COMMUNICATION
Ionic bonding driven unusual crystal growth of tubular organic nitrate salts†
*
Prasanna S. Ghalsasi and Ashok K. Vishwakarma
Received 25th September 2009, Accepted 13th January 2010
First published as an Advance Article on the web 27th January 2010
DOI: 10.1039/b920059b
aluminium. The observed crystals showed a signature of presence of
anilinium nitrate (AnHNO3-I) by liquid state NMR, and FT-IR
spectroscopy. Unfortunately, AnHNO3-I crystals were too thin for
single-crystal X-ray analysis. That means that in this system,
Al(NO3)3 is acting as a source of nitrate anion, an environmentally
friendly nitrating reagent. Normally, concentrate or diluted HNO3 is
used to make nitrate salts. Thus, anilinium nitrate was synthesized by
treating aniline with concentrated HNO3. In the latter case, we
obtained very good quality sugar-like crystals (AnHNO3-II). Single-
crystal X-ray analysis of AnHNO3-II at room temperature, shows
that it crystallizes in orthorhombic system having space group Pbca
We report the unusual crystal growth of 10–15 cm long tubular
anilinium nitrate salts. The tubes resemble the bamboo structure, in
which each shoot is 100–200 mm in length and around 10 mm in
diameter. This growth is observed after mixing aniline and
aluminium nitrate [Al(NO3)3] in methanol.
Inorganic nitrate salts are commonly used in agriculture and high
energy materials research. These compounds are well studied in the
literature for their ferroelectric behaviour and phase transitions.
Surprisingly, organic nitrate salts are scarcely reported. The standard
method of nitration is to treat the compound with concentrated or
dilute nitric acid. Here, we report the use of a milder synthetic route
for nitration, that is the use of aluminium nitrate [Al(NO3)3]. When
aniline is treated with Al(NO3)3 in methanol, a translucent gel is
obtained. The gel, on standing for a few hours in an open atmo-
sphere, gave unusual white hair-like crystal growth of 10–15 cm. Even
though the formation of aluminium hydroxide is expected during the
reaction of Al(NO3)3 and the base, the observed growth was found to
be anilinium nitrate (AnHNO3). The white hair crystals show
bamboo-like growth. This process also yielded rosette shaped
alumina from starting aluminium nitrate. We present a detailed
investigation of this growth using solid-state NMR, DSC, powder
X-ray diffraction and SEM techniques.
Recently, studies on polymorphism1 and different structural
modifications of the same chemical compound, have generated a lot
of interest in applied as well as basic research. The occurrence of
polymorphic modification in the compounds is manifested not just as
a consequence of minimum free energy of the crystalline phases but
also by crystal nucleation and growth.2 On the other hand, alumina
has been a subject of interest due to its potential applications in
catalysis,3 adsorptive/separation materials,4 and optical applications.5
The motivation of the present work lies in our attempts to grow
crystals in porous solids, especially for layered organic–inorganic
hybrid compounds for multiferroic behaviour.6 The literature has
shown that mesoporous/nanoporous alumina can be synthesized
using a gel of Al(NO3)3 or AlCl3 in the presence of a proton
acceptor.7 We thought aniline could act as a good proton acceptor
source. Therefore, aniline was added directly into the methanolic
solution of Al(NO3)3(3 : 1 molar ratio). This yielded a translucent gel.
The gel was filtered and kept aside. To our surprise, white, hair-like
crystals developed from it after 1 h, growing up to 10–15 cm long in
about 8–10 h, as shown in Fig. 1. This growth is observed only using
Al(NO3)3 as a substrate8 and not for AlCl3 or other simple salts of
9
ꢀ
(no. 61), Z ¼ 8, a ¼ 10. 158(2), b ¼ 9.277(2), c ¼ 16.177(3) A. The
primitive unit cell contains eight molecules of anilinium nitrate.
Mono-protonated anilinium cation interacts through weak hydrogen
bonds with three neighbouring nitrate anions. There are six N–H/O
hydrogen bonds, which, can be divided in two groups. One group of
ꢀ
three bonds with similar lengths equal to 2.81, 2.86 and 2.88 A, gives
ꢀ
average value equals 2.85 A. Three other hydrogen bonds with
ꢀ
lengths 3.13, 3.17 and 3.20 A constitute the second group with
ꢀ
average value of lengths equal to 3.17 A. These two groups of N/O
hydrogen bond distances corresponds to essentially linear or bent
N–H/O interactions, respectively. These N/O hydrogen bonds
helps alternately stacked and mutually perpendicular anilinium
cations to form a chain along the crystallographic c axis, as shown in
Fig. 2. The calculated powder X-ray pattern of AnHNO3-II from
powder crystal X-ray diffraction data matches well with the
AnHNO3-I see Fig. 3. Both these compounds have largely similar
powder X-ray diffraction patterns. Powder X-ray diffraction data,
obtained from single-crystal X-ray data, show that AnHNO3-II has
one major peak at 22.1ꢀ 2q, which was indexed to the 212 plane. The
corresponding plane is also observed in AnHNO3-I. Apart from this
very strong peaks at 19 and 28ꢀ 2q were observed for AnHNO3-I,
which were indexed for the 020 and 302 planes. Even though latter
peaks were present in AnHNO3-II, their intensities were quite less.
Apart from this, the major differences in both the compounds are the
presence of 301 plane and absence of 204, 023, and 114 planes for
AnHNO3-I. The small difference in the indexed cell parameters for
Department of Chemistry, The M. S. University of Baroda, Vadodara,
India 390 002. E-mail: prasanna.ghalsasi@gmail.com
† Electronic supplementary information (ESI) available: Solid-state
NMR, powder X-ray data, single-crystal X-ray data and SEM images.
See DOI: 10.1039/b920059b
Fig. 1 Crystal growth of AnHNO3-I from the gel on filter paper after 8 h.
CrystEngComm, 2010, 12, 1693–1695 | 1693
This journal is ª The Royal Society of Chemistry 2010