because the large dipolar shifts provide improved resolution over diamagnetic forms and allow distinction between the two isomeric heme orientations [Peyton, D. H., La Mar, G. N. & Gersonde, K. (1988) Biochim. Biophys. Acta 954, 82–94]. The crystal structure for the low-pH form of the hemoglobin III derivative, moreover, has been reported and showed that the functionally implicated distal His58 side chain adopts alternative orientations, either in or out of the pocket [Steigemann, W. & Weber, E. (1979) J. Mol. Biol. 127, 309–338]. All heme pocket residues for the low-pH forms of the two hemoglobins were located, at least in part, and positioned in the heme cavity on the basis of nuclear Overhauser effects to the heme and each other, dipolar shifts, and Paramagnetic-induced relaxation. The resulting structure yielded the orientation of the major axis of the Paramagnetic Susceptibility tensor. The heme pocket structure of the cyanomet hemoglobins III and IV were found to be indistinguishable, with both exhibiting a distal His58 oriented solely into the heme cavity and in contact with the ligand, and with two residues, Phe100 and Phe38, exhibiting small but significant displacements in solution relative to hemoglobin III in the crystal.
P Porcher - One of the best experts on this subject based on the ideXlab platform.
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optical spectroscopy and crystal field effects on the Paramagnetic Susceptibility of rare earth germanates garge2o7 r pr nd
Principles and Practice of Constraint Programming, 2000Co-Authors: C Cascales, G Lozano, C Zaldo, P PorcherAbstract:Abstract Optical absorption and photoluminescence measurements at 9 K were performed on stoichiometric GaRGe2O7 and on R-doped LaGaGe2O7, R=Pr3+, Nd3+, polycrystalline samples. In this monoclinic matrix, space group P21/c (no. 14), the lanthanide ion occupies a single crystallographic point site with symmetry C1. From the crystal-field analysis of the optical spectra, energy level schemes and an expression of the associated wavefunctions for the 4f2 and 4f3 configurations were derived. In both cases, the fitting of experimental Stark level energies and the phenomenological calculation of free-ion as well as crystal-field parameters (CFPs) were performed for the approximate Cs (C2) symmetry. Despite the low symmetry, very satisfactory correlations between experimental and simulated energy level schemes were obtained, with root mean square deviations σ=10.1 and 14.8 cm−1 for Pr3+ and Nd3+, respectively. Energy levels and the composition of their wavefunctions were checked for both configurations through a calculation of the thermal evolution of the Paramagnetic Susceptibility χ, according to the Van Vleck formula. The same calculation was accomplished with CFPs resulting from the semi-empirical simple overlap model (SOM), considering the real point symmetry of R and C1. Over the whole measured range, 1.7–300 K, a very good reproduction of the observed χ vs. T is yielded by each of the phenomenological and SOM calculated collections of CFPs.
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simulation of the Paramagnetic Susceptibility in rare earth oxychlorides
Journal of Alloys and Compounds, 2000Co-Authors: Jorma Holsa, P Porcher, Ralfjohan Lamminmaki, Mika Lastusaari, R SaezpucheAbstract:The average magnetic susceptibilities of polycrystalline rare earth oxychlorides (REOCl; RE=Ce, Pr, Nd, Sm, Tb, Dy and Ho) were measured between 2 and 300 K. The Susceptibility of most REOCl samples follows the Paramagnetic Curie–Weiss behaviour down to low temperatures (<20 K). However, TbOCl, SmOCl, and DyOCl show additional antiferromagnetic ordering at 4, 8, and 11 K, respectively. The experimental Paramagnetic susceptibilities were simulated with the aid of the van Vleck formalism based on the wave functions and energy level values obtained from a previous phenomenological simulations of spectroscopic data. The deviation of the magnetic susceptibilities of several REOCl from the Curie–Weiss behaviour at low temperatures can be explained by the gradual population of the crystal field (c.f.) components of the ground 2S+1LJ level. At high temperatures the difference between the calculated and experimental data is due to the modification of the c.f. effect as a result of the lattice expansion. The strong preferred orientation of the REOCl crystallites may affect the experimental data because of the strong anisotropy of the Paramagnetic Susceptibility.
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Paramagnetic Susceptibility simulations from crystal field effects on nd3 in magnesium borate mgnd bo2 5
Principles and Practice of Constraint Programming, 1999Co-Authors: C Cascales, Saez R Puche, P PorcherAbstract:Abstract From the crystal field analysis of optical absorption spectra on monoclinic S.G. P21/c MgNd(BO2)5, where Nd3+ occupies a single crystallographic position with no symmetry elements, energy level schemes and an expression of the associated wavefunctions for the 4f3 configuration of Nd3+ have been derived, considering approximate C2v and Cs (C2) symmetries. Despite of the low symmetry of Nd3+, resulting rms deviations between calculated and experimental levels are very satisfactory. The composition of the crystal field wavefunctions from both sets of phenomenological free-ion and crystal field parameters has been checked through a calculation of the thermal evolution of the Paramagnetic Susceptibility χ, according to the Van Vleck formula. The same simulation was performed with crystal field parameters (CFPs) resulting from the ab initio simple overlap model (SOM), considering the Nd3+ real point symmetry, C1. Over the whole measured range, 1.7–375 K, and especially at low temperatures, a very good reproduction of the experimental χ vs. T is yielded by each of the experimental and SOM calculated collections of CFPs. The oversimplified SOM model seems thus to be useful providing reasonable estimations of crystal field strength parameters.
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Paramagnetic Susceptibility simulations from crystal field effects on rare earth antimonates r3sb5o12
Journal of Alloys and Compounds, 1997Co-Authors: Concepción Cascales, P Porcher, R SaezpucheAbstract:Abstract The magnetic susceptibilities of polycrystalline rare earth antimonates R3Sb5O12 (R=rare earth), with cubic structure, space group I43m (No. 217), where the point site symmetry of R is S4, have been measured from 4.2 to 800 K. Using the wave functions and energy levels derived from standard free ion and crystal field parameters deduced from the analysis of the optical spectra of pure and/or doped Pr, Nd, Eu and Er compounds, the calculation of the temperature dependent Paramagnetic Susceptibility has been carried out according to the Van Vleck formalism. The same calculation through crystal field parameters resulting from the ab initio simple overlap model over the entire R3Sb5O12 series has been performed. Curves of both kinds were very similar for each configuration, with the exception of those for the Pr compound. Very good agreements with experimental data are found, especially below around 400 K, even when the approximate D2d (near S4) potential is considered. The standard crystal field treatment of 4fN configurations is shown to explain the magnetic properties of these compounds with no need to consider any sort of magnetic interaction between the magnetic ions.
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Paramagnetic Susceptibility simulations from crystal field effects on rare earth double molybdate and tungstate
Journal of Physics: Condensed Matter, 1996Co-Authors: Concepción Cascales, P Porcher, R SaezpucheAbstract:Magnetic susceptibilities of polycrystalline samples of two series of double rare-earth molybdate and tungstate compounds, (RE = rare earth, X = Mo or W), with scheelite-type structure, space group (No 88), where RE atoms adopt point symmetry, have been measured in the temperature range 1.7 - 400 K. Using the wavefunctions and energy levels derived from standard free-ion and crystal-field parameters deduced from the analysis of the optical spectra of some selected (Pr, Nd and Eu) compounds, a systematic calculation of the temperature-dependent Paramagnetic Susceptibility, for all rare-earth configurations on the two series, has been carried out according to the Van Vleck formula. Very good agreements with experimental data are found over the whole temperature range. The standard crystal-field treatment of ions is shown to explain the magnetic properties of these compounds, for which no kind of magnetic interaction among the rare-earth ions has been detected.
Bao D Nguyen - One of the best experts on this subject based on the ideXlab platform.
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the use of chemical shift temperature gradients to establish the Paramagnetic Susceptibility tensor orientation implication for structure determination refinement in Paramagnetic metalloproteins
Journal of Biomolecular NMR, 2000Co-Authors: Zhicheng Xia, Bao D Nguyen, Gerd N La MarAbstract:The use of dipolar shifts as important constraints in refining molecular structure of Paramagnetic metalloproteins by solution NMR is now well established. A crucial initial step in this procedure is the determination of the orientation of the anisotropic Paramagnetic Susceptibility tensor in the molecular frame which is generated interactively with the structure refinement. The use of dipolar shifts as constraints demands knowledge of the diamagnetic shift, which, however, is very often not directly and easily accessible. We demonstrate that temperature gradients of dipolar shifts can serve as alternative constraints for determining the orientation of the magnetic axes, thereby eliminating the need to estimate the diamagnetic shifts. This approach is tested on low-spin, ferric sperm whale cyanometmyoglobin by determining the orientation, anisotropies and anisotropy temperature gradients by the alternate routes of using dipolar shifts and dipolar shift gradients as constraints. The alternate routes ultimately lead to very similar orientation of the magnetic axes, magnetic anisotropies and magnetic anisotropy temperature gradients which, by inference, would lead to an equally valid description of the molecular structure. It is expected that the use of the dipolar shift temperature gradients, rather than the dipolar shifts directly, as constraints will provide an accurate shortcut in a solution structure determination of a Paramagnetic metalloprotein.
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solution nmr determination of the anisotropy and orientation of the Paramagnetic Susceptibility tensor as a function of temperature for metmyoglobin cyanide implications for the population of excited electronic states
Journal of the American Chemical Society, 1999Co-Authors: Bao D Nguyen, K Vyas, And Helen DeagueroAbstract:Comprehensive 1H NMR assignments of the heme cavity proton resonances of sperm whale metmyoglobin cyanide have provided the dipolar shifts for nonligated residues which, together with the crystal coordinates of carbonyl myoglobin, allow accurate determination of both the anisotropies and orientation of the Paramagnetic Susceptibility tensor, χ, in the molecular framework. The resulting axial, Δχax = 2.48 × 10-8 m3/mol, and rhombic anisotropy, Δχrh = −0.58 × 10-8 m3/mol, values at 25 °C determined from the most complete set of dipolar shifts are determined to 2% and 6% uncertainty, respectively, and agree well with theoretical estimates (Horrocks, W. D., Jr. and Greenberg, E. S. Mol. Phys. 1974, 27, 993−999). Numerically and spatially restricted input data sets lead to larger uncertainties in Δχax and Δχrh, but do not systematically bias the orientation of the tensor. Determination of the anisotropies and orientation over the temperature range 5−50 °C shows that the Susceptibility tensor orientation is min...
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solution nmr determination of the anisotropy and orientation of the Paramagnetic Susceptibility tensor as a function of temperature for metmyoglobin cyanide implications for the population of excited electronic states
Journal of the American Chemical Society, 1999Co-Authors: Bao D Nguyen, Zhicheng Xia, K Vyas, And Helen Deaguero, Deok Cheon Yeh, Gerd N La MarAbstract:Comprehensive 1H NMR assignments of the heme cavity proton resonances of sperm whale metmyoglobin cyanide have provided the dipolar shifts for nonligated residues which, together with the crystal c...
R Saezpuche - One of the best experts on this subject based on the ideXlab platform.
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simulation of the Paramagnetic Susceptibility in rare earth oxychlorides
Journal of Alloys and Compounds, 2000Co-Authors: Jorma Holsa, P Porcher, Ralfjohan Lamminmaki, Mika Lastusaari, R SaezpucheAbstract:The average magnetic susceptibilities of polycrystalline rare earth oxychlorides (REOCl; RE=Ce, Pr, Nd, Sm, Tb, Dy and Ho) were measured between 2 and 300 K. The Susceptibility of most REOCl samples follows the Paramagnetic Curie–Weiss behaviour down to low temperatures (<20 K). However, TbOCl, SmOCl, and DyOCl show additional antiferromagnetic ordering at 4, 8, and 11 K, respectively. The experimental Paramagnetic susceptibilities were simulated with the aid of the van Vleck formalism based on the wave functions and energy level values obtained from a previous phenomenological simulations of spectroscopic data. The deviation of the magnetic susceptibilities of several REOCl from the Curie–Weiss behaviour at low temperatures can be explained by the gradual population of the crystal field (c.f.) components of the ground 2S+1LJ level. At high temperatures the difference between the calculated and experimental data is due to the modification of the c.f. effect as a result of the lattice expansion. The strong preferred orientation of the REOCl crystallites may affect the experimental data because of the strong anisotropy of the Paramagnetic Susceptibility.
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Paramagnetic Susceptibility simulations from crystal field effects on rare earth antimonates r3sb5o12
Journal of Alloys and Compounds, 1997Co-Authors: Concepción Cascales, P Porcher, R SaezpucheAbstract:Abstract The magnetic susceptibilities of polycrystalline rare earth antimonates R3Sb5O12 (R=rare earth), with cubic structure, space group I43m (No. 217), where the point site symmetry of R is S4, have been measured from 4.2 to 800 K. Using the wave functions and energy levels derived from standard free ion and crystal field parameters deduced from the analysis of the optical spectra of pure and/or doped Pr, Nd, Eu and Er compounds, the calculation of the temperature dependent Paramagnetic Susceptibility has been carried out according to the Van Vleck formalism. The same calculation through crystal field parameters resulting from the ab initio simple overlap model over the entire R3Sb5O12 series has been performed. Curves of both kinds were very similar for each configuration, with the exception of those for the Pr compound. Very good agreements with experimental data are found, especially below around 400 K, even when the approximate D2d (near S4) potential is considered. The standard crystal field treatment of 4fN configurations is shown to explain the magnetic properties of these compounds with no need to consider any sort of magnetic interaction between the magnetic ions.
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Paramagnetic Susceptibility simulations from crystal field effects on rare earth double molybdate and tungstate
Journal of Physics: Condensed Matter, 1996Co-Authors: Concepción Cascales, P Porcher, R SaezpucheAbstract:Magnetic susceptibilities of polycrystalline samples of two series of double rare-earth molybdate and tungstate compounds, (RE = rare earth, X = Mo or W), with scheelite-type structure, space group (No 88), where RE atoms adopt point symmetry, have been measured in the temperature range 1.7 - 400 K. Using the wavefunctions and energy levels derived from standard free-ion and crystal-field parameters deduced from the analysis of the optical spectra of some selected (Pr, Nd and Eu) compounds, a systematic calculation of the temperature-dependent Paramagnetic Susceptibility, for all rare-earth configurations on the two series, has been carried out according to the Van Vleck formula. Very good agreements with experimental data are found over the whole temperature range. The standard crystal-field treatment of ions is shown to explain the magnetic properties of these compounds, for which no kind of magnetic interaction among the rare-earth ions has been detected.
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analysis of the optical spectra and Paramagnetic Susceptibility of dyof
Journal of Physics: Condensed Matter, 1996Co-Authors: Jorma Holsa, R Saezpuche, Eija Kestila, Pia Ylha, P J Deren, W Strek, P PorcherAbstract:The optical absorption spectra of the rhombohedral DyOF were measured at selected temperatures between 9 and 300 K. The emission spectra of the ion in the LaOF and GdOF matrices were obtained at 77 K and room temperature. The energy level scheme of ( electron configuration) was simulated with a Hamiltonian of 20 parameters. The diagonalization of the energy matrices including simultaneously the free ion and crystal field (c.f.) interactions was carried out for the point symmetry of the site. Good correlation with an rms deviation of was obtained between the experimental and calculated energy level schemes of 153 Kramers doublets. The c.f. parameters show only slight distortion from symmetry. A comparison to , , , , and electron configuration) in other REOF matrices showed smooth evolution of the c.f. effect which was discussed in terms of the possible interactions involved. The x-ray powder diffraction pattern of DyOF between was analysed by the Rietveld profile refinement method. The structural data were used to calculate the c.f. parameters by the modified electrostatic point charge model yielding , and values close to the experimental ones, whereas the , , and values were too large. Using the experimental free ion and c.f. wave functions, the Paramagnetic Susceptibility of DyOF as a function of temperature was simulated. Above the Neel temperature (3.6 K), good agreement was obtained between the calculated and experimental susceptibilities in the Paramagnetic range.
N.o. Gopal - One of the best experts on this subject based on the ideXlab platform.
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mixed alkali effect in li2o na2o b2o3 glasses containing cuo an epr and optical study
Journal of Non-crystalline Solids, 2006Co-Authors: R.p. Sreekanth Chakradhar, B. Yasoda, N.o. Gopal, Lakshmana J RaoAbstract:Electron Paramagnetic Resonance (EPR) and optical absorption spectra of 0.5 mol% CuO doped $xLi_{2}O-(30-x)Na_{2}O-{69.5}B_{2}O_{3}$ (5 \leq x \leq 25) mixed alkali glasses have been investigated. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of $Cu^{2+}$ ions in octahedral sites with tetragonal distortion. It is found that the spin-Hamiltonian parameters do not vary much with x. It is interesting to observe that the number of $Cu^{2+}$ ions participating in resonance (N) and its Paramagnetic Susceptibility (χ) exhibits the mixed alkali effect with composition. It is observed that the temperature dependence of Paramagnetic Susceptibility (χ) obeys Curie–Weiss law. The Paramagnetic Curie temperature $(\theta_p)$ is negative for the investigated sample, which suggests that the copper ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. A broad band corresponding to the transition $(^{2}B_{1g}\longrightarrow^{2}B_{2g})$ in the optical absorption spectrum shows a blue shift with x. By correlating the EPR and optical absorption data, the molecular orbital coefficients $\alpha^{2}$ and $\beta^{2}_{2}$ have been evaluated. It is interesting to observe that the optical band gap $(E_{opt})$ and Urbach energies $(\Delta{E})$ exhibit the mixed alkali effect. The theoretical values of optical basicity $({\Delta}_{th})$ have also been evaluated.
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mixed alkali effect in li2o na2o b2o3 glasses containing fe2o3 an epr and optical absorption study
Materials Research Bulletin, 2006Co-Authors: R.p. Sreekanth Chakradhar, B. Yasoda, Lakshmana J Rao, N.o. GopalAbstract:This paper reports the interesting results on mixed alkali effect (MAE) in $xLi_2O–(30-x)Na_2O-{69.5}B_2O_3$ (5\leq x\leq 28) glasses containing Fe2O3 studied by electron Paramagnetic resonance (EPR) and optical absorption techniques. The EPR spectra in these glasses exhibit three resonance signals at g = 7.60, 4.20 and 2.02. The resonance signal at g = 7.60 has been attributed to $Fe^{3+}$ ions in axial symmetry sites whereas the resonance signal at g = 4.20 is due to isolated $Fe^{3+}$ ions in rhombic symmetry site. The resonance signal at g = 2.02 is due to $Fe^{3+}$ ions coupled by exchange interaction. It is interesting to observe that the number of spins participating in resonance (N) and its Paramagnetic Susceptibility (χ) exhibits the mixed alkali effect with composition. The present study also gives an indication that the size of alkali ions we choose in mixed alkali glasses is also an important contributing factor in showing the mixed alkali effect. It is observed that the variation of N with temperature obeys Boltzmann law. A linear relationship is observed between 1/χ and T in accordance with Curie–Weiss law. The Paramagnetic Curie temperature $(\theta_p)$ is negative for the investigated sample, which suggests that the iron ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. The optical absorption spectra exhibit only one weak band corresponding to the transition $(^{6}A_{1g}(S)\longrightarrow^{4}A_{1g}(G))$; $^4{E}_g{(G)}$ at 446 nm which is a characteristic of $Fe_{3+}$ ions in octahedral symmetry.
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optical absorption epr infrared and raman spectral studies of clinochlore mineral
Journal of Physics and Chemistry of Solids, 2004Co-Authors: N.o. Gopal, K V Narasimhulu, Lakshmana J RaoAbstract:Abstract Optical absorption, EPR, Infrared and Raman spectral studies have been carried out on natural clinochlore mineral. The optical absorption spectrum exhibits bands characteristic of Fe 2+ and Fe 3+ ions. A band observed in the NIR region is attributed to an intervalence charge transfer (Fe 2+ –Fe 3+ ) band. The room temperature EPR spectrum of single crystal of clinochlore mineral reveals the dominance of Fe 3+ ion exhibiting resonance signals at g =2.66; 3.68 and 4.31 besides one isotropic resonance signal at g =2.0. The EPR studies have been carried out for a polycrystalline sample in the temperature range from 103 to 443 K and for a single crystal of clinochlore mineral in the temperature range 123–297 K. The number of spins ( N ) participating in resonance at g =4.3 signal of the single crystal of clinochlore mineral has been calculated at different temperatures. The Paramagnetic Susceptibility ( χ ) is calculated from the EPR data at different temperatures for single crystal of clinochlore mineral. The Curie constant and Curie temperature values are evaluated from 1/ χ versus T graph. The infrared spectral studies reveal the formation of Fe 3+ –OH complexes due to the presence of higher amount of iron in this mineral. The Raman spectrum exhibits bands characteristic of Si–O–Si stretching and Si–O bending modes.
