EPR and optical absorption studies of V O2+ ions in L-asparagine monohydrate

Solid State Communications 142 (2007) 412–416 www.elsevier.com/locate/ssc

EPR and optical absorption studies of VO2+ ions in L-asparagine monohydrate Ram Kripal ∗ , Pragya Singh EPR Laboratory, Department of Physics, University of Allahabad, Allahabad, India Received 28 March 2006; received in revised form 1 February 2007; accepted 6 March 2007 by J. Hsu Available online 12 March 2007

Abstract Electron paramagnetic resonance (EPR) studies of VO2+ ions in L-asparagine monohydrate single crystals are reported at room temperature. It is found that the VO2+ ion takes up an interstitial site. The angular variations of the EPR spectra in three mutually perpendicular planes are used to determine the principal g and A values and their direction cosines. The values of g and A parameters are: gx = 1.9011, g y = 2.1008, gz = 1.9891 and A x = 100, A y = 78, A z = 126 (×10−4 ) cm−1 . The optical absorption spectrum of VO2+ ions in L-asparagine monohydrate is also studied at room temperature. The band positions are calculated using the energy expressions and compared with the observed band positions to confirm the transitions. The best-fit values of the crystal field (Dq) and tetragonal (Ds and Dt) parameters are evaluated from the observed band positions. c 2007 Elsevier Ltd. All rights reserved.

PACS: 33.15.Fm; 42.25.Bs; 61.72.Ss; 71.15.Ap; 71.20.-b; 71.70.Ch; 76.30.-v; 78.20.Ci Keywords: A. Electron paramagnetic resonance; A. Organic crystals; B. Crystal growth; D. Crystal and ligand fields

1. Introduction EPR spectra of impurity-doped single crystals provide valuable information about the structure and the dynamics of the host lattice. It also enables us to identify site symmetry around the metal ion. Optical spectra give knowledge of the energy levels and crystal field surrounding the metal ion. Thus EPR and optical absorption are two powerful supplementary tools for investigating the exact site occupancy and dynamics of the substituted metal ion in the lattice. A number of papers on the EPR of VO2+ in single crystals have been published in recent years [1–4]. The importance of studying coordination complexes of amino acids for understanding the more complex protein and enzyme system has been emphasized by several workers [5–7] in response to the biological significance of such complexes. In the present investigation, the EPR and optical absorption of VO2+ ion doped L-asparagine monohydrate are presented to

∗ Corresponding author. Tel.: +91 532 2641312.

E-mail address: ram [email protected] (R. Kripal). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.03.007

obtain information on the site symmetry of the metal ion and to have better knowledge of the physical interactions present in the system. The study is further used to find the energy-level structure of the metal ion as well as to describe the nature of bonding in the crystal. 2. Crystal structure L-asparagine monohydrate, C4 H8 N2 O3 ·H2 O (LAM) single crystals are orthorhombic [8] and belong to the space group P21 21 21 . There are four molecules in a unit cell. Cell ˚ b = 9.827 A ˚ and c = 11.808 A. ˚ parameters are a = 5.593 A, The molecule is in the zwitterion form and is linked together by seven distinct hydrogen bonds forming a three-dimensional network. 3. Experimental Colourless crystals of VO2+ L-asparagine monohydrate with well-developed faces are obtained by the slow evaporation of an aqueous solution of L-asparagine monohydrate with 0.01 mol % of vanadyl sulphate. The EPR measurements are carried

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R. Kripal, P. Singh / Solid State Communications 142 (2007) 412–416 Table 1 Spin Hamiltonian parameters for vanadyl ions in L-asparagine monohydrate gx

gy

gz

A x (×10−4 ) cm−1

A y (×10−4 ) cm−1

A z (×10−4 ) cm−1

1.9011 ± 0.0002 Powder (gk ) 1.9982 ± 0.0002

2.1008 ± 0.0002

1.9891 ± 0.0002

100 ± 2

78 ± 2

126 ± 2

(A⊥ )80 ± 2

(Ak )126 ± 2

(g⊥ )2.0013 ± 0.0002

Fig. 1. EPR spectrum of VO2+ -doped L-asparagine monohydrate single crystal when the magnetic field is parallel to the ‘a’ axis.

Fig. 2. Powder EPR spectrum of VO2+ -doped L-asparagine monohydrate.

out on a Varian X-band EPR spectrometer (9.5 GHz) at room temperature. Single crystals are mounted at the end of a goniometer device with the help of a quick fix along the crystallographic a, b and c axes, and spectra are recorded at every 10◦ rotation about these axes. The optical absorption spectra are recorded at room temperature on a Unicam5625 UV–visible spectrophotometer in the wavelength range 325–925 nm. 4. Results and discussion The EPR spectrum of VO2+ -ion-doped LAM recorded for a magnetic field parallel to the ‘a’ axis is shown in Fig. 1. The spectrum shows the presence of only one site of VO2+ in the crystal. The powder EPR spectrum shown in Fig. 2 also indicates a single site of a VO2+ ion. In order to obtain the spin Hamiltonian parameters, the crystal was rotated for every 10◦ rotations in the three orthogonal planes. The line positions of all the lines are plotted against the rotation angle in three mutually perpendicular planes, as shown in Figs. 3(a), 3(b) and 3(c). The spin Hamiltonian parameters have been fitted with the following Hamiltonian: H = µ B (gx Bx Sx + g y B y S y + gz Bz Sz ) + A z Iz Sz + A y I y S y + A x I x Sx

Fig. 3(a). Angular variation of EPR line positions of VO2+ -doped L-asparagine monohydrate in the ab plane.

(1)

which includes only electronic Zeeman and hyperfine interaction terms. Additional terms of quadrupole and nuclear Zeeman interactions are neglected as these are sufficiently small. The Schonland [9] procedure has been adopted to obtain spin Hamiltonian parameters. The angular variation of the g 2

Fig. 3(b). Angular variation of EPR line positions of VO2+ -doped L-asparagine monohydrate in the bc plane.

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R. Kripal, P. Singh / Solid State Communications 142 (2007) 412–416 Table 2 Direction cosines of different bonds and distortion axis in L-asparagine monohydrate

Fig. 3(c). Angular variation of EPR line positions of VO2+ -doped L-asparagine monohydrate in the ca plane.

Bond

l

m

n

N(2)–N(1) O(1)–N(2) N(2)–O(2) N(2)–O(3) O(4)–N(2) O(2)–O(1) O(3)–O(1) O(4)–O(1) N(1)–O(2) O(3)–O(2) O(2)–O(4) O(3)–N(1) O(4)–O(3) O(1)–N(1) N(1)–O(4) Distortion axis

±0.6763 ±0.1408 ±0.2437 ±0.0140 ±0.3056 ±0.2841 ±0.2007 ±0.6053 ±0.6247 ±0.3991 ±0.4883 ±0.9526 ±0.2834 ±0.6327 ±0.7208 0.7769

±0.5029 ±0.0478 ±0.4569 ±0.7382 ±0.4521 ±0.9584 ±0.4507 ±0.6475 ±0.0668 ±0.2110 ±0.8250 ±0.1617 ±0.8731 ±0.5393 ±0.6815 0.6200

±0.5381 ±0.9888 ±0.8554 ±0.6744 ±0.8379 ±0.0245 ±0.8697 ±0.4629 ±0.7779 ±0.8922 ±0.2842 ±0.2573 ±0.3965 ±0.5556 ±0.1259 0.1098

have given the following expressions: 4 3 Ak = −Pk − β22 P − (ge − gk )P − (ge − g⊥ ) 7 7 11 2 A⊥ = −Pk − β22 P − (ge − g⊥ )P 7 14

(2) (3)

where P = 2gn βn βhr −3 i is the dipolar term which accounts for the direct dipole–dipole interaction of the electron moment and nuclear moment. The hyperfine coupling constants are related to a direct dipolar term and an indirect dipolar interaction due to anisotropy in the g value. Using the relation Fig. 4. Angular variation of the g 2 values of VO2+ -doped L-asparagine monohydrate in three mutually perpendicular planes.

values in three mutually perpendicular planes is shown in Fig. 4. The spin Hamiltonian parameters obtained using a computer are shown in Table 1 together with data obtained from the powder spectrum [10]. The involvement of ligands in hydrogen bonding results in a decrease in the oxygen to vanadium donation along the z-axis, with an increase in the in-plane donation. This results in a decrease in hyperfine splitting. A reduction in the hyperfine splitting is justified, as the water molecules are involved in very strong hydrogen bonding. To ascertain whether the vanadyl ion takes up the interstitial position in LAM, the direction cosines of different bonds are computed using the X-ray data [8] and compared with the value of the direction cosine of the distortion axis obtained from the EPR study. These data are given in Table 2. The observed site can be correlated with the different metal ligand distances and the V = O orientations are confirmed by comparing the direction cosines. From Table 2 it is concluded that the V = O orientation is along the N(1)–O(4) bond direction. To estimate the covalency, the Fermi contact term k is calculated, which relates the amount of unpaired electron density at the nucleus and the bonding coefficients neglecting the spin–orbit effects of the ligands. Maki and McGarvey [11]

g0 =

1 (gk + 2g⊥ ) 3

(4)

and 1 (Ak + 2A⊥ ) (5) 3 and combining with Eqs. (2) and (3) one gets the following expression [12]: A0 =

A0 = −Pk − (ge − g0 )P.

(6)

In order to compute the value of the Fermi contact term, Ak and A⊥ are taken to be negative and the value of the spin–orbit coupling constant is taken to be 136 G [13]. The value of the Fermi contact term is found to be 0.43. This value is smaller than the values obtained for the VO(H2 O)2+ 5 complex, where no covalency effects are observed. This is in agreement with the values obtained in complexes with appreciable covalency effects. The non-zero value of the Fermi contact term is assumed to arise from spin polarization. With an increase in covalency, the value of k decreases. Therefore in the present case the decrease in the k value may be expected due to the involvement of water molecules in strong hydrogen bonding. 5. Optical spectra The optical absorption spectrum of vanadyl ions in single crystals of LAM is shown in Fig. 5. There are four bands at

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Table 3 Band positions and assignments of transitions of vanadyl ions in L-asparagine monohydrate Observed band positions

Calculated band positions (cm−1 )

Assignment

2 B → 2 E (d → d xy x z,yz ) 2 2 B → 2 B (d → d x 2 −y 2 ) 2 1 xy 2 B → 2 A (d → d ) z2 2 1 xy

(nm)

(cm−1 )

821 675

12 180(22) 14 815(20)

12 180 14 882

487 354

20 534(9) 28 249(12)

20 601

CT band filled bonding levels → dx y {eπb → 2 B2 (b2 )}

Uncertainties are given in brackets.

Fig. 5. Absorption spectrum of VO2+ in L-asparagine monohydrate in the wavelength range 325–925 nm.

cm−1 ,

cm−1 ,

cm−1

cm−1 ,

12 180 14 815 20 534 and 28 249 respectively. Except for the higher energy band at 28 249 cm−1 , all other bands are attributed to the d–d transitions. Using crystal field theory the first three bands at 12 180 cm−1 , 14 815 cm−1 and 20 534 cm−1 are assigned to transitions 2B 2 2 2 2g → E2g (dx y → dx z,yz ), B2g → B1g (dx y → dx 2 −y 2 ) and 2 B2g → 2 A1g (dx y → dz 2 ), respectively. The higher energy band at 28 249 cm−1 is probably the charge transfer band [14] arising due to the promotion of an electron from the filled bonding level eπb to the non-bonding level b2 . The octahedral field parameter (Dq) and tetragonal field parameters (Ds and Dt) are evaluated using the following expressions [14]:

β12

β22

γ2

k

P(×10−4 ) cm−1

(1 − β12 )

(1 − γ 2 )

∆gk /∆g⊥

0.15

1

0.07

0.43

125

0.85

0.93

9.90

β2∗2 = 1 (see Table 4). β1∗2 is taken to be 1 when the b2 orbital is strictly non-(in-plane) bonding, as predicted by Ballhausen and Gray [15] and accepted by others [16,17]. The expressions (1−β1∗2 )2 and (1−γ 2 )2 are the covalency rates [16]: the former gives an indication of the influence of the σ bonding between the vanadium atom and the equatorial ligands, while the latter indicates the influence of π bonding with the vanadyl oxygen. The values of these parameters suggest that both the in-plane σ and out-of-plane π bonding are partly covalent.

B2g → 2 E2g : −3Ds + 5Dt

6. Conclusions

B2g → 2 B1g : 10Dq

EPR and optical absorption studies of VO2+ -dopedLasparagine monohydrate have been made at room temperature. By comparing the direction cosines obtained from singlecrystal EPR data with those calculated from crystal structure data, VO2+ ions are expected to take up the interstitial site in the lattice. The crystal field (Dq) and tetragonal distortion (Ds and Dt) parameters have been evaluated by assigning the crystal field transitions. The bonding parameters have been evaluated to estimate the covalency. The values of bonding parameters suggest that both the in-plane σ and out-of-plane π bonding are partly covalent.

2 2

Table 4 Fermi contact term and molecular orbital coefficients of VO2+ -doped L-asparagine monohydrate

2

B2g → 2 A1g : 10Dq − 4Ds − 5Dt.

(7)

The evaluated values of the parameters are Dq = 1482, Ds = −2557 and Dt = 902 cm−1 . The energy matrices have been solved using a computer to confirm these assignments. The calculated band positions shown in Table 3 are in good agreement with the observed ones. Using the energy differences obtained from the optical absorption spectrum and EPR data, the bonding parameters β1∗2 and γ 2 can be estimated with the help of the following expressions [11]: ge − gk =

4ge λβ1∗2 β2∗2 ∆k

ge − g⊥ =

ge λγ 2 β2∗2 ∆⊥

Acknowledgement The authors are thankful to Dr T.K. Gundu Rao, SAIF, I.I.T. Powai, Mumbai for providing the facility of the EPR spectrometer.

(8)

where λ is the free ion value of the spin–orbit coupling constant and is taken as 170 cm−1 , and ∆k and ∆⊥ correspond to the energy differences 2 B2g → 2 B1g and 2 B2g → 2 E2g , respectively. Taking the values of gk and g⊥ from EPR data and the energy values from Table 3, the bonding parameters have been found to be β1∗2 = 0.15 and γ 2 = 0.07 with

References [1] S.K. Misra, J. Sun, Phys. B 162 (1990) 331. [2] A.K. Vishwanath, S. Radhakrishna, J. Phys. Chem. Solids 52 (1991) 227. [3] B. Deva Prasad Raju, K.V. Narsimhulu, N.O. Gopal, J. Lakshman Rao, J. Phys. Chem. Solids 64 (2003) 1339. [4] R. Tapramaz, B. Karabulut, F. Koksal, J. Phys. Chem. Solids 61 (2000) 1367. [5] R. Osterberg, Coord. Chem. Rev. 12 (1974) 309.

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[6] J.A. Fee, Struct. Bond. 23 (1975) 1. [7] B. Jezoska-Trzebiatowska, Pure Appl. Chem. 38 (1974) 367. [8] J.J. Verbist, M.S. Lehman, T.F. Koetzla, W.C. Hamilton, Acta Crystallogr., B 28 (1972) 3006. [9] D.S. Schonland, Proc. Phys. Soc. 73 (1989) 788. [10] J. Lakshman Rao, R. Murali, S.V.J. Lakshman, Solid State Commun. 67 (1988) 531. [11] A.H. Maki, B.R. McGarvey, J. Chem. Phys. 29 (1958) 35.

[12] S.C. Sathyanarayan, V.G. Krishnan, G.S. Sastry, J. Chem. Phys. 65 (1976) 418. [13] B.R. McGarvey, J. Phys. Chem. 71 (1967) 51. [14] N. Satyanarayana, S. Radhakrishna, J. Chem. Phys. 83 (1985) 529. [15] C.J. Ballhausen, H.B. Gray, Inorg. Chem. 1 (1962) 111. [16] V.P. Seth, V.K. Jain, A. Yadav, R.S. Bansal, Phys. Status Solidi (b) 132 (1985) K139. [17] V.K. Jain, J. Phys. Soc. Jpn. 46 (1979) 1250.