Single crystal EPR and X-ray studies of exchange interactions in Hexaaquocopper (II) picrylsulphonate tetrahydrate

 

K. Vijayaraj¹, A. Jawahar¹* , R. Anantharam² , M. Kumara Dhas³

¹Department of Chemistry, NMSSVN College, Nagamalai, Madurai-625019, Tamilnadu, India

²Department of Chemistry, ANJA College, Sivakasi, Tamilnadu, India

³Department of Physics, Mahendra Arts and Science College, Kalippatti, Namakkal-637501, Tamilnadu, India

*Corresponding Author E-mail: nmdhas@gmail.com

Single crystals of hexaaquocopper(II)picrylsulphonate tetrahydrate, [Cu(H₂O)₆(picsul)₂.4H₂O and the corresponding copper(II) doped zinc and cadmium analogues have been prepared and subjected to EPR studies. X-ray diffraction studies of the pure complex reveal the structure to be triclinic and z=1, with a two dimensional Cu-picsul-Cu-picsul- chain running along b-axis, super exchange interactions are taking place through the chains, linked by H-bonds from uncoordinated water. The line width and line shape analysis indicate the system to be a quasi two dimensional one. A probe into the study of the metal ligand bonding has also been made.

 

 

INTRODUCTION:

The transition metal ion like Cu²⁺ ion as a probe can be used to determine the symmetry environments of the complexes in host lattice by electron paramagnetic technique [1-3]. The study of the exchange interaction between metal centers is a subject of continuous interest [4-6]. Although techniques, such as magnetic susceptibility and specific heat measurements can be used to probe these  interactions, electron paramagnetic resonance (EPR) spectroscopy is unique in allowing one to evaluate complete information on the exchange mechanism, especially in systems, where the magnitudes of the exchange coupling J are similar to those  of the Zeeman energies [7-9]. For example, exchange interaction between copper(II) ions through glutamic acid molecules has been recently investigated by single crystal EPR study [10].

 

Much work, both theoretical and experimental, has been done on exchange coupled systems to understand the factors influencing super exchange mechanism. Nevertheless, the limit on the exchange integral imposed by the distance between the coupled ions is still unclear [11]. The Electron Paramagnetic Resonance studies of Cu²⁺ ion in teraaqua-di(nicotinamide)Ni(II)-Saccharinates single crystals show that the Cu²⁺  ion enters Ni²⁺ sites in the lattice and distorted local environment of Ni²⁺ site [12]. The EPR studies on divalent copper ions embedded in kainite crystals reveals the presence of two magnetically inquivalent Cu²⁺ sites in as orthorhombic symmetry of the copper ions enter the lattice substitutionly at Mg²⁺ sites, also the variation of paramagnetic susceptibitities with temperature clearly indicte that the Cu²⁺ ion in the system are dominat in dynamic Jahn-Teller type distortion [13]. The EPR parameters indicate that Cu²⁺ ions in alkali lead tetraborate glasses have octahedral coordination with a strong tetragonal distortion [14].

 

The single crystal EPR study of Cu²⁺  ion doped in BTCC reveals that the copper exhibit orthorhombic symmetry with slight deviation from axial symmetry and Cu²⁺  ion in the host lattice enhance the optical property of the crystal [15]. The EPR and optical absorption spectra of single crystal shows the presence of two complex groups, the in-plane and out- plane π-bonding is significantly covalent and in-plane σ-bonding is nearly ionic [16-21]. The single crystal XRD and EPR studies confirms the presence of in the lattice [22-27].

 

Systems with hydrogen bonding network are shown to be quite suitable to clarify this point [11]. In search of such a system, we have come across hexaaquocopper(II) picrylsulphonate tetrahydrate, [Cu(H₂O)₆(picsul)₂.4H₂O] (hereafter referred to as CUPS; picsul = picrylsulphonate C₆H₂N₃SO₉). The present work on CUPS demonstrates that hydrogen bonding by even uncoordinated lattice water can provide an efficient pathway for magnetic exchange between Cu(II) ions.

 

In this work the synthesis, X-ray crystal structure and EPR study of electronic properties and magnetic interactions in copper(II) complexes of picrylsulphonic acid, both in pure crystals and  in isomorphously diluted crystals are reported.

 

Additionally, the bonding characteristics of Cu(II)-doped zinc and cadmium complexes of picrylsulphonic acid [Zn(H₂O)₆](picsul)₂.4H₂O and [Cd(H₂O)₆] (picsul)₂. 4H₂O (hereafter referred to as ZNPS and CDPS respectively) are also investigated. EPR results indicate the ground state of Cu(II) to be an

 

MATERIAL AND METHODS:

Preparation of single crystals:

Single crystals of CUPS were prepared by dissolving copper(II) carbonate (90 mg) in a hot aqueous solution (30 ml) containing a slight excess of picrylsulphonic acid (500 mg) in the molar ratio 1:2 and allowing the solution to evaporate slowly at 300 K. Large parallel pipedal crystals, yellowish green in colour, were obtained within a week. A similar procedure was adopted to prepare single crystals of Cu(II)-doped [M(H₂O)₆] (picsul)₂.4H₂O(M = Zn, Cd) using copper and the corresponding metal (Zn or Cd) carbonates in the molar ratio of 1:99. The crystals show a tendency to lose the water molecules on exposure to atmosphere. The crystals were, therefore, stored in sealed thin-walled glass capillaries.

 

X-ray data collection and refinement:

X-ray data for CUPS were collected at 293 K on an Enraf-Nonium CAD-4 four-circle diffractometer with a graphite monochromated MoKα radiation (λ = 0.71037 Å) and an ω-2θ scan technique. Details of crystal data and data collections are listed in Table 1. The data were corrected for Lonrentz and polarization effects as well as for absorption. The structure was solved by direct methods, using SHELXS-86 program [32]. The structure was refined, using SHELXL-93 program [33] with the use of a full-matrix least squares method based on minimization of the function

 

EPR measurements

EPR measurements were performed at X-band with a Varian E-112 spectrometer having 100 kHz field modulation and phase sensitive detection. The 77 K spectra were recorded using a liquid nitrogen dewar. Angular variation of the spectra was measured by rotating the crystal about three mutually orthogonal axes. DPPH was used as an internal field marker.

 

RESULTS AND DISCUSSION:                 

XRD Analysis

The structure of CUPS is triclinic, space group P1, Z = 1 with lattice parameters, a = 8.0668(9) Å, b = 8.1871(6) Å, c = 11.920(2) Å, α = 83.054(9)˚, β=75.504(12)˚ and g=78.224(8)˚. The crystal structure consists of [Cu(H₂O)₆]²⁺ cation and [C₆H₂N₃SO₉]^- anion units. The structure of CUPS showing the labeling scheme is shown in Figure 1. A list of bond distances and angles are given in Table 2. The coordination geometry of the copper(II) center can be described as distorted octahedron. Examination of the copper-oxygen distances (Table 2) reveals that the distortion of the CuO₆ octahedron occurs along Cu-O₃ bond direction which is almost perpendicular to the crystal b-axis. The packing of the molecules in the unit cell reveals the presence of only one copper per unit cell (Figure 2).

 

H-bond distances and angles are detailed in Table 3. The picrylsulphonate group, acting as a counter ion, is bridged to the oxygen atom (O₁) on Cu(II) ion, through the oxygen atoms on sulphonate group (O₄) and nitro group (O₈). This generates a two dimensional –Cu-picsul-Cu-picsul-chain along b-axis. The chains are linked by H-bridges that provide electronic paths for superexchange interactions. Water bridged low dimensional systems of hydrated copper complexes have already been reported [34]. Powder X-ray data reveal that the doped crystals of ZNPS and CDPS are isomorphous with the copper analogue, CUPS.

 

Table 1 Crystal and Data Collection Parameters for [Cu(H₂O)₆](picsul)₂.4H₂O

Empirical formula	C12H20Cu1O26N6S2
Formula weight	792.00
Wave length	0.71037
Temp	293(2) K
Crystal class	Triclinic
Space group	P1
Unit cell dimensions	a = 8.0668(9) Å                                                               b = 8.1871(6) Å                                                                                                          c = 11.920(2) Å                                                                                                      α = 83.054(9)°                                                                                  β = 75.504(12)°                                                                                    γ = 78.224
Volume	744.1(2)A3
Z	1
Density (calculated)	1.794 gcm-3
Absorption coefficient	0.996 gcm-3
R factor-absorbed	0.0471
WR factor-absorbed	0.1499
R factor (all data)	R1 = 0.0475, wR2 = 0.1505
F (000)	399
Theta min.	1.77
Theta max.	25.64
Total reflections	2952
Observed reflections	2917
Index ranges	0<=h<9,-9<=k<9, -14<=l<=13
Refinement method	Full matrix least square on F2
Structure factor coef.	Fsqd
Matrix type	full type
Extinction method	SHELXL
Extinction coef	0.3416(129)
Goodness of fit – all	1.112
Goodness of fit-observed	1.11

 

Fig. 1       ORTEP view of [Cu(H₂O)₆](picsul)₂.4H₂O with atomic numbering scheme.

 

Table  2   Selected Bond Distances (Å) and Bond Angles (deg) of CUPS

 

 

Fig. 2       Unit cell of CUPS

Table 3    Hydrogen Bond Distances (Å) for CUPS

Donor-H	Donor…Accepter	H…Accepter	Donor-H…. Accepter
01  –H201 0.652(.051)	01  ….04 ( 0) 2.967(.003)	H201….04  (0)       2.466(.045)	01     -H201 ….04 ( 0)           135.68 ( 4.99)
03  –H103 0.892(.078)	03 ….07 ( 0) 2.9531(.003)	H103….07  (0)       2.095(.078)	03    -H103 ….07 ( 0)          160.97 ( 7.36)
0w1  –H1W1 0.581 (.048)	0w1  ….04 ( 0) 3.362(.004)	H1W1….04  (0)       2.981(.057)	0w1    -H1W1….04 ( 0)            126.91 ( 5.69)
0w1  –H1W1 0.581 (.048)	0w1  ….05 ( 0) 2.836(.004)	H1W1….05  (0)       2.256(.048)	0w1    -H1W1….05 ( 0)            175.89 ( 6.12)
0w2  –H2W2 0.734 (.060)	0w2  ….03 ( 0) 2.911(.004)	H2W2….03  (0)       2.236(.060)	0w2    -H2W2….03 ( 0)            153.46 ( 5.59)
03 –H203 0.727 (.047)	03  ….05 ( 1) 2.954(.004)	H203….05  (1)       2.242(.049)	03      -H203….05 ( 1)            166.29 ( 4.71)
02 –H102 0.646 (.044)	02  ….06 ( 2) 2.771(.004)	H102….06  (2)       2.128(.044)	02      -H102….06 ( 1) 173.82 ( 5.16)

Exchange Interactions

The single crystal EPR spectra of CUPS at X-band generally showed a single broad line in all the three orthogonal planes. The observation of the broad signal may be rationalized as below.

 

EPR Parameters

The principal values of magnetic tensors could be extracted from powder data, provided the g-values are not too close and there is only one paramagnetic species. EPR spectra of powder samples of the undiluted copper complex, CUPS were recorded at 300 K and 77 K. The spectrum of CUPS shows a broad four-line hyperfine pattern arising from copper (Figure 3) with g₁= 2.4421 and g₂ = g₃ = 2.0895. The EPR spectra of CUPS at 300 K and 77 K were not very different from each other. EPR parameters, derived from the computer simulation of the X-band powder spectra, are listed in Table 4.

 

To study the orientation of the magnetic tensors, single crystal EPR spectra of CUPS were recorded at 300 K. At any orientation, generally the hyperfine splitting from copper was not resolved and only one broad line was noticed. The Schonland diagonalisatian procedure [38] was adopted to determine the principal values of the g-tensor (Table 4). Figure 4 gives a comparison of the experimental and theoretical angular variation, where the agreement is good.

 

 

Fig. 3       X-band powder spectrum of [Cu(H₂O)₆](picsul)₂.4H₂O at 300 K (a) and 77K (b).

 

Fig. 4       Calculated (—) and experimental (■■■■) angular variation of the g-factor for rotation in plane for ……[Cu(H₂O)₆](picsul)₂.4H₂O

 

 

Line Width Data 

Due to different interactions in magnetically concentrated systems, the resonance may be broad, owing to dipolar interaction or narrow, arising from exchange interaction. In general, the line width is narrow for three dimensional systems where exchange effects dominate. When the exchange effect is less, as in the case of low dimensional systems, line width of intermediate nature is also obtained [39]. The dimensionality of a system can be predicated by the angular dependence of the line width. While a three dimensional system follows a ( 1 + cos²θ)  angular dependence, the two and one dimensional systems follow a  ( 3 cos²θ-1)² or ( 3 cos²θ-1)^4/3  pattern [40].

 

The angular variation of the EPR line width at X-band for three different planes of CUPS is shown in Figure 5. An 180˚ periodicity for the line width data is seen in all the three planes. The 180˚ periodicity may be caused by hyperfine, antisymmetric exchange and contributions from either the dipolar or the anisotropic exchange interactions [41].  The angular dependence of the line width of the single exchange collapsed resonance was least-square fitted to the function [42].

                                                                                                                                                                                    (6)

With a = 48.5 mT, b = -10.9 mT and c = - 4.7 mT (correlation coefficient,  r = 0.983). Here θ is the angle between the magnetic field and the normal to the magnetic plane. The first term in the expansion is dominant while the second and third coefficients are smaller. As shown in Figure 6, there is a good agreement between the simulated curve (using eq 6) and the experimental values. The applicability of eq (6) to the line width data is indicative of the quasi two dimensional characteristics of the present system [40].

 

 

 

 

Table 4 Spin Hamiltonian parameters for Cu(II)/ZNPS and CU(II)/CDPS and CUPS (A in units of 10⁴ cm^-1)

CUPS		g1	g2	g3	A1	A2	A3
Powder	300 K	2.442	2.0895	2.0895	121.6	28.3	25.6
	77 K	2.4376	2.0826	2.0826	121.3	27.5	23.0
Single crystal		2.4533	2.0815	2.0556	-	-	-
Cu(II)/ZNPS
Powder	300 K	2.4399	2.0951	2.0713	111.0	13.5	13.3
	77 K	2.4416	2.0899	2.0651	120.83	8.5	12.5
single crystal		2.4470	2.0995	2.0812	110.9	21.9	8.5
Cu(II)/CDPS
Powder	300 K	2.4375	2.091	2.0678	112.7	12.4	26.8
single crystal		2.4470	2.0910	2.081	108.1	22.4	12.3

 

Fig. 5 Linewidth variation of different planes

 

The position of the copper atoms in the magnetic plane can be deduced from crystal structure data. The lattice sum Σ (3cos²θ_ij – 1)² r_ij^-6 for every 10˚ rotation along ab plane, was calculated from the various positions and angles of the atoms relative to the copper centre. An attempt to correlate the theoretically calculated local fields with the experimental line widths yielded no satisfactory fit, revealing that dipolar interaction is not the only factor contributing to second moment.

 

Fig. 7 X-band spectra of powder samples of Cu(II)/[Zn(H₂O)₆](picsul)₂.4H₂O and at a) 300 K and b) 77 K.

 

The angular variation of the second moment is given by

Fig. 8 X – band EPR spectra of Cu(II)/[Zn(H₂O)₆](picsul)₂.4H₂O crystal at 300 K showing anglular dependence in plane

 

Doped Systems

In contrast to the concentrated systems, molecular bonding parameters can be conveniently studied only in diluted systems. Hence, EPR measurements were done on the diluted systems, Cu(II)/ZNPS and Cu(II)/CDPS. Compared to those for the undiluted complex, the spectra for the diluted systems have narrower line widths.

 

EPR spectrum of the powder sample of Cu(II)-doped ZNPS is presented in Figure 7a. A similar spectrum was obtained for the corresponding cadmium analogue. On cooling to 77 K, the spectrum becomes highly anisotropic, indicating an orthorhombic g-tensor (Figure 7b), the magnetic parameters do not vary significantly with temperature and are collected in Table 4.

 

EPR measurements were done on single crystals of Cu(II)/ZNPS and Cu(II)/CDPS . The angular variations of EPR spectra have been studied at 300 K, and the magnetic parameters, derived using the Schonland procedure, are listed in Table 4. Typical spectra of Cu(II)/ZNPS about b-axis are shown in Figure 8. The number of sites that could be seen at any orientation was only one.

 

 

 

The EPR behaviour can be described by the Hamiltonian

                                            (11)

 

Where, μ_B is the Bohr Magneton. B and A are the external Zeeman field and the hyperfine interaction matrix respectively. Angular variation plot indicates clearly the presence of only one site. Angular variation of experimental and calculated g-tensors for the single site can be described with an orthorhombic g-tensor

                 (12)

 

 

The plots of the angular variation of the g-factors in all the three planes at  X-band are shown in Figure 9. The g-factors calculated using eq (12) agree well with the experimental values

 

Fig. 9 Calculated (—) and experimental (○○○○○) angular variation of the g-factors for rotation in plane for Cu(II)/[Zn(H₂O)₆](picsul)₂.4H₂O.

 

Evaluation of admixture Coefficients

As seen from Table 4, copper hyperfine values are very much smaller. Such systems, showing very low hyperfine coupling are relatively rare. The reason for low magnitudes for hyperfine coupling is attributed to the admixture of   and . In the present system also, the observed facts can be rationalized in terms of a formal   ground state with a finite admixture of  state. Such an admixture of   and  states result in one the lowest ever reported values of copper hyperfine tensor (Table 4). 

 

 

To obtain quantitative information, we have used the method of  Salen et a l[43], where the ground state orbital were function is written as a linear combination of the five d-orbitals.

 

Table  5 d-orbit coefficients for Cu(II)/ZNPS and CU(II)/CDPS and some related Cu(II) systems in low symmetry environments

Compound	A1	a	b	c	d	e	Investigation
Cu/ZNPS	105	0.0738	0.9952	0.0561	0.0233	-0.0233	This study
Cu/CDPS	108.1	0.0326	0.9977	0.0555	0.0213	-0.0213	This study
Cu/SAIC	137.1	0.1512	0.9885	0.0411	0.0192	-0.0144	[44]
Cu/Zn(nap)2 (4-mepy)	145.8	0.0665	0.9980	0.0379	0.0203	-0.0158	[45]
Cu/Zn(hfa)2 (2,2-bipy)	162.0	0.0000	1.0000	0.0373	0.0145	-0.0145	[45]
Cu/Zn(hfa)2 (py)2	151.0	0.0093	1.0000	0.0433	0.0226	-0.0186	[45]
Cu/ZnF2	-	0.9752	0.1977	0.0481	0.0614	-0.0614	[46]

 

 

Molecular Orbital Coefficients

EPR parameters can be used to calculate in the molecular orbital coefficients to gain deeper insight into metal-ligand bonding. In general, the covalency is measured in terms of α, the d-orbital coefficient for in-plane bonding. A value of α² = 0.5 indicates the presence of pure covalent bonding and values nearing 1.0 shows complete ionic character in the metal-ligand bonding. The bonding parameters for Cu(II)/ZNPS and Cu(II)/CDPS were estimated using the expressions [47] available in the literature.

(16)

 

The spin Hamiltonian parameters and the coefficients a and b were taken from Table 4 and 5 respectively. The α² values 0.8103 and 0.7704 for Cu(II)/ZNPS and Cu(II)/CDPS respectively suggest a fairly covalent character in them.

 

CONCLUSIONS

X-ray studies of CUPS reveal the presence of only copper per unit cell, which is also confirmed by the EPR spectra. A two dimensional–Cu-picsul-Cu-picsul-chain is running along the crystals perpendicular to the   b-axis and they are linked by hydrogen bridges, through which the superexchange interactions take place. Line width and second moment’s data confirm with those of a quasi two dimensional system. Using the g values of the doped system, the d-orbital coefficients of the Kramers doublet have been determined. These values suggest a good admixture of and orbitals. The molecular orbital coefficient values suggest a fairly covalent character.

 

ACKNOWLEDGEMENT:

The authors K. Vijayaraj, and  A. Jawahar, thank the management of NMSSVN College for encouragement and permission to carry out this work.

 

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

 

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Received on 15.02.2017         Modified on 28.04.2017

Accepted on 10.05.2017         © AJRC All right reserved