epr and optical absorption studies of cu2+-doped bis(l-asparaginato)mg(ii)

EPR and Optical Absorption Studies of Cu2+-DopedBis(L-Asparaginato)Mg(II)

Prashant Dwivedi • Ram Kripal

Received: 30 October 2009 / Published online: 18 May 2010

� Springer 2010

Abstract The electron paramagnetic resonance study of Cu2?-doped bis(L-as-

paraginato)Mg(II) is performed at room temperature. Two magnetically non-

equivalent sites for Cu2? are observed. The spin-Hamiltonian parameters evaluated

by fitting spectra to the crystalline field of rhombic symmetry are as follows:

gx = 2.0420 ± 0.0002, gy = 2.0808 ± 0.0002, gz = 2.3600 ± 0.0002, Ax =

(99 ± 2) 9 10-4 cm-1, Ay = (108 ± 2) 9 10-4 cm-1, Az = (140 ± 2) 9 10-4

cm-1. The ground state wave function of Cu2? is also determined. The g-anisotropy

is estimated and compared with the experimental value. Further, with the help of

optical study the nature of bonding of a metal ion with different ligands in the

complex is discussed.

1 Introduction

Electron paramagnetic resonance (EPR) studies of transition ion-doped single

crystals can lead to a detailed description of the electronic structure of these

compounds [1]. Ideally, EPR spectroscopy enables to identify the oxidation and spin

states of the metal as well as its binding site and symmetry [2]. The parameters

obtainable by EPR are the magnitudes and directions of magnetic parameters such

as the g tensor and the hyperfine structure (hfs) tensor associated with the central

metal and ligand atoms. If a simple molecular orbital model is then assumed for

metal–ligand bonding, it is possible to obtain atomic-orbital mixing coefficients and

hence a measure of metal–ligand bond covalency [3, 4].

P. Dwivedi

Department of Physics, Kali Charan Nigam Institute of Technology, Banda 210001, UP, India

e-mail: [email protected]

R. Kripal (&)

EPR Laboratory, Department of Physics, University of Allahabad, Allahabad 211002, India

e-mail: [email protected]

123

Appl Magn Reson (2010) 38:403–416

DOI 10.1007/s00723-010-0125-0

Applied

Magnetic Resonance

The optical absorption study can provide information about the energy level

ordering, the structure of the complexes and the site symmetry of the metal ion.

Thus, optical absorption study is complementary to EPR technique [5]. Therefore,

site symmetry and the dynamic behavior of the metal ion in the host lattice can be

investigated using EPR and optical absorption studies. Cu2? ions having 3d9

configuration are widely used as paramagnetic probes as they represent a one-hole

magnetic system that can be used to obtain information about the electron wave

function in a ligand field of low symmetry. The Cu2? impurity has been well

characterized in ionic crystals [6, 7].

Knowledge of amino acid sequence and its geometry is of importance in

nutritional and medical technology [8]. The enzyme L-asparaginase has been

identified as the anticancer agent. If the enzyme L-asparaginase is given to humans,

various types of leukaemias can be controlled [http://Drugs-Anti-cancer.htm, 9].

The enzyme asparaginase AG is effective in antitumor activity in mice [10]. We

reported EPR and optical absorption of Cu2?-doped bis(L-asparaginato)Zn(II)

(LAZn) in an earlier study [11]. Bis(L-asparaginato)Mg(II) (LAMg) is used in

treatment of depression, bi-polar disorder, attention deficit disorder and many other

hyperemotional mental conditions [12, 13]. It is used as the Extress formula to

provide safe nutrition support for patients dealing with stress and has been found to

be exceptionally effective as an alternative fibromyalgia medication for stress,

helping patients with the stress, tension, anxiety, minor phobic reactions and gen-

eralized patterns of discomfort [http://www.back-fibromyalgia-pain.com, 14]. Sig-

nificant advances have been made recently in the field of non-linear optics in the

area of materials engineering and optoelectronic device technologies. Organic

materials are quite relevant in this context because the delocalized electronic

structure of p-bonded organic compound provides a number of opportunities in

applications as non-linear optical materials. The molecules consisting of delocalized

p electron systems interacting with suitably substituted electron donor and acceptor

groups show large second-order polarizability [15]. This developed our interest in

Cu2?-doped LAMg. In the present study, we report the results of EPR and optical

absorption of Cu2?-doped LAMg at room temperature. The purpose of this study is

to investigate the ground state of the impurity ion, the symmetry of the crystal field

around the impurity ion and the nature of bonding of an impurity ion with its different

ligands. This is useful in understanding the interactions between amino acids and

non-transition metal ions in biological systems. Moreover, the knowledge of the

nature of bonding in the crystal provides information about its technical application.

2 Crystal Structure

LAMg is isomorphous to LAZn [16]. As the detailed crystal structure data of LAMg

are not known, the structural data of LAZn [16] are used in the present study for the

discussion of results. It is monoclinic with space group P21 and unit cell dimensions

a = 12.323(1), b = 5.027(2), c = 9.702(2) A, b = 99.12(4)0 and Z = 2. The Mg

atom is in a distorted octahedral environment. A carboxylic O atom and the a-amino

N atom from each ligand coordinate to the Mg atom in a trans-square-planar

404 P. Dwivedi, R. Kripal

123

http://Drugs-Anti-cancer.htm

http://www.back-fibromyalgia-pain.com

configuration (Mg–O, 2.09; Mg–N, 2.08 A). The octahedral environment is

completed by carbonyl O atoms from neighboring molecules (Mg–O 2.28,

2.48 A) creating infinite chains linked in the [011] direction.

3 Experimental

The crystal of LAMg, C8H14N4O6Mg, was prepared by adding a stoichiometric

amount of MgCl2 in aqueous solution to a dilute aqueous solution of L-asparagine.

Slow evaporation yielded colorless plate-type single crystals. Cu2? doping was done

by adding 0.1 wt% of cupric chloride to the solution.

EPR spectra were recorded on an X-band Varian E4 spectrometer at room

temperature operating at the microwave frequencies of 9.52 GHz. A Varian flux

meter with proton probe, having 0.2 cm3 of 0.25 M solution of GdCl3 in H2O, was

used for magnetic field measurement along with a Hewlett-Packard frequency

counter. The measurements have been carried out by rotating the crystal about the

three mutually perpendicular axes a*, b and c in steps of 100. The optical absorption

spectra were recorded on a Unicam-5625 spectrophotometer in the wavelength

range of 195–925 nm at room temperature.

4 Results and Discussion

The EPR spectra of Cu2?-doped LAMg consist of eight hyperfine lines as shown in

Fig. 1. As given in the crystal structure, the unit cell contains two molecules per unit

cell and hence two sets of four hyperfine lines are observed. Figure 2a–c shows the

variation of hyperfine lines in the three planes a*b, bc and ca*, respectively. The

unequal separation of lines may be due to second-order effects [17] and magnetic

field modulation amplitude greater than the line width gives an asymmetric line

shape. The shape and positions of lines show that the spectra of the two Cu isotopes

Fig. 1 EPR spectrum of Cu2?-doped LAMg when B is appliedin the ca* plane

EPR and Optical Absorption Studies 405

123

2700

2800

2900

3000

3100

3200

3300

3400

3500

3600

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Angle (degree)

Mag

netic

fiel

d (G

)

Site I

Site II

a* b a*

2700

2800

2900

3000

3100

3200

3300

3400

3500

Angle (degree)

Mag

netic

fiel

d (G

)

Site I

b b C

2700

2800

2900

3000

3100

3200

3300

3400

3500

3600

3700

Angle (degree)

Mag

netic

fiel

d (G

)

Site I

SiteII

c c a*

a

b

c

Fig. 2 Angular variation of the EPR spectra of the Cu2? ion in LAMg crystal for rotation in a a*b, b bc,c ca* planes


123

[63Cu (69.09%), I = 3/2; 65Cu (30.91%), I = 3/2] are not resolved. Also,

sufficiently small nuclear Zeeman and nuclear quadrupole interactions indicate

forbidden transitions to be absent [18]. Figure 2a shows that the system contains

two sites, which become magnetically equivalent along the b and c axes. In Fig. 2b

the hyperfine splitting was slightly larger than that in Fig. 2a. In Fig. 2b the second

site did not appear at any orientation and Fig. 2c shows that system contains two

sites. This is consistent with the monoclinic crystal symmetry where only single

quartet of hyperfine lines should be observed when the crystal is rotated about the

twofold axis [19, 20]. The angular variation plots of g2 (Fig. 3) indicate a rhombic

local electric field symmetry for Cu2? in the host lattice. From the angular variation

of the hyperfine pattern upon rotation about the a*, b and c axes, the principal values

of g and A tensors for Cu2? in LAMg were evaluated by Schonland procedure [19]

with the spin Hamiltonian:

H ¼ lBBgSþ SAI: ð1ÞThe spin-Hamiltonian parameters (Eq. (1)) are given in Table 1. As the spectra of

the two Cu isotopes are not resolved, the obtained hyperfine constants correspond to

average values of these constants for different isotopes. The values of these

parameters of Cu2? obtained here are similar to the results of Refs. [2–4]. The Cu2?

ion can enter the lattice as a substitutional impurity instead of Mg2?. Therefore, the

coordinating oxygens and nitrogens must determine the local electric field

symmetry around the Cu2? ion. A closer examination of the crystal structure of

LAMg [16] shows that the unit cell is monoclinic and accommodates two

molecules. The two L-asparagine ligands each coordinate to the Mg atom via a

carboxylic O and the a-amino N atom forming a trans-square-planar configuration,

with the Mg–O and Mg–N distances being 2.09 and 2.08 A, respectively. A

distorted octahedral coordination about the Mg atom is completed via carbonyl O

3.8

4

4.2

4.4

4.6

4.8

5

5.2

5.4

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270

Angle (degree)

g 2

a* b c a*

Fig. 3 Angular variation of the g2 values of Cu2?-doped LAMg crystal for rotation in a*b, bc and ca*planes


123

atoms from neighboring molecules linked in the [011] direction. The Mg–O

distances completing the octahedron are not equivalent (2.278 and 2.481 A) and are

both significantly longer than the in-plane Mg–O distances.

In LAMg the cation exhibits the sixfold coordination. The coordination

polyhedra are distorted octahedra as shown in Fig. 4a, b. The distorted octahedra

around the Mg atom are equivalent from the point of view of the Mg–O and Mg–N

distances. Thus, they provide ideal sites for the Cu2? ion bestowing rhombic

symmetry for the immediate environment of Cu2?. The calculations, as given in

Table 2, show that the direction cosines of the Mg–O (12I) vector agree, within

experimental error, with those of gy. This confirms that the impurity ion Cu2? has

entered the lattice substitutionally instead of Mg2?. The other direction cosines of

the g tensors do not agree with any other vectors of the octahedron. This is expected

considering the distorted nature of the sixfold coordination around the impurity ion.

In most of the copper complexes, the ligand field may be considered as a strong

axial component with a perturbation that lowers the symmetry. The perturbation in

the complexes can be due to different types of ligands or asymmetrical local

environment. The major effect of the symmetry lowering is the introduction of the

in-plane anisotropy in g and A values [18]. However, the values of Az may not

change due to the above-mentioned perturbation being relatively unimportant in

comparison with the strong axial internal molecular field. The deviation from the

axial symmetry, as can be seen from the spin Hamiltonian parameters, is perhaps

related to one or the other effects discussed above.

Kripal et al. [11] have reported EPR study of Cu2?-doped LAZn with spin-

Hamiltonian parameters gx = 2.0341, gy = 2.0649, gz = 2.2390, Ax = 51 9

10-4 cm-1, Ay = 75 9 10-4 cm-1, Az = 169 9 10-4 cm-1, respectively. Their

results indicate that the Cu(II) impurities replace Zn(II) ions in the host lattice,

which supports our study of Cu2?-doped LAMg at 9.52 GHz frequency. A close

look at the hfs A indicates that Az in Cu2?-doped LAMg is smaller than that of the

corresponding Cu2?-doped Zn(II) host. This low value in Mg(II) host may be due to

slightly larger admixture of |3z2 - r2i with the |x2 - y2i orbital (discussed later)

and metal–ligand covalent character [21, 22] as compared with the corresponding

Zn(II) host. In general, the symmetry and orientation of the g tensor are related to

the symmetry of the effective ligand field. It may safely be assumed that the orbital

half-occupied in the ground state is essentially |x2 - y2i with its four lobes pointing

approximately towards the four ligand atoms. Then a very small admixture of

Table 1 Principal g and A values and their direction cosines of Cu2? in LAMg

Principal gvalues

Direction cosines in the a*,

b, c axes system

Principal A values

(10-4 cm-1)

Direction cosines in the a*,

b, c axes system

a* b c a* b c

2.0420 0.4898 -0.8266 0.2771 99 0.8923 -0.2939 -0.3426

2.0808 -0.6466 -0.1312 0.7515 108 0.2795 -0.2365 0.9306

2.3600 0.5849 0.5472 0.5987 140 0.3545 0.9261 0.1289

Estimated errors for g and A values are ±0.0002 and ±2 9 10-4 cm-1, respectively


123

|3z2 - r2i will tend to increase the difference between the two principal in-plane gvalues and to keep the principal axes of the g tensor directed towards the ligand. If

|xzi is oriented towards the ligands, then one could describe the effective ligand field

as being generally D2h, the principal axes of g would coincide with the bonds, and

the difference (gy - gx) could be quite large [18]. The larger value of (gy - gx) in

the Mg(II) host as compared with the corresponding Zn(II) host is consistent with

Fig. 4 a Molecular environment about the Mg atom and the labelling of the atoms (from the courtesy ofStephens et al. [16]). b Molecular packing in the crystal in the ca* plane. Proposed hydrogen bonds arerepresented by thin lines (from the courtesy of Stephens et al. [16])


123

this interpretation. The larger value of gz in the present study may be due to smaller

value of Exy (given later) and considerable p-as well as r-bonding [23] (discussed

later). The spin Hamiltonian parameters of Cu2?-doped LAMg are slightly different

as compared to those in the corresponding Zn(II) host. This indicates that though

LAMg is isomorphous to LAZn, there may be some difference in its crystal

structure and/or local symmetry as compared with LAZn. The detailed crystal

structure study of LAMg is in progress and the result will be published soon.

Santana et al. [24] reported the EPR study of Cu2?-doped LAZn at 33.9 GHz and

room temperature showing two groups of four peaks arising from the hyperfine

interaction with the nuclear spin of copper. Their results indicate that the Cu(II)

impurities replace Zn(II) ions in the host lattice, which is consistent with our study

at 9.52 GHz.

In the study of Cu2? in LAMg, it is found that in this crystal the electric field

symmetry around the Cu2? ion is rhombic. The ground state wave function in such

crystals has been determined by the formula a|x2 - y2i ? b|3z2 - r2i [25, 26],

where a is very close to unity and b is much less than unity. Using the theory of

Bleaney et al. [27], several authors [28–31] have studied the ground state wave

function of the Cu2? ion in different lattices. A similar procedure has been adopted

in the present study.

The ground state wave functions of Cu2? in LAMg thus obtained is (Eq. (2))

0:878 x2 � y2��

�

þ 0:330 3z2 � r2��

�

: ð2ÞAn attempt has also been made to obtain dg = gy - gx as well as the spin-

exchange polarization/Fermi contact parameter K of the Cu2? ion which represents

the admixture of configurations with s-electrons caused by the spin-exchange

polarization. The difference between gy and gx (dgexp) has been compared with the

calculated value dgcal. dg is caused by the following factors:

(i) By mixing of the |x2 - y2i orbital with the |3z2 - r2i orbital. dg1cal is given by

dg1cal ¼ 2

p3b Dgx þ Dgy

� �

: ð3Þ

(ii) By energy splitting of the |xzi and |yzi states. dg2cal is given by

dg2cal ¼ ð1=2Þp3b Dgx þ Dgy

� �

: ð4Þ

Table 2 Distance and direction cosines of the Mg–O and Mg–N vectors LAMg

Bond Distance (A) Direction cosines in the a*, b, c axes system

a* b c

Mg–O(11) 2.102 -0.5018 -0.5583 0.6605

Mg–O(12I) 2.278 -0.6069 -0.3331 0.7215

Mg–O(21) 2.086 -0.1624 0.5862 -0.7937

Mg–O(22II) 2.481 0.1485 0.3016 -0.9417

Mg–N(11) 2.071 -0.9121 0.3727 0.1703

Mg–N(21) 2.092 0.5421 -0.7179 -0.4367


123

(iii) By different covalency of the |xzi and |yzi states. dg3calis given by

dg3cal ¼ 1=2ð Þp3b Dgx þ Dgy

� �

E1=E2ð Þ ð5Þ

where E1 is the energy difference of the |x2 - y2i and |3z2 - r2i states, E2 = Edt

- Epp, Edt is the energy level of a single dt electron on the Cu2? ion and Epp is the

energy level of a single pp electron on the attached ligand ion.

(iv) By mixing of the |yzi orbital with the |xyi orbital of the first excited state. dg4cal

has the same effect as in (iii).

One can take the contribution (i) (Eq. (3)) first and if dg1cal 6¼ dgexp; contributions

(ii) (Eq. (4)) and (iii) (Eq. (5)) must be considered. In the present crystal dgcal1 =

-0.135, dgexp = 0.038, indicating poor agreement between them. One can find a

good agreement between dgcalc and dgexp by adding the contributions (ii) (Eq. (4))

and (iii) (Eq. (5)) above [27]. However, the actual calculation of (iii) could not be

done as the energy E2 was not known but with the help of expression for (iii) (Eq.

(5)) one can predict approximate value of dgcal. The parameter P represents [18, 32]

the dipole–dipole interaction of the electronic and nuclear moments

(P = gegNbbNhr-3i). The measured value of P, Pexp, can be obtained by

multiplying Pfi (fi = free ion, Pfi = 0.036 cm-1 for copper) by the probability of

the electrons actually being in the 2D state around the Cu2? ion, i.e.,

Pexp ¼ a2Pfi: ð6ÞThe parameters a, b, K, Pexp (Eq. (6)) and dgcalc, dgexp for Cu2? in LAMg

together with other host lattices are given in Table 3. The estimated value of Pexp in

the present system is 0.027 cm-1 that is considerably reduced from the free ion

value of 0.036 cm-1, which indicates a significant covalent bonding in the complex

[5]. The value of Pexp in the present host is slightly smaller than that of the

corresponding Zn(II) host (Pexp = 0.031 cm-1). This indicates a slightly larger

amount of the covalent bonding in the Mg(II) host.

5 Optical Spectrum

The optical absorption spectrum of Cu2? in LAMg crystal, in the wavelength range

of 195–925 nm at room temperature is exhibited in Fig. 5a, b. There are four bands

Table 3 Parameters a, b, K, Pexp and dgcalc, dgexp for Cu2? in LAMg and other host lattices

Host a b K Pexp (cm-1) dg1cal dg2

cal dgcal ¼ dg1cal þ dg2

cal dgexp

LAMga 0.878 0.330 0.353 0.027 -0.135 -0.033 -0.168 0.038

LAZnb 0.931 0.018 0.168 0.031 -0.006 -0.001 -0.007 0.031

TCDNc 0.769 0.140 0.469 0.021 -0.061 -0.015 -0.076 0.032

NMZnd 0.868 0.094 0.350 0.027 -0.045 -0.011 -0.056 0.034

LAMg (L-asparaginato)Mg(II), LAZn (L-asparaginato)Zn(II), TCDN tris(glycine)calcium(II)dinitrate,

NMZn bis(x-nitroacetophenonato)bis(4-methylpyridine)Zn(II)a Present study, bRef. [10], cRef. [19], dRef. [19]


123

in the visible range, occurring at m1 = 12,247 cm-1, m2 = 14,833 cm-1, m3 =

20,292 cm-1 and m4 = 27,602 cm-1 and four bands in the ultraviolet (UV) range,

which are weak in intensity occurring at about m5 = 32,768 cm-1, m6 =

35,136 cm-1, m7 = 37,149 cm-1 and m8 = 44,422 cm-1. Considering the first-

order perturbation, splitting into five energy levels of the Cu2? ion is caused by the

tetragonally distorted octahedral field together with the spin–orbit coupling [33]. As

the spin–orbit coupling of the Cu2? ion is rather large, a significant effect of the

spin–orbit coupling on the absorption spectrum of the Cu2? complex is expected.

An analysis of the optical spectrum is done using the above approximation. From

the nature of the absorption spectrum in the visible region, the observed bands at m2

and m3 can be regarded, respectively, as the d–d transfer bands between the ground

state dx2�y2 and the excited states dxz and dyz, into which the twofold degenerate

level dxz,yz is split by the crystal field and spin–orbit coupling. Thus, the band at

Fig. 5 Optical absorption spectra of Cu2?-doped LAMg in a wavelength range of 325–925 nm (a) and195–325 nm (b)


123

17,562 cm-1, which is equal to the average value of the main band m3 and m2, can be

assigned as the d–d transfer band dxz;yz $ dx2�y2 , being usually the most intense

band [34]. The other two bands observed at m1 and m4 are assigned as dxy $ dx2�y2

and d3z2�r2 $ dx2�y2 transfer bands, respectively.

These optical data can be used to calculate the crystal field and tetragonal

parameters Dq, Ds and Dt of the Cu2? complexes from the relation given by

Ballhausen and Gray [35]. The calculated parameters for Cu2? in LAMg are:

Dq = -1,224 cm-1, Ds = -4,702 cm-1 and Dt = -1,758 cm-1. The values of

Dq, Ds and Dt obtained in the present case are quite comparable to the values for

Cu2? in LAZn [11] and tris(glycine)calcium(II)dinitrate (TCDN) [21, 22], where

the values of Dq, Ds and Dt are -1,472, -3,349, -1,478, -1,500, -3,364,

-1,488 cm-1, respectively. These values suggest that the crystal field of the Cu2?

complexes under consideration is rather highly distorted tetragonal. Both the

experimental energies and calculated ones using the above values of Dq, Ds and Dt

[36, 37] of the d–d transfer bands are given in Table 4.

The four observed absorption bands in the UV range, occurring at

m5 = 32,768 cm-1, m6 = 35,136 cm-1, m7 = 37,149 cm-1 and m8 = 44,422 cm-1

are, probably, charge-transfer transition bands, because they arise from the higher-

lying energy levels. The present results can be compared with those for the CuCl42-

complex [38] for which there have been observed four charge-transfer transitions in

the UV range, they have been assigned as 1a2g $ 3b1g, 4eu $ 3b1g, 3eu $ 3b1g

and 2a1g $ 3b1g transitions, in the order of decreasing wavelengths. Using these

results for CuCl42-, the four transitions 1a2g $ 3b1g, 4eu $ 3b1g, 3eu $ 3b1g and

2a1g $ 3b1g correspond to the observed frequencies m5 = 32,768 cm-1,

m6 = 35,136 cm-1, m7 = 37,149 cm-1 and m8 = 44,422 cm-1.

6 Molecular Orbital (MO) Analysis of the EPR Data

The spin-Hamiltonian parameters indicate the dx2�y2 ground state of the Cu2? ions.

The non-axial symmetry of the g and A tensors suggest a rhombic crystal field

symmetry. Assuming overlap integrals to be zero and omitting the small terms in the

general expressions for spin-Hamiltonian parameters [21, 22], we have

gz ¼ 2:0023� 8a20b

21k0

Exyð7Þ

Table 4 Observed and

calculated energies, and

assignments of the bands for

Cu2? in LAMg

Transition Band positions (cm-1)

Calculated Observed

dxy $ dx2�y2 12,177 m1 = 12,247

dxz;yz $ dx2�y2 17,217 m2 = 14,833

17,977 m3 = 20,292

d3z2�r2 $ dx2�y2 27,602 m4 = 27,602


123

gy ¼ 2:0023� 2a20b

20k0

Exzð8Þ

gx ¼ 2:0023� 2a20b02k0

Eyzð9Þ

Az

P0

¼ �K � 4

7

� �

a20 þ Dgz þ

3

14

� �

Dgy þ3

14

� �

Dgx ð10Þ

Ay

P0

¼ �K þ 2

7

� �

a20 þ Dgy �

3

14

� �

Dgx ð11Þ

Ax

P0

¼ �K þ 2

7

� �

a20 þ Dgx �

3

14

� �

Dgy ð12Þ

where k0 = -829 cm-1, P0 = 0.036 cm-1, dgi = gi - 2.0023 [21, 22]. The a20 and

Fermi contact parameter K can be calculated from equations

a20 ¼

7

12Ax þ Ay � 2Az

� �

=P0 þ 2Dgz �5

14

� �

Dgx þ Dgy

� ��

ð13Þ

K ¼ 1

a20

�Az

P0

� 4

7

� �

a20 þ Dgz þ

3

14

� �

Dgx þ Dgy

� ��

ð14Þ

Using different permutations of the signs of Ax, Ay and Az, we have found that

only Ax [ 0, Ay [ 0 and Az \ 0 give the acceptable value of a20. Taking observed

values of Eyz, Exz and Exy, the estimated MO coefficients (Eqs. (7)–(13)) are given in

Table 5. MO coefficients in some other similar lattices are also given in Table 5 for

comparison.

The value of a20 indicates that the r-bonding is partly covalent in nature. b2

1 and

b20 indicate that in-plane and out-of-plane p-bondings are weaker. b2

0 ¼ 1 and

b02 = 0.76 indicate that there is a ligand atom located close to apical positions of

the Cu(II) complex.

The a20 value of 0.86 in the present case is slightly smaller than the value in the

corresponding Zn(II) host (a20 ¼ 0:91 [11]). This indicates a slightly larger covalent

r-bonding in the present host as compared to the corresponding Zn(II) host. The

values of b21and b2

0 in the Mg(II) host are 1.0 and 0.76 while these are 0.57 and 0.71

in the corresponding Zn(II) host [11], which indicates slightly weaker p-bonding in

the present host [18].

Table 5 MO coefficients for Cu2? in LAMg and other host lattices

Host a20 b2

1 b20 b

02 K

LAMg 0.86 0.7 1.0 0.76 0.32

LAZn 0.91 0.57 0.71 0.36 0.15

TCDN 0.73 0.80 1.0 0.67 0.26

NMZn 0.71 0.98 0.91 0.91 0.49


123

The parameter K is a measure of polarization produced by uneven distribution

of d-electron density on the inner core s-electrons and it has been suggested [39,

40] that for 3d transition metal ions, K & 0.3. The value of K obtained (Eq. (14))

here is consistent with this. The small increase in the value of K in the present

case as compared to Cu2?-doped LAZn may be due to larger contribution of the

empty 4s orbital on the metal in the r-bonding to the ligands. For low symmetry,

both the d levels (dx2�y2 and d3z2�r2 ) mix with the 4s orbital. This would, of

course, introduce 4s character into the r-bonding system of the complex for both

the axial and in-plane r ligand-to-metal bonds. The 4s contribution to K should be

proportional to the metal electron density in the filled orbital containing the

contribution from the 4s orbital. The energy separation of the bonding and anti-

bonding orbitals for the 4s ligand interaction is inversely proportional to the

indirect 4s contribution to K [18]. This energy separation should be a function of

the in-plane ligand field. As the in-plane ligand field increases, the 4s bonding

contribution to the isotropic contact term decreases. A direct contribution to K can

arise from dx2�y2 , 4s mixing in low-symmetry complexes. This is of opposite sign

to the indirect 4s contribution. The relatively small values of hfs constant for low-

symmetry complexes of oxygen and nitrogen donors perhaps arise from this effect

[21, 22].

Moderate p-bonding in Cu2?: LAMg indicates that it can be used as a non-linear

optical material [15]. The absorption band at 492 nm suggests that there can be

emission of green radiation using appropriate laser and thus the second harmonic

generation [41]. This also supports the above statement regarding the use of the

present crystal as a non-linear optical material. Further studies to confirm the non-

linear optical properties of the crystal are in progress and will be published soon.

7 Conclusion

The EPR study of bis(L-asparaginato)Mg(II):Cu2? has been done at room

temperature. Various spin-Hamiltonian parameters have been determined. Two

magnetically non-equivalent sites for Cu2? have been observed. The ground state

wave function for Cu2? in the lattice has been constructed. The g-anisotropy, Fermi

contact and molecular orbital parameters have been determined. Optical spectra

have been explained taking into account the crystal field and spin–orbit coupling.

The MO coefficients indicate that the r-bonding is partly covalent, whereas in-plane

and out-of-plane p-bondings are moderate. The comparison with the results of

Cu2?-doped bis(L-asparaginato)Zn(II) indicates a slightly larger covalent r-bonding

and slightly weaker p-bonding in the present host. Moderate p-bonding and

absorption band at 492 nm in bis(L-asparaginato)Mg(II):Cu2? suggest that it can be

used as a non-linear optical material.

Acknowledgments We are grateful to Dr. T.K. Gundu Rao (Sophisticated Analytical Instruments

Facility, Indian Institute of Technology, Powai, Mumbai) for providing the facility of EPR spectrometer.

P. D. is grateful to the Head of Physics Department, University of Allahabad, Allahabad, for providing

departmental facilities.


123

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epr and optical absorption studies of cu2+-doped bis(l-asparaginato)mg(ii)

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