Bis(2,3-diamino­pyridinium) phthalate dihydrate

Hemamalini, M.; Goh, J.H.; Fun, H.-K.

doi:10.1107/S1600536811055838

organic compounds

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ISSN: 2056-9890

Volume 68| Part 2| February 2012| Page o402

doi:10.1107/S1600536811055838

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Bis(2,3-diaminopyridinium) phthalate dihydrate

Madhukar Hemamalini,^a Jia Hao Goh ^a ‡ and Hoong-Kun Fun ^a ^*§

^aX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
^*Correspondence e-mail: hkfun@usm.my

(Received 20 December 2011; accepted 27 December 2011; online 14 January 2012)

The complete anion of the title hydrated molecular salt, 2C₅H₈N₃⁺·C₈H₄O₄⁻·2H₂O, is generated by a crystallographic twofold axis. In the crystal, the cations, anions and water molecules are connected by N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds, forming a three-dimensional network. The crystal structure also features C—H⋯π interactions.

Related literature

For background to hydrogen-bonding patterns of 2-aminopyridine derivatives, see: Gellert & Hsu (1988 ); Banerjee & Murugavel (2004 ). For related structures, see: Hemamalini & Fun (2010a ,b ,c ). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986 ).

Experimental

Crystal data

2C₅H₈N₃⁺·C₈H₄O₄⁻·2H₂O
M_r = 420.43
Monoclinic, C 2/c
a = 15.795 (5) Å
b = 13.083 (4) Å
c = 11.012 (4) Å
β = 115.194 (5)°
V = 2059.2 (11) Å³
Z = 4
Mo Kα radiation
μ = 0.10 mm⁻¹
T = 100 K
0.42 × 0.36 × 0.03 mm

Data collection

Bruker APEXII DUO CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2009 ) T_min = 0.958, T_max = 0.997
10343 measured reflections
2955 independent reflections
2148 reflections with I > 2σ(I)
R_int = 0.042

Refinement

R[F² > 2σ(F²)] = 0.043
wR(F²) = 0.125
S = 1.04
2955 reflections
184 parameters
All H-atom parameters refined
Δρ_max = 0.33 e Å⁻³
Δρ_min = −0.26 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

Cg1 and Cg2 are the centroids of the C6–C8/C6A–C8A and N1/C1–C5 rings, respectively.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N2—H5⋯O2ⁱ	0.90 (2)	2.03 (2)	2.9155 (19)	169.0 (18)
N1—H6⋯O2ⁱⁱ	0.98 (2)	1.790 (19)	2.7565 (18)	170.3 (19)
N2—H7⋯O1ⁱⁱ	0.92 (2)	1.99 (2)	2.8976 (19)	168.1 (19)
N3—H9⋯O1Wⁱⁱⁱ	0.89 (2)	2.09 (2)	2.978 (2)	179 (3)
N3—H10⋯O2ⁱ	0.89 (2)	2.23 (2)	3.078 (2)	158.8 (17)
O1W—H11⋯O1^iv	0.95 (3)	1.86 (3)	2.7878 (18)	165 (3)
O1W—H12⋯O1^v	0.92 (2)	1.99 (2)	2.8641 (18)	158.2 (19)
C3—H2⋯O1W^vi	0.984 (17)	2.516 (18)	3.369 (2)	145.0 (14)
C4—H8⋯Cg1ⁱⁱⁱ	1.01 (2)	2.76 (2)	3.629 (2)	144.3 (15)
C6—H3⋯Cg2^vii	0.959 (18)	2.568 (18)	3.497 (2)	163.4 (14)
Symmetry codes: (i) $[x, -y+1, z+{\script{1\over 2}}]$ ; (ii) x, y+1, z+1; (iii) $[-x+{\script{1\over 2}}, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]$ ; (iv) x, y, z+1; (v) -x+1, -y, -z+1; (vi) $[x-{\script{1\over 2}}, y+{\script{1\over 2}}, z]$ ; (vii) $[-x+{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1]$ .

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008 ); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536811055838/hb6574sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536811055838/hb6574Isup2.hkl

Supporting information file. DOI: 10.1107/S1600536811055838/hb6574Isup3.cml

checkCIF report

Comment top

2-Aminopyridine and its derivatives are amomg the most frequently used synthons in supramolecular chemistry based on hydrogen bonds (Gellert & Hsu, 1988; Banerjee & Murugavel, 2004). A series of similar complexes formed from 2-aminopyridine and carboxylates has been reported previously (Hemamalini & Fun, 2010a,b,c). The present work, 2,3-diaminopyridinium phthalate dihydrate, (2/1/1) (I), is a continuation of our structural study of complexes of the 2,3-diaminopyridinium system.

The asymmetric unit of the title compound, (I), contains one 2,3-diamino- pyridinium cation, a half of phthalate anion (which lies on a twofold axis; -x+1, y, -z+1/2) and a water molecule as shown in Fig. 1. The dihedral angle between the pyridine (N1/C1–C5) and benzene (C6–C8/ C6A–C8A) ring is 80.61 (7)°.

In the crystal structure (Fig. 2), the ion pairs and water molecules are connected via N—H···O, O—H···O and C—H···O (Table 1) hydrogen bonds forming a three-dimensional network. The crystal structure is further stabilized by C—H···π interactions involving the centroids of the C6–C8/C6A–C8A (Cg1) and N1/C1–C5 (Cg2) rings.

Related literature top

For background to hydrogen-bonding patterns of 2-aminopyridine derivatives, see: Gellert & Hsu (1988); Banerjee & Murugavel (2004). For related structures, see: Hemamalini & Fun (2010a,b,c). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986).

Experimental top

A hot methanol solution (20 ml) of 2,3-diaminopyridine (27 mg, Aldrich) and phthalic acid (42 mg, Merck) were mixed and warmed over a magnetic stirrer hotplate for a few minutes. The resulting solution was allowed to cool slowly at room temperature and brown plates of the title compound appeared after a few days.

Computing details top

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); 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, 2009).

Figures top

	Fig. 1. The molecule of (I) with 30% probability displacement ellipsoids. Atoms labelled A are generated by -x+1, y, -z+1/2.
	Fig. 2. The crystal packing of the title compound showing hydrogen-bonded (dashed lines) networks. H atoms not involved in hydrogen bond interactions are omitted for clarity.

Bis(2,3-diaminopyridinium) benzene-1,2-dicarboxylate dihydrate top

Crystal data top

2C₅H₈N₃⁺·C₈H₄O₄⁻·2H₂O	F(000) = 888
M_r = 420.43	D_x = 1.356 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 3390 reflections
a = 15.795 (5) Å	θ = 2.8–29.8°
b = 13.083 (4) Å	µ = 0.10 mm⁻¹
c = 11.012 (4) Å	T = 100 K
β = 115.194 (5)°	Plate, brown
V = 2059.2 (11) Å³	0.42 × 0.36 × 0.03 mm
Z = 4

Data collection top

Bruker APEXII DUO CCD diffractometer	2955 independent reflections
Radiation source: fine-focus sealed tube	2148 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.042
ϕ and ω scans	θ_max = 29.9°, θ_min = 2.1°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −22→22
T_min = 0.958, T_max = 0.997	k = −18→18
10343 measured reflections	l = −14→14

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.043	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.125	All H-atom parameters refined
S = 1.04	w = 1/[σ²(F_o²) + (0.0653P)² + 0.7423P] where P = (F_o² + 2F_c²)/3
2955 reflections	(Δ/σ)_max < 0.001
184 parameters	Δρ_max = 0.33 e Å⁻³
0 restraints	Δρ_min = −0.26 e Å⁻³

Crystal data top

2C₅H₈N₃⁺·C₈H₄O₄⁻·2H₂O	V = 2059.2 (11) Å³
M_r = 420.43	Z = 4
Monoclinic, C2/c	Mo Kα radiation
a = 15.795 (5) Å	µ = 0.10 mm⁻¹
b = 13.083 (4) Å	T = 100 K
c = 11.012 (4) Å	0.42 × 0.36 × 0.03 mm
β = 115.194 (5)°

Data collection top

Bruker APEXII DUO CCD diffractometer	2955 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2009)	2148 reflections with I > 2σ(I)
T_min = 0.958, T_max = 0.997	R_int = 0.042
10343 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.043	0 restraints
wR(F²) = 0.125	All H-atom parameters refined
S = 1.04	Δρ_max = 0.33 e Å⁻³
2955 reflections	Δρ_min = −0.26 e Å⁻³
184 parameters

Special details top

Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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

	x	y	z	U_iso*/U_eq
N1	0.23077 (8)	0.94380 (9)	1.03746 (11)	0.0227 (3)
N2	0.30262 (8)	0.94315 (10)	0.89459 (13)	0.0256 (3)
N3	0.17815 (10)	0.78666 (11)	0.74285 (14)	0.0330 (3)
C1	0.23819 (9)	0.90400 (10)	0.93023 (13)	0.0192 (3)
C2	0.17516 (9)	0.82430 (9)	0.85688 (13)	0.0210 (3)
C3	0.11106 (9)	0.79131 (11)	0.90331 (14)	0.0249 (3)
C4	0.10585 (11)	0.83570 (13)	1.01540 (15)	0.0313 (3)
C5	0.16621 (11)	0.91224 (13)	1.08149 (15)	0.0310 (3)
O1	0.43751 (6)	0.07071 (7)	0.10193 (9)	0.0225 (2)
O2	0.32273 (6)	0.12167 (7)	0.15491 (9)	0.0225 (2)
C6	0.45346 (10)	0.41720 (10)	0.20469 (14)	0.0242 (3)
C7	0.40590 (9)	0.32537 (10)	0.16012 (13)	0.0201 (3)
C8	0.45249 (8)	0.23274 (9)	0.20552 (12)	0.0160 (2)
C9	0.40094 (8)	0.13405 (9)	0.15149 (12)	0.0168 (2)
O1W	0.45043 (8)	0.10881 (8)	0.86130 (12)	0.0315 (3)
H1	0.3388 (12)	0.3242 (12)	0.0959 (17)	0.028 (4)*
H2	0.0692 (11)	0.7346 (13)	0.8557 (17)	0.026 (4)*
H3	0.4209 (12)	0.4807 (14)	0.1749 (17)	0.032 (4)*
H4	0.1689 (13)	0.9496 (15)	1.160 (2)	0.043 (5)*
H5	0.3138 (13)	0.9165 (14)	0.8275 (19)	0.033 (5)*
H6	0.2697 (12)	1.0029 (15)	1.0806 (18)	0.036 (5)*
H7	0.3468 (13)	0.9879 (15)	0.9506 (19)	0.039 (5)*
H8	0.0557 (13)	0.8149 (14)	1.044 (2)	0.042 (5)*
H9	0.1404 (14)	0.7328 (17)	0.712 (2)	0.046 (5)*
H10	0.2265 (14)	0.7967 (14)	0.723 (2)	0.040 (5)*
H11	0.4555 (16)	0.1035 (17)	0.950 (3)	0.064 (7)*
H12	0.4828 (16)	0.0542 (18)	0.850 (2)	0.062 (7)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
N1	0.0235 (6)	0.0244 (6)	0.0181 (5)	−0.0068 (5)	0.0070 (4)	−0.0020 (4)
N2	0.0237 (6)	0.0281 (6)	0.0260 (6)	−0.0104 (5)	0.0116 (5)	−0.0096 (5)
N3	0.0323 (7)	0.0332 (7)	0.0364 (7)	−0.0153 (6)	0.0174 (6)	−0.0188 (6)
C1	0.0186 (6)	0.0182 (6)	0.0175 (6)	−0.0001 (4)	0.0043 (5)	0.0016 (4)
C2	0.0195 (6)	0.0181 (6)	0.0197 (6)	−0.0001 (5)	0.0028 (5)	0.0004 (5)
C3	0.0232 (6)	0.0223 (6)	0.0224 (6)	−0.0060 (5)	0.0033 (5)	0.0024 (5)
C4	0.0312 (7)	0.0392 (8)	0.0237 (7)	−0.0138 (6)	0.0119 (6)	0.0001 (6)
C5	0.0348 (8)	0.0393 (8)	0.0223 (7)	−0.0128 (6)	0.0153 (6)	−0.0043 (6)
O1	0.0229 (5)	0.0220 (5)	0.0222 (5)	−0.0006 (4)	0.0093 (4)	−0.0059 (4)
O2	0.0205 (4)	0.0227 (5)	0.0253 (5)	−0.0041 (4)	0.0106 (4)	−0.0029 (4)
C6	0.0252 (7)	0.0173 (6)	0.0297 (7)	0.0038 (5)	0.0113 (6)	0.0033 (5)
C7	0.0191 (6)	0.0200 (6)	0.0196 (6)	0.0017 (5)	0.0068 (5)	0.0022 (4)
C8	0.0180 (6)	0.0171 (6)	0.0140 (5)	−0.0005 (4)	0.0078 (4)	−0.0002 (4)
C9	0.0178 (5)	0.0173 (6)	0.0124 (5)	0.0007 (4)	0.0036 (4)	0.0006 (4)
O1W	0.0361 (6)	0.0276 (6)	0.0363 (6)	0.0102 (4)	0.0208 (5)	0.0120 (4)

Geometric parameters (Å, º) top

N1—C1	1.3404 (17)	C4—H8	1.01 (2)
N1—C5	1.3663 (18)	C5—H4	0.98 (2)
N1—H6	0.98 (2)	O1—C9	1.2596 (15)
N2—C1	1.3388 (17)	O2—C9	1.2623 (15)
N2—H5	0.899 (19)	C6—C6ⁱ	1.382 (3)
N2—H7	0.92 (2)	C6—C7	1.3910 (19)
N3—C2	1.3683 (19)	C6—H3	0.959 (18)
N3—H9	0.89 (2)	C7—C8	1.3954 (17)
N3—H10	0.89 (2)	C7—H1	0.993 (17)
C1—C2	1.4304 (18)	C8—C8ⁱ	1.400 (2)
C2—C3	1.383 (2)	C8—C9	1.5072 (17)
C3—C4	1.398 (2)	O1W—H11	0.95 (3)
C3—H2	0.983 (17)	O1W—H12	0.91 (2)
C4—C5	1.360 (2)

C1—N1—C5	123.61 (12)	C5—C4—H8	119.7 (11)
C1—N1—H6	117.6 (11)	C3—C4—H8	121.0 (11)
C5—N1—H6	118.5 (10)	C4—C5—N1	119.44 (14)
C1—N2—H5	122.2 (12)	C4—C5—H4	126.7 (11)
C1—N2—H7	120.2 (11)	N1—C5—H4	113.8 (11)
H5—N2—H7	116.0 (16)	C6ⁱ—C6—C7	120.25 (8)
C2—N3—H9	111.0 (13)	C6ⁱ—C6—H3	119.9 (10)
C2—N3—H10	122.9 (13)	C7—C6—H3	119.8 (10)
H9—N3—H10	121.6 (18)	C6—C7—C8	120.03 (12)
N2—C1—N1	118.16 (12)	C6—C7—H1	121.1 (10)
N2—C1—C2	123.13 (12)	C8—C7—H1	118.9 (10)
N1—C1—C2	118.67 (12)	C7—C8—C8ⁱ	119.70 (7)
N3—C2—C3	122.93 (12)	C7—C8—C9	119.26 (11)
N3—C2—C1	119.54 (12)	C8ⁱ—C8—C9	120.95 (6)
C3—C2—C1	117.49 (12)	O1—C9—O2	124.30 (11)
C2—C3—C4	121.61 (13)	O1—C9—C8	117.67 (11)
C2—C3—H2	118.0 (10)	O2—C9—C8	117.99 (11)
C4—C3—H2	120.4 (10)	H11—O1W—H12	105.6 (19)
C5—C4—C3	119.14 (13)

C5—N1—C1—N2	178.22 (13)	C3—C4—C5—N1	−0.3 (2)
C5—N1—C1—C2	0.2 (2)	C1—N1—C5—C4	0.6 (2)
N2—C1—C2—N3	−1.5 (2)	C6ⁱ—C6—C7—C8	−0.9 (2)
N1—C1—C2—N3	176.36 (12)	C6—C7—C8—C8ⁱ	−0.9 (2)
N2—C1—C2—C3	−179.27 (13)	C6—C7—C8—C9	−177.35 (12)
N1—C1—C2—C3	−1.39 (18)	C7—C8—C9—O1	126.40 (13)
N3—C2—C3—C4	−175.91 (14)	C8ⁱ—C8—C9—O1	−50.01 (19)
C1—C2—C3—C4	1.8 (2)	C7—C8—C9—O2	−51.38 (16)
C2—C3—C4—C5	−0.9 (2)	C8ⁱ—C8—C9—O2	132.22 (15)

Symmetry code: (i) −x+1, y, −z+1/2.

Hydrogen-bond geometry (Å, º) top

Cg1 and Cg2 are the centroids of the C6–C8/C6A–C8A and N1/C1–C5 rings, respectively.

D—H···A	D—H	H···A	D···A	D—H···A
N2—H5···O2ⁱⁱ	0.90 (2)	2.03 (2)	2.9155 (19)	169.0 (18)
N1—H6···O2ⁱⁱⁱ	0.98 (2)	1.790 (19)	2.7565 (18)	170.3 (19)
N2—H7···O1ⁱⁱⁱ	0.92 (2)	1.99 (2)	2.8976 (19)	168.1 (19)
N3—H9···O1W^iv	0.89 (2)	2.09 (2)	2.978 (2)	179 (3)
N3—H10···O2ⁱⁱ	0.89 (2)	2.23 (2)	3.078 (2)	158.8 (17)
O1W—H11···O1^v	0.95 (3)	1.86 (3)	2.7878 (18)	165 (3)
O1W—H12···O1^vi	0.92 (2)	1.99 (2)	2.8641 (18)	158.2 (19)
C3—H2···O1W^vii	0.984 (17)	2.516 (18)	3.369 (2)	145.0 (14)
C4—H8···Cg1^iv	1.01 (2)	2.76 (2)	3.629 (2)	144.3 (15)
C6—H3···Cg2^viii	0.959 (18)	2.568 (18)	3.497 (2)	163.4 (14)

Symmetry codes: (ii) x, −y+1, z+1/2; (iii) x, y+1, z+1; (iv) −x+1/2, y+1/2, −z+3/2; (v) x, y, z+1; (vi) −x+1, −y, −z+1; (vii) x−1/2, y+1/2, z; (viii) −x+1/2, −y+3/2, −z+1.

Experimental details

Crystal data
Chemical formula	2C₅H₈N₃⁺·C₈H₄O₄⁻·2H₂O
M_r	420.43
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	15.795 (5), 13.083 (4), 11.012 (4)
β (°)	115.194 (5)
V (Å³)	2059.2 (11)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.10
Crystal size (mm)	0.42 × 0.36 × 0.03

Data collection
Diffractometer	Bruker APEXII DUO CCD diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2009)
T_min, T_max	0.958, 0.997
No. of measured, independent and observed [I > 2σ(I)] reflections	10343, 2955, 2148
R_int	0.042
(sin θ/λ)_max (Å⁻¹)	0.702

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.043, 0.125, 1.04
No. of reflections	2955
No. of parameters	184
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.33, −0.26

Computer programs: APEX2 (Bruker, 2009), SAINT (Bruker, 2009), SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Hydrogen-bond geometry (Å, º) top

Cg1 and Cg2 are the centroids of the C6–C8/C6A–C8A and N1/C1–C5 rings, respectively.

D—H···A	D—H	H···A	D···A	D—H···A
N2—H5···O2ⁱ	0.90 (2)	2.03 (2)	2.9155 (19)	169.0 (18)
N1—H6···O2ⁱⁱ	0.98 (2)	1.790 (19)	2.7565 (18)	170.3 (19)
N2—H7···O1ⁱⁱ	0.92 (2)	1.99 (2)	2.8976 (19)	168.1 (19)
N3—H9···O1Wⁱⁱⁱ	0.89 (2)	2.09 (2)	2.978 (2)	179 (3)
N3—H10···O2ⁱ	0.89 (2)	2.23 (2)	3.078 (2)	158.8 (17)
O1W—H11···O1^iv	0.95 (3)	1.86 (3)	2.7878 (18)	165 (3)
O1W—H12···O1^v	0.92 (2)	1.99 (2)	2.8641 (18)	158.2 (19)
C3—H2···O1W^vi	0.984 (17)	2.516 (18)	3.369 (2)	145.0 (14)
C4—H8···Cg1ⁱⁱⁱ	1.01 (2)	2.76 (2)	3.629 (2)	144.3 (15)
C6—H3···Cg2^vii	0.959 (18)	2.568 (18)	3.497 (2)	163.4 (14)

Symmetry codes: (i) x, −y+1, z+1/2; (ii) x, y+1, z+1; (iii) −x+1/2, y+1/2, −z+3/2; (iv) x, y, z+1; (v) −x+1, −y, −z+1; (vi) x−1/2, y+1/2, z; (vii) −x+1/2, −y+3/2, −z+1.

Footnotes

‡Thomson Reuters ResearcherID: C-7576-2009.

§Thomson Reuters ResearcherID: A-3561-2009.

Acknowledgements

MH, JHG and HKF thank the Malaysian Government and Universiti Sains Malaysia for the Research University grant No. 1001/PFIZIK/811160. MH also thanks Universiti Sains Malaysia for a post-doctoral research fellowship.

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