organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
azines that we have investigated [R = Me, X = OCH3, Y = Br,
CN or NO2 (Glaser et al., 1995); R = Me, X = NH2, Y = F
(Lewis et al., 1998)] have an NÐN gauche conformation about
the azine bridge. The NÐN gauche conformation, in
conjunction with a phenyl-ring twist, causes the two phenyl
rings to be nearly orthogonal and this allows for the arene±
arene double T-contact throughout the crystal structures of
the previously studied mixed azines. A T-contact describes the
edge-to-face interaction between two phenyl rings. Such
arrangements are typically characterized by distances of 3.5±
ISSN 0108-2701
4-Chloroacetophenone [1-(4-methoxy-
phenyl)ethylidene]hydrazone1
Michael Lewis, Charles L. Barnes and Rainer Glaser*
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5.0 A between the midpoints of the benzene rings (Hobza et
al., 1994). A double T-contact occurs when the two phenyl
rings of one spacer-connected biphenyl system interact with
the two phenyl rings of another such molecule to form two
intermolecular arene±arene T-contacts. Scheme 2 illustrates
four types of double arene±arene contacts; I±III are all double
T-contacts and IV is a double face-to-face contact. Type I has a
planar spacer and the two phenyl rings in each molecule are
coplanar. Types II and III contain a twisted spacer and the two
phenyl rings in each molecule are twisted with respect to each
other. Note that the double T-contacts II and III are structural
isomers and differ only in the choice of the benzene edges
used in the double T-contact. The double face-to-face contact,
illustrated by IV, occurs when the two phenyl rings of each
molecule interact in a face-to-face fashion. The double
T-contacts present in all of the mixed azines that we have
prepared to date have the conformations shown in II and III,
Department of Chemistry, University of Missouri±Columbia, Columbia, Missouri
65211, USA
Correspondence e-mail: glaserr@missouri.edu
Received 23 April 1999
Accepted 14 September 1999
The crystal structure of the title mixed azine, C17H17ClN2O,
contains four independent molecules, A±D, and molecule B is
disordered. All four molecules have an NÐN gauche
conformation, with CÐNÐNÐC torsion angles of 136.5 (4),
137.0 (4), 134.7 (4) and 134.7 (4)ꢀ, respectively. The phenyl
rings are also somewhat twisted with respect to the plane
de®ned by Cipso and the imine bond. On average, the
combined effect of these twists results in an angle of 64.7ꢀ
between the best planes of the two phenyl rings. Arene±arene
double T-contacts are the dominant intermolecular inter-
action. The methoxy-substituted phenyl ring of one azine
molecule interacts to form a T-contact with the methoxy-
substituted phenyl ring of an adjacent molecule and, similarly,
two chloro-substituted phenyl rings of neighboring molecules
interact to form another T-contact. The only exception is for
molecule B, for which the disorder leads to the formation of
T-contacts between methoxy- and chloro-substituted phenyl
rings. The prevailing structural motif of T-contact formation
between like-substituted arene rings results in a highly dipole-
parallel-aligned crystal structure.
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with inter-ring distances of 3.5±5.0 A.
Comment
We have been investigating the stereochemistry (Glaser et al.,
1993, 1995; Chen et al., 1995), electronics (Glaser et al., 1993;
Chen et al., 1995; Glaser & Chen, 1998) and crystal packing
We are interested in mixed azines with the structure shown
in Scheme 1 because of their promise as non-linear optical
(NLO) materials. In order for a crystal to exhibit an NLO
response, there are two necessary conditions. First, the crystals
must be non-centrosymmetric or chiral, and this is a require-
ment for all types of polar effects of crystals (NLO activity,
piezo- and pyroelectricity, ferroelectricity, ferroelasticity and
many others). Non-centrosymmetry, in fact, occurs in
approximately 20% of organic crystals (Sakamoto, 1997). The
second condition, the central issue in the aggregation of
crystals with macroscopic polarizations, is the parallel align-
ment of the dipole moments of the individual chromophores,
and this is a challenge orders of magnitude greater than the
simple quest for non-centrosymmetry. We have previously
(Glaser et al., 1993; Chen et al., 1995; Lewis et al., 1998, 1999) of
symmetric and unsymmetric azines. An azine is the conden-
sation product of two carbonyl molecules and hydrazine. The
azines that we have studied have the general structure shown
in Scheme 1 and they are called symmetric if X equals Y and
unsymmetric or mixed if X does not equal Y. All of the mixed
1 Part 11 in the series `Stereochemistry and Stereoelectronics of Azines'. For
Part 10, see Lewis et al. (1999).
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Acta Cryst. (2000). C56, 393±396
# 2000 International Union of Crystallography
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