organic compounds
N-[(E)-4-Chlorobenzylidene]-2,4-dimethylaniline
aX-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia, and bDepartment of Chemistry, Mangalore University, Karnataka, India
*Correspondence e-mail: hkfun@usm.my
The title molecule, C15H14ClN, exists in a trans configuration with respect to the C=N bond [1.2813 (16) Å]. The dihedral angle between the benzene rings is 52.91 (6)°. The is stabilized by weak intermolecular C—H⋯π interactions.
Related literature
For general background to and the pharmacological activity of Schiff base compounds, see: Ittel et al. (2000); Shah et al. (1992); Cimerman et al. (2000); Pandeya et al. (1999); More et al. (2001); Cimerman & Stefanac (2001); Galic et al. (1997). For the stability of the temperature controller used in the data collection, see: Cosier & Glazer (1986). For standard bond-length data, see: Allen et al. (1987).
Experimental
Crystal data
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Data collection: APEX2 (Bruker, 2009); cell 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
10.1107/S1600536811026109/lh5274sup1.cif
contains datablocks global, I. DOI:Structure factors: contains datablock I. DOI: 10.1107/S1600536811026109/lh5274Isup2.hkl
Supporting information file. DOI: 10.1107/S1600536811026109/lh5274Isup3.cml
Equimolar amounts of of 4-chloro benzaldehyde and 2,4 dimethyl aniline were dissolved in a minimum amount of ethanol, followed by addition of 2 ml glacial acetic acid. The solution was refluxed for 8 h then cooled to room temperature and poured into ice cold water. The solid product was collected through filtration and then dried at 353 K. The product was dissolved in ethanol, recrystallized and then dried to give colourless crystals. Yield: 75%, m.p. 432-435 K.
All H atoms were positioned geometrically and refined using a riding model with C–H = 0.95 or 0.98 Å and Uiso(H) = 1.2 or 1.5 Ueq(C). A rotating-group model was applied for the methyl group. The highest residual electron density peak is located at 0.69 Å from C12 and the deepest hole is located at 0.15 Å from H15B.
Data collection: APEX2 (Bruker, 2009); cell
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).Fig. 1. The molecular structure of the title compound showing 50% probability displacement ellipsoids for non-H atoms. |
C15H14ClN | F(000) = 512 |
Mr = 243.72 | Dx = 1.269 Mg m−3 |
Monoclinic, Cc | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: C -2yc | Cell parameters from 9896 reflections |
a = 7.2852 (1) Å | θ = 2.7–31.0° |
b = 15.2715 (2) Å | µ = 0.28 mm−1 |
c = 11.5382 (1) Å | T = 100 K |
β = 96.304 (1)° | Block, colourless |
V = 1275.93 (3) Å3 | 0.40 × 0.24 × 0.20 mm |
Z = 4 |
Bruker SMART APEXII CCD area-detector diffractometer | 3975 independent reflections |
Radiation source: fine-focus sealed tube | 3800 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.020 |
ϕ and ω scans | θmax = 31.1°, θmin = 2.7° |
Absorption correction: multi-scan (SADABS; Bruker, 2009) | h = −10→10 |
Tmin = 0.898, Tmax = 0.946 | k = −17→22 |
14547 measured reflections | l = −16→16 |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.031 | H-atom parameters constrained |
wR(F2) = 0.078 | w = 1/[σ2(Fo2) + (0.0435P)2 + 0.4511P] where P = (Fo2 + 2Fc2)/3 |
S = 1.04 | (Δ/σ)max = 0.001 |
3975 reflections | Δρmax = 0.32 e Å−3 |
156 parameters | Δρmin = −0.20 e Å−3 |
2 restraints | Absolute structure: Flack (1983), 1919 Friedel pairs |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: 0.04 (4) |
C15H14ClN | V = 1275.93 (3) Å3 |
Mr = 243.72 | Z = 4 |
Monoclinic, Cc | Mo Kα radiation |
a = 7.2852 (1) Å | µ = 0.28 mm−1 |
b = 15.2715 (2) Å | T = 100 K |
c = 11.5382 (1) Å | 0.40 × 0.24 × 0.20 mm |
β = 96.304 (1)° |
Bruker SMART APEXII CCD area-detector diffractometer | 3975 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2009) | 3800 reflections with I > 2σ(I) |
Tmin = 0.898, Tmax = 0.946 | Rint = 0.020 |
14547 measured reflections |
R[F2 > 2σ(F2)] = 0.031 | H-atom parameters constrained |
wR(F2) = 0.078 | Δρmax = 0.32 e Å−3 |
S = 1.04 | Δρmin = −0.20 e Å−3 |
3975 reflections | Absolute structure: Flack (1983), 1919 Friedel pairs |
156 parameters | Absolute structure parameter: 0.04 (4) |
2 restraints |
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. |
x | y | z | Uiso*/Ueq | ||
Cl1 | 1.00011 (4) | 0.620805 (19) | 0.24699 (3) | 0.02374 (8) | |
N1 | 0.98639 (15) | 0.21574 (7) | 0.00510 (9) | 0.0164 (2) | |
C1 | 1.10819 (17) | 0.36449 (8) | 0.25068 (11) | 0.0164 (2) | |
H1A | 1.1660 | 0.3226 | 0.3036 | 0.020* | |
C2 | 1.09740 (18) | 0.45134 (9) | 0.28532 (11) | 0.0174 (2) | |
H2A | 1.1469 | 0.4693 | 0.3612 | 0.021* | |
C3 | 1.01230 (18) | 0.51134 (8) | 0.20614 (11) | 0.0172 (2) | |
C4 | 0.93749 (17) | 0.48677 (9) | 0.09481 (11) | 0.0180 (2) | |
H4A | 0.8786 | 0.5288 | 0.0425 | 0.022* | |
C5 | 0.95032 (17) | 0.39987 (8) | 0.06141 (11) | 0.0166 (2) | |
H5A | 0.9011 | 0.3823 | −0.0147 | 0.020* | |
C6 | 1.03527 (17) | 0.33777 (8) | 0.13907 (11) | 0.0152 (2) | |
C7 | 1.04654 (18) | 0.24507 (8) | 0.10622 (11) | 0.0163 (2) | |
H7A | 1.1011 | 0.2049 | 0.1628 | 0.020* | |
C8 | 0.99097 (17) | 0.12365 (8) | −0.01051 (11) | 0.0153 (2) | |
C9 | 1.05963 (17) | 0.08957 (8) | −0.11026 (11) | 0.0151 (2) | |
C10 | 1.05936 (17) | −0.00135 (8) | −0.12576 (11) | 0.0164 (2) | |
H10A | 1.1084 | −0.0250 | −0.1920 | 0.020* | |
C11 | 0.98889 (18) | −0.05863 (8) | −0.04642 (11) | 0.0177 (2) | |
C12 | 0.91927 (18) | −0.02310 (9) | 0.05076 (11) | 0.0182 (2) | |
H12A | 0.8702 | −0.0608 | 0.1052 | 0.022* | |
C13 | 0.92063 (18) | 0.06723 (8) | 0.06920 (11) | 0.0172 (2) | |
H13A | 0.8735 | 0.0905 | 0.1363 | 0.021* | |
C14 | 1.1312 (2) | 0.14980 (9) | −0.19821 (12) | 0.0212 (3) | |
H14A | 1.1739 | 0.1151 | −0.2615 | 0.032* | |
H14B | 1.2342 | 0.1844 | −0.1603 | 0.032* | |
H14C | 1.0321 | 0.1892 | −0.2301 | 0.032* | |
C15 | 0.9863 (2) | −0.15650 (9) | −0.06663 (13) | 0.0244 (3) | |
H15A | 0.8998 | −0.1840 | −0.0183 | 0.037* | |
H15B | 1.1104 | −0.1803 | −0.0457 | 0.037* | |
H15C | 0.9469 | −0.1686 | −0.1490 | 0.037* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Cl1 | 0.03157 (16) | 0.01487 (12) | 0.02478 (15) | 0.00105 (13) | 0.00319 (11) | −0.00480 (12) |
N1 | 0.0188 (5) | 0.0143 (5) | 0.0163 (5) | −0.0001 (4) | 0.0033 (4) | −0.0014 (4) |
C1 | 0.0181 (6) | 0.0165 (5) | 0.0144 (5) | −0.0008 (4) | 0.0007 (4) | −0.0001 (4) |
C2 | 0.0194 (6) | 0.0181 (5) | 0.0144 (5) | −0.0012 (5) | 0.0011 (4) | −0.0029 (4) |
C3 | 0.0189 (6) | 0.0133 (5) | 0.0198 (6) | −0.0009 (4) | 0.0045 (5) | −0.0034 (4) |
C4 | 0.0188 (6) | 0.0163 (6) | 0.0187 (6) | 0.0012 (4) | 0.0010 (4) | 0.0005 (4) |
C5 | 0.0187 (6) | 0.0161 (5) | 0.0149 (5) | 0.0001 (4) | 0.0015 (4) | −0.0014 (4) |
C6 | 0.0159 (5) | 0.0141 (5) | 0.0157 (5) | −0.0004 (4) | 0.0025 (4) | −0.0017 (4) |
C7 | 0.0168 (5) | 0.0145 (5) | 0.0178 (5) | 0.0002 (4) | 0.0024 (4) | −0.0004 (4) |
C8 | 0.0162 (5) | 0.0134 (5) | 0.0160 (6) | −0.0001 (4) | 0.0009 (4) | −0.0009 (4) |
C9 | 0.0164 (5) | 0.0149 (5) | 0.0138 (5) | −0.0010 (4) | 0.0011 (4) | −0.0012 (4) |
C10 | 0.0178 (5) | 0.0160 (5) | 0.0155 (5) | 0.0003 (4) | 0.0016 (4) | −0.0025 (4) |
C11 | 0.0185 (6) | 0.0146 (5) | 0.0191 (6) | −0.0012 (5) | −0.0019 (5) | −0.0005 (4) |
C12 | 0.0198 (6) | 0.0168 (6) | 0.0180 (6) | −0.0029 (5) | 0.0011 (5) | 0.0026 (4) |
C13 | 0.0204 (6) | 0.0173 (6) | 0.0141 (5) | −0.0008 (5) | 0.0024 (4) | −0.0005 (4) |
C14 | 0.0272 (7) | 0.0173 (6) | 0.0205 (6) | −0.0027 (5) | 0.0088 (5) | −0.0008 (5) |
C15 | 0.0305 (7) | 0.0135 (6) | 0.0284 (7) | −0.0016 (5) | −0.0005 (6) | −0.0020 (5) |
Cl1—C3 | 1.7418 (13) | C8—C9 | 1.4043 (17) |
N1—C7 | 1.2813 (16) | C9—C10 | 1.3999 (17) |
N1—C8 | 1.4187 (15) | C9—C14 | 1.5045 (18) |
C1—C2 | 1.3900 (17) | C10—C11 | 1.4036 (18) |
C1—C6 | 1.3988 (17) | C10—H10A | 0.9500 |
C1—H1A | 0.9500 | C11—C12 | 1.3906 (19) |
C2—C3 | 1.3908 (18) | C11—C15 | 1.5125 (18) |
C2—H2A | 0.9500 | C12—C13 | 1.3957 (18) |
C3—C4 | 1.3906 (17) | C12—H12A | 0.9500 |
C4—C5 | 1.3881 (18) | C13—H13A | 0.9500 |
C4—H4A | 0.9500 | C14—H14A | 0.9800 |
C5—C6 | 1.4012 (17) | C14—H14B | 0.9800 |
C5—H5A | 0.9500 | C14—H14C | 0.9800 |
C6—C7 | 1.4702 (17) | C15—H15A | 0.9800 |
C7—H7A | 0.9500 | C15—H15B | 0.9800 |
C8—C13 | 1.3980 (18) | C15—H15C | 0.9800 |
C7—N1—C8 | 116.85 (11) | C10—C9—C14 | 121.06 (11) |
C2—C1—C6 | 121.02 (12) | C8—C9—C14 | 120.48 (11) |
C2—C1—H1A | 119.5 | C9—C10—C11 | 122.03 (12) |
C6—C1—H1A | 119.5 | C9—C10—H10A | 119.0 |
C1—C2—C3 | 118.36 (11) | C11—C10—H10A | 119.0 |
C1—C2—H2A | 120.8 | C12—C11—C10 | 118.34 (12) |
C3—C2—H2A | 120.8 | C12—C11—C15 | 120.68 (12) |
C4—C3—C2 | 122.02 (12) | C10—C11—C15 | 120.98 (12) |
C4—C3—Cl1 | 118.84 (10) | C11—C12—C13 | 120.76 (12) |
C2—C3—Cl1 | 119.13 (10) | C11—C12—H12A | 119.6 |
C5—C4—C3 | 118.88 (12) | C13—C12—H12A | 119.6 |
C5—C4—H4A | 120.6 | C12—C13—C8 | 120.39 (12) |
C3—C4—H4A | 120.6 | C12—C13—H13A | 119.8 |
C4—C5—C6 | 120.52 (11) | C8—C13—H13A | 119.8 |
C4—C5—H5A | 119.7 | C9—C14—H14A | 109.5 |
C6—C5—H5A | 119.7 | C9—C14—H14B | 109.5 |
C1—C6—C5 | 119.20 (11) | H14A—C14—H14B | 109.5 |
C1—C6—C7 | 119.47 (11) | C9—C14—H14C | 109.5 |
C5—C6—C7 | 121.33 (11) | H14A—C14—H14C | 109.5 |
N1—C7—C6 | 123.24 (12) | H14B—C14—H14C | 109.5 |
N1—C7—H7A | 118.4 | C11—C15—H15A | 109.5 |
C6—C7—H7A | 118.4 | C11—C15—H15B | 109.5 |
C13—C8—C9 | 120.00 (11) | H15A—C15—H15B | 109.5 |
C13—C8—N1 | 120.80 (12) | C11—C15—H15C | 109.5 |
C9—C8—N1 | 119.14 (11) | H15A—C15—H15C | 109.5 |
C10—C9—C8 | 118.47 (11) | H15B—C15—H15C | 109.5 |
C6—C1—C2—C3 | −0.07 (19) | C7—N1—C8—C9 | −133.76 (13) |
C1—C2—C3—C4 | 0.46 (19) | C13—C8—C9—C10 | −1.42 (18) |
C1—C2—C3—Cl1 | −179.11 (10) | N1—C8—C9—C10 | −178.57 (11) |
C2—C3—C4—C5 | −0.81 (19) | C13—C8—C9—C14 | 178.43 (12) |
Cl1—C3—C4—C5 | 178.77 (10) | N1—C8—C9—C14 | 1.29 (18) |
C3—C4—C5—C6 | 0.76 (19) | C8—C9—C10—C11 | 1.54 (18) |
C2—C1—C6—C5 | 0.04 (19) | C14—C9—C10—C11 | −178.31 (12) |
C2—C1—C6—C7 | −178.99 (12) | C9—C10—C11—C12 | −0.63 (19) |
C4—C5—C6—C1 | −0.39 (19) | C9—C10—C11—C15 | 178.60 (11) |
C4—C5—C6—C7 | 178.62 (12) | C10—C11—C12—C13 | −0.40 (18) |
C8—N1—C7—C6 | −174.26 (12) | C15—C11—C12—C13 | −179.64 (13) |
C1—C6—C7—N1 | −178.57 (12) | C11—C12—C13—C8 | 0.50 (19) |
C5—C6—C7—N1 | 2.4 (2) | C9—C8—C13—C12 | 0.44 (19) |
C7—N1—C8—C13 | 49.12 (17) | N1—C8—C13—C12 | 177.53 (12) |
Cg1 and Cg2 are the centroids of the C1–C6 and C8–C13 benzene rings, respectively. |
D—H···A | D—H | H···A | D···A | D—H···A |
C12—H12A···Cg1i | 0.95 | 2.67 | 3.3885 (14) | 132 |
C14—H14A···Cg1ii | 0.98 | 2.86 | 3.4853 (14) | 124 |
C2—H2A···Cg2iii | 0.95 | 2.73 | 3.4371 (14) | 132 |
C4—H4A···Cg2iv | 0.95 | 2.80 | 3.5534 (15) | 134 |
Symmetry codes: (i) x+1/2, −y+1/2, z+1/2; (ii) x−1/2, y+1/2, z; (iii) x−1/2, y−1/2, z; (iv) x+1/2, −y+1/2, z−1/2. |
Experimental details
Crystal data | |
Chemical formula | C15H14ClN |
Mr | 243.72 |
Crystal system, space group | Monoclinic, Cc |
Temperature (K) | 100 |
a, b, c (Å) | 7.2852 (1), 15.2715 (2), 11.5382 (1) |
β (°) | 96.304 (1) |
V (Å3) | 1275.93 (3) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.28 |
Crystal size (mm) | 0.40 × 0.24 × 0.20 |
Data collection | |
Diffractometer | Bruker SMART APEXII CCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Bruker, 2009) |
Tmin, Tmax | 0.898, 0.946 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 14547, 3975, 3800 |
Rint | 0.020 |
(sin θ/λ)max (Å−1) | 0.727 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.031, 0.078, 1.04 |
No. of reflections | 3975 |
No. of parameters | 156 |
No. of restraints | 2 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.32, −0.20 |
Absolute structure | Flack (1983), 1919 Friedel pairs |
Absolute structure parameter | 0.04 (4) |
Computer programs: APEX2 (Bruker, 2009), SAINT (Bruker, 2009), SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).
Cg1 and Cg2 are the centroids of the C1–C6 and C8–C13 benzene rings, respectively. |
D—H···A | D—H | H···A | D···A | D—H···A |
C12—H12A···Cg1i | 0.95 | 2.67 | 3.3885 (14) | 132 |
C14—H14A···Cg1ii | 0.98 | 2.86 | 3.4853 (14) | 124 |
C2—H2A···Cg2iii | 0.95 | 2.73 | 3.4371 (14) | 132 |
C4—H4A···Cg2iv | 0.95 | 2.80 | 3.5534 (15) | 134 |
Symmetry codes: (i) x+1/2, −y+1/2, z+1/2; (ii) x−1/2, y+1/2, z; (iii) x−1/2, y−1/2, z; (iv) x+1/2, −y+1/2, z−1/2. |
Acknowledgements
HKF and CKQ thank Universiti Sains Malaysia for the Research University Grant (No. 1001/PFIZIK/811160).
References
Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1–19. CrossRef Web of Science Google Scholar
Bruker (2009). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Cimerman, Z., Miljanic, S. & Galic, N. (2000). Croat. Chem. Acta, 73, 81–95. CAS Google Scholar
Cimerman, Z. & Stefanac, Z. (2001). Polyhedron, 4, 1755–1760. CrossRef Web of Science Google Scholar
Cosier, J. & Glazer, A. M. (1986). J. Appl. Cryst. 19, 105–107. CrossRef CAS Web of Science IUCr Journals Google Scholar
Flack, H. D. (1983). Acta Cryst. A39, 876–881. CrossRef CAS Web of Science IUCr Journals Google Scholar
Galic, N., Cimerman, Z. & Tomisic, V. (1997). Anal. Chim. Acta, 343, 135–143. CrossRef CAS Web of Science Google Scholar
Ittel, S. D., Johnson, L. K. & Brookhart, M. (2000). Chem. Rev. 100, 1169–1203. Web of Science CrossRef PubMed CAS Google Scholar
More, P. G., Bhalvankar, R. B. & Patter, S. C. (2001). J. Indian Chem. Soc. 78, 474–475. CAS Google Scholar
Pandeya, S. N., Sriram, D., Nath, G. & De Clercq, E. (1999). Eur. J. Pharm. Sci. 9, 25–31. Web of Science CrossRef PubMed CAS Google Scholar
Shah, S., Vyas, R. & Mehta, R. H. (1992). J. Indian Chem. Soc. 69, 590–596. CAS Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar
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The chemistry of the carbon-nitrogen double bond plays a vital role in the progress of science. Schiff-base compounds have been used as fine chemicals and medical substrates. Recently, multi-dentate complexes of iron and nickel showed high activities of ethylene oligomerization and polymerization (Ittel et al., 2000). Schiff bases have a wide variety of applications in many fields, e.g., biological, inorganic and analytical chemistry (Cimerman et al., 2000). They are known to exhibit potent antibacterial, anticonvulsant, anti-inflammatory activities (Shah et al., 1992). In addition, some Schiff bases show pharmacologically useful activities like anticancer (Pandeya et al., 1999), anti-hypertensive and hypnotic (More et al., 2001) properties. Unfortunately, most Schiff bases are chemically unstable and show a tendency to be involved in various equilibria, like tautomeric interconversions, hydrolysis or formation of ionized species (Cimerman & Stefanac, 2001; Galic et al., 1997). Therefore, successful application of Schiff bases requires a careful study of their characteristics.
The molecular structure of the title compound is shown in Fig. 1. The bond lengths (Allen et al., 1987) and angles are within normal ranges. The title compound exists in trans configuration with respect to the C7═N1 bond [C7═N1 = 1.2813 (16) Å]. The benzene rings (C1-C6 and C8-C13) form a dihedral angle of 52.91 (6)°.
The crystal structure is stabilized by weak intermolecular C12–H12A···Cg1i, C14–H14A···Cg1ii, C2–H2A···Cg2iii and C4–H4A···Cg2iv interactions (see Table 1 for symmetry codes), where Cg1 and Cg2 are the centroid of C1-C6 and C8-C13 benzene rings, respectively. No significant classical intermolecular hydrogen bonds are observed.