organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
crystal has no odd rank tensor property, such as SHG (second
harmonic generation) and piezoelecticity, the design of non-
centrosymmetric crystals is an important and dif®cult problem
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
within the class 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.
ISSN 0108-2701
CÐHÁ Á Áp, p±p and CÐHÁ Á ÁCl inter-
actions in chloro-substituted Schiff
bases and 4-chloro-N-[4-(dimethyl-
amino)benzylidene]aniline1
These crystals contain no strong-hydrogen-bond-forming
groups, such as ±COOH, ±NH2CO± or ±NO2; various weak
interactions, such as CÐHÁ Á Áꢀ (Dmezawa, 1998), ꢀ±ꢀ
(Sharma, 1993) and weak hydrogen-bonding interactions, and
the interplay between these interactions, must therefore play
an important role in determining the crystal structures.
Crystal packing analyses were carried out using OPEC
(Gavezzotti, 1983), which was locally modi®ed with additional
calculation routines. Given a reference molecule (FM), and
with the distance between interacting molecules limited to
Xiu-Li You, Cheng-Rong Lu, Yong Zhang and De-Chun
Zhang*
Department of Chemistry, Suzhou University, Suzhou 215006, People's Republic
of China
Correspondence e-mail: dczhang@suda.edu.cn
Received 30 March 2004
Accepted 21 July 2004
Online 21 August 2004
Ê
within 15 A, the program calculated approximately 150
surrounding molecules (SM), which form the crystal model.
Molecular packing analyses were carried out on 15 crystal data
sets of chloro-substituted Schiff bases, including that of the
title compound, C15H15ClN2. CÐHÁ Á Áꢀ and ꢀ±ꢀ 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, ꢀ±ꢀ interactions favor a parallel, but
not usually antiparallel, mode of packing. Weak CÐHÁ Á ÁX
hydrogen bonds (X = Cl or Br) and XÁ Á ÁX 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 CÐHÁ Á Áꢀ and ꢀ±ꢀ
interactions, forming a `dimer' and further forming double
chains along [001]. The double chains are extended along [101]
through CÐHÁ Á ÁCl 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).
The main results are listed in Table 1S (supplementary
material). From Table 1S, we can see the following:
(i) CÐHÁ Á Áꢀ interactions play a major role in controlling
the molecular packing. 15 CÐHÁ Á Áꢀ 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-centro-
symmetric space groups.
(ii) ꢀ±ꢀ interactions play the same important role as the
CÐHÁ Á Áꢀ interaction. Out of 19 listed, half of the interactions
assemble the molecules in a translation-related mode, favoring
a non-centrosymmetric crystal. This mode appears to occur
when the molecular dipole is relatively small. When the
molecular dipole is relatively large, the ꢀ±ꢀ interactions cause
the molecules to pack in an antiparallel fashion into centro-
symmetric crystals.
Comment
Benzylideneanilines are an important class of Schiff bases
which have been widely used in coordinate, medical and
biological chemistry for some time (Metzler et al., 1980;
Tarafder et al., 2002). Recently, the thermochromism (Pistolis
et al., 1996), photochromism (Jalali-Heravi, 2000) and non-
linear optical properties of these compounds have found
applications in modern technologies (Sekikawa et al., 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 play a role in crystal packing and how
they interplay (Desiraju, 1989). Since any centrosymmetric
(iii) Weak interactions, such as CÐHÁ Á ÁCl (Thallapally &
Nangia, 2001), also favor the assembling of molecules with the
®rst kind of symmetric operators (screw 10 and translation 13
in the 27 important interactions listed), but the percentage of
packing energy is relatively small.
(iv) To our surprise, the role played by X atoms (X = Cl or
Br) through so-called XÁ Á ÁX 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 prop-
erties of crystals designed for similar purposes.
1 Supported by the Key Subject Programme of Jiangsu Province (grant No.
S1109001).
During our study of the design of organic functional
materials, the title compound, (I) (Fig. 1), was obtained, and
Acta Cryst. (2004). C60, o693±o695
DOI: 10.1107/S0108270104017871
# 2004 International Union of Crystallography o693
You, Xiu-Li
