3-Acetyl-4-hy­droxy-6,7-dimethyl-2H-chromen-2-one

Asad, M.; Oo, C.-W.; Osman, H.; Goh, J.H.; Fun, H.-K.

doi:10.1107/S1600536810045010

organic compounds

CRYSTALLOGRAPHIC
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ISSN: 2056-9890

Volume 66| Part 12| December 2010| Pages o3129-o3130

https://doi.org/10.1107/S1600536810045010

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3-Acetyl-4-hydroxy-6,7-dimethyl-2H-chromen-2-one

Mohammad Asad,^a Chuan-Wei Oo,^a ‡ Hasnah Osman,^a Jia Hao Goh ^b § and Hoong-Kun Fun ^b ^*¶

^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 28 October 2010; accepted 3 November 2010; online 13 November 2010)

In the title coumarin derivative, C₁₃H₁₂O₄, the 2H-chromene ring system is essentially planar [maximum deviation = 0.047 (1) Å]. An intramolecular hydrogen bond is observed between the hydroxy and the ketonic O atoms. In the crystal, pairs of intermolecular C—H⋯O hydrogen bonds link inversion-related molecules into dimers. Additional intermolecular C—H⋯O hydrogen bonds further interconnect these dimers into two-dimensional arrays incorporating R₂²(9) ring motifs.

Related literature

For general background to and applications of coumarin derivatives, see: Eisenhauer & Link (1953 ); Franz et al. (1981 ); Frontiera et al. (2009 ); Maurer & Arlt (1998 ). Tamura et al. (1982 ); Wang et al. (2007 ). For graph-set theory of hydrogen-bond ring motifs, see: Bernstein et al. (1995 ). For bond-length data, see: Allen et al. (1987 ). For a related coumaric structure, see: Mechi et al. (2009 ). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986 ).

Experimental

Crystal data

C₁₃H₁₂O₄
M_r = 232.23
Monoclinic, P 2₁ /c
a = 3.9491 (4) Å
b = 12.1359 (11) Å
c = 22.101 (2) Å
β = 90.563 (1)°
V = 1059.16 (17) Å³
Z = 4
Mo Kα radiation
μ = 0.11 mm⁻¹
T = 100 K
0.32 × 0.19 × 0.13 mm

Data collection

Bruker APEXII DUO CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2009 ) T_min = 0.966, T_max = 0.986
13172 measured reflections
3139 independent reflections
2539 reflections with I > 2σ(I)
R_int = 0.030

Refinement

R[F² > 2σ(F²)] = 0.048
wR(F²) = 0.148
S = 1.05
3139 reflections
161 parameters
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.64 e Å⁻³
Δρ_min = −0.28 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O2—H1O2⋯O3	1.277 (18)	1.183 (18)	2.4299 (14)	162.0 (16)
C6—H6A⋯O3ⁱ	0.93	2.58	3.4603 (17)	159
C11—H11B⋯O2ⁱⁱ	0.96	2.59	3.5458 (18)	172
C12—H12A⋯O4ⁱⁱⁱ	0.96	2.53	3.4751 (17)	168
Symmetry codes: (i) $[-x+2, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (ii) $[-x+2, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (iii) -x, -y+1, -z.

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: https://doi.org/10.1107/S1600536810045010/rz2512sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536810045010/rz2512Isup2.hkl

checkCIF report

Comment top

We report here a new 4-hydroxycoumarin derivative, which has been synthesized by acetylation process (Eisenhauer & Link, 1953). Recently, coumarin and its derivatives have been extensively used in industrial products as dyes/laser materials (Wang et al., 2007), photosensitizers (Frontiera et al., 2009), pesticides (Franz et al., 1981), in pharmacology (Maurer & Arlt, 1998) and in enzymology as biological probes (Tamura et al., 1982).

In the title coumarin compound, (Fig. 1), the 2H-chromene ring system (C1-C9/O1) is essentially planar, as indicated by the maximum deviation of -0.047 (1) Å at atom C1. Bond length of C10═O3 [1.2590 (16) Å] is longer than normal value due to the delocalization of atom H1O2 between the hydroxyl oxygen atom (O2) and the ketonic oxygen atom (O3), as observed in a related structure (Mechi et al., 2009). However, the bond lengths of O2—H1O2 = 1.277 (18) and O3—H1O2 = 1.183 (18) Å are inconsistent with the respective values observed in Mechi et al., 2009. All other bond lengths (Allen et al., 1987) and angles are within normal ranges. In the crystal structure, (Fig. 2), pairs of intermolecular C12—H12A···O4 hydrogen bonds (Table 1) link inversion-related molecules into dimers, producing R₂²(16) ring motifs (Bernstein et al., 1995). Intermolecular C6—H16A···O3 and C11—H11B···O2 hydrogen bonds (Table 1) further interconnect these dimers into two-dimensional arrays incorporating R₂²(9) hydrogen bond ring motifs (Bernstein et al., 1995).

Related literature top

For general background to and applications of coumarin derivatives, see: Eisenhauer & Link (1953); Franz et al. (1981); Frontiera et al. (2009); Maurer & Arlt (1998). Tamura et al. (1982); Wang et al. (2007). For graph-set theory of hydrogen-bond ring motifs, see: Bernstein et al. (1995). For bond-length data, see: Allen et al. (1987). For a related coumaric structure, see: Mechi et al. (2009). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986).

Experimental top

Acetyl chloride (1 ml) was added to a solution of 4-hydroxy-6,7-dimethylcoumarin (5.2 mmol, 1.0 g) in pyridine (10 ml) which contains piperidine (one drop) on ice bath. The reaction mixture was kept at room temperature for 7 days. The solution was then poured into ice-cold water and hydrochloric acid was added to afford the precipitate, which was washed with water, dried and recrystallized from ethanol to get the pure title compound in 70% yield.

Structure description top

For general background to and applications of coumarin derivatives, see: Eisenhauer & Link (1953); Franz et al. (1981); Frontiera et al. (2009); Maurer & Arlt (1998). Tamura et al. (1982); Wang et al. (2007). For graph-set theory of hydrogen-bond ring motifs, see: Bernstein et al. (1995). For bond-length data, see: Allen et al. (1987). For a related coumaric structure, see: Mechi et al. (2009). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986).

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 molecular structure of the title coumarin compound, showing 50 % probability displacement ellipsoids for non-H atoms and the atom-numbering scheme.
	Fig. 2. The crystal structure of the title coumarin compound, viewed along an arbitrary axis, showing dimers being linked into a two-dimensional array. H atoms not involved in intermolecular hydrogen bonds (dashed lines) have been omitted for clarity.

3-Acetyl-4-hydroxy-6,7-dimethyl-2H-chromen-2-one top

Crystal data top

C₁₃H₁₂O₄	F(000) = 488
M_r = 232.23	D_x = 1.456 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 5636 reflections
a = 3.9491 (4) Å	θ = 3.2–30.2°
b = 12.1359 (11) Å	µ = 0.11 mm⁻¹
c = 22.101 (2) Å	T = 100 K
β = 90.563 (1)°	Block, brown
V = 1059.16 (17) Å³	0.32 × 0.19 × 0.13 mm
Z = 4

Data collection top

Bruker APEXII DUO CCD area-detector diffractometer	3139 independent reflections
Radiation source: fine-focus sealed tube	2539 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.030
φ and ω scans	θ_max = 30.2°, θ_min = 3.2°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −5→5
T_min = 0.966, T_max = 0.986	k = −17→17
13172 measured reflections	l = −29→31

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)] = 0.048	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.148	H atoms treated by a mixture of independent and constrained refinement
S = 1.05	w = 1/[σ²(F_o²) + (0.0758P)² + 0.5033P] where P = (F_o² + 2F_c²)/3
3139 reflections	(Δ/σ)_max < 0.001
161 parameters	Δρ_max = 0.64 e Å⁻³
0 restraints	Δρ_min = −0.28 e Å⁻³

Crystal data top

C₁₃H₁₂O₄	V = 1059.16 (17) Å³
M_r = 232.23	Z = 4
Monoclinic, P2₁/c	Mo Kα radiation
a = 3.9491 (4) Å	µ = 0.11 mm⁻¹
b = 12.1359 (11) Å	T = 100 K
c = 22.101 (2) Å	0.32 × 0.19 × 0.13 mm
β = 90.563 (1)°

Data collection top

Bruker APEXII DUO CCD area-detector diffractometer	3139 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2009)	2539 reflections with I > 2σ(I)
T_min = 0.966, T_max = 0.986	R_int = 0.030
13172 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.048	0 restraints
wR(F²) = 0.148	H atoms treated by a mixture of independent and constrained refinement
S = 1.05	Δρ_max = 0.64 e Å⁻³
3139 reflections	Δρ_min = −0.28 e Å⁻³
161 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 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

	x	y	z	U_iso*/U_eq
O1	0.2856 (2)	0.47223 (7)	0.06747 (4)	0.0226 (2)
O2	0.8349 (3)	0.53534 (8)	0.22188 (4)	0.0257 (2)
O3	0.8887 (3)	0.33852 (8)	0.23973 (5)	0.0279 (2)
O4	0.2728 (3)	0.29458 (8)	0.08591 (5)	0.0282 (2)
C1	0.3765 (3)	0.38338 (10)	0.10303 (6)	0.0207 (2)
C2	0.3684 (3)	0.57902 (10)	0.08348 (6)	0.0194 (2)
C3	0.2653 (3)	0.66119 (10)	0.04395 (6)	0.0203 (2)
H3A	0.1509	0.6432	0.0083	0.024*
C4	0.3335 (3)	0.77060 (10)	0.05780 (6)	0.0193 (2)
C5	0.5009 (3)	0.79852 (10)	0.11250 (6)	0.0198 (2)
C6	0.6079 (3)	0.71492 (10)	0.15068 (6)	0.0203 (2)
H6A	0.7232	0.7324	0.1863	0.024*
C7	0.5447 (3)	0.60395 (10)	0.13639 (6)	0.0192 (2)
C8	0.6577 (3)	0.51334 (10)	0.17322 (5)	0.0193 (2)
C9	0.5807 (3)	0.40436 (10)	0.15620 (5)	0.0184 (2)
C10	0.7117 (3)	0.31530 (11)	0.19379 (6)	0.0218 (3)
C11	0.6506 (4)	0.19714 (11)	0.18029 (7)	0.0259 (3)
H11B	0.7690	0.1525	0.2094	0.039*
H11C	0.7309	0.1805	0.1405	0.039*
H11D	0.4124	0.1820	0.1822	0.039*
C12	0.2309 (3)	0.85786 (11)	0.01304 (6)	0.0249 (3)
H12A	0.0781	0.8268	−0.0164	0.037*
H12B	0.4284	0.8854	−0.0069	0.037*
H12C	0.1207	0.9171	0.0338	0.037*
C13	0.5604 (4)	0.91704 (11)	0.12989 (7)	0.0264 (3)
H13A	0.6985	0.9200	0.1658	0.040*
H13B	0.3471	0.9522	0.1373	0.040*
H13C	0.6735	0.9544	0.0976	0.040*
H1O2	0.886 (4)	0.4360 (15)	0.2387 (8)	0.029 (4)*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0297 (5)	0.0181 (4)	0.0201 (4)	−0.0013 (3)	−0.0067 (3)	0.0018 (3)
O2	0.0335 (5)	0.0222 (5)	0.0210 (5)	−0.0004 (4)	−0.0084 (4)	−0.0013 (4)
O3	0.0345 (5)	0.0247 (5)	0.0244 (5)	0.0003 (4)	−0.0086 (4)	0.0041 (4)
O4	0.0363 (5)	0.0201 (5)	0.0282 (5)	−0.0049 (4)	−0.0071 (4)	−0.0018 (4)
C1	0.0241 (5)	0.0180 (5)	0.0201 (6)	−0.0002 (4)	−0.0010 (4)	0.0013 (4)
C2	0.0222 (5)	0.0157 (5)	0.0203 (6)	0.0000 (4)	0.0006 (4)	0.0000 (4)
C3	0.0235 (5)	0.0182 (5)	0.0191 (6)	−0.0003 (4)	−0.0017 (4)	0.0001 (4)
C4	0.0213 (5)	0.0169 (5)	0.0198 (6)	0.0010 (4)	−0.0008 (4)	0.0015 (4)
C5	0.0222 (5)	0.0173 (5)	0.0197 (6)	0.0000 (4)	−0.0006 (4)	0.0000 (4)
C6	0.0225 (5)	0.0202 (5)	0.0182 (6)	−0.0002 (4)	−0.0020 (4)	0.0002 (4)
C7	0.0214 (5)	0.0182 (5)	0.0179 (6)	0.0014 (4)	0.0000 (4)	0.0019 (4)
C8	0.0214 (5)	0.0201 (5)	0.0163 (5)	−0.0002 (4)	−0.0016 (4)	−0.0002 (4)
C9	0.0211 (5)	0.0163 (5)	0.0177 (5)	0.0004 (4)	−0.0007 (4)	0.0013 (4)
C10	0.0229 (5)	0.0214 (6)	0.0212 (6)	0.0006 (4)	0.0000 (4)	0.0029 (4)
C11	0.0293 (6)	0.0178 (6)	0.0304 (7)	0.0002 (4)	−0.0035 (5)	0.0051 (5)
C12	0.0304 (6)	0.0196 (6)	0.0245 (6)	0.0015 (5)	−0.0060 (5)	0.0039 (5)
C13	0.0331 (6)	0.0182 (6)	0.0279 (7)	−0.0021 (5)	−0.0050 (5)	−0.0015 (5)

Geometric parameters (Å, º) top

O1—C1	1.3797 (15)	C6—C7	1.4050 (17)
O1—C2	1.3819 (15)	C6—H6A	0.9300
O2—C8	1.3048 (15)	C7—C8	1.4366 (17)
O2—H1O2	1.277 (18)	C8—C9	1.4074 (17)
O3—C10	1.2590 (16)	C9—C10	1.4550 (17)
O3—H1O2	1.183 (18)	C10—C11	1.4840 (18)
O4—C1	1.2121 (15)	C11—H11B	0.9600
C1—C9	1.4415 (17)	C11—H11C	0.9600
C2—C3	1.3843 (17)	C11—H11D	0.9600
C2—C7	1.3884 (17)	C12—H12A	0.9600
C3—C4	1.3884 (17)	C12—H12B	0.9600
C3—H3A	0.9300	C12—H12C	0.9600
C4—C5	1.4133 (17)	C13—H13A	0.9600
C4—C12	1.5022 (17)	C13—H13B	0.9600
C5—C6	1.3831 (17)	C13—H13C	0.9600
C5—C13	1.5066 (17)

C1—O1—C2	121.83 (10)	C9—C8—C7	120.19 (11)
C8—O2—H1O2	97.4 (8)	C8—C9—C1	120.10 (11)
C10—O3—H1O2	101.7 (9)	C8—C9—C10	118.09 (11)
O4—C1—O1	115.58 (11)	C1—C9—C10	121.82 (11)
O4—C1—C9	126.59 (12)	O3—C10—C9	119.06 (12)
O1—C1—C9	117.83 (10)	O3—C10—C11	117.79 (11)
O1—C2—C3	116.52 (11)	C9—C10—C11	123.15 (11)
O1—C2—C7	122.39 (11)	C10—C11—H11B	109.5
C3—C2—C7	121.09 (11)	C10—C11—H11C	109.5
C2—C3—C4	119.64 (11)	H11B—C11—H11C	109.5
C2—C3—H3A	120.2	C10—C11—H11D	109.5
C4—C3—H3A	120.2	H11B—C11—H11D	109.5
C3—C4—C5	120.40 (11)	H11C—C11—H11D	109.5
C3—C4—C12	118.59 (11)	C4—C12—H12A	109.5
C5—C4—C12	121.00 (11)	C4—C12—H12B	109.5
C6—C5—C4	118.91 (11)	H12A—C12—H12B	109.5
C6—C5—C13	119.92 (11)	C4—C12—H12C	109.5
C4—C5—C13	121.17 (11)	H12A—C12—H12C	109.5
C5—C6—C7	120.90 (11)	H12B—C12—H12C	109.5
C5—C6—H6A	119.6	C5—C13—H13A	109.5
C7—C6—H6A	119.6	C5—C13—H13B	109.5
C2—C7—C6	118.99 (11)	H13A—C13—H13B	109.5
C2—C7—C8	117.43 (11)	C5—C13—H13C	109.5
C6—C7—C8	123.57 (11)	H13A—C13—H13C	109.5
O2—C8—C9	121.68 (11)	H13B—C13—H13C	109.5
O2—C8—C7	118.12 (11)

C2—O1—C1—O4	176.00 (11)	C5—C6—C7—C2	−0.86 (19)
C2—O1—C1—C9	−3.75 (17)	C5—C6—C7—C8	178.13 (11)
C1—O1—C2—C3	179.59 (11)	C2—C7—C8—O2	177.43 (11)
C1—O1—C2—C7	−0.62 (18)	C6—C7—C8—O2	−1.57 (19)
O1—C2—C3—C4	178.64 (11)	C2—C7—C8—C9	−1.77 (18)
C7—C2—C3—C4	−1.15 (19)	C6—C7—C8—C9	179.23 (11)
C2—C3—C4—C5	−1.35 (18)	O2—C8—C9—C1	178.30 (11)
C2—C3—C4—C12	177.66 (11)	C7—C8—C9—C1	−2.53 (18)
C3—C4—C5—C6	2.68 (18)	O2—C8—C9—C10	−1.45 (18)
C12—C4—C5—C6	−176.30 (11)	C7—C8—C9—C10	177.72 (11)
C3—C4—C5—C13	−176.65 (12)	O4—C1—C9—C8	−174.46 (13)
C12—C4—C5—C13	4.36 (19)	O1—C1—C9—C8	5.26 (17)
C4—C5—C6—C7	−1.57 (19)	O4—C1—C9—C10	5.3 (2)
C13—C5—C6—C7	177.78 (11)	O1—C1—C9—C10	−175.00 (11)
O1—C2—C7—C6	−177.53 (11)	C8—C9—C10—O3	−0.23 (18)
C3—C2—C7—C6	2.25 (19)	C1—C9—C10—O3	−179.98 (12)
O1—C2—C7—C8	3.42 (18)	C8—C9—C10—C11	−179.43 (12)
C3—C2—C7—C8	−176.80 (11)	C1—C9—C10—C11	0.82 (19)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O2—H1O2···O3	1.277 (18)	1.183 (18)	2.4299 (14)	162.0 (16)
C6—H6A···O3ⁱ	0.93	2.58	3.4603 (17)	159
C11—H11B···O2ⁱⁱ	0.96	2.59	3.5458 (18)	172
C12—H12A···O4ⁱⁱⁱ	0.96	2.53	3.4751 (17)	168

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

Experimental details

Crystal data
Chemical formula	C₁₃H₁₂O₄
M_r	232.23
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	3.9491 (4), 12.1359 (11), 22.101 (2)
β (°)	90.563 (1)
V (Å³)	1059.16 (17)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.11
Crystal size (mm)	0.32 × 0.19 × 0.13

Data collection
Diffractometer	Bruker APEXII DUO CCD area-detector
Absorption correction	Multi-scan (SADABS; Bruker, 2009)
T_min, T_max	0.966, 0.986
No. of measured, independent and observed [I > 2σ(I)] reflections	13172, 3139, 2539
R_int	0.030
(sin θ/λ)_max (Å⁻¹)	0.708

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.048, 0.148, 1.05
No. of reflections	3139
No. of parameters	161
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.64, −0.28

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

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O2—H1O2···O3	1.277 (18)	1.183 (18)	2.4299 (14)	162.0 (16)
C6—H6A···O3ⁱ	0.93	2.58	3.4603 (17)	159
C11—H11B···O2ⁱⁱ	0.96	2.59	3.5458 (18)	172
C12—H12A···O4ⁱⁱⁱ	0.96	2.53	3.4751 (17)	168

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

Footnotes

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

§Thomson Reuters ResearcherID: C-7576-2009.

¶Thomson Reuters ResearcherID: A-3561-2009.

Acknowledgements

The authors are thankful to Universiti Sains Malaysia (USM) for providing the necessary research facilities and RU research funding under grant No. 1001/PKIMIA/811134. JHG and HKF also thank USM for the Research University Grant (No. 1001/PFIZIK/811160) and MA also thanks USM for the award of a post doctoral fellowship.

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