The influence of weak interactions on phase transformations and polymorphism in distributed n-aryl -formamides and -thioamides
Date
2008-12-22T07:46:47Z
Authors
Omondi, Bernard
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
A series of arylformamides and arylthioamides has been synthesized
and analyzed using nuclear magnetic resonance spectroscopy (NMR),
differential scanning calorimetry (DSC), powder and single crystal X-ray
diffraction. The work involved the study of hydrogen bonding, weak
intermolecular interactions, phase changes and co-crystallization in aryl -
formamides and -thioamides resulting in the structure determination of twenty
four crystals.
Three sets of isomorphic compounds were identified from the 24 solid
state structures: set one; 2,6-difluorophenylformamide (1a), 2,6-
dichlorophenylformamide (2a) and 2-chloro-6-methylphenylformamide (4a);
set two; 2,6-dimethylphenylthioamide (17) and 2-chloro-6-
methylphenylthioamide (18) and set three; 2,6-diisopropylphenylformamide
(6) and 2,6-diisopropylphenylthioamide (20). In the first two sets, 1a, 2a and
4a, and 17 and 18, there are similar regions of halogen interactions and
hydrocarbon interactions with disorder in the chloro-methyl substituents in
structures 4a and 18. As for compounds 6 and 20, both the chemical and
geometrical effects (size and volume of the isopropyl substituents) play a role
in their isomorphism.
A mixture of 2,6-dichlorophenylformamide (2a) and 2,6-
dimethylphenylformamide (3) yielded a co-crystal 22 in which there was one
molecule in the asymmetric unit, same as co-crystal 23 [derived from 2,6-
dichlorophenylthioamide (17) and 2,6-dimethylphenylthioamide (18)]. The
molecules of the two co-crystals displayed disorder in the substituents on the
2 and 6 positions of the aryl ring as a result of the occurrence of chlorine and
methyl groups in the same crystallographic sites. Co-crystal 22 adopted the
structure of 2,6-dichlorophenylformamide 2a. Co-crystal 23 also had a
ii
structure similar to that of 2a and co-crystal 22. Co-crystal 24 derived from a
mixture of 2,6-diisopropylphenylformamide (6) and 2,6-diisopropylphenylthioamide
(20), and also had one molecule in the asymmetric unit which
showed disorder in the position occupied by oxygen and sulfur atoms.
The 24 structures studied exhibited a variety of motifs formed from
weak intermolecular interactions. Investigation of these weak intermolecular
interactions revealed four different categories1 for the arylformamides and
only one category for the arylthioamides. The categories were different in
their formation of N-H…O/S hydrogen bonds (in which adjacent molecules
are related by 21-screw axes, glide planes or by translation) forming chains
(as in category 1, 2 and 5), sheets (as in Category 3) or dimers and tetramers
(as in category 4). The chains in categories 1, 2 and 5 are in the for form of
spirals (molecules along the chain are related by 21-screw axes or glide
planes) or stacks (molecules along the chains are related by translation).
Compounds from the different categories had certain interactions that
contributed most to the stabilizations of their crystals. Apart from the N-H…O/
S hydrogen bonds, π…π, C-H…π, C-F…π, C-H…F, C-H…Cl, C-H…O,
Cl…Cl, Br…Br, Cl…O and Br…O interactions also had a role to play in the
stabilization of the different structures. Lattice energies and the energies
relating to different molecular arrangements were calculated using
Gavezzottis’ OPIX program suit. This showed that the N-H…O/S hydrogen
bonds and π…π interactions were the most important interactions amongst
the 24 structures discussed in this work.
The crystal structures, thermal behaviour and phase transformations of
all arylformamides and arylthioamides have shown that a phase
transformation was only observed when a halogen atom was one of the
substituents and only for some of the formamides. 2,6-dichlorophenylformamide
2a and 2-chloro-6-methylphenylformamide 4a transform to a hightemperature
form at 155 and 106 °C, respectively. The high-temperature
forms 2b and 4b (grown by sublimation) are both monoclinic but not
isomorphous, with one short axis of about 4.3 A°, and consist of chains of N–
H…O hydrogen-bonded molecules stacked along the short axis, related by
translation. 1a and 1b are related to the above polymorphs in their formation
of N-H…O hydrogen bonding patterns.
Finally, this contribution has analyzed the role of weak interactions on
the structural and thermal properties of the compounds studied. In addition, a
mechanism for the phase change in 2,6-dichlorophenylformamide has been
proposed and rationalized through the examination of the structures
themselves together with lattice energy calculations.
(1. Category = different types of hydrogen bonding patterns formed by disubstituted phenyl
-formamides and -thioamides discussed in this thesis).
Description
Keywords
arylformamides, arylthioamides, weak interactions, polymorphism