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Molecular packing analyses were carried out on 15 crystal data sets of chloro-substituted Schiff bases, including that of the title compound, C15H15ClN2. C—H...π and π–π interactions play a major role in the molecular self-assembly in the crystal. The former interactions favor mol­ecules assembling into a screw, with a non-centrosymmetric crystal structure. When the molecular dipole is small, π–π interactions favor a parallel, but not usually antiparallel, mode of packing. Weak C—H...X hydrogen bonds (X = Cl or Br) and X...X interactions seem to be a secondary driving force in packing. The title mol­ecule takes the trans form and the two benzene rings are twisted around the central linkage in opposite directions. In the crystal structure, mol­ecules interact through C—H...π and π–π interactions, forming a `dimer' and further forming double chains along [001]. The double chains are extended along [10\overline 1] through C—H...Cl hydrogen bonds, forming double layers in (010). In the third direction, there are only ordinary, weaker, van der Waals interactions, which explains the crystal habit (i.e. thin plate).

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104017871/de1245sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270104017871/de1245Isup2.hkl
Contains datablock I

pdf

Portable Document Format (PDF) file https://doi.org/10.1107/S0108270104017871/de1245sup3.pdf
Supplementary material

CCDC reference: 251337

Comment top

Benzylideneanilines are an important class of Schiff bases, which have been widely used in coordinate, medical and biological chemistry for some time (Metzler, 1980; Tarafder, 2002). Recently, the thermochromism (Pistolis, 1996), photochromism (Jalali-Heravi, 2000) and non-linear optical properties of these compounds have found applications in modern technologies (Sekikawa, 1997). In the design of solid materials, one of the key steps is to understand how the constituent molecules are packed, what kinds of interactions are playing a role in crystal packing and how they are interplaying (Desiraju, 1989). Since any centrosymmetric crystal has no odd rank tensor property, such as SHG and piezoelecticity, the design of non-centrosymmetric crystals is one of the important and also difficult problems in crystal engineering. As an example, molecular packing analyses were carried out for 14 chloro-substituted Schiff bases retrieved from the Cambridge Structural Database (CSD; Allen, 2002); this class of compounds was chosen because the percentage of non-centrosymmetric space groups for these compounds is 57.1%, much higher than that for general organic compounds (about 25%). The crystal data of these 14 compounds are listed in Table 3.

These crystals contain no strong hydrogen-bond-forming groups, such as-COOH, –NH2CO– or –NO2; various weak interactions, such as CH/π (Dmezawa, 1998), π···π (Sharma, 1993) and weak hydrogen bonds, and the interplay between these interactions, must therefore play an important role in determining the crystal structures.

The crystal packing analysis was carried out using OPEC (Gavezzotti, 1983), which was locally modified with additional calculation routines. Given a reference molecule (FM), and with the distance of interacting molecules limited to within 15 Å, the program calculated approximate 150 surrounding molecules (SM), which form the crystal model.

The main results are listed in Table 1S (supplementary material). From Table 1S, we can see the following:

(i) The CH/π interactions play a major role in controlling the molecular packing. 15 CH/π interactions, out of 18 listed for the most important intermolecular interactions, assemble molecules into a screw, which might explain why the crystals studied are more likely to crystallize in non-centrosymmetric groups.

2. The π···π interactions almost equally play the same role. Out of 19 listed, half of the interactions assemble the molecules in a translation-related mode, favouring an acentric crystal. This mode appears to occur when the molecular dipole is relatively small. When the molecular dipole is relatively large, the π···π interactions cause the molecules to packed in an antiparallel fashion into centric crystals.

3. Weak interactions, such as C—H···Cl (Thallapally & Nangia, 2001), also favour the assembling of molecules with the first kind of symmetric operators (screw 10 and traslation 13 in the 27 important interactions listed), but the percentage of packing energy is relatively small.

4. To our suprize, the role played by X atoms (X = Cl or Br) through so-called X···X interactions (Desiraju, 1994) seems to be only a secondary one, perhaps because the atomic fractions are not large enough. However, the? replacement of substituents like NO2 by X atoms? can also modify the properties of crystals designed for similar purposes.

During our study on the design of organic functional materials, the title compound, (I), was obtained, and its crystal structure is reported here. The selected geometric parameters are listed in Table 1.

The title molecule has a trans configuration and the C1—C7—N1—C8 (plane 3) torsion angle is 179.9°. A dihedral angle of 50.4° exists between planes 1 and 2 (plane 1 is the plane directly attached to the central C atom and plane 2 is the plane directly attached to the central N atom), which are twisted ?in opposite directions? around the central linkage by 9.6 and 41.3° for planes 1 and 2, respectively. For the title crystal, there are no strong hydrogen-bond-forming groups, such as –COOH, –NH2CO– or –OH, –NO2, so weak interactions must play determining roles in the crystal packing. The molecules that interacte strongly with the FM are listed in Tables 2 and 4. As shown by these tables, molecules interact through CH/π and π···π interactions, forming a `dimer' and further forming double chains along [001]. The double chains are connected by a C14—H14B···Cl1 hydrogen bond (3.729 Å and 174.5 °), extending along [10–1], forming double layers (010). In the third direction, there are only ordinary, weaker, van der Waals interactions, consistent with the formation of thin plates.

Experimental top

To a solution of 4-(N,N-dimethyl)benzaldehyde (10 mmol) in ethanol (10 ml), 4-chlor-aniline (11 mmol) was added. The solution was refluxed for 30 min at 363 K and then cooled to ambient temperature, yielding a pale-yellow product. The product was recrystallized three times from 85% ethanol, and colorless plate-like crystals were obtained from acetone solution by slow evaporation at ambient temperature for a week. Analysis calculated for C15H15N2Cl: C 69.63, H 5.84, N 10.82%; found: C 69.73, H 5.79, N 10.84%. IR (KBr pellets, cm−1): 1649, 1582, 1519, 1420, 765,726. 1H NMR (CDCl3, 399.97 MHz): δ 3.08 [s, 6H, –N(CH3)2], 6.73–6.75 (d, 2H, Ph), 7.15 (s, 2H, Ph), 7.32–7.34 (d, 2H, Ph), 7.76 (s, 2H, Ph), 8.28 (s, 1H, –CH=N–).

Refinement top

H atoms were positioned geometrically and treated as riding, with C—H distances of 0.98 Å (methyl H atoms) and 0.95 Å (other H atoms), and with Uiso(H) values of 1.5Ueq(C) (methyl) and 1.2Ueq(C) (other H atoms).

Computing details top

Data collection: CrystalClear (Rigaku/MSC, 2001); cell refinement: CrystalClear; data reduction: CrystalStructure (Rigaku/MSC & Rigaku Corporation, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: reference?; software used to prepare material for publication: reference?.

Figures top
[Figure 1] Fig. 1. A view of the title molecule, showing 30% probability displacement ellipsoids
[Figure 2] Fig. 2. A packing diagram, viewed down the b axis.
4-chloro-N-[4-(dimethylamino)benzylidene]aniline top
Crystal data top
C15H15ClN2F(000) = 544
Mr = 258.74Dx = 1.301 Mg m3
Monoclinic, P21/cMelting point: 378 K
Hall symbol: -P 2ybcMo Kα radiation, λ = 0.71070 Å
a = 9.852 (6) ÅCell parameters from 3935 reflections
b = 16.268 (9) Åθ = 3.5–27.5°
c = 9.512 (6) ŵ = 0.27 mm1
β = 119.904 (6)°T = 193 K
V = 1321.5 (14) Å3Thin plate, colorless
Z = 40.70 × 0.70 × 0.30 mm
Data collection top
Mercury CCD
diffractometer
2956 independent reflections
Radiation source: fine-focus sealed tube2773 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.029
Detector resolution: 7.31 pixels mm-1θmax = 27.5°, θmin = 3.5°
ω scansh = 1212
Absorption correction: multi-scan
(Jacobson, 1998)
k = 2117
Tmin = 0.832, Tmax = 0.923l = 1112
10180 measured reflections
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)] = 0.056Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.143H-atom parameters constrained
S = 1.16 w = 1/[σ2(Fo2) + (0.0677P)2 + 0.3993P]
where P = (Fo2 + 2Fc2)/3
2956 reflections(Δ/σ)max < 0.001
166 parametersΔρmax = 0.22 e Å3
0 restraintsΔρmin = 0.23 e Å3
Crystal data top
C15H15ClN2V = 1321.5 (14) Å3
Mr = 258.74Z = 4
Monoclinic, P21/cMo Kα radiation
a = 9.852 (6) ŵ = 0.27 mm1
b = 16.268 (9) ÅT = 193 K
c = 9.512 (6) Å0.70 × 0.70 × 0.30 mm
β = 119.904 (6)°
Data collection top
Mercury CCD
diffractometer
2956 independent reflections
Absorption correction: multi-scan
(Jacobson, 1998)
2773 reflections with I > 2σ(I)
Tmin = 0.832, Tmax = 0.923Rint = 0.029
10180 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0560 restraints
wR(F2) = 0.143H-atom parameters constrained
S = 1.16Δρmax = 0.22 e Å3
2956 reflectionsΔρmin = 0.23 e Å3
166 parameters
Special details top

Experimental. Elemental analysis (Perkin-Elmer 240 C elemental analyzer) IR (FT—IR spectrometer with KBr pellets, cm−1) 1H NMR (Bruker AV-400 NMR spectrometer, CDCl3 as solvent, 1H (399.97 MHz) NMR, Ambient temperature)

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 using reflections with F2 > 3.0 σ(F2). The weighted R-factor (wR) and goodness of fit (S) are based on F2. R-factor (gt) are based on F. The threshold expression of F2 > 2.0 σ(F2) is used only for calculating R-factor (gt).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Cl10.89087 (6)0.37831 (4)0.21784 (6)0.0580 (2)
N10.60073 (17)0.38891 (8)0.63361 (17)0.0362 (3)
N20.18129 (18)0.37376 (10)0.97868 (17)0.0417 (4)
C10.40283 (18)0.35404 (9)0.69945 (19)0.0340 (4)
C20.47823 (19)0.38991 (10)0.8534 (2)0.0356 (4)
H20.58130.41080.89510.043*
C30.40763 (19)0.39577 (10)0.94605 (19)0.0351 (4)
H30.46320.41971.05090.042*
C40.25288 (19)0.36664 (9)0.88783 (19)0.0335 (3)
C50.17790 (19)0.32941 (10)0.7333 (2)0.0385 (4)
H50.07480.30840.69050.046*
C60.2521 (2)0.32317 (10)0.6435 (2)0.0389 (4)
H60.19930.29710.54050.047*
C70.47312 (19)0.35135 (10)0.5968 (2)0.0356 (4)
H70.42260.32060.49890.043*
C80.66290 (18)0.38361 (9)0.52806 (19)0.0324 (3)
C90.66878 (19)0.31082 (10)0.4542 (2)0.0382 (4)
H90.62490.26200.46960.046*
C100.7380 (2)0.30911 (11)0.3584 (2)0.0416 (4)
H100.74200.25930.30830.050*
C110.80115 (19)0.38026 (11)0.3362 (2)0.0389 (4)
C120.7964 (2)0.45362 (10)0.4078 (2)0.0403 (4)
H120.83900.50250.39060.048*
C130.7288 (2)0.45433 (10)0.5045 (2)0.0381 (4)
H130.72720.50400.55610.046*
C140.0142 (2)0.36234 (16)0.9064 (3)0.0590 (6)
H14A0.04070.40690.82920.089*
H14B0.01430.36290.99140.089*
H14C0.01570.30950.84950.089*
C150.2598 (3)0.41126 (16)1.1376 (2)0.0572 (5)
H15A0.36710.39031.19920.086*
H15B0.20320.39791.19510.086*
H15C0.26210.47101.12630.086*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Cl10.0501 (3)0.0794 (4)0.0518 (3)0.0009 (2)0.0309 (2)0.0057 (2)
N10.0375 (7)0.0308 (6)0.0378 (7)0.0027 (5)0.0169 (6)0.0006 (5)
N20.0350 (7)0.0525 (9)0.0339 (7)0.0057 (6)0.0145 (6)0.0014 (6)
C10.0342 (8)0.0264 (7)0.0355 (8)0.0023 (6)0.0128 (7)0.0027 (6)
C20.0288 (7)0.0332 (8)0.0364 (8)0.0020 (6)0.0099 (6)0.0007 (6)
C30.0326 (8)0.0339 (8)0.0299 (7)0.0028 (6)0.0089 (6)0.0004 (6)
C40.0327 (8)0.0287 (7)0.0318 (8)0.0012 (6)0.0105 (6)0.0058 (6)
C50.0332 (8)0.0379 (8)0.0370 (8)0.0082 (6)0.0119 (7)0.0009 (7)
C60.0374 (8)0.0357 (8)0.0353 (8)0.0053 (6)0.0119 (7)0.0050 (6)
C70.0358 (8)0.0304 (7)0.0351 (8)0.0043 (6)0.0134 (7)0.0012 (6)
C80.0290 (7)0.0305 (7)0.0308 (7)0.0049 (6)0.0097 (6)0.0021 (6)
C90.0358 (8)0.0311 (8)0.0418 (9)0.0017 (6)0.0147 (7)0.0045 (6)
C100.0368 (8)0.0388 (8)0.0430 (9)0.0016 (7)0.0154 (7)0.0100 (7)
C110.0288 (7)0.0488 (9)0.0323 (8)0.0056 (7)0.0100 (6)0.0003 (7)
C120.0401 (9)0.0348 (8)0.0419 (9)0.0020 (7)0.0174 (7)0.0041 (7)
C130.0435 (9)0.0277 (7)0.0405 (8)0.0040 (6)0.0191 (7)0.0002 (6)
C140.0356 (10)0.0907 (16)0.0478 (11)0.0047 (10)0.0186 (8)0.0038 (10)
C150.0519 (11)0.0780 (14)0.0446 (10)0.0163 (10)0.0263 (9)0.0139 (10)
Geometric parameters (Å, º) top
Cl1—C111.744 (2)C7—H70.9500
N1—C71.280 (2)C8—C131.393 (2)
N1—C81.415 (2)C8—C91.393 (2)
N2—C41.366 (2)C9—C101.384 (3)
N2—C141.445 (3)C9—H90.9500
N2—C151.445 (3)C10—C111.379 (3)
C1—C61.397 (2)C10—H100.9500
C1—C21.397 (2)C11—C121.387 (2)
C1—C71.452 (2)C12—C131.379 (3)
C2—C31.371 (2)C12—H120.9500
C2—H20.9500C13—H130.9500
C3—C41.419 (2)C14—H14A0.9800
C3—H30.9500C14—H14B0.9800
C4—C51.411 (2)C14—H14C0.9800
C5—C61.377 (3)C15—H15A0.9800
C5—H50.9500C15—H15B0.9800
C6—H60.9500C15—H15C0.9800
C7—N1—C8119.36 (14)C10—C9—C8120.50 (16)
C4—N2—C14120.87 (16)C10—C9—H9119.8
C4—N2—C15121.45 (15)C8—C9—H9119.8
C14—N2—C15116.13 (17)C11—C10—C9119.46 (16)
C6—C1—C2117.26 (16)C11—C10—H10120.3
C6—C1—C7120.36 (15)C9—C10—H10120.3
C2—C1—C7122.33 (15)C10—C11—C12121.24 (17)
C3—C2—C1121.85 (15)C10—C11—Cl1119.74 (14)
C3—C2—H2119.1C12—C11—Cl1119.01 (14)
C1—C2—H2119.1C13—C12—C11118.75 (16)
C2—C3—C4121.10 (15)C13—C12—H12120.6
C2—C3—H3119.5C11—C12—H12120.6
C4—C3—H3119.5C12—C13—C8121.28 (15)
N2—C4—C5121.90 (15)C12—C13—H13119.4
N2—C4—C3121.19 (15)C8—C13—H13119.4
C5—C4—C3116.90 (16)N2—C14—H14A109.5
C6—C5—C4120.90 (16)N2—C14—H14B109.5
C6—C5—H5119.5H14A—C14—H14B109.5
C4—C5—H5119.5N2—C14—H14C109.5
C5—C6—C1121.95 (15)H14A—C14—H14C109.5
C5—C6—H6119.0H14B—C14—H14C109.5
C1—C6—H6119.0N2—C15—H15A109.5
N1—C7—C1122.31 (15)N2—C15—H15B109.5
N1—C7—H7118.8H15A—C15—H15B109.5
C1—C7—H7118.8N2—C15—H15C109.5
C13—C8—C9118.75 (16)H15A—C15—H15C109.5
C13—C8—N1117.70 (14)H15B—C15—H15C109.5
C9—C8—N1123.46 (14)
C6—C1—C2—C30.8 (2)C6—C1—C7—N1170.00 (16)
C7—C1—C2—C3176.70 (14)C2—C1—C7—N17.4 (2)
C1—C2—C3—C41.1 (2)C7—N1—C8—C13140.53 (16)
C14—N2—C4—C515.4 (3)C7—N1—C8—C943.0 (2)
C15—N2—C4—C5179.36 (18)C13—C8—C9—C100.4 (2)
C14—N2—C4—C3165.29 (18)N1—C8—C9—C10176.86 (15)
C15—N2—C4—C30.1 (3)C8—C9—C10—C110.1 (2)
C2—C3—C4—N2178.80 (15)C9—C10—C11—C120.1 (3)
C2—C3—C4—C51.9 (2)C9—C10—C11—Cl1179.16 (12)
N2—C4—C5—C6179.76 (15)C10—C11—C12—C130.9 (3)
C3—C4—C5—C60.9 (2)Cl1—C11—C12—C13178.39 (12)
C4—C5—C6—C10.9 (3)C11—C12—C13—C81.4 (2)
C2—C1—C6—C51.7 (2)C9—C8—C13—C121.2 (2)
C7—C1—C6—C5175.78 (15)N1—C8—C13—C12177.87 (15)
C8—N1—C7—C1179.93 (13)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C14—H14B···Cl1i0.982.753.729 (3)175
C2—H2···Cl1ii0.953.113.812 (3)132
C6—H6···N2iii0.952.833.481 (3)127
C14—H14A···N1iv0.983.083.602 (4)115
C10—H10···N1iii0.952.873.728 (3)152
Symmetry codes: (i) x1, y, z+1; (ii) x, y, z+1; (iii) x, y+1/2, z1/2; (iv) x1, y, z.

Experimental details

Crystal data
Chemical formulaC15H15ClN2
Mr258.74
Crystal system, space groupMonoclinic, P21/c
Temperature (K)193
a, b, c (Å)9.852 (6), 16.268 (9), 9.512 (6)
β (°) 119.904 (6)
V3)1321.5 (14)
Z4
Radiation typeMo Kα
µ (mm1)0.27
Crystal size (mm)0.70 × 0.70 × 0.30
Data collection
DiffractometerMercury CCD
diffractometer
Absorption correctionMulti-scan
(Jacobson, 1998)
Tmin, Tmax0.832, 0.923
No. of measured, independent and
observed [I > 2σ(I)] reflections
10180, 2956, 2773
Rint0.029
(sin θ/λ)max1)0.649
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.056, 0.143, 1.16
No. of reflections2956
No. of parameters166
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.22, 0.23

Computer programs: CrystalClear (Rigaku/MSC, 2001), CrystalClear, CrystalStructure (Rigaku/MSC & Rigaku Corporation, 2001), SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), reference?.

Selected geometric parameters (Å, º) top
Cl1—C111.744 (2)N2—C141.445 (3)
N1—C71.280 (2)N2—C151.445 (3)
N1—C81.415 (2)C1—C71.452 (2)
N2—C41.366 (2)
C7—N1—C8119.36 (14)C5—C4—C3116.90 (16)
C4—N2—C14120.87 (16)N1—C7—C1122.31 (15)
C4—N2—C15121.45 (15)C13—C8—C9118.75 (16)
C14—N2—C15116.13 (17)C13—C8—N1117.70 (14)
C6—C1—C2117.26 (16)C9—C8—N1123.46 (14)
C6—C1—C7120.36 (15)C10—C11—C12121.24 (17)
C2—C1—C7122.33 (15)C10—C11—Cl1119.74 (14)
N2—C4—C5121.90 (15)C12—C11—Cl1119.01 (14)
N2—C4—C3121.19 (15)
C7—C1—C2—C3176.70 (14)C8—N1—C7—C1179.93 (13)
C14—N2—C4—C515.4 (3)C6—C1—C7—N1170.00 (16)
C15—N2—C4—C5179.36 (18)C2—C1—C7—N17.4 (2)
C14—N2—C4—C3165.29 (18)C7—N1—C8—C13140.53 (16)
C15—N2—C4—C30.1 (3)C7—N1—C8—C943.0 (2)
C7—C1—C6—C5175.78 (15)N1—C8—C9—C10176.86 (15)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C14—H14B···Cl1i0.9792.7533.729 (3)174.5
C2—H2···Cl1ii0.9503.1103.812 (3)132.0
C6—H6···N2iii0.9502.8283.481 (3)126.8
C14—H14A···N1iv0.9813.0783.602 (4)114.9
C10—H10···N1iii0.9512.8653.728 (3)151.5
Symmetry codes: (i) x1, y, z+1; (ii) x, y, z+1; (iii) x, y+1/2, z1/2; (iv) x1, y, z.
The chloro-substituted Schiff bases in this study top
RefcodeSpace groupSS'µ(D)
1BADDAL10P21/c2-OH4'-Cl2.20
2BEYQEBP2121212-OH5'-Cl,2'-Me3.03
3CBZYANP2121212,4-Cl1.10
4CHLSANP2121212-OH2'-Cl3.73
5CSALAN02Pca212-OH, 5-Cl2.35
6FAKDIEP21/n4-Cl2'-OH2.76
7RONKEKP214-Cl3'-Cl0.52
8RONKOUP214-Br3'-Cl0.57
9RONKUAP214-Cl2'-Br0.48
10RONLAHP21/c3-Cl4'-Br2.78
11WEMJIHP2121213-Br3'-Cl1.46
12YICNONP21/c3,5-Cl, 2-OH4'-NEt26.22
13YICPABP21/c3,5-Cl, 2-OH4'-NMe26.16
14ZAMMEFP21/c2,3-OH2'-Cl3.73
15(I)P21/c4-NMe24'-Cl6.20
Notes: plane 1 is the plane directly attached to the central C atom; plane 2 is the plane directly attached to the central N atom; S and S' refer to the substituent groups in planes 1 and 2, respectively; µ is the molecular dipole moment calculated by MOPAC (Stewert, 1989).
C—H···π and ππ interactions in (I) (Å, °) top
E(SM)%InteractionH···PC—H···PSymmetry code
15.3C—H12···P13.47126.9-x + 1,-y + 1, −z + 1
15.3C—H13···P33.65121.7-x + 1,-y + 1, −z + 1
15.3Plane 3–34.244.66-x + 1,-y + 1, −z + 1
12.6C—H10···P33.63138.8x, −y + 1/2, z + 1/2
Notes: plane 3–3 for the ππ interaction between the two planes 3 in the interacting molecules. The parameters followed are the distances between the planes and between their centers, respectively; E(SM)% is the percentage interaction energy between the two interacting molecules in the total packing energy, calculated by OPEC (Gavezzotti, 1983)
 

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