1-Methyl-2-({[(2-methyl­phen­yl)meth­yl]disulfan­yl}meth­yl)benzene

Tayamon, S.; Ravoof, T.B.S.A.; Tahir, M.I.M.; Crouse, K.A.; Tiekink, E.R.T.

doi:10.1107/S160053681202418X

organic compounds

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ISSN: 2056-9890

Volume 68| Part 6| June 2012| Page o1953

doi:10.1107/S160053681202418X

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1-Methyl-2-({[(2-methylphenyl)methyl]disulfanyl}methyl)benzene

Shahedeh Tayamon,^a Thahira Begum S. A. Ravoof,^a Mohamed Ibrahim Mohamed Tahir,^a Karen A. Crouse ^a ‡ and Edward R. T. Tiekink ^b ^*

^aDepartment of Chemistry, Universiti Putra Malaysia, 43400 Serdang, Malaysia, and ^bDepartment of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
^*Correspondence e-mail: edward.tiekink@gmail.com

(Received 24 May 2012; accepted 27 May 2012; online 31 May 2012)

In the title disulfide, C₁₆H₁₈S₂, the molecule is twisted about the central S—S bond [the C—S—S—C torsion angle = 93.24 (7)°] and the dihedral angle between the benzene rings is 72.84 (7)°, indicating an almost orthogonal relationship; the methyl groups are orientated to the same side of the molecule. The crystal packing features C—H⋯.π interactions which consolidate a three-dimensional architecture.

Related literature

For background to the coordination chemistry of dithiocarbazate derivatives, see: Crouse et al. (2004 ); Ravoof et al. (2010 ). For the synthesis and methodology, see: Tarafder et al. (2000 ). For the structure of bis(benzyl)disulfide, see: van Dijk & Visser (1971 ).

Experimental

Crystal data

C₁₆H₁₈S₂
M_r = 274.42
Monoclinic, P 2₁ /c
a = 10.3640 (4) Å
b = 7.6408 (3) Å
c = 18.1106 (7) Å
β = 91.099 (3)°
V = 1433.90 (10) Å³
Z = 4
Cu Kα radiation
μ = 3.18 mm⁻¹
T = 100 K
0.56 × 0.38 × 0.21 mm

Data collection

Oxford Diffraction Xcalibur Eos Gemini diffractometer
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2011 ) T_min = 0.299, T_max = 0.513
9993 measured reflections
2773 independent reflections
2672 reflections with I > 2σ(I)
R_int = 0.023

Refinement

R[F² > 2σ(F²)] = 0.035
wR(F²) = 0.096
S = 1.06
2773 reflections
165 parameters
H-atom parameters constrained
Δρ_max = 0.41 e Å⁻³
Δρ_min = −0.22 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

Cg1 and Cg2 are the centroids of the C2–C7 and C10–C15 rings, respectively.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C1—H1B⋯Cg1ⁱ	0.99	2.91	3.4605 (16)	116
C16—H16B⋯Cg2ⁱⁱ	0.98	2.85	3.7392 (18)	151
C16—H16C⋯Cg1ⁱⁱⁱ	0.98	2.97	3.7764 (18)	140
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) $[-x, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (iii) $[x, -y+{\script{1\over 2}}, z-{\script{3\over 2}}]$ .

Data collection: CrysAlis PRO (Agilent, 2011); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997 ) and DIAMOND (Brandenburg, 2006 ); software used to prepare material for publication: publCIF (Westrip, 2010 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S160053681202418X/hb6818sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S160053681202418X/hb6818Isup2.hkl

Supporting information file. DOI: 10.1107/S160053681202418X/hb6818Isup3.cml

checkCIF report

Comment top

Our interest in investigating the coordination properties of ligands containing the H—N—C═S moiety and our desire to expand the study of this class of biologically important compounds has lead us to synthesize series of related ligands (Tarafder et al., 2000; Crouse et al., 2004; Ravoof et al., 2010). The title compound, [(2-methyl)benzyl]disulfide, (I), was obtained during the attempt to prepare the phenyl hydrazine analogue of S-benzyldithiocarbazate.

In (I), Fig. 1, the molecule is twisted about the central S1—S2 bond as seen in the value of the C1—S1—S2—C9 torsion angle of 93.24 (7) Å. The dihedral between the benzene rings is 72.84 (7)°, indicating an almost orthogonal relationship, and the methyl groups are orientated to the same side of the molecule. The overall conformation in (I) contrasts that found in the parent compound, bis(benzyl)disulfide (van Dijk & Visser, 1971), which adopts an open conformation with both phenyl rings directed away from the sulfur atoms. In (I), the S1-bound benzyl is directed away having an anti disposition [the S2—S1—C1—C2 torsion angle is 176.23 (9)°] whereas the S2-bound benzyl residue in bis(benzyl)disulfide (van Dijk & Visser, 1971) has a syn conformation [S1—C1—C2—C7 = 91.90 (14)°].

The crystal packing is dominated by C—H···.π interactions, Table 1, which consolidates a three-dimensional architecture, Fig. 2.

Related literature top

For background to the coordination chemistry of dithiocarbazate derivatives, see: Crouse et al. (2004); Ravoof et al. (2010). For the synthesis and methodology, see: Tarafder et al. (2000). For the structure of bis(benzyl)disulfide, see: van Dijk & Visser (1971).

Experimental top

[(2-Methyl)benzyl]disulfide was isolated as a by-product from the synthesis of the phenylhydrazine analog of S-benzyldithiocarbazate (Tarafder et al., 2000). Potassium hydroxide (0.2 mol, 11.2 g) was completely dissolved in absolute ethanol (70 ml) and phenylhydrazine (0.2 mol, 21.6 g) was added to the solution cooled in an ice-salt bath producing a dark-yellow solution. Carbon disulfide (0.2 mol, 15.2 g) was added drop-wise with constant stirring over one hour. 2-Methylbenzyl chloride (0.1 mol, 13.2 ml) was then added drop-wise with vigorous stirring. The temperature of reaction was maintained below 278 K. The high yield yellow-white product was filtered and dried in a dessicator over anhydrous silica gel, dissolved in absolute ethanol and kept in a freezer. A few colourless blocks were harvested on the third day and washed with cold n-hexane.

Computing details top

Data collection: CrysAlis PRO (Agilent, 2011); cell refinement: CrysAlis PRO (Agilent, 2011); data reduction: CrysAlis PRO (Agilent, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top

	Fig. 1. The molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.
	Fig. 2. A view in projection down the b axis of the unit-cell contents for (I). The C—H···π interactions are shown as purple dashed lines.

1-Methyl-2-({[(2-methylphenyl)methyl]disulfanyl}methyl)benzene top

Crystal data top

C₁₆H₁₈S₂	F(000) = 584
M_r = 274.42	D_x = 1.271 Mg m⁻³
Monoclinic, P2₁/c	Cu Kα radiation, λ = 1.54180 Å
Hall symbol: -P 2ybc	Cell parameters from 5536 reflections
a = 10.3640 (4) Å	θ = 4–71°
b = 7.6408 (3) Å	µ = 3.18 mm⁻¹
c = 18.1106 (7) Å	T = 100 K
β = 91.099 (3)°	Block, colourless
V = 1433.90 (10) Å³	0.56 × 0.38 × 0.21 mm
Z = 4

Data collection top

Oxford Diffraction Xcalibur Eos Gemini diffractometer	2773 independent reflections
Radiation source: fine-focus sealed tube	2672 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.023
Detector resolution: 16.1952 pixels mm^-1	θ_max = 71.6°, θ_min = 4.3°
ω scans	h = −12→12
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2011)	k = −8→9
T_min = 0.299, T_max = 0.513	l = −21→22
9993 measured reflections

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)] = 0.035	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.096	H-atom parameters constrained
S = 1.06	w = 1/[σ²(F_o²) + (0.0629P)² + 0.5578P] where P = (F_o² + 2F_c²)/3
2773 reflections	(Δ/σ)_max = 0.001
165 parameters	Δρ_max = 0.41 e Å⁻³
0 restraints	Δρ_min = −0.22 e Å⁻³

Crystal data top

C₁₆H₁₈S₂	V = 1433.90 (10) Å³
M_r = 274.42	Z = 4
Monoclinic, P2₁/c	Cu Kα radiation
a = 10.3640 (4) Å	µ = 3.18 mm⁻¹
b = 7.6408 (3) Å	T = 100 K
c = 18.1106 (7) Å	0.56 × 0.38 × 0.21 mm
β = 91.099 (3)°

Data collection top

Oxford Diffraction Xcalibur Eos Gemini diffractometer	2773 independent reflections
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2011)	2672 reflections with I > 2σ(I)
T_min = 0.299, T_max = 0.513	R_int = 0.023
9993 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.035	0 restraints
wR(F²) = 0.096	H-atom parameters constrained
S = 1.06	Δρ_max = 0.41 e Å⁻³
2773 reflections	Δρ_min = −0.22 e Å⁻³
165 parameters

Special details top

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
S1	0.19303 (3)	0.79536 (5)	0.531391 (19)	0.01839 (13)
S2	0.16204 (3)	0.97650 (5)	0.450468 (19)	0.01857 (13)
C1	0.32793 (14)	0.6661 (2)	0.49591 (8)	0.0197 (3)
H1A	0.3014	0.6058	0.4497	0.024*
H1B	0.4019	0.7435	0.4853	0.024*
C2	0.36554 (14)	0.5340 (2)	0.55430 (8)	0.0175 (3)
C3	0.45586 (14)	0.5739 (2)	0.61072 (8)	0.0190 (3)
C4	0.48816 (15)	0.4427 (2)	0.66150 (8)	0.0231 (3)
H4	0.5498	0.4670	0.6996	0.028*
C5	0.43266 (16)	0.2782 (2)	0.65765 (9)	0.0252 (4)
H5	0.4571	0.1908	0.6925	0.030*
C6	0.34159 (17)	0.2409 (2)	0.60306 (10)	0.0254 (3)
H6	0.3022	0.1287	0.6007	0.030*
C7	0.30838 (15)	0.3688 (2)	0.55173 (8)	0.0216 (3)
H7	0.2457	0.3434	0.5143	0.026*
C8	0.51809 (16)	0.7523 (2)	0.61716 (10)	0.0258 (4)
H8A	0.5715	0.7574	0.6623	0.039*
H8B	0.4509	0.8424	0.6191	0.039*
H8C	0.5722	0.7729	0.5742	0.039*
C9	0.03670 (14)	0.8731 (2)	0.39237 (8)	0.0197 (3)
H9A	−0.0361	0.8389	0.4239	0.024*
H9B	0.0039	0.9600	0.3561	0.024*
C10	0.08223 (14)	0.71442 (19)	0.35152 (8)	0.0168 (3)
C11	0.15632 (14)	0.7299 (2)	0.28736 (8)	0.0187 (3)
C12	0.19310 (14)	0.5778 (2)	0.25128 (9)	0.0232 (3)
H12	0.2407	0.5869	0.2071	0.028*
C13	0.16219 (16)	0.4131 (2)	0.27805 (9)	0.0265 (4)
H13	0.1895	0.3111	0.2528	0.032*
C14	0.09121 (16)	0.3982 (2)	0.34191 (10)	0.0262 (4)
H14	0.0706	0.2861	0.3610	0.031*
C15	0.05054 (15)	0.5485 (2)	0.37768 (9)	0.0209 (3)
H15	0.0002	0.5382	0.4208	0.025*
C16	0.19659 (16)	0.9069 (2)	0.25888 (9)	0.0263 (4)
H16A	0.2582	0.9603	0.2939	0.039*
H16B	0.1205	0.9822	0.2534	0.039*
H16C	0.2373	0.8930	0.2108	0.039*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
S1	0.0218 (2)	0.0188 (2)	0.0146 (2)	0.00273 (13)	0.00327 (14)	−0.00021 (12)
S2	0.0244 (2)	0.0138 (2)	0.0175 (2)	−0.00018 (13)	−0.00079 (14)	−0.00113 (12)
C1	0.0196 (7)	0.0225 (8)	0.0173 (7)	0.0042 (6)	0.0039 (5)	−0.0003 (6)
C2	0.0176 (7)	0.0187 (8)	0.0164 (7)	0.0027 (6)	0.0046 (5)	−0.0009 (5)
C3	0.0176 (7)	0.0222 (8)	0.0175 (7)	0.0020 (6)	0.0036 (5)	−0.0031 (6)
C4	0.0223 (7)	0.0301 (9)	0.0169 (7)	0.0060 (6)	0.0009 (6)	−0.0017 (6)
C5	0.0297 (8)	0.0245 (9)	0.0217 (8)	0.0077 (6)	0.0065 (6)	0.0058 (6)
C6	0.0290 (8)	0.0188 (8)	0.0285 (8)	−0.0014 (6)	0.0074 (7)	0.0007 (6)
C7	0.0206 (7)	0.0231 (8)	0.0211 (7)	0.0001 (6)	0.0019 (6)	−0.0029 (6)
C8	0.0257 (8)	0.0254 (9)	0.0263 (8)	−0.0030 (7)	−0.0003 (6)	−0.0047 (7)
C9	0.0194 (7)	0.0202 (8)	0.0194 (7)	0.0015 (6)	−0.0010 (5)	−0.0025 (6)
C10	0.0167 (7)	0.0182 (8)	0.0155 (7)	0.0006 (5)	−0.0024 (5)	−0.0006 (5)
C11	0.0167 (7)	0.0221 (8)	0.0171 (7)	−0.0019 (6)	−0.0023 (5)	−0.0001 (6)
C12	0.0186 (7)	0.0313 (9)	0.0196 (7)	0.0003 (6)	0.0000 (6)	−0.0070 (6)
C13	0.0249 (8)	0.0229 (9)	0.0316 (9)	0.0054 (6)	−0.0061 (6)	−0.0096 (7)
C14	0.0306 (8)	0.0151 (8)	0.0325 (9)	−0.0022 (6)	−0.0087 (7)	0.0021 (6)
C15	0.0228 (7)	0.0216 (8)	0.0184 (7)	−0.0031 (6)	−0.0017 (6)	0.0023 (6)
C16	0.0305 (8)	0.0281 (9)	0.0203 (8)	−0.0084 (7)	0.0024 (6)	0.0026 (6)

Geometric parameters (Å, º) top

S1—C1	1.8377 (15)	C8—H8C	0.9800
S1—S2	2.0368 (5)	C9—C10	1.501 (2)
S2—C9	1.8349 (15)	C9—H9A	0.9900
C1—C2	1.508 (2)	C9—H9B	0.9900
C1—H1A	0.9900	C10—C15	1.395 (2)
C1—H1B	0.9900	C10—C11	1.410 (2)
C2—C7	1.395 (2)	C11—C12	1.390 (2)
C2—C3	1.406 (2)	C11—C16	1.509 (2)
C3—C4	1.397 (2)	C12—C13	1.389 (3)
C3—C8	1.511 (2)	C12—H12	0.9500
C4—C5	1.384 (2)	C13—C14	1.387 (3)
C4—H4	0.9500	C13—H13	0.9500
C5—C6	1.383 (3)	C14—C15	1.388 (2)
C5—H5	0.9500	C14—H14	0.9500
C6—C7	1.387 (2)	C15—H15	0.9500
C6—H6	0.9500	C16—H16A	0.9800
C7—H7	0.9500	C16—H16B	0.9800
C8—H8A	0.9800	C16—H16C	0.9800
C8—H8B	0.9800

C1—S1—S2	102.94 (5)	H8B—C8—H8C	109.5
C9—S2—S1	102.71 (5)	C10—C9—S2	113.87 (10)
C2—C1—S1	107.61 (10)	C10—C9—H9A	108.8
C2—C1—H1A	110.2	S2—C9—H9A	108.8
S1—C1—H1A	110.2	C10—C9—H9B	108.8
C2—C1—H1B	110.2	S2—C9—H9B	108.8
S1—C1—H1B	110.2	H9A—C9—H9B	107.7
H1A—C1—H1B	108.5	C15—C10—C11	119.47 (14)
C7—C2—C3	119.79 (14)	C15—C10—C9	119.22 (14)
C7—C2—C1	118.60 (14)	C11—C10—C9	121.31 (13)
C3—C2—C1	121.61 (14)	C12—C11—C10	118.41 (14)
C4—C3—C2	118.07 (15)	C12—C11—C16	120.55 (14)
C4—C3—C8	120.01 (14)	C10—C11—C16	121.04 (14)
C2—C3—C8	121.92 (14)	C13—C12—C11	121.77 (15)
C5—C4—C3	121.66 (15)	C13—C12—H12	119.1
C5—C4—H4	119.2	C11—C12—H12	119.1
C3—C4—H4	119.2	C14—C13—C12	119.66 (15)
C4—C5—C6	120.03 (15)	C14—C13—H13	120.2
C4—C5—H5	120.0	C12—C13—H13	120.2
C6—C5—H5	120.0	C13—C14—C15	119.47 (15)
C5—C6—C7	119.40 (16)	C13—C14—H14	120.3
C5—C6—H6	120.3	C15—C14—H14	120.3
C7—C6—H6	120.3	C14—C15—C10	121.18 (15)
C6—C7—C2	121.02 (15)	C14—C15—H15	119.4
C6—C7—H7	119.5	C10—C15—H15	119.4
C2—C7—H7	119.5	C11—C16—H16A	109.5
C3—C8—H8A	109.5	C11—C16—H16B	109.5
C3—C8—H8B	109.5	H16A—C16—H16B	109.5
H8A—C8—H8B	109.5	C11—C16—H16C	109.5
C3—C8—H8C	109.5	H16A—C16—H16C	109.5
H8A—C8—H8C	109.5	H16B—C16—H16C	109.5

C1—S1—S2—C9	93.24 (7)	S1—S2—C9—C10	−68.30 (11)
S2—S1—C1—C2	176.23 (9)	S2—C9—C10—C15	102.14 (14)
S1—C1—C2—C7	91.90 (14)	S2—C9—C10—C11	−77.83 (16)
S1—C1—C2—C3	−87.97 (15)	C15—C10—C11—C12	1.4 (2)
C7—C2—C3—C4	1.9 (2)	C9—C10—C11—C12	−178.67 (13)
C1—C2—C3—C4	−178.24 (13)	C15—C10—C11—C16	−177.79 (14)
C7—C2—C3—C8	−178.44 (14)	C9—C10—C11—C16	2.2 (2)
C1—C2—C3—C8	1.4 (2)	C10—C11—C12—C13	−2.0 (2)
C2—C3—C4—C5	−0.7 (2)	C16—C11—C12—C13	177.16 (14)
C8—C3—C4—C5	179.60 (15)	C11—C12—C13—C14	0.9 (2)
C3—C4—C5—C6	−0.8 (2)	C12—C13—C14—C15	0.9 (2)
C4—C5—C6—C7	1.1 (2)	C13—C14—C15—C10	−1.5 (2)
C5—C6—C7—C2	0.1 (2)	C11—C10—C15—C14	0.4 (2)
C3—C2—C7—C6	−1.6 (2)	C9—C10—C15—C14	−179.62 (14)
C1—C2—C7—C6	178.51 (14)

Hydrogen-bond geometry (Å, º) top

Cg1 and Cg2 are the centroids of the C2–C7 and C10–C15 rings, respectively.

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1B···Cg1ⁱ	0.99	2.91	3.4605 (16)	116
C16—H16B···Cg2ⁱⁱ	0.98	2.85	3.7392 (18)	151
C16—H16C···Cg1ⁱⁱⁱ	0.98	2.97	3.7764 (18)	140

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

Experimental details

Crystal data
Chemical formula	C₁₆H₁₈S₂
M_r	274.42
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	10.3640 (4), 7.6408 (3), 18.1106 (7)
β (°)	91.099 (3)
V (Å³)	1433.90 (10)
Z	4
Radiation type	Cu Kα
µ (mm⁻¹)	3.18
Crystal size (mm)	0.56 × 0.38 × 0.21

Data collection
Diffractometer	Oxford Diffraction Xcalibur Eos Gemini diffractometer
Absorption correction	Multi-scan (CrysAlis PRO; Agilent, 2011)
T_min, T_max	0.299, 0.513
No. of measured, independent and observed [I > 2σ(I)] reflections	9993, 2773, 2672
R_int	0.023
(sin θ/λ)_max (Å⁻¹)	0.615

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.035, 0.096, 1.06
No. of reflections	2773
No. of parameters	165
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.41, −0.22

Computer programs: CrysAlis PRO (Agilent, 2011), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 1997) and DIAMOND (Brandenburg, 2006), publCIF (Westrip, 2010).

Hydrogen-bond geometry (Å, º) top

Cg1 and Cg2 are the centroids of the C2–C7 and C10–C15 rings, respectively.

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1B···Cg1ⁱ	0.99	2.91	3.4605 (16)	116
C16—H16B···Cg2ⁱⁱ	0.98	2.85	3.7392 (18)	151
C16—H16C···Cg1ⁱⁱⁱ	0.98	2.97	3.7764 (18)	140

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

Footnotes

‡Additional correspondence author, e-mail: kacrouse@gmail.com.

Acknowledgements

Support for the project came from Universiti Putra Malaysia (UPM) through the purchase of the diffractometer and under their Research University Grant Scheme (RUGS No. 9174000), the Malaysian Ministry of Science, Technology and Innovation (grant No. 09-02-04-0752-EA001) and the Malaysian Fundamental Research Grant Scheme (FRGS No. 01-13-11-986FR). The authors also thank the Ministry of Higher Education (Malaysia) for funding structural studies through the High-Impact Research scheme (UM.C/HIR/MOHE/SC/12).

References

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