Molecular structure and ring distortions of 1,3-dicyanobenzene in the gas phase and in the crystal

Journal of Molecular Structure 553 (2000) 157–166 www.elsevier.nl/locate/molstruc

Molecular structure and ring distortions of 1,3-dicyanobenzene in the gas phase and in the crystal J. Janczak, R. Kubiak* W Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Oko´lna Str., P.O. Box 1410, 50-950 Wroclaw, Poland Received 12 January 2000; revised 6 April 2000; accepted 6 April 2000

Abstract The crystals of 1,3-dicyanobenzene have been obtained by a sublimation method. 1,3-Dicyanobenzene crystallizes in the monoclinic system. The X-ray geometry of the 1,3-dicyanobenzene molecule has been compared with the ab initio (HF/631⫹G(d) basis sets level) calculated structure in the gas-phase. The difference between equivalent parameters obtained by these methods is a consequence of the intermolecular interactions present in the crystal. The distortion of the benzene ring from the D6h symmetry is consistent with the substitution effect caused by two highly polar groups. The distortion of the benzene ring in 1,3-dicyanonbenzene has been compared and discussed in relation to the o- and p-isomers of dicyanobenzene. The differences in the C–C bond lengths of the phenyl ring in the gas-phase have been analyzed in terms of the distribution of the charge density. The calculated charge density and its Laplacian function were analyzed in term of the topological properties at the (3,1) critical points. Some comments on the crystal packing and intermolecular interactions present in the crystals of o-, m- and pdicyanobenzene and their melting points have also been presented. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Molecular structure; Ring distortions; 1,3-Dicyanobenzene

1. Introduction X-ray investigations of o- [1] and p-dicyanobenzene [2–6] have shown that the strong intermolecular interactions between the p-electron system of the aromatic ring and two very electrophilic cyano groups give rise to intermolecular charge compensation, which leads to the interesting packing principles. The crystal structure study of the third isomer of dicyanobenzene (m-isomer) gives complete crystal * Corresponding author. Tel.: ⫹48-71-343-5021; fax: ⫹48-71441029. E-mail addresses: [email protected] (J. Janczak), [email protected] (R. Kubiak).

structure information on the dicyanobenzene isomers, and provide a basis for further investigations in our laboratory. For a synthesis of metallophthalocyanines we use the o-dicyanobenzene [7–21]. The formation of the phthalocyaninato(2⫺) ring from o-dicyanonbenzene molecules requires a transformation of both cyano groups with concomitant change of the hybridization of the carbon atoms from sp to sp 2 and incorporation of the two electrons, released from the metal that has been oxidized, to the common p-electron conjugated ring system (Scheme 1). On the syntheses of the metallophthalocyanines we have found that depending on the reaction conditions, only one of the cyano groups of o-dicyanobenzene

0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(00)00578-0

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Scheme 1.

undergoes transformation with the change from sp to sp 2 hybridization yielding the triazine derivative (Scheme 2) [22]. A similar trimerization of m- and p-dicyanobenzene may be expected and the formation of the appropriate analogue of molecule (II) is possible. Therefore, we decided to determine at first the crystal structure of mdicyanobenzene. Additionally, the distortions of the benzene ring caused by the two –CN substituents in the crystal have been supported by ab initio calculation. Moreover, the substitution effect of the two cyano groups in m-dicyabobenzene has been discussed in relation to that in the o- and p-isomers.

2. Experimental 2.1. Crystallization The single crystals of the 1,3-dicyanobenzene suitable for X-ray diffraction studies were obtained by sublimation. Powdered 1,3-dicyanobenzene was heated under vacuum in a glass ampoule with the

temperature gradient. The single crystals of 1,3-dicyanobenzene grew in a cooler part of glass ampoule.

2.2. Data collection A colorless single crystal of 1,3-dicyanobenzene of approximate dimensions of 0:44 × 0:34 × 0:22 mm3 was used for data collection on a four circle KUMA KM4 diffractometer, with monochromatized MoKa  Preliminary examination radiation …l ˆ 0:71073 A†: of the crystal by rotation and Weissenberg photographs indicated monoclinic symmetry. The orientation matrix and unit cell dimensions were refined by the least-squares method on 25 accurately centered reflections measured in the range 4 ⬍ 2u ⬍ 20⬚: A total of 2076 reflections were measured in the range 4 ⱕ 2u ⱕ 60⬚; using the v –2u scan technique. Two standard reflections were monitored every 50 (intensity variation 0.8%). Intensities and their standard deviations were corrected for Lorentz and polarization effects. Face-indexed analytical absorption was calculated using the shelxtl-program [23].

Scheme 2.

J. Janczak, R. Kubiak / Journal of Molecular Structure 553 (2000) 157–166 Table 1 Crystallographic data for 1,3-dicyanobenzene and refinement details Formula Mol. wt. Crystal system, space group ˚) Unit cell dimensions a, b, c (A b (⬚) ˚ 3) Volume, V (A Z F(000) Dcalc. (g/cm 3) Dobs. (measured, floation), (g/ cm 3) ˚) Radiation, MoKa, (A 2u range Refls. collected Independent refls. Observed refls. Absorption coefficient, m (mm ⫺1) Correction

Refinement on F 2 R (F 2 ⬎ 2a(F 2)) wR (F 2 all reflections) Goodness-of-fit, S Largest D/s ˚ ⫺3 Residual electron density, e A P P R ˆ 储F o 兩 ⫺ 兩Fc 储= Fo W ˆ ‰s 2 …Fo2 † ⫹ …0:025P†2 Š where P ˆ …Fo2 ⫹ Fo2 †=3

C6H4(CN)2 128.13 Monoclinic, P21/c 3.873(1), 11.826(2), 14.524(3) 94.64(3) 663.0(2) 4 264 1.284 1.28

l ˆ 0:71073 4–60 2076 1073 582 (with I ⬎ 2s ) 0.081 Lorentz, polarization, face indexed analytical absorption, T max: ˆ 0:9825; Tmin: ˆ 0:9654; Extinction: shelxl97, k ˆ 0:0060…9† 0.0286 0.0279 0.835 0.001 ⫹ 0.073 ⫺0.073 wR…F 2 † ˆ P P { …w…Fo2 ⫺ Fc2 †2 Š= wFo4 }1=2

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Table 2 Final atomic coordinates and equivalent isotropic displacement P P parameter for 1,3-dicyanobenzene, Ueq ˆ …1=3† i j Uij ai aj 2  ai a j …A † Atom

x

y

z

Ueq

Cl C2 C3 C4 C5 C6 C7 C8 N1 N2 H2 H4 H5 H6

⫺0.0835(4) ⫺0.1213(4) ⫺0.0065(4) 0.1481(5) 0.1877(5) 0.0723(4) ⫺0.2091(4) 0.3472(4) ⫺0.3159(4) 0.4759(4) 0.101(3) 0.230(3) ⫺0.033(4) ⫺0.238(3)

0.3020(1) 0.3489(1) 0.2916(2) 0.1876(2) 0.1413(2) 0.1986(2) 0.3610(2) 0.0328(2) 0.4071(2) ⫺0.0534(1) 0.167(1) 0.147(1) 0.322(1) 0.420(1)

0.0555(1) 0.1414(1) 0.2198(1) 0.2141(1) 0.1274(1) 0.0481(1) ⫺0.0267(1) 0.1210(1) ⫺0.0925(1) 0.1162(1) ⫺0.012(1) 0.266(1) 0.279(1) 0.144(1)

0.0509(4) 0.0612(5) 0.0672(5) 0.0645(5) 0.0514(4) 0.0520(4) 0.0605(5) 0.0600(5) 0.0834(5) 0.0761(4) 0.062 0.077 0.081 0.073

gence. The final difference Fourier maps showed no peaks of chemical significance. Scattering factors for neutral atoms and calculations for anomalous dispersion were as in shelxl97 program, which was used for all crystal structure calculations. The drawing preparation was made by XP-graphics program of the shelxtl package [23]. Details of data collection and final agreement parameters are collected in Table 1. The final positional parameters and equivalent thermal parameters are listed in Table 2.

3. Details of calculations 2.3. Structure determination and refinement The structure was solved by direct methods and refined on F 2 by full-matrix least square techniques using the shelxl97 program [24]. Initially the structure was refined with isotropic, then with anisotropic displacement parameters. A difference Fourier synthesis showed maxima in positions consistent with expected locations of the hydrogen atoms. The positional parameters of H atoms were refined, but their displacement parameters were fixed as U ˆ 1.2Uiso of the carbon atom directly linked the H atom. The final unweighted and weighted factors converged to R ˆ 0.0286 and wR ˆ 0.0279, a goodness-of-fit calculation resulted in a value of 0.835 at final conver-

Ab initio calculations were performed with the gaussian program package [25] at the Hartree– Fock level of theory. Full geometry optimizations were carried out with the different basis sets functions starting from the X-ray structural geometry. As a convergence criterion the threshold limits of 0.00045 and 0.0018 a.u. were applied for the maximum force and displacement, respectively. The analysis of the theoretical charge density r (r) and Laplacian 7 2r (r) at the bond critical points were calculated using the aimpac program [26]. The topological properties of the charge density (r (r)) or its Laplacian 7 2r (r) are summarized in term of their critical points rc, i.e. the points where 7r or 7(7 2r ) are equal to zero. The critical points are classified according to

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their type (m, n). The rank m is equal to the number of the non-zero eigenvalues of the Hessian matrix of r (or 7 2r ) at rc. The signature n equals the algebraic sum of the signs of the eigenvalues of r (or 7 2r ) at rc. A bonding interaction between two atoms is characterized by a line linking the nuclei (bond path) along which the maximum of the charge density is present. If the three eigenvalues l 1, l 2 and l 3 of the Hessian matrix of the charge density are non-zero and two are negative (l 1, l 2 and l 1 ⬍ l 2) the critical point is a bond critical point (3,⫺1). The positive curvature (l 3) is associated with the eigenvector v3 of the Hessian matrix of r at rb while the two negative curvatures (l 1 and l 2) are associated with the eigenvectors perpendicular to the bond path. The eigenvectors perpendicular to the bond path (v1 and v2) define the interatomic surface between the bonded atoms. The value of the charge r (rb) at the bond critical point indicates the bond order [27,28], and the ratio of the two negative curvatures define the bond ellipticity …e ˆ l 1 =l2 ⫺ 1†: The ellipticity is a measure of the p-character of bonds. The location of the bond critical points (CP) correlates with the polar character of the bond [29]. The sum of the three curvatures (l 1, l 2 and l 3) equals the Laplacian of r at rb and its sign determines the concentration of the charge density [30]. The distribution of the charge density provides some information on the reactivity of the molecule and allows insight into the nature of the chemical bonds.

The calculation exposed only one minimum energy on the PES. The benzene ring of m-dicyanobenzene in the crystal is not perfectly hexagonal. The deviation from the D6h symmetry is highly significant. Both internal angles at the carbon atoms that directly joined the cyano groups are larger than 120⬚ in X-ray as well as in the ab initio geometry. Similar correlation can be found in the molecule of p-dicyanobenzene [6], but in o-dicyanobenzene molecule these angles are smaller than 120⬚ in X-ray [1] and in the gas-phase. This is likely due to the strong intramolecular interaction (in o-dicyanobenzene) between the polar cyano groups joined directly to the neighbouring carbon atoms of the ring. The bond distances and angles for the all isomers of dicyanobenzene obtained by X-ray experiments are in sufficiently good agreement with those obtained from ab initio optimized molecules (see Table 3). The differences of the C–C bond lengths in the benzene ring of m-dicyanobenzene can be explain by the contribution of the canonical structures as presented in Scheme 3. The shortest C–C bond in the benzene ring of the m-dicyanobenzene obtained by X-ray experiment is ˚ ), and is in agreement with the C5–C6 (1.368(2) A canonical structures (in the four with the seven canonical structures the double bond between C5 and C6 atoms is present). The differences in bond lengths of benzene ring in the X-ray structure of o- and p-dicyanobenzene are also in a good agreement with the

4. Results and discussion 4.1. Molecular geometry and ring deformation The molecular geometry of the m-dicyanobenzene obtained by the X-ray diffraction study with the atomic labeling is shown in Fig. 1. The X-ray geometrical parameters are shown in Table 3, which also comprises the ab initio full optimized parameters, using the HF/6-31⫹G(d) basis sets level. The X-ray and ab-initio gas-phase full optimized parameters for the o- and p-isomers of dicyanobenzene are also listed in the Table 3, for a comparison. The absolute energy calculated for m-dicyanobenzene using the HF/6-31⫹G(d) basis functions was found to be ⫺414.2621 Hartrees and is a global minimum on the potential energy surface (PES).

Fig. 1. Molecule of 1,3-dicyanobenzene. Thermal ellipsoids of the non-hydrogen atoms are scaled to the 50% probability level.

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Table 3 X-ray and optimized HF/6-31⫹G(d) geometrical parameter of dicyanobenzene molecule

Table 3 (continued) Bond/angle

X-ray

HF/6-31⫹G(d)

Bond/angle

C5–C6 C6–C1 C1–C7 C2–C8 C7–N1 C8–N2 C1–C2–C3 C6–C1–C2 C1–C2–C8 C2–C1–C7 C2–C3–C4 C5–C6–C1 C3–C4–C5 C4–C5–C6 C6–C1–C7 C3–C1–C8 C1–C7–N1 C2–C8–N2

1.397(11) 1.378(2) 1.430(7) 1.430(7) 1.149(8) 1.149(8) 119.5(2) 119.5(2) 119.7(2) 119.7(2) 120.3(4) 120.3(4) 120.2(3) 120.3(3) 120.7(3) 120.7(3) 179.5(2) 179.5(2)

1.384 1.387 1.443 1.443 1.129 1.129 119.8 119.8 121.3 121.3 120.0 120.0 120.2 120.2 118.9 118.9 179.3 179.3

X-ray

HF/6-31⫹G(d)

1.372(2) 1.379(2) 1.393(2) 1.373(2) 1.368(2) 1.384(2) 1.434(2) 1.430(2) 1.148(2) 1.139(2) 119.2(2) 120.6(2) 119.2(2) 120.5(2) 120.1(2) 120.5(2) 120.1(2) 119.4(2) 120.0(2) 119.4(2) 178.6(2) 179.6(2)

1.387 1.387 1.390 1.383 1.383 1.390 1.443 1.443 1.130 1.130 119.2 120.5 119.9 120.1 119.9 120.5 119.8 119.7 119.7 119.9 179.8 179.8

(a) 1,3-dicyanobenzene

C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C6–C1 C1–C7 C3–C8 C7–N1 C8–N2 C1–C2–C3 C2–C3–C4 C3–C4–C5 C4–C5–C6 C5–C6–C1 C6–C1–C2 C6–C1–C7 C2–C1–C7 C2–C3–C8 C4–C3–C8 C1–C7–N1 C3–C8–N2

(b) 1,2-dicyanobenzene a

C1–C2 C2–C3 C3–C4 C4–C5

1.401(6) 1.378(2) 1.354(6) 1.397(11)

1.396 1.387 1.384 1.384

(c) 1,4-dicyanobenzene b

C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C6–C1 C1–C7 C4–C8 C7–N1 C8–N2 C1–C2–C3 C2–C3–C4 C3–C4–C5 C4–C5–C6 C5–C6–C1 C6–C1–C2 C2–C1–C7 C6–C1–C7 C3–C4–C8 C5–C4–C8 C1–C7–N1 C4–C8–N2 a b

1.378(3) 1.374(2) 1.378(2) 1.378(3) 1.374(2) 1.378(2) 1.441(2) 1.441(2) 1.133(2) 1.133(2) 119.3(2) 119.2(2) 121.1(1) 119.3(2) 119.2(2) 121.1(1) 119.8(2) 119.1(2) 119.1(2) 119.8(2) 178.7(2) 178.7(2)

X-ray data from Ref. [1]. X-ray data from Ref. [6].

1.389 1.380 1.389 1.389 1.380 1.389 1.444 1.444 1.130 1.130 119.7 119.7 120.7 119.7 119.7 120.7 119.7 119.7 119.7 119.7 180.0 180.0

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Table 4 Charge density topological parameters calculated using HF/6-311⫹⫹G(d,p) Bond

Distance

(a) o-dicyanobenzene C1–C2 1.396 C2–C3 1.387 C3–C4 1.384 C4–C5 1.384 C5–C6 1.384 C6–C1 1.387 C1–C7 1.443 C7–N1 1.129 C2–C8 1.443 C8–N2 1.129 (b) m-dicyanobenzene C1–C2 1.387 C2–C3 1.387 C3–C4 1.390 C4–C5 1.391 C5–C6 1.391 C6–C1 1.390 C1–C7 1.443 C7–N1 1.130 C3–C8 1.443 C8–N2 1.130 (c) p-dicyanobenzene C1–C2 1.389 C2–C3 1.380 C3–C4 1.389 C4–C5 1.389 C5–C6 1.380 C6–C1 1.389 C1–C7 1.443 C7–N1 1.130 C4–C8 1.443 C8–N2 1.130

˚ ⫺3) r (e A

˚ ⫺5) 7 2r (e A

l1

l2

l3

e

AT1–CP

CP–AT2

2.145 2.183 2.198 2.201 2.198 2.183 1.916 3.304 1.916 3.404

⫺23.768 ⫺24.771 ⫺25.147 ⫺25.195 ⫺25.147 ⫺24.771 ⫺22.043 4.383 ⫺22.043 4.383

⫺16.56 ⫺16.71 ⫺16.84 ⫺16.87 ⫺16.84 ⫺16.76 ⫺13.61 ⫺28.00 ⫺13.61 ⫺28.00

⫺13.01 ⫺13.43 ⫺13.81 ⫺13.95 ⫺13.81 ⫺13.43 ⫺12.89 ⫺27.42 ⫺12.89 ⫺27.42

5.80 5.42 5.51 5.52 5.51 5.42 4.46 59.80 4.46 59.80

0.273 0.248 0.219 0.218 0.219 0.248 0.056 0.021 0.056 0.021

0.698 0.726 0.701 0.692 0.701 0.661 0.622 0.393 0.622 0.393

0.698 0.661 0.683 0.692 0.683 0.726 0.821 0.737 0.821 0.737

2.186 2.186 2.174 2.202 2.202 2.174 1.908 3.403 1.908 3.403

⫺24.735 ⫺24.735 ⫺24.574 ⫺25.201 ⫺25.201 ⫺24.574 ⫺21.972 3.752 ⫺21.972 3.752

⫺16.85 ⫺16.85 ⫺16.69 ⫺16.88 ⫺16.88 ⫺16.69 ⫺13.49 ⫺28.09 ⫺13.49 ⫺28.09

⫺13.47 ⫺13.47 ⫺13.48 ⫺13.79 ⫺13.79 ⫺13.48 ⫺12.72 ⫺27.54 ⫺12.72 ⫺27.54

5.59 5.59 5.58 5.47 5.47 5.58 4.24 59.37 4.24 59.37

0.250 0.250 0.237 0.224 0.224 0.237 0.061 0.020 0.061 0.020

0.704 0.683 0.715 0.716 0.677 0.675 0.612 0.393 0.612 0.393

0.683 0.704 0.675 0.677 0.716 0.715 0.831 0.737 0.831 0.737

2.176 2.210 2.176 2.176 2.210 2.176 1.908 3.403 1.908 3.403

⫺24.628 ⫺25.299 ⫺24.631 ⫺24.631 ⫺25.299 ⫺24.631 ⫺22.075 3.804 ⫺22.075 3.804

⫺16.70 ⫺16.98 ⫺16.70 ⫺16.70 ⫺16.98 ⫺16.70 ⫺13.47 ⫺28.08 ⫺13.47 ⫺28.08

⫺13.46 ⫺13.79 ⫺13.46 ⫺13.46 ⫺13.79 ⫺13.46 ⫺12.85 ⫺27.59 ⫺12.85 ⫺27.59

5.53 5.47 5.53 5.53 5.47 5.53 4.25 59.48 4.25 59.48

0.240 0.231 0.240 0.240 0.231 0.240 0.048 0.018 0.048 0.018

0.720 0.690 0.669 0.720 0.690 0.669 0.612 0.393 0.612 0.393

0.669 0.690 0.720 0.669 0.690 0.720 0.831 0.737 0.831 0.737

canonical structures presented in Schemes 4 and 5, respectively. Additionally, in the benzene ring of the o-dicyanobenzene the C1–C2 bond clearly evidences the substitution effect of the electrophilic cyano groups. The strong intramolecular interaction (repulsive forces) between the atoms of both cyano groups in o-dicyanobenzene lead to extension of the C1–C2 bond in relation to the other C–C bonds in the phenyl ring. The differences in the C–C bonds of the benzene ring in o-, m- and p-isomers of dicyanobenzene are also evidenced in the charge density at the bond CPs and correlates with their ellipticity …e ˆ l1 =l2 ⫺ 1†; (see Table 4). The value of r (r) at the bond CPs correlate

with the bond order [27,28], while the location of the bond CP correlates with its polar character [29] and the ellipticity determined the contribution of the pcharacter of the bond [30]. The shortening of the central C–C bonds of the phenyl ring observed in pdicyanobenzene may arise from appreciable contribution of the canonical forms like p-2 and p-5 presented in Scheme 5. The mean C–CN bond length in m-dicyanobenzene ˚ ) is not significantly different from the value (1.437 A ˚ ). A similar relation can calculated ab initio (1.443 A be found in the o- and p-isomers. In the all isomers of dicyanobenzene the C–CN bond lengths are a little ˚ shorter that those observed in benzonitrile 1.451(1) A

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163

Scheme 3.

[31], but they are in a good agreement with those observed in 2,3-dicyanonaphthalene [32] and in cyanoaniline [33]. The C–CN bond length in the dicyanobenzene molecules is intermediate between ˚ and sp–sp 3 1.459 A ˚ the expected sp–sp 2 1.419 A carbon–carbon bond length [34]. The optimized bond lengths of the CxN are shorter than those obtained by X-ray experiment in all dicyanobenzene isomers (see Table 3). The greatest difference in the CxN bond lengths between the X-ray and ab initio methods are observed in o-dicyanobenzene, while the smallest is seen in the p-isomer. This is likely due to the intermolecular interactions present in the crystal, since the ab initio calculated parameters refer to the

isolated and non-interacting molecule. The location of the bond CPs in the C–CN and CxN bonds correlates with their polar character [29], while the value of r (rb) determines the bond strength. Looking at these X-ray experimental and ab initio results obtained for all the isomers of dicyanobenzene in more detail it appears that the X-ray bond lengths are usually longer than they are in ab initio calculated ones. Additionally it has been stated that the optimized geometry of dicyanobenzene molecules has a higher symmetry than the molecules present in the crystal. The o-, m- and p-isomers of dicyanobenzene in the crystal have C2, C1 and Ci symmetry, while in the gas-phase they have C2v, C2v and D2h symmetry,

Scheme 4.

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Scheme 5.

respectively. Thus the geometry of the molecules obtained by X-ray experiment clearly demonstrated the intermolecular interaction present in the crystal. 4.2. Crystal packing The packing of the o-, m- and p-dicyanobenzene molecules in the crystal with the important intermolecular contacts is shown in Fig. 2a–c, respectively. The crystals of the three isomers have similar characteristic packing. The crystals are built up from two stacks of parallel molecules along the shortest lattice parameters, in the m- and p-isomers along the x-axis and in the o-isomer along the y-axis. The angle between the molecular planes of the neighbouring stacks is equal to 128.2, 127.2 and 129.8⬚ for isomer o-, m- and p-, respectively. Recently we have stated that the p-isomer of dicyanobenzene crystallizes in two different crystalline modifications, triclinic and monoclinic [6]. The triclinic modification is a low-temperature phase, while the monoclinic is a high-temperature phase. The triclinic modification transforms to the monoclinic form at about 155⬚C and the phase is stable up to the melting point (223–226⬚C). The suggestion by Dru¨ck and Littke [3] that the main reason for the

high melting point of p-dicyanobenzene is the packing of almost planar and parallel molecules existing in the crystal of the triclinic form seems to be not true, since at the temperature about 70⬚C lower the triclinic form is not stable and transforms to the monoclinic phase. Thus the highest melting point of p-dicyanobenzene is associated with the molecular packing of the monoclinic crystal. The interplanar distances between the molecules in ˚ in the the stack are equal to 3.504, 3.469 and 3.463 A crystal of o-, m- and p-isomers, respectively. The distance between the planes in the stack correlate with the melting point of o- (141–142⬚C), m- (162–163⬚C) and p-isomer (223–226⬚C). Besides the interplanar distance, the C–H···N intermolecular interactions (see Fig. 2) also give rise to the temperature of the melting point. In the crystal of monoclinic form of pdicyanobenzene the molecules are displaced with respect to the next in such a way that the neighbouring cyano groups interact together. However, the dipole moment of p-dicyanobenzene in contrast to the o- and m-isomers (the calculated dipole moment using the HF/6-31⫹G(d) basis sets level for o- and m-isomers equals 7.69 and 4.67 D, respectively) is equal to 0, the local dipole moment of the cyano group is non-zero. Thus in the crystal of p-dicyanobenzene the two neighboring molecules are located antiparallel in relation to the local moment of the cyano groups. In this orientation of the p-dicyanobenzene molecules both the CN groups interact electrostatically, since the negatively charged N atom of one cyano group lies opposite to the positively charged C atoms of the second CN group. This kind of the intermolecular interaction and charge compensation gives rise to the highest melting point of the p-isomer in relation to the o- and m-isomers. The packing role is clearly seen in the crystal volume per one molecule of dicya˚ 3 in the nobenzene, 170.40, 165.75 and 164.74 A crystal of o-, m- and p-isomers, respectively. Thus the molecular volume of dicyanobenzene in the crystal also correlate with the melting points of the three isomers. Supplementary material. Additional material comprising atomic coordinates, anisotropic thermal parameters, bond lengths and angles have been deposited with the Cambridge Crystallographic Data Center as supplementary publications no. CCDC 138957.

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Fig. 2. Molecular architecture of the: (a) o-; (b) m-; and (c) p-dicyanobenzene with the closest intermolecular contacts. View along the shortest lattice parameter (left) and perpendicular to the stack direction (right).

Acknowledgements We would like to thank Prof. P. Luger, Institut fu¨r Kristallographie, Freie Universita¨t Berlin, for the opportunity to make all calculations using the gaussian program and Prof. Z. Galdecki, Technical University of Lo´dz, Poland, for the opportunity to make drawing preparation using the shelxtl program system.

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