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ISSN: 2056-9890

2,3,4,9-Tetra­hydro-1H-carbazole

aDepartment of Physics, Thanthai Periyar Government Institute of Technology, Vellore 632 002, India, bDepartment of Physics, SMK Fomra Institute of Technology, Thaiyur, Chennai 603 103, India, cDepartment of Physics, Presidency College (Autonomous), Chennai 600 005, India, dDepartment of Chemistry, Sathyabama University, Jeppiaar Nagar, Chennai 600 119, India, and eDepartment of Chemistry, Mohamed Sathak A. J. College of Engineering, Egattur, Chennai 603 103, India
*Correspondence e-mail: a_spandian@yahoo.com

(Received 31 October 2008; accepted 19 November 2008; online 26 November 2008)

In the title compound, C12H13N, two methyl­ene C atoms of the cyclo­hexene ring are disordered over two sites with occupancies of 0.591 (10) and 0.409 (10); both disorder components adopt half-chair conformations. The crystal structure is stabilized by inter­molecular N—H⋯π and C—H⋯π inter­actions.

Related literature

For a related structure, see: Arulmozhi et al. (2008[Arulmozhi, R., Vennila, J. P., Babu, S. M., Kavitha, H. P. & Manivannan, V. (2008). Acta Cryst. E64, o1208.]). For general background, see: Mi et al. (2003[Mi, B. X., Wang, P. F., Liu, M. W., Kwong, H. L., Wong, N. B., Lee, C. S. & Lee, S. T. (2003). Chem. Mater. 15, 3148-3151.]); Hewlins et al. (1984[Hewlins, J. M. E., Oliveira-Campos, A. M. & Shannon, P. V. R. (1984). Synthesis, pp. 289-302.]); Mohanakrishnan & Srinivasan (1995a[Mohanakrishnan, A. K. & Srinivasan, P. C. (1995a). Indian J. Chem. Sect. B, 35, 838-841.],b[Mohanakrishnan, A. K. & Srinivasan, P. C. (1995b). J. Org. Chem. 60, 1939-1946.]); Kansal & Potier (1986[Kansal, V. K. & Potier, P. (1986). Tetrahedron, 42, 2389-2408.]); Phillipson & Zenk (1980[Phillipson, J. D. & Zenk, M. H. (1980). Editors. Indole and Biogenitically Related Alkaloids, ch. 3. New York: Academic Press.]); Saxton (1983[Saxton, J. E. (1983). Editor. Heterocyclic Compounds, Vol. 25, The Monoterpenoid Indole Alkaloids, chs. 8 and 11. New York: Wiley.]); Abraham (1975[Abraham, D. J. (1975). The Catharanthus Alkaloids, edited by W. I. Taylor & N. R. Farnsworth, chs. 7 and 8. New York: Marcel Decker.]).

[Scheme 1]

Experimental

Crystal data
  • C12H13N

  • Mr = 171.23

  • Orthorhombic, P 21 21 21

  • a = 6.1067 (4) Å

  • b = 7.9488 (5) Å

  • c = 19.4512 (12) Å

  • V = 944.18 (10) Å3

  • Z = 4

  • Mo Kα radiation

  • μ = 0.07 mm−1

  • T = 293 (2) K

  • 0.26 × 0.15 × 0.15 mm

Data collection
  • Bruker Kappa APEXII area-detector diffractometer

  • Absorption correction: none

  • 13269 measured reflections

  • 1777 independent reflections

  • 1323 reflections with I > 2σ(I)

  • Rint = 0.036

Refinement
  • R[F2 > 2σ(F2)] = 0.048

  • wR(F2) = 0.123

  • S = 1.07

  • 1777 reflections

  • 137 parameters

  • 15 restraints

  • H-atom parameters constrained

  • Δρmax = 0.14 e Å−3

  • Δρmin = −0.20 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
N1—H1ACg2i 0.86 2.62 3.327 (1) 140
C4—H4⋯Cg1i 0.93 2.86 3.645 (1) 143
C12—H12BCg2ii 0.97 2.83 3.577 (2) 135
C12—H12DCg2ii 0.96 2.72 3.577 (2) 149
Symmetry codes: (i) [x+{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+2]; (ii) [x-{\script{1\over 2}}, -y+{\script{1\over 2}}, -z+2].Cg1 and Cg2 are the centroids of the N1/C5–C8 and C1–C6 rings, respectively.

Data collection: APEX2 (Bruker, 2004[Bruker (2004). APEX2 and SAINT. Bruker AXS Inc., Madison Wisconsin, USA.]); cell refinement: SAINT (Bruker, 2004[Bruker (2004). APEX2 and SAINT. Bruker AXS Inc., Madison Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: ORTEP-3 (Farrugia, 1997[Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.]); software used to prepare material for publication: SHELXL97 and PLATON (Spek, 2003[Spek, A. L. (2003). J. Appl. Cryst. 36, 7-13.]).

Supporting information


Comment top

Carbazole derivatives exhibit good charge transfer and hole transporting properties, which are being explored for a multitude of optoelectronic and photocatalytic applications, including organic light emitting diodes (OLEDs) (Mi et al., 2003). In carbazole derivatives, the preliminary study shows that the presence of oxygenated substituents increases their biological activity (Hewlins et al., 1984). The 2,3-disubstituted indoles have been used as bidentate synthons for the synthesis of various medicinally important carbazole alkaloids (Mohanakrishnan & Srinivasan, 1995a,b). Intercalation between the base pairs in DNA has been implicated for their anticancer activity. It was conceived that the benzo[b] carbazoles as isosteric analogs of pyrido[4,3-b]carbazoles, with oxygenated D-ring could mimic the anti-cancer activity of ellipticine. So it was of interest to study the anticancer activity of D-ring oxygenated benzo[b]carbazoles as it is believed that these molecules could form a stable intercalation complex with DNA (Kansal & Potier, 1986). Tetrahydrocarbazole derivatives are present in the framework of indole-type alkaloids of biological interest (Phillipson & Zenk, 1980; Saxton, 1983; Abraham, 1975). We report here the crystal structure of the title compound (Fig. 1).

Bond lengths are normal and are comparable to the corresponding values observed in 1-naphthyl-9H-carbazole-4-sulfonate (Arulmozhi et al., 2008). The dihedral angle between the C1–C6 and N1/C5—C8 rings is 0.6 (1)°. Both the major and minor conformers of the disordered cyclohexene ring adopt half-chair conformations.

The crystal structure is stabilized by intermolecular N—H···π and C–H···π interactions (Table 1).

Related literature top

For a related structure, see: Arulmozhi et al. (2008). For general background, see: Mi et al. (2003); Hewlins et al. (1984); Mohanakrishnan & Srinivasan (1995a,b); Kansal & Potier (1986); Phillipson & Zenk (1980); Saxton (1983); Abraham (1975). Cg1 and Cg2 are the centroids of the N1/C5–C8 and C1–C6 rings,

respectively.

Experimental top

A mixture of cyclohexanone (0.12 mol) and glacial acetic acid (40 ml) was heated and then redistilled phenylhydrazine (0.1 mol) was added dropwise for 30 min. The mixture was refluxed on a water bath for a further period of 30 min. The reaction mixture was poured into ice-cold water with continuous stirring and brown-coloured solid separated out. It was filtered, washed repeatedly with water and recrystallized from methanol in the presence of a little decolorized carbon to give the title compound. Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a methanol solution.

Refinement top

Atoms C10 and C11 of the cyclohexene ring are disordered over two positions (C10A/C10B and C11A/C11B) with refined occupancies of 0.591 (10) and 0.409 (10). The corresponding bond distances involving the disordered atoms were restrained to be equal. H atoms were positioned geometrically (C—H = 0.93Å and N—H = 0.86%A) and were treated as riding on their parent atoms, with Uiso(H)=1.2Ueq(C,N). In the absence of significant anomalous dispersion effects, Friedel pairs were merged before the final refinement.

Computing details top

Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2 (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2003).

Figures top
[Figure 1] Fig. 1. The molecular structure of title compound, showing 30% probability displacement ellipsoids. Both disorder components are shown.
2,3,4,9-Tetrahydro-1H-carbazole top
Crystal data top
C12H13NF(000) = 368
Mr = 171.23Dx = 1.205 Mg m3
Orthorhombic, P212121Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 1778 reflections
a = 6.1067 (4) Åθ = 2.1–31.1°
b = 7.9488 (5) ŵ = 0.07 mm1
c = 19.4512 (12) ÅT = 293 K
V = 944.18 (10) Å3Block, colourless
Z = 40.26 × 0.15 × 0.15 mm
Data collection top
Bruker Kappa APEXII area-detector
diffractometer
1323 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.036
Graphite monochromatorθmax = 31.1°, θmin = 2.1°
ω and ϕ scansh = 88
13269 measured reflectionsk = 1111
1777 independent reflectionsl = 2828
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)] = 0.048Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.123H-atom parameters constrained
S = 1.07 w = 1/[σ2(Fo2) + (0.0603P)2 + 0.0496P]
where P = (Fo2 + 2Fc2)/3
1777 reflections(Δ/σ)max = 0.001
137 parametersΔρmax = 0.14 e Å3
15 restraintsΔρmin = 0.20 e Å3
Crystal data top
C12H13NV = 944.18 (10) Å3
Mr = 171.23Z = 4
Orthorhombic, P212121Mo Kα radiation
a = 6.1067 (4) ŵ = 0.07 mm1
b = 7.9488 (5) ÅT = 293 K
c = 19.4512 (12) Å0.26 × 0.15 × 0.15 mm
Data collection top
Bruker Kappa APEXII area-detector
diffractometer
1323 reflections with I > 2σ(I)
13269 measured reflectionsRint = 0.036
1777 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.04815 restraints
wR(F2) = 0.123H-atom parameters constrained
S = 1.07Δρmax = 0.14 e Å3
1777 reflectionsΔρmin = 0.20 e Å3
137 parameters
Special details top

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*/UeqOcc. (<1)
N10.7448 (2)0.5987 (2)0.95685 (8)0.0547 (4)
H1A0.86560.65130.94950.066*
C10.3210 (3)0.4486 (2)1.06567 (10)0.0557 (5)
H10.19450.38461.06040.067*
C20.3803 (4)0.5087 (3)1.12920 (10)0.0634 (5)
H20.29210.48541.16700.076*
C30.5689 (4)0.6033 (3)1.13826 (10)0.0619 (5)
H30.60460.64191.18200.074*
C40.7035 (3)0.6410 (2)1.08415 (10)0.0568 (5)
H40.82980.70451.09040.068*
C50.6448 (3)0.5811 (2)1.01954 (9)0.0457 (4)
C60.4535 (3)0.4849 (2)1.00903 (9)0.0429 (4)
C70.4433 (3)0.4464 (2)0.93742 (8)0.0429 (4)
C80.6210 (3)0.5184 (2)0.90741 (9)0.0473 (4)
C90.6730 (3)0.5153 (3)0.83301 (10)0.0668 (6)
H9A0.82520.48410.82670.080*0.591 (10)
H9B0.65210.62670.81380.080*0.591 (10)
H9C0.78930.43710.82400.080*0.409 (10)
H9D0.71930.62490.81820.080*0.409 (10)
C10A0.5287 (9)0.3918 (9)0.7958 (4)0.0674 (15)0.591 (10)
H10A0.58760.27950.80190.101*0.591 (10)
H10B0.53180.41710.74700.101*0.591 (10)
C11A0.2927 (8)0.3943 (9)0.8204 (2)0.0638 (13)0.591 (10)
H11A0.20830.31330.79410.096*0.591 (10)
H11B0.23080.50490.81220.096*0.591 (10)
C10B0.4709 (17)0.4587 (13)0.7953 (5)0.076 (2)0.409 (10)
H10C0.36990.55270.79240.114*0.409 (10)
H10D0.51170.42820.74870.114*0.409 (10)
C11B0.3543 (15)0.3125 (11)0.8276 (3)0.0651 (19)0.409 (10)
H11C0.23130.28060.79890.098*0.409 (10)
H11D0.45340.21730.83000.098*0.409 (10)
C120.2741 (3)0.3519 (3)0.89757 (10)0.0579 (5)
H12A0.12930.38170.91410.069*0.591 (10)
H12B0.29430.23200.90430.069*0.591 (10)
H12C0.14280.41790.89410.069*0.409 (10)
H12D0.23900.24940.92120.069*0.409 (10)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
N10.0455 (8)0.0620 (9)0.0565 (9)0.0161 (8)0.0044 (7)0.0019 (7)
C10.0518 (10)0.0556 (11)0.0597 (11)0.0079 (9)0.0078 (9)0.0039 (9)
C20.0707 (12)0.0693 (12)0.0502 (10)0.0015 (12)0.0120 (9)0.0040 (9)
C30.0752 (13)0.0599 (11)0.0507 (11)0.0054 (11)0.0054 (10)0.0048 (9)
C40.0577 (11)0.0495 (10)0.0631 (12)0.0038 (9)0.0095 (10)0.0022 (9)
C50.0447 (8)0.0402 (8)0.0522 (9)0.0020 (7)0.0005 (7)0.0042 (7)
C60.0425 (8)0.0374 (7)0.0488 (8)0.0012 (7)0.0007 (7)0.0036 (7)
C70.0422 (8)0.0379 (8)0.0487 (9)0.0013 (7)0.0007 (7)0.0010 (7)
C80.0440 (8)0.0468 (9)0.0511 (9)0.0018 (8)0.0023 (7)0.0017 (8)
C90.0601 (11)0.0876 (15)0.0526 (10)0.0017 (12)0.0084 (9)0.0054 (11)
C10A0.061 (3)0.082 (4)0.059 (2)0.010 (3)0.004 (2)0.007 (3)
C11A0.057 (2)0.076 (3)0.058 (2)0.003 (2)0.0071 (18)0.010 (2)
C10B0.084 (6)0.097 (6)0.046 (3)0.009 (5)0.008 (4)0.005 (4)
C11B0.071 (4)0.067 (4)0.057 (3)0.003 (4)0.011 (3)0.018 (3)
C120.0515 (10)0.0590 (11)0.0632 (11)0.0084 (9)0.0038 (9)0.0028 (9)
Geometric parameters (Å, º) top
N1—C51.371 (2)C9—H9B0.97
N1—C81.379 (2)C9—H9C0.96
N1—H1A0.86C9—H9D0.96
C1—C21.374 (3)C10A—C11A1.519 (7)
C1—C61.397 (2)C10A—H10A0.97
C1—H10.93C10A—H10B0.97
C2—C31.387 (3)C11A—C121.542 (5)
C2—H20.93C11A—H11A0.97
C3—C41.369 (3)C11A—H11B0.97
C3—H30.93C10B—C11B1.501 (10)
C4—C51.391 (3)C10B—H10C0.97
C4—H40.93C10B—H10D0.97
C5—C61.411 (2)C11B—C121.480 (6)
C6—C71.427 (2)C11B—H11C0.97
C7—C81.359 (2)C11B—H11D0.97
C7—C121.494 (2)C12—H12A0.97
C8—C91.482 (3)C12—H12B0.97
C9—C10B1.505 (9)C12—H12C0.96
C9—C10A1.505 (6)C12—H12D0.96
C9—H9A0.97
C5—N1—C8109.19 (14)H9C—C9—H9D108.3
C5—N1—H1A125.4C9—C10A—C11A113.3 (5)
C8—N1—H1A125.4C9—C10A—H10A108.9
C2—C1—C6118.99 (18)C11A—C10A—H10A108.9
C2—C1—H1120.5C9—C10A—H10B108.9
C6—C1—H1120.5C11A—C10A—H10B108.9
C1—C2—C3121.49 (19)H10A—C10A—H10B107.7
C1—C2—H2119.3C10A—C11A—C12112.0 (5)
C3—C2—H2119.3C10A—C11A—H11A109.2
C4—C3—C2121.30 (18)C12—C11A—H11A109.2
C4—C3—H3119.3C10A—C11A—H11B109.2
C2—C3—H3119.3C12—C11A—H11B109.2
C3—C4—C5117.72 (18)H11A—C11A—H11B107.9
C3—C4—H4121.1C11B—C10B—C9114.6 (7)
C5—C4—H4121.1C11B—C10B—H10C108.6
N1—C5—C4130.84 (17)C9—C10B—H10C108.6
N1—C5—C6107.17 (15)C11B—C10B—H10D108.6
C4—C5—C6121.99 (17)C9—C10B—H10D108.6
C1—C6—C5118.50 (16)H10C—C10B—H10D107.6
C1—C6—C7134.42 (16)C12—C11B—C10B112.2 (7)
C5—C6—C7107.08 (15)C12—C11B—H11C109.2
C8—C7—C6107.10 (14)C10B—C11B—H11C109.2
C8—C7—C12122.77 (16)C12—C11B—H11D109.2
C6—C7—C12130.10 (15)C10B—C11B—H11D109.2
C7—C8—N1109.45 (15)H11C—C11B—H11D107.9
C7—C8—C9125.70 (17)C11B—C12—C7110.8 (3)
N1—C8—C9124.85 (17)C7—C12—C11A110.1 (2)
C8—C9—C10B107.8 (4)C11B—C12—H12A131.1
C8—C9—C10A110.8 (3)C7—C12—H12A109.6
C8—C9—H9A109.5C11A—C12—H12A109.6
C10B—C9—H9A130.4C11B—C12—H12B82.7
C10A—C9—H9A109.5C7—C12—H12B109.6
C8—C9—H9B109.5C11A—C12—H12B109.6
C10B—C9—H9B88.7H12A—C12—H12B108.1
C10A—C9—H9B109.5C11B—C12—H12C109.1
H9A—C9—H9B108.1C7—C12—H12C109.8
C8—C9—H9C110.3C11A—C12—H12C82.8
C10B—C9—H9C109.0H12B—C12—H12C130.7
C10A—C9—H9C85.6C11B—C12—H12D109.5
H9B—C9—H9C128.2C7—C12—H12D109.4
C8—C9—H9D109.9C11A—C12—H12D131.9
C10B—C9—H9D111.5H12A—C12—H12D81.1
C10A—C9—H9D128.3H12C—C12—H12D108.1
H9A—C9—H9D84.9
C6—C1—C2—C30.4 (3)C5—N1—C8—C71.0 (2)
C1—C2—C3—C40.1 (3)C5—N1—C8—C9177.78 (18)
C2—C3—C4—C50.0 (3)C7—C8—C9—C10B14.1 (5)
C8—N1—C5—C4178.92 (19)N1—C8—C9—C10B164.5 (5)
C8—N1—C5—C60.7 (2)C7—C8—C9—C10A11.7 (4)
C3—C4—C5—N1179.77 (19)N1—C8—C9—C10A169.7 (3)
C3—C4—C5—C60.2 (3)C8—C9—C10A—C11A40.6 (8)
C2—C1—C6—C50.6 (3)C10B—C9—C10A—C11A46.8 (11)
C2—C1—C6—C7179.34 (19)C9—C10A—C11A—C1259.6 (9)
N1—C5—C6—C1179.87 (15)C8—C9—C10B—C11B43.8 (11)
C4—C5—C6—C10.4 (3)C10A—C9—C10B—C11B57.5 (12)
N1—C5—C6—C70.20 (18)C9—C10B—C11B—C1261.8 (14)
C4—C5—C6—C7179.48 (17)C10B—C11B—C12—C742.9 (10)
C1—C6—C7—C8179.52 (19)C10B—C11B—C12—C11A51.4 (8)
C5—C6—C7—C80.39 (18)C8—C7—C12—C11B14.2 (5)
C1—C6—C7—C121.6 (3)C6—C7—C12—C11B168.1 (5)
C5—C6—C7—C12178.35 (17)C8—C7—C12—C11A17.0 (4)
C6—C7—C8—N10.84 (19)C6—C7—C12—C11A160.7 (3)
C12—C7—C8—N1178.98 (16)C10A—C11A—C12—C11B51.7 (7)
C6—C7—C8—C9177.92 (18)C10A—C11A—C12—C745.1 (7)
C12—C7—C8—C90.2 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···Cg2i0.862.623.327 (1)140
C4—H4···Cg1i0.932.863.645 (1)143
C12—H12B···Cg2ii0.972.833.577 (2)135
C12—H12D···Cg2ii0.962.723.577 (2)149
Symmetry codes: (i) x+1/2, y+3/2, z+2; (ii) x1/2, y+1/2, z+2.

Experimental details

Crystal data
Chemical formulaC12H13N
Mr171.23
Crystal system, space groupOrthorhombic, P212121
Temperature (K)293
a, b, c (Å)6.1067 (4), 7.9488 (5), 19.4512 (12)
V3)944.18 (10)
Z4
Radiation typeMo Kα
µ (mm1)0.07
Crystal size (mm)0.26 × 0.15 × 0.15
Data collection
DiffractometerBruker Kappa APEXII area-detector
diffractometer
Absorption correction
No. of measured, independent and
observed [I > 2σ(I)] reflections
13269, 1777, 1323
Rint0.036
(sin θ/λ)max1)0.727
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.048, 0.123, 1.07
No. of reflections1777
No. of parameters137
No. of restraints15
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.14, 0.20

Computer programs: APEX2 (Bruker, 2004), SAINT (Bruker, 2004), SHELXS97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 1997), SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2003).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···Cg2i0.862.623.327 (1)140
C4—H4···Cg1i0.932.863.645 (1)143
C12—H12B···Cg2ii0.972.833.577 (2)135
C12—H12D···Cg2ii0.962.723.577 (2)149
Symmetry codes: (i) x+1/2, y+3/2, z+2; (ii) x1/2, y+1/2, z+2.
 

Acknowledgements

The authors are grateful to Dr J. Jothi Kumar, Principal of Presidency College (Autonomous), Chennai, for providing computer and internet facilities. Dr Babu Vargheese, SAIF, IIT-Madras, India, is thanked for his help with the data collection.

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