organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

Journal logoCRYSTALLOGRAPHIC
COMMUNICATIONS
ISSN: 2056-9890

3-{(E)-[(4-Formyl­phen­yl)iminium­yl]meth­yl}naphthalen-2-olate

aSchool of Chemical Sciences, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia, and bX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
*Correspondence e-mail: hkfun@usm.my

(Received 8 December 2010; accepted 10 December 2010; online 15 December 2010)

The title Schiff base compound, C18H13NO2, is a zwitterion, with the naphthol hy­droxy group deprotonated and the imine N atom protonated. It adopts an E configuration about the central C=N double bond. The dihedral angle between the naphthyl ring system and the benzene ring is 1.73 (11)°. An intra­molecular N—H⋯O hydrogen bond generates an S(6) ring motif. In the crystal, adjacent mol­ecules are connected by inter­molecular C—H⋯O hydrogen bonds, forming a supra­molecular ribbon along the b axis.

Related literature

For details and applications of condensation reactions, see: Alsalim et al. (2010[Alsalim, T. A., Hadi, J. S., Al-Nasi, E. A., AbboH, S. & Titinch, S. J. (2010). Catal Lett. 136, 228-233.]); Wadher et al. (2009[Wadher, S. J., Puranik, M. P., Karande, N. A. & Yeole, P. G. (2009). Int. J. Pharm. Tech. Res. 1, 22-33.]); Abou-Melha & Faruk (2008[Abou-Melha, K. S. & Faruk, H. (2008). J. Iran. Chem. Soc. 5, 122-134.]); Sondhi et al. (2006[Sondhi, S. M., Singh, N., Kumar, A., Lozach, O. & Meijer, L. (2006). Bioorg. Med. Chem. 14, 3758-3765.]). For 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.]).

[Scheme 1]

Experimental

Crystal data
  • C18H13NO2

  • Mr = 275.29

  • Monoclinic, P 21 /c

  • a = 7.3685 (8) Å

  • b = 12.7437 (13) Å

  • c = 14.4586 (15) Å

  • β = 91.979 (7)°

  • V = 1356.9 (2) Å3

  • Z = 4

  • Mo Kα radiation

  • μ = 0.09 mm−1

  • T = 296 K

  • 0.86 × 0.08 × 0.07 mm

Data collection
  • Bruker SMART APEXII CCD area-detector diffractometer

  • Absorption correction: multi-scan (SADABS; Bruker, 2009[Bruker (2009). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.928, Tmax = 0.994

  • 11574 measured reflections

  • 2394 independent reflections

  • 1416 reflections with I > 2σ(I)

  • Rint = 0.054

Refinement
  • R[F2 > 2σ(F2)] = 0.064

  • wR(F2) = 0.171

  • S = 1.04

  • 2394 reflections

  • 194 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • Δρmax = 0.13 e Å−3

  • Δρmin = −0.23 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
N1—H1N1⋯O1 1.00 (3) 1.68 (3) 2.551 (3) 143 (2)
C8—H7⋯O2i 0.93 2.54 3.399 (4) 153
C17—H17A⋯O2i 0.93 2.57 3.453 (4) 159
Symmetry code: (i) [-x+2, y+{\script{1\over 2}}, -z+{\script{1\over 2}}].

Data collection: APEX2 (Bruker, 2009[Bruker (2009). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 2009[Bruker (2009). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; 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, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]).

Supporting information


Comment top

Condensation reactions between carbonyl compounds and primary amines have provided one of the most important and widely studied classes of chelating ligand. The ligands, usually obtained via Schiff base condensation reactions, show variations in flexibility and electronic properties (Alsalim et al., 2010). Schiff bases are a class of important compounds in the medicinal and pharmaceutical fields. They exhibit biological properties, including antibacterial, antifungal (Wadher et al., 2009; Abou-Melha & Faruk, 2008), anticancer, herbicidal (Wadher et al., 2009), anti-inflammatory, analgesic and kinase inhibitory activities (Sondhi et al., 2006).

The asymmetric unit of the title compound is shown in Fig. 1. The molecule is a zwitterion in the crystal, with the naphthol hydroxy group deprotonated and the imine N atom protonated. It adopts an E configuration about the central C11N1 double bond [1.329 (3) Å] with the torsion angle C10-C11-N1-C12 = -179.4 (3)°. The dihedral angle between the naphthyl (C1–C10) ring system and the benzene (C12–C17) ring is 1.73 (11)°.

In the crystal structure (Fig. 2), an intramolecular N1—H1N1···O1 hydrogen bonding generates an S(6) ring motif (Bernstein et al., 1995). Furthermore, the adjacent molecules are connected by intermolecular C8—H7···O2 and C17—H17A···O2 (Table 1) hydrogen bonds forming a supramolecular ribbon along the b-axis.

Related literature top

For details and applications of condensation reactions, see: Alsalim et al. (2010); Wadher et al. (2009); Abou-Melha & Faruk (2008); Sondhi et al. (2006). For hydrogen-bond motifs, see: Bernstein et al. (1995).

Experimental top

2-Hydroxy-1-naphthaldehyde (0.172 g, 1 mmol) was added to the solution of 4-aminobenzaldehyde (0.121 g, 1mmol) in ethanol (30 ml) following which the mixture was refluxed with stirring for 1 h. The resultant orange needle-shaped single crystals suitable for X-ray structure determination which formed was then filtered and washed with ethanol

Refinement top

Atom H1N1 was located from a difference Fourier map and refined freely [N—H = 1.00 (3) Å]. The remaining H atoms were positioned geometrically [C—H = 0.93 Å] and were refined using a riding model, with Uiso(H) = 1.2 Ueq(C).

Structure description top

Condensation reactions between carbonyl compounds and primary amines have provided one of the most important and widely studied classes of chelating ligand. The ligands, usually obtained via Schiff base condensation reactions, show variations in flexibility and electronic properties (Alsalim et al., 2010). Schiff bases are a class of important compounds in the medicinal and pharmaceutical fields. They exhibit biological properties, including antibacterial, antifungal (Wadher et al., 2009; Abou-Melha & Faruk, 2008), anticancer, herbicidal (Wadher et al., 2009), anti-inflammatory, analgesic and kinase inhibitory activities (Sondhi et al., 2006).

The asymmetric unit of the title compound is shown in Fig. 1. The molecule is a zwitterion in the crystal, with the naphthol hydroxy group deprotonated and the imine N atom protonated. It adopts an E configuration about the central C11N1 double bond [1.329 (3) Å] with the torsion angle C10-C11-N1-C12 = -179.4 (3)°. The dihedral angle between the naphthyl (C1–C10) ring system and the benzene (C12–C17) ring is 1.73 (11)°.

In the crystal structure (Fig. 2), an intramolecular N1—H1N1···O1 hydrogen bonding generates an S(6) ring motif (Bernstein et al., 1995). Furthermore, the adjacent molecules are connected by intermolecular C8—H7···O2 and C17—H17A···O2 (Table 1) hydrogen bonds forming a supramolecular ribbon along the b-axis.

For details and applications of condensation reactions, see: Alsalim et al. (2010); Wadher et al. (2009); Abou-Melha & Faruk (2008); Sondhi et al. (2006). For hydrogen-bond motifs, see: Bernstein et al. (1995).

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
[Figure 1] Fig. 1. The asymmetric unit of the title compound, showing 50% probability displacement ellipsoids and the atom-numbering scheme. Intramolecular interaction is shown as dashed lines
[Figure 2] Fig. 2. A molecular ribbon generated by C—H···O hydrogen bonds.
3-{(E)-[(4-Formylphenyl)iminiumyl]methyl}naphthalen-2-olate top
Crystal data top
C18H13NO2F(000) = 576
Mr = 275.29Dx = 1.348 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybcCell parameters from 2561 reflections
a = 7.3685 (8) Åθ = 3.2–30.0°
b = 12.7437 (13) ŵ = 0.09 mm1
c = 14.4586 (15) ÅT = 296 K
β = 91.979 (7)°Needle, orange
V = 1356.9 (2) Å30.86 × 0.08 × 0.07 mm
Z = 4
Data collection top
Bruker SMART APEXII CCD area-detector
diffractometer
2394 independent reflections
Radiation source: fine-focus sealed tube1416 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.054
φ and ω scansθmax = 25.0°, θmin = 2.1°
Absorption correction: multi-scan
(SADABS; Bruker, 2009)
h = 88
Tmin = 0.928, Tmax = 0.994k = 1514
11574 measured reflectionsl = 1617
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)] = 0.064Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.171H atoms treated by a mixture of independent and constrained refinement
S = 1.04 w = 1/[σ2(Fo2) + (0.0799P)2 + 0.244P]
where P = (Fo2 + 2Fc2)/3
2394 reflections(Δ/σ)max = 0.001
194 parametersΔρmax = 0.13 e Å3
0 restraintsΔρmin = 0.23 e Å3
Crystal data top
C18H13NO2V = 1356.9 (2) Å3
Mr = 275.29Z = 4
Monoclinic, P21/cMo Kα radiation
a = 7.3685 (8) ŵ = 0.09 mm1
b = 12.7437 (13) ÅT = 296 K
c = 14.4586 (15) Å0.86 × 0.08 × 0.07 mm
β = 91.979 (7)°
Data collection top
Bruker SMART APEXII CCD area-detector
diffractometer
2394 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2009)
1416 reflections with I > 2σ(I)
Tmin = 0.928, Tmax = 0.994Rint = 0.054
11574 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0640 restraints
wR(F2) = 0.171H atoms treated by a mixture of independent and constrained refinement
S = 1.04Δρmax = 0.13 e Å3
2394 reflectionsΔρmin = 0.23 e Å3
194 parameters
Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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
O10.6211 (3)0.00216 (15)0.67024 (13)0.0694 (7)
O20.9842 (4)0.31405 (18)0.16917 (18)0.0913 (8)
N10.7418 (3)0.02531 (16)0.50829 (17)0.0496 (6)
C10.6323 (4)0.0967 (2)0.65940 (19)0.0527 (7)
C20.5864 (4)0.1658 (2)0.73351 (19)0.0578 (8)
H20.54960.13730.78900.069*
C30.5952 (4)0.2702 (2)0.72465 (19)0.0563 (8)
H30.56340.31190.77430.068*
C40.6515 (4)0.3201 (2)0.64209 (19)0.0503 (7)
C50.6622 (4)0.4289 (2)0.6367 (2)0.0673 (9)
H40.63020.46920.68720.081*
C60.7185 (5)0.4774 (2)0.5590 (2)0.0804 (11)
H50.72550.55020.55640.096*
C70.7654 (5)0.4169 (2)0.4837 (2)0.0752 (10)
H60.80340.44960.43020.090*
C80.7564 (4)0.3100 (2)0.4871 (2)0.0587 (8)
H70.78840.27120.43560.070*
C90.7000 (3)0.25724 (19)0.56633 (17)0.0450 (7)
C100.6907 (3)0.14332 (19)0.57427 (17)0.0448 (7)
C110.7430 (3)0.07881 (19)0.50285 (18)0.0463 (7)
H11A0.78070.10980.44850.056*
C120.7929 (3)0.09668 (18)0.44023 (18)0.0440 (7)
C130.7754 (4)0.20274 (19)0.46048 (19)0.0524 (7)
H13A0.73290.22360.51750.063*
C140.8209 (4)0.2768 (2)0.39614 (19)0.0537 (7)
H14A0.80920.34760.41040.064*
C150.8834 (3)0.24824 (19)0.31103 (18)0.0455 (7)
C160.9013 (4)0.1417 (2)0.29133 (19)0.0534 (7)
H16A0.94430.12120.23440.064*
C170.8566 (4)0.0671 (2)0.35455 (18)0.0513 (7)
H17A0.86880.00370.34030.062*
C180.9279 (4)0.3289 (2)0.2450 (2)0.0625 (8)
H18A0.91150.39820.26290.075*
H1N10.698 (4)0.048 (2)0.570 (2)0.086 (10)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0902 (17)0.0565 (13)0.0634 (14)0.0048 (11)0.0285 (12)0.0080 (10)
O20.116 (2)0.0838 (16)0.0757 (17)0.0014 (13)0.0242 (15)0.0122 (12)
N10.0528 (15)0.0482 (14)0.0486 (15)0.0021 (10)0.0130 (12)0.0011 (11)
C10.0472 (18)0.0561 (18)0.0552 (18)0.0041 (13)0.0085 (14)0.0025 (14)
C20.0542 (19)0.074 (2)0.0461 (17)0.0002 (15)0.0125 (14)0.0011 (14)
C30.053 (2)0.065 (2)0.0511 (18)0.0029 (14)0.0072 (15)0.0126 (14)
C40.0432 (17)0.0557 (17)0.0520 (17)0.0027 (12)0.0037 (14)0.0088 (13)
C50.080 (2)0.0574 (19)0.065 (2)0.0052 (15)0.0082 (18)0.0145 (15)
C60.115 (3)0.0487 (18)0.078 (2)0.0010 (17)0.016 (2)0.0039 (17)
C70.104 (3)0.055 (2)0.068 (2)0.0024 (17)0.0191 (19)0.0091 (16)
C80.072 (2)0.0506 (18)0.0543 (18)0.0065 (14)0.0099 (16)0.0014 (13)
C90.0373 (16)0.0488 (15)0.0487 (16)0.0020 (11)0.0018 (13)0.0020 (12)
C100.0379 (16)0.0494 (16)0.0475 (16)0.0032 (11)0.0058 (13)0.0001 (12)
C110.0440 (16)0.0482 (16)0.0470 (16)0.0035 (12)0.0053 (13)0.0025 (12)
C120.0374 (16)0.0447 (15)0.0500 (17)0.0019 (11)0.0053 (13)0.0008 (12)
C130.0563 (19)0.0534 (17)0.0479 (16)0.0013 (13)0.0088 (14)0.0058 (13)
C140.0603 (19)0.0427 (15)0.0581 (18)0.0008 (12)0.0048 (15)0.0005 (13)
C150.0385 (16)0.0478 (15)0.0501 (17)0.0002 (12)0.0022 (13)0.0032 (12)
C160.0525 (19)0.0583 (18)0.0500 (17)0.0030 (13)0.0124 (14)0.0004 (13)
C170.0570 (18)0.0440 (15)0.0539 (17)0.0007 (12)0.0152 (14)0.0032 (12)
C180.071 (2)0.0581 (18)0.059 (2)0.0023 (15)0.0134 (17)0.0067 (14)
Geometric parameters (Å, º) top
O1—C11.272 (3)C7—H60.9300
O2—C181.201 (3)C8—C91.404 (4)
N1—C111.329 (3)C8—H70.9300
N1—C121.401 (3)C9—C101.458 (3)
N1—H1N11.00 (3)C10—C111.385 (3)
C1—C21.437 (4)C11—H11A0.9300
C1—C101.446 (3)C12—C131.390 (3)
C2—C31.338 (4)C12—C171.392 (3)
C2—H20.9300C13—C141.374 (4)
C3—C41.427 (4)C13—H13A0.9300
C3—H30.9300C14—C151.378 (4)
C4—C51.390 (4)C14—H14A0.9300
C4—C91.413 (3)C15—C161.394 (3)
C5—C61.359 (4)C15—C181.448 (4)
C5—H40.9300C16—C171.367 (3)
C6—C71.389 (4)C16—H16A0.9300
C6—H50.9300C17—H17A0.9300
C7—C81.364 (4)C18—H18A0.9300
C11—N1—C12127.2 (2)C4—C9—C10119.3 (2)
C11—N1—H1N1110.3 (17)C11—C10—C1119.3 (2)
C12—N1—H1N1122.6 (17)C11—C10—C9121.2 (2)
O1—C1—C2119.8 (2)C1—C10—C9119.5 (2)
O1—C1—C10122.3 (2)N1—C11—C10123.1 (2)
C2—C1—C10117.9 (2)N1—C11—H11A118.5
C3—C2—C1121.7 (3)C10—C11—H11A118.5
C3—C2—H2119.2C13—C12—C17119.2 (2)
C1—C2—H2119.2C13—C12—N1117.0 (2)
C2—C3—C4122.7 (3)C17—C12—N1123.8 (2)
C2—C3—H3118.7C14—C13—C12119.9 (2)
C4—C3—H3118.7C14—C13—H13A120.1
C5—C4—C9120.3 (3)C12—C13—H13A120.1
C5—C4—C3120.7 (3)C13—C14—C15121.4 (2)
C9—C4—C3119.0 (2)C13—C14—H14A119.3
C6—C5—C4121.3 (3)C15—C14—H14A119.3
C6—C5—H4119.3C14—C15—C16118.4 (2)
C4—C5—H4119.3C14—C15—C18119.5 (2)
C5—C6—C7119.1 (3)C16—C15—C18122.1 (3)
C5—C6—H5120.5C17—C16—C15120.9 (2)
C7—C6—H5120.5C17—C16—H16A119.5
C8—C7—C6120.9 (3)C15—C16—H16A119.5
C8—C7—H6119.6C16—C17—C12120.2 (2)
C6—C7—H6119.6C16—C17—H17A119.9
C7—C8—C9121.6 (3)C12—C17—H17A119.9
C7—C8—H7119.2O2—C18—C15125.7 (3)
C9—C8—H7119.2O2—C18—H18A117.2
C8—C9—C4116.8 (2)C15—C18—H18A117.2
C8—C9—C10123.9 (2)
O1—C1—C2—C3179.6 (3)C4—C9—C10—C11177.8 (2)
C10—C1—C2—C30.6 (4)C8—C9—C10—C1179.4 (3)
C1—C2—C3—C40.4 (4)C4—C9—C10—C10.1 (3)
C2—C3—C4—C5178.7 (3)C12—N1—C11—C10179.6 (2)
C2—C3—C4—C90.0 (4)C1—C10—C11—N10.2 (4)
C9—C4—C5—C60.2 (5)C9—C10—C11—N1177.9 (2)
C3—C4—C5—C6178.9 (3)C11—N1—C12—C13178.2 (2)
C4—C5—C6—C70.3 (5)C11—N1—C12—C170.8 (4)
C5—C6—C7—C80.3 (5)C17—C12—C13—C140.0 (4)
C6—C7—C8—C90.1 (5)N1—C12—C13—C14179.1 (2)
C7—C8—C9—C40.6 (4)C12—C13—C14—C150.3 (4)
C7—C8—C9—C10179.0 (3)C13—C14—C15—C160.5 (4)
C5—C4—C9—C80.6 (4)C13—C14—C15—C18179.4 (3)
C3—C4—C9—C8179.3 (2)C14—C15—C16—C170.5 (4)
C5—C4—C9—C10178.9 (2)C18—C15—C16—C17179.3 (3)
C3—C4—C9—C100.2 (4)C15—C16—C17—C120.3 (4)
O1—C1—C10—C112.4 (4)C13—C12—C17—C160.0 (4)
C2—C1—C10—C11177.4 (2)N1—C12—C17—C16179.1 (2)
O1—C1—C10—C9179.8 (2)C14—C15—C18—O2179.6 (3)
C2—C1—C10—C90.3 (4)C16—C15—C18—O20.5 (5)
C8—C9—C10—C111.7 (4)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1N1···O11.00 (3)1.68 (3)2.551 (3)143 (2)
C8—H7···O2i0.932.543.399 (4)153
C17—H17A···O2i0.932.573.453 (4)159
Symmetry code: (i) x+2, y+1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaC18H13NO2
Mr275.29
Crystal system, space groupMonoclinic, P21/c
Temperature (K)296
a, b, c (Å)7.3685 (8), 12.7437 (13), 14.4586 (15)
β (°) 91.979 (7)
V3)1356.9 (2)
Z4
Radiation typeMo Kα
µ (mm1)0.09
Crystal size (mm)0.86 × 0.08 × 0.07
Data collection
DiffractometerBruker SMART APEXII CCD area-detector
Absorption correctionMulti-scan
(SADABS; Bruker, 2009)
Tmin, Tmax0.928, 0.994
No. of measured, independent and
observed [I > 2σ(I)] reflections
11574, 2394, 1416
Rint0.054
(sin θ/λ)max1)0.594
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.064, 0.171, 1.04
No. of reflections2394
No. of parameters194
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.13, 0.23

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

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1N1···O11.00 (3)1.68 (3)2.551 (3)143 (2)
C8—H7···O2i0.932.543.399 (4)153
C17—H17A···O2i0.932.573.453 (4)159
Symmetry code: (i) x+2, y+1/2, z+1/2.
 

Footnotes

Additional correspondence author, e-mail: sgteoh@usm.my.

§Thomson Reuters ResearcherID: A-3561-2009.

Acknowledgements

AMF, SGT and HO thank the Malaysian Government and Universiti Sains Malaysia for the RU research grant (815002). AMF also thanks the Libyan Government for providing a schol­arship. HKF and MH 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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