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ISSN: 2056-9890

4,4′-[2,2-Di­methyl­propane-1,3-diyl­bis­(nitrilo­methyl­­idyne)]dibenzo­nitrile

aX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia, and bDepartment of Chemistry, School of Science, Payame Noor University (PNU), Ardakan, Yazd, Iran
*Correspondence e-mail: hkfun@usm.my

(Received 16 June 2008; accepted 17 June 2008; online 19 June 2008)

The title compound, C21H20N4, is a bidentate Schiff base ligand. An intra­molecular C—H⋯N hydrogen bond forms a five-membered ring, producing an S(5) ring motif. The cyano and imino [–C(H2)—N=C–] functional groups are coplanar with the benzene ring in each half of the mol­ecule. The packing of the mol­ecules is controlled by C—H⋯π and ππ inter­actions [centroid-to-centroid distance = 3.6944 (8) Å].

Related literature

For related literature on hydrogen-bond motifs, see Bernstein et al. (1995[Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555-1573.]). For values of bond lengths, see Allen et al. (1987[Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1-19.]). For related structures, see, for example: Li et al. (2005[Li, Y.-G., Zhu, H.-L., Chen, X.-Z. & Song, Y. (2005). Acta Cryst. E61, o4156-o4157.]); Bomfim et al. (2005[Bomfim, J. A. S., Wardell, J. L., Low, J. N., Skakle, J. M. S. & Glidewell, C. (2005). Acta Cryst. C61, o53-o56.]); Glidewell et al. (2005[Glidewell, C., Low, J. N., Skakle, J. M. S. & Wardell, J. L. (2005). Acta Cryst. E61, o3551-o3553.], 2006[Glidewell, C., Low, J. N., Skakle, J. M. S. & Wardell, J. L. (2006). Acta Cryst. C62, o1-o4.]); Sun et al. (2004[Sun, Y.-X., You, Z.-L. & Zhu, H.-L. (2004). Acta Cryst. E60, o1707-o1708.]); Habibi et al. (2007[Habibi, M. H., Mokhtari, R., Harrington, R. W. & Clegg, W. (2007). Acta Cryst. E63, o2881.]); Fun et al. (2008[Fun, H.-K., Kia, R. & Kargar, H. (2008). Acta Cryst. E64. Submitted]).

[Scheme 1]

Experimental

Crystal data
  • C21H20N4

  • Mr = 328.41

  • Monoclinic, P 21 /c

  • a = 6.3833 (2) Å

  • b = 34.0679 (8) Å

  • c = 8.5190 (2) Å

  • β = 100.654 (2)°

  • V = 1820.65 (8) Å3

  • Z = 4

  • Mo Kα radiation

  • μ = 0.07 mm−1

  • T = 100.0 (1) K

  • 0.42 × 0.20 × 0.19 mm

Data collection
  • Bruker SMART APEXII CCD area-detector diffractometer

  • Absorption correction: multi-scan (SADABS; Bruker 2005[Bruker (2005). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.970, Tmax = 0.986

  • 21813 measured reflections

  • 5329 independent reflections

  • 3711 reflections with I > 2σ(I)

  • Rint = 0.046

Refinement
  • R[F2 > 2σ(F2)] = 0.056

  • wR(F2) = 0.139

  • S = 1.04

  • 5329 reflections

  • 228 parameters

  • H-atom parameters constrained

  • Δρmax = 0.24 e Å−3

  • Δρmin = −0.20 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

Cg1 is the centroid of the C12–C17 benzene ring.

D—H⋯A D—H H⋯A DA D—H⋯A
C20—H20B⋯N2 0.96 2.57 2.923 (2) 102
C21—H21CCg1i 0.96 2.95 3.7193 (16) 138
Symmetry code: (i) x, y, z-1.

Data collection: APEX2 (Bruker, 2005[Bruker (2005). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: APEX2; data reduction: SAINT (Bruker, 2005[Bruker (2005). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); program(s) used to solve structure: SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2003[Spek, A. L. (2003). J. Appl. Cryst. 36, 7-13.]).

Supporting information


Comment top

Schiff bases are one of most prevalent mixed-donor ligands in the field of coordination chemistry. They play an important role in the development of coordination chemistry related to catalysis and enzymatic reactions, magnetism, and supramolecular architectures. Structures of Schiff bases derived from substituted benzaldehydes and closely related to the title compound have been reported (Li et al., 2005; Bomfim et al., 2005; Glidewell et al., 2005, 2006; Sun et al., 2004; Habibi et al., 2007; Fun et al., 2008).

An intramolecular C—H···N hydrogen bond forms a five-membered ring, producing an S(5) ring motif (Bernstein et al., 1995). The bond lengths and angles in the molecule (Fig. 1) are within normal ranges (Allen et al., 1987). The cyano and imino (—C(H2)—NC—) functional groups are coplanar with the benzene ring in each half of the molcule. The torsion angles of C6—C7—N1—C8 and C12—C11—N2—C10 are -178.95 (13) and -179.43 (12)°, respectively. The packing of the molecule, (Fig. 2), is controlled by C—H···π and ππ interactions [centroid-centroid distance being 3.6944 (8) Å] (Table 1).

Related literature top

For related literature on hydrogen-bond motifs, see Bernstein et al. (1995). For values of bond lengths, see Allen et al. (1987). For related structures, see, for example: Li et al. (2005); Bomfim et al. (2005); Glidewell et al. (2005, 2006); Sun et al., (2004); Habibi et al. (2007); Fun et al., (2008).

Experimental top

The synthetic method has been described earlier (Fun et al., 2008). Single crystals suitable for X-ray diffraction were obtained by evaporation of an ethanol solution at room temperature.

Refinement top

H atoms were positioned geometrically and refined using a riding model with C—H = 0.93 Å for aromatic H and 0.97 Å for methylene and 0.96 Å for methyl H atoms. The Uiso values were constrained to be 1.5Ueq of the carrier atom for the methyl H atoms and 1.2Ueq for the remaining H atoms. A rotating-group model was used for the methyl groups.

Computing details top

Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2 (Bruker, 2005); data reduction: SAINT (Bruker, 2005); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2003).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound with atom labels and 50% probability ellipsoids for non-H atoms. Intramolecular hydrogen bond is shown as a dashed line.
[Figure 2] Fig. 2. The crystal packing, viewed down the a axis, showing stacking of the benzene rings. Intramolecular H bonds are drawn as dashed lines.
4,4'-[2,2-Dimethylpropane-1,3-diylbis(nitrilomethylidyne)]dibenzonitrile top
Crystal data top
C21H20N4F(000) = 696
Mr = 328.41Dx = 1.198 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybcCell parameters from 3870 reflections
a = 6.3833 (2) Åθ = 2.4–29.3°
b = 34.0679 (8) ŵ = 0.07 mm1
c = 8.5190 (2) ÅT = 100 K
β = 100.654 (2)°Block, colourless
V = 1820.65 (8) Å30.42 × 0.20 × 0.19 mm
Z = 4
Data collection top
Bruker SMART APEXII CCD area-detector
diffractometer
5329 independent reflections
Radiation source: fine-focus sealed tube3711 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.046
ϕ and ω scansθmax = 30.2°, θmin = 1.2°
Absorption correction: multi-scan
(SADABS; Bruker 2005)
h = 97
Tmin = 0.970, Tmax = 0.986k = 4848
21813 measured reflectionsl = 1212
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.056Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.139H-atom parameters constrained
S = 1.04 w = 1/[σ2(Fo2) + (0.0568P)2 + 0.5007P]
where P = (Fo2 + 2Fc2)/3
5329 reflections(Δ/σ)max < 0.001
228 parametersΔρmax = 0.24 e Å3
0 restraintsΔρmin = 0.20 e Å3
Crystal data top
C21H20N4V = 1820.65 (8) Å3
Mr = 328.41Z = 4
Monoclinic, P21/cMo Kα radiation
a = 6.3833 (2) ŵ = 0.07 mm1
b = 34.0679 (8) ÅT = 100 K
c = 8.5190 (2) Å0.42 × 0.20 × 0.19 mm
β = 100.654 (2)°
Data collection top
Bruker SMART APEXII CCD area-detector
diffractometer
5329 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker 2005)
3711 reflections with I > 2σ(I)
Tmin = 0.970, Tmax = 0.986Rint = 0.046
21813 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0560 restraints
wR(F2) = 0.139H-atom parameters constrained
S = 1.04Δρmax = 0.24 e Å3
5329 reflectionsΔρmin = 0.20 e Å3
228 parameters
Special details top

Experimental. The low-temperature data was collected with the Oxford Cyrosystem Cobra low-temperature attachment.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
N10.8180 (2)0.14310 (4)1.22220 (14)0.0240 (3)
N20.7297 (2)0.07939 (4)0.76666 (14)0.0243 (3)
N30.1075 (3)0.23585 (4)1.75196 (17)0.0346 (4)
N40.2208 (2)0.01242 (4)0.06341 (15)0.0278 (3)
C10.4959 (3)0.16261 (4)1.41380 (17)0.0234 (3)
H1A0.48440.13761.36890.028*
C20.3530 (3)0.17457 (4)1.50763 (17)0.0243 (3)
H2A0.24620.15761.52670.029*
C30.3698 (3)0.21224 (4)1.57374 (16)0.0226 (3)
C40.5288 (3)0.23767 (4)1.54525 (17)0.0254 (3)
H4A0.53860.26281.58860.031*
C50.6719 (3)0.22545 (4)1.45229 (17)0.0237 (3)
H5A0.77890.24241.43350.028*
C60.6573 (3)0.18785 (4)1.38618 (16)0.0211 (3)
C70.8171 (3)0.17624 (4)1.29000 (16)0.0224 (3)
H7A0.92300.19421.27850.027*
C80.9911 (3)0.13680 (4)1.13468 (17)0.0245 (3)
H8A1.07610.11471.18160.029*
H8B1.08230.15981.14690.029*
C90.9149 (3)0.12886 (4)0.95573 (16)0.0215 (3)
C100.7881 (3)0.09017 (4)0.93463 (16)0.0232 (3)
H10A0.87360.06940.99230.028*
H10B0.66000.09300.97960.028*
C110.5405 (3)0.06825 (4)0.71580 (16)0.0226 (3)
H11A0.44450.06750.78570.027*
C120.4686 (3)0.05644 (4)0.54744 (16)0.0212 (3)
C130.6067 (3)0.05915 (4)0.43861 (17)0.0231 (3)
H13A0.74420.06870.47190.028*
C140.5402 (3)0.04776 (4)0.28203 (17)0.0233 (3)
H14A0.63220.04970.20960.028*
C150.3345 (3)0.03329 (4)0.23295 (16)0.0212 (3)
C160.1937 (3)0.03068 (4)0.33940 (17)0.0256 (3)
H16A0.05610.02120.30580.031*
C170.2624 (3)0.04247 (4)0.49654 (17)0.0255 (3)
H17A0.16940.04100.56850.031*
C180.2220 (3)0.22512 (4)1.67256 (18)0.0270 (3)
C190.2687 (3)0.02115 (4)0.06860 (17)0.0227 (3)
C201.1140 (3)0.12547 (5)0.88075 (19)0.0287 (4)
H20A1.18950.15000.89200.043*
H20B1.07290.11920.76950.043*
H20C1.20460.10510.93370.043*
C210.7745 (3)0.16266 (4)0.87814 (18)0.0274 (4)
H21A0.85070.18700.89860.041*
H21B0.64660.16380.92220.041*
H21C0.73810.15840.76500.041*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
N10.0265 (8)0.0254 (6)0.0199 (6)0.0005 (5)0.0037 (5)0.0025 (5)
N20.0282 (8)0.0233 (6)0.0222 (6)0.0009 (5)0.0070 (5)0.0030 (5)
N30.0335 (10)0.0336 (8)0.0379 (8)0.0006 (6)0.0099 (7)0.0063 (6)
N40.0268 (8)0.0305 (7)0.0270 (6)0.0013 (6)0.0071 (6)0.0028 (5)
C10.0282 (9)0.0186 (7)0.0219 (7)0.0002 (6)0.0011 (6)0.0015 (5)
C20.0257 (9)0.0222 (7)0.0244 (7)0.0010 (6)0.0028 (6)0.0011 (5)
C30.0230 (9)0.0247 (7)0.0193 (6)0.0030 (6)0.0015 (6)0.0003 (5)
C40.0319 (10)0.0203 (7)0.0234 (7)0.0002 (6)0.0034 (6)0.0040 (5)
C50.0275 (9)0.0212 (7)0.0222 (7)0.0033 (6)0.0037 (6)0.0006 (5)
C60.0236 (9)0.0214 (7)0.0168 (6)0.0013 (6)0.0002 (6)0.0006 (5)
C70.0232 (9)0.0225 (7)0.0206 (6)0.0012 (6)0.0019 (6)0.0007 (5)
C80.0250 (9)0.0235 (7)0.0244 (7)0.0015 (6)0.0029 (6)0.0025 (5)
C90.0232 (9)0.0200 (7)0.0220 (6)0.0002 (6)0.0059 (6)0.0009 (5)
C100.0275 (9)0.0234 (7)0.0191 (6)0.0005 (6)0.0056 (6)0.0007 (5)
C110.0286 (10)0.0193 (7)0.0218 (7)0.0010 (6)0.0095 (6)0.0000 (5)
C120.0277 (9)0.0152 (6)0.0214 (6)0.0012 (6)0.0063 (6)0.0000 (5)
C130.0234 (9)0.0226 (7)0.0240 (7)0.0016 (6)0.0058 (6)0.0023 (5)
C140.0269 (9)0.0225 (7)0.0226 (7)0.0003 (6)0.0101 (6)0.0013 (5)
C150.0266 (9)0.0161 (6)0.0211 (6)0.0020 (6)0.0051 (6)0.0004 (5)
C160.0256 (9)0.0258 (7)0.0257 (7)0.0031 (6)0.0054 (6)0.0004 (6)
C170.0274 (10)0.0289 (8)0.0227 (7)0.0025 (7)0.0107 (6)0.0002 (6)
C180.0300 (10)0.0236 (7)0.0268 (7)0.0002 (7)0.0037 (7)0.0020 (6)
C190.0240 (9)0.0199 (7)0.0253 (7)0.0007 (6)0.0075 (6)0.0000 (5)
C200.0284 (10)0.0290 (8)0.0309 (8)0.0013 (7)0.0112 (7)0.0014 (6)
C210.0306 (10)0.0242 (7)0.0274 (7)0.0020 (7)0.0053 (7)0.0009 (6)
Geometric parameters (Å, º) top
N1—C71.2687 (18)C9—C211.532 (2)
N1—C81.459 (2)C9—C101.539 (2)
N2—C111.264 (2)C10—H10A0.9700
N2—C101.4576 (17)C10—H10B0.9700
N3—C181.143 (2)C11—C121.4786 (19)
N4—C191.1486 (19)C11—H11A0.9300
C1—C21.380 (2)C12—C171.391 (2)
C1—C61.395 (2)C12—C131.396 (2)
C1—H1A0.9300C13—C141.379 (2)
C2—C31.398 (2)C13—H13A0.9300
C2—H2A0.9300C14—C151.392 (2)
C3—C41.389 (2)C14—H14A0.9300
C3—C181.444 (2)C15—C161.392 (2)
C4—C51.379 (2)C15—C191.446 (2)
C4—H4A0.9300C16—C171.389 (2)
C5—C61.396 (2)C16—H16A0.9300
C5—H5A0.9300C17—H17A0.9300
C6—C71.476 (2)C20—H20A0.9600
C7—H7A0.9300C20—H20B0.9600
C8—C91.536 (2)C20—H20C0.9600
C8—H8A0.9700C21—H21A0.9600
C8—H8B0.9700C21—H21B0.9600
C9—C201.529 (2)C21—H21C0.9600
C7—N1—C8115.55 (14)N2—C10—H10B109.4
C11—N2—C10117.85 (13)C9—C10—H10B109.4
C2—C1—C6120.21 (13)H10A—C10—H10B108.0
C2—C1—H1A119.9N2—C11—C12121.33 (14)
C6—C1—H1A119.9N2—C11—H11A119.3
C1—C2—C3119.66 (15)C12—C11—H11A119.3
C1—C2—H2A120.2C17—C12—C13119.54 (13)
C3—C2—H2A120.2C17—C12—C11120.10 (14)
C4—C3—C2120.41 (14)C13—C12—C11120.36 (15)
C4—C3—C18119.30 (14)C14—C13—C12120.32 (15)
C2—C3—C18120.29 (15)C14—C13—H13A119.8
C5—C4—C3119.66 (14)C12—C13—H13A119.8
C5—C4—H4A120.2C13—C14—C15119.65 (14)
C3—C4—H4A120.2C13—C14—H14A120.2
C4—C5—C6120.45 (15)C15—C14—H14A120.2
C4—C5—H5A119.8C16—C15—C14120.88 (13)
C6—C5—H5A119.8C16—C15—C19120.37 (15)
C1—C6—C5119.60 (14)C14—C15—C19118.75 (14)
C1—C6—C7122.45 (13)C17—C16—C15118.87 (16)
C5—C6—C7117.95 (14)C17—C16—H16A120.6
N1—C7—C6123.57 (15)C15—C16—H16A120.6
N1—C7—H7A118.2C16—C17—C12120.73 (14)
C6—C7—H7A118.2C16—C17—H17A119.6
N1—C8—C9113.72 (13)C12—C17—H17A119.6
N1—C8—H8A108.8N3—C18—C3178.73 (19)
C9—C8—H8A108.8N4—C19—C15177.74 (17)
N1—C8—H8B108.8C9—C20—H20A109.5
C9—C8—H8B108.8C9—C20—H20B109.5
H8A—C8—H8B107.7H20A—C20—H20B109.5
C20—C9—C21109.99 (12)C9—C20—H20C109.5
C20—C9—C8107.02 (13)H20A—C20—H20C109.5
C21—C9—C8110.40 (12)H20B—C20—H20C109.5
C20—C9—C10110.25 (12)C9—C21—H21A109.5
C21—C9—C10109.88 (13)C9—C21—H21B109.5
C8—C9—C10109.26 (11)H21A—C21—H21B109.5
N2—C10—C9111.35 (11)C9—C21—H21C109.5
N2—C10—H10A109.4H21A—C21—H21C109.5
C9—C10—H10A109.4H21B—C21—H21C109.5
C6—C1—C2—C30.6 (2)C21—C9—C10—N264.21 (16)
C1—C2—C3—C40.2 (2)C8—C9—C10—N2174.52 (13)
C1—C2—C3—C18179.80 (14)C10—N2—C11—C12179.43 (12)
C2—C3—C4—C50.6 (2)N2—C11—C12—C17176.85 (14)
C18—C3—C4—C5179.36 (15)N2—C11—C12—C133.1 (2)
C3—C4—C5—C60.3 (2)C17—C12—C13—C140.6 (2)
C2—C1—C6—C50.8 (2)C11—C12—C13—C14179.35 (13)
C2—C1—C6—C7178.55 (14)C12—C13—C14—C150.4 (2)
C4—C5—C6—C10.4 (2)C13—C14—C15—C160.9 (2)
C4—C5—C6—C7179.02 (14)C13—C14—C15—C19179.39 (13)
C8—N1—C7—C6178.95 (13)C14—C15—C16—C170.5 (2)
C1—C6—C7—N11.7 (2)C19—C15—C16—C17179.79 (14)
C5—C6—C7—N1178.92 (14)C15—C16—C17—C120.4 (2)
C7—N1—C8—C9119.58 (14)C13—C12—C17—C161.0 (2)
N1—C8—C9—C20176.60 (12)C11—C12—C17—C16178.95 (13)
N1—C8—C9—C2156.91 (17)C4—C3—C18—N319 (9)
N1—C8—C9—C1064.04 (16)C2—C3—C18—N3161 (9)
C11—N2—C10—C9134.31 (14)C16—C15—C19—N4149 (5)
C20—C9—C10—N257.17 (17)C14—C15—C19—N431 (5)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C20—H20B···N20.962.572.923 (2)102
C21—H21C···Cg1i0.962.953.7193 (16)138
Symmetry code: (i) x, y, z1.

Experimental details

Crystal data
Chemical formulaC21H20N4
Mr328.41
Crystal system, space groupMonoclinic, P21/c
Temperature (K)100
a, b, c (Å)6.3833 (2), 34.0679 (8), 8.5190 (2)
β (°) 100.654 (2)
V3)1820.65 (8)
Z4
Radiation typeMo Kα
µ (mm1)0.07
Crystal size (mm)0.42 × 0.20 × 0.19
Data collection
DiffractometerBruker SMART APEXII CCD area-detector
diffractometer
Absorption correctionMulti-scan
(SADABS; Bruker 2005)
Tmin, Tmax0.970, 0.986
No. of measured, independent and
observed [I > 2σ(I)] reflections
21813, 5329, 3711
Rint0.046
(sin θ/λ)max1)0.707
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.056, 0.139, 1.04
No. of reflections5329
No. of parameters228
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.24, 0.20

Computer programs: APEX2 (Bruker, 2005), SAINT (Bruker, 2005), SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2003).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C20—H20B···N20.96002.57002.923 (2)102.00
C21—H21C···Cg1i0.96002.953.7193 (16)138
Symmetry code: (i) x, y, z1.
 

Footnotes

Additional correspondence author, e-mail: hadi_kargar@yahoo.com.

§Additional correspondence author, e-mail: zsrkk@yahoo.com.

Acknowledgements

HKF and RK thank the Malaysian Government and Universiti sains Malaysia for Science Fund Grant No. 305/PFIZIK/613312. RK thanks Universiti Sains Malaysia for a post-doctoral research fellowship. HK thanks PNU for financial support.

References

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