A comparison of the intramolecular and intermolecular hydrogen bonding of N,N′-ethylenebis(aminobenzylidene) in the solid state with its salen analogue

Article abstract of DOI:10.1016/j.molstruc.2004.10.032

The crystal structure of H₂amben has been elucidated for the first time. The close NH?N contact between neighbouring molecules supports strongly the notion that intermolecular hydrogen bonding exists within the crystal lattice, creating a series of molecular chains, which are crosslinked in a three-dimensional array. This intermolecular bonding is suggested to account for the anomalously high melting point of H ₂amben (176-178°C) compared to its H₂salen counterpart (126-128°C). Although strong intramolecular forces are present in both ligands, H₂salen contains no intermolecular hydrogen bonds.

Full text of DOI:10.1016/j.molstruc.2004.10.032

Journal of Molecular Structure 737 (2005) 69–74
www.elsevier.com/locate/molstruc
A comparison of the intramolecular and intermolecular hydrogen
bonding of N,N0-ethylenebis(aminobenzylidene) in the solid
state with its salen analogue
a
a,
a
a
*
Samantha Gakias , Colin Rix , Alan Fowless , Graham Wills-Johnson ,
Kay Lathama, Jonathan Whiteb
aDepartment of Applied Chemistry, School of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne, Vic. 3001, Australia
bSchool of Chemistry, Melbourne University, Parkville, Melbourne, Vic. 3010, Australia
Received 4 September 2004; revised 23 October 2004; accepted 26 October 2004
Available online 19 November 2004
Abstract
The crystal structure of H2amben has been elucidated for the first time. The close NH/N contact between neighbouring molecules
supports strongly the notion that intermolecular hydrogen bonding exists within the crystal lattice, creating a series of molecular chains,
which are crosslinked in a three-dimensional array. This intermolecular bonding is suggested to account for the anomalously high melting
point of H2amben (176–178 8C) compared to its H2salen counterpart (126–128 8C). Although strong intramolecular forces are present in both
ligands, H2salen contains no intermolecular hydrogen bonds.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Aminobenzylidene; Intermolecular; Intramolecular; Hydrogen bonding; Amben; Salen; Schiff base
1. Introduction
are prepared in situ, rather than being isolated in the solid
state, and then reacted with the metal ion to yield the desired
complex. As a result, most of the crystallographic data
reported for H2salen and its derivatives are those of metal
complexes rather than the free ligands themselves.
Schiff bases, or imines, R2CaNR—the condensation
products of aldehydes and ketones reacting with primary
amines—have been used extensively in the preparation of
metal complexes [1–3]. Such ligands possess interesting
photo-physical properties with potential applications as
organic materials for reversible optical data storage [4].
Since these photo-physical properties, including thermo-
chromism and photochromism, are governed strongly by the
presence of intramolecular hydrogen bonds, several studies
have focused on the nature of the hydrogen bonding in such
materials [5–9]. A large number of Schiff base ligands,
pertaining to the salicylidene family, have been prepared,
with modification of the simple H2salen ligand (N,N0-
ethylenebis(salicylideneimine) proving relatively straight-
forward in most cases. Generally, the salicylidene ligands
For the N4 quadridentate ligand, N,N0-ethylenebis(ami-
nobenzylidene), or H2amben, modifications to its aromatic
groups have been limited, and unlike the analogous
salicylidene Schiff bases, the chemical and physical
properties of H2amben and its derivatives are relatively
unexplored. Whereas strong intramolecular hydrogen bond-
ing is reported to exist within the crystalline lattice of
H2salen, governing a number of its properties [10], no
crystallographic data have been reported thus far for
H2amben, although strong intramolecular hydrogen bonds
have been reported for macrocyclic amben derivatives and
N4 porphyrins [11,12]. A renewed interest in these systems
has resulted in the preparation of ligands derived from tosyl-
aminobenzaldehyde and various diamines, where crystal-
lographic data indicate the presence of both intramolecular
and intermolecular interactions [13–15].
* Corresponding author. Tel.: C613 9925 2628; fax: C613 9639 1321.
E-mail address: colin.rix@rmit.edu.au (C. Rix).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.10.032

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S. Gakias et al. / Journal of Molecular Structure 737 (2005) 69–74
2. Experimental
3. Results and discussion
3.1. Molecular structure of H2amben
2.1. Methodology
After many unsuccessful attempts using a variety of
mixed solvents, crystallization of H2amben (in a form
suitable for analysis by single crystal X-ray diffraction) was
finally achieved using a 1:9 tetrahydrofuran:diethyl ether
mixture. An X-ray analysis of H2amben shows the amino
groups to be trans, as are the hydroxy groups in H2salen
[10], and confirms the long-held belief that it is an extended
molecule with the two aromatic rings lying in parallel planes
(0.28 displacement) [22,23]. The distance between these
An ethanolic solution of 30% w/w ethylenediamine
hydrate (16 mmol) and 2-aminobenzaldehyde (34 mmol)
was refluxed for one hour. The solution was then cooled and
allowed to stand overnight. The white, plate-like crystals
of H2amben which separated were filtered and allowed to
air-dry as reported previously [16,17]. Recrystallization
from a solution of tetrahydrofuran and ether (1:9) yielded
white plates that were dried in vacuum over P2O5 (3.14 g,
70%).
˚
planes is 1.37 A. The molecular point group symmetry is Ci
(previously termed S2) with an inversion center (Fig. 1).
Selected bond lengths and angles from the analysis are
presented in Table 2. Bond lengths within the aromatic rings
are consistent with those expected for sp2 aromatic carbon
atoms [24–26]. The slightly longer bond lengths of the
carbon atoms connected to the imine and amine moieties
2.2. X-ray diffraction analyses
Data were collected on a Bruker Smart Apex diffract-
ometer, equipped with a CCD detector. The structures were
solved by the direct method, SHELXTL [18], and refined by
a full matrix least squares method, SHELXL97 [19].
Thermal ellipsoid plots were drawn using ORTEP [20],
whilst diagrams of the unit cell were created using
Mercury [21]. Details of the data collected are provided in
Table 1.
˚
˚
[C(2)–C(1) (1.409(3) A), C(11)–C(12), (1.408(3) A)] are
not unexpected for Schiff bases, as substitution of aromatic
rings can lead to minor distortions within the aromatic
moiety [5,10,27,28].
Bond angles, which range from 118.38 to 121.808 in the
aromatic moieties, are consistent with mean literature
˚
values [24]. The C(1)–C(7) (1.454(3) A) and C(10)–C(11)
Table 1
Details of the X-ray crystal structure analysis for the H2amben molecule
˚
(1.462(3) A) distances are consistent with single bonds
between sp2 hybridized carbon atoms, whilst the bond
Empirical formula
Formula weight (g/mol)
Melting point (8C)
Crystal color
C16H18N4
266.35
˚
length of 1.277(2) A for each imine group, C(7)–N(2) and
176–178
C(10)–N(3), is in agreement with those reported for
H2salen, H2tosylamben and H2salphen [10,14,29].
Colorless
Crystal description
Crystal class
Plate
Monoclinic
The imine nitrogen atoms, N(2) and N(3), bond angles of
117.49(17) and 117.05(17)8, respectively, confirms their sp2
character, whilst the bond lengths of 1.463(2) and
Space group
˚
a (A)
˚
b (A)
P21/n
5.7472(11)
7.7478(14)
˚
1.458(2) A for C(8)–N(2) and C(9)–N(3), respectively, are
˚
c (A)
31.595(6)
consistent with C(sp3)–N(sp2) single bonds. Bond lengths of
a (8)
b (8)
90.00
2
1.372(2) and 1.370(2) A for the C(sp )–N(sp ) single bonds
3
˚
94.245(3)
g (8)
Density (g/m3)
Crystal size (mm)
90.00
1.261
0.40, 0.20, 0.03
3
˚
Volume (A )
1403.0
Z
4
m (mmK1
)
0.078
F(000) (e)
q range (8)
Index ranges
568
2.59–25.0
K6%h%6, K8%k%9,
K24%l%37,
Reflections collected/unique
Data/restraints/parameters
Goodness of fit on F2
6940/2472 [R(int) Z0.1129)
2472/0/197
0.982
Final R indices [IO2s(I)]
R indices (all data)
R1Z0.0579, wR2Z0.1464
R1Z0.0813, wR2Z0.1584
Crystallographic data for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as deposition
No. 248810. Copies of the data can be obtained, free of charge, via www.
ccdc.cam.ac.uk or on application to Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: C44 1233
336033).
Fig. 1. Molecular structure of H2amben (50% ellipsoid probability).

S. Gakias et al. / Journal of Molecular Structure 737 (2005) 69–74
71
Table 2
˚
Selected bond lengths (A) and angles (8) for H2amben
whilst the dihedral angle for N(2)–C(8)–C(9)–N(3) is G
1798. Torsion angles indicate that the molecule is strictly
planar about the aromatic ring and imine group, although
free rotation is possible about C(8)–C(9) in the ethylene-
diamine bridge.
C1–C2
1.409(3)
1.454(3)
1.372(2)
1.277(2)
1.463(2)
1.513(3)
1.459(2)
1.277(2)
1.462(3)
1.408(3)
1.370(2)
C1–C7
C2–N1
C7–N2
C8–N2
C8–C9
On average, the bond angles around the two amino
nitrogen atoms, N(1) and N(4) are closer to sp2 than to sp3
hybridization. For example, the angles C(2)–N(1)–H(2A)
[113.04(15)8] and C(2)–N(1)–H(2B), [116.91(13)8] and
C(12)–N(4)–H(4A) [115.95(14)8] and C(12)–N(4)–H(4B)
[117.57(14)8], indicate considerable flattening of the –NH2
moiety in order to optimize both intra- and inter-molecular
hydrogen bonding.
C9–N3
C10–N3
C10–C11
C11–C12
C12–N4
C6–C1–C7
117.96(18)
120.06(18)
120.95(18)
109.83(16)
109.87(16)
117.45(18)
120.46(19)
121.13(18)
117.49(17)
117.05(17)
113.04(15)
116.91(13)
115.95(14)
117.57(14)
128.72(19)
124.70(02)
N1–C2–C3
The distances between N(1)–N(2) and N(4)–N(3), of
˚
2.731 and 2.745 A, respectively, and the close proximity of
N1–C2–C1
N2–C8–C9
N3–C9–C8
˚
˚
H(2A)/N(2) (Z1.994 A) and H(4A)/N(3) (Z2.037 A)
in Fig. 1, suggest strong intramolecular hydrogen bonding.
Similar distances are observed between the aminic (NH2)
and iminic nitrogen (CaN) atoms in the free ligands of
N,N0-bis(2-tosylaminobenzylidene)1,2-ethanediamine and
N,N0-bis(2-tosylaminobenzylidene)-1,3-propanediamine
[13,14]. Likewise, the molecular interactions in H2salen are
dominated by two strong intramolecular hydrogen bonds
within the unit cell, indicated by the close approach of the
C16–C11–C10
N4–C12–C13
N4–C12–C11
C7–N2–C8
C10–N3–C9
C2–N1–H2A
C2–NI–H2B
C12–N4–H4A
C12–N4–H4B
H2A–N1–H2B
H4A–N4–H2B
˚
phenolic oxygens and imine nitrogens (2.596 and 2.598 A
for the two pairs) [10].
Esd’s are given in parentheses.
3.2. The unit cell
for C(2)–N(1) and C(12)–N(4), respectively, are in excellent
˚
agreement with the mean bond length of 1.375 A reported
Four molecules are contained within the unit cell of
H2amben (Fig. 2), which is marginally larger than the
analogous H2salen unit cell. For the pair of molecules on the
previously [24]. The ethylene bridge is in the trans
˚
conformation and the C(8)–C(9) distance of 1.513(3) A is
consistent with a single bond between sp3 carbon atoms.
Furthermore, C(8) and C(9) deviate from C(1)–C(6) by
right of the unit cell, the distances are 6.405 and 3.542 A
between N(1)–N(4) at their right- and left-hand ends,
respectively. These distances are reversed for the pair of
˚
˚
G0.11 A and C(11)–C(16) by G0.12 A, respectively,
˚
Fig. 2. Intermolecular hydrogen bonding (H(2B)/N(4)) within the unit cell of H2amben.

72
S. Gakias et al. / Journal of Molecular Structure 737 (2005) 69–74
molecules on the left of the unit cell. It is noteworthy, in
contrast to H2salen, that two intermolecular hydrogen
bonds, (which are parallel to, and separated by, (002)
planes) are contained fully within each unit cell. On the left
half of Fig. 2, the hydrogen bond is from H(2B) in the upper
molecule to N(4) in the lower, while on the right half, it is
from H(2B) in the lower to N(4) in the upper molecule. The
distance between the intermolecular hydrogen-bonded
˚
molecules within the unit cell is 2.673 A (H(2B)/N(4)).
The complete hydrogen-bonding pattern, taking into
account intermolecular hydrogen bonds which are shared
between adjacent unit cells, is much more extensive. Indeed,
each molecule participates in four hydrogen bonds
involving N(1), H(2B), N(4) and H(4B), as indicated in
Fig. 3. All the resulting hydrogen bonds are in directions
parallel to the (002) crystallographic planes. This enables
the extended hydrogen-bond structure to be described
completely by referring only to the left-hand side of Fig. 2
(this side is reproduced as the middle section of Fig. 3). As
an aid to interpretation, the molecules have been numbered
in each of Figs. 2–4 in the same way. A molecule with the
same orientation as the upper molecule on the left-hand side
of the unit cell will be found in the unit cell immediately
below (only the upper left half of this unit cell is shown),
while one with the same orientation as the lower molecule
will be found in the unit cell immediately above (only the
lower left half of this unit cell is shown). Thus, a chain of
molecules, 8-1-2-7, can be traced by their intermolecular
hydrogen bonding from N(1) in the upper unit cell to H(4B)
in the lower unit cell, which is then repeated throughout the
crystal lattice. In this chain, it can be seen that the resulting
hydrogen bonds alternate between N(1)/H(4B)
and H(2B)/N(4) in opposite parts of the molecule.
Fig. 4. Crosslinked intermolecular hydrogen bonding within the crystal
lattice of H2amben.
The direction in which the molecular chain propagates
is [110], and chains extend throughout the solid crystal.
Fig. 4 depicts three molecular chains. As in Fig. 3, the chain
8-1-2-7 propagates in the [110] direction. Thus, any given
molecule that is hydrogen-bonded via nitrogen H(2B)/
N(4) in the [110] direction is also simultaneously hydrogen-
bonded to a molecule in another chain, that being 5-1-3 and
ꢀ
6-2-4 which propagate in the [110] direction (as seen in
Fig. 4). These crosslinked hydrogen-bonded chains con-
stitute a two-dimensional network.
The long axes of the H2amben molecules are close to
perpendicular to the plane of the bonded network (bZ948).
The lattice of the H2amben crystal is monoclinic, where the
˚
dimensions of the unit cell are 5.747 (A) (a direction),
˚
˚
7.747 (A) (b direction) and 31.595 (A) (c direction). This
compares to the marginally-smaller H2salen unit cell with
˚
dimensions, 6.094 (A) (a direction), 7.567 (A) (b direction)
˚
˚
and 30.680 (A) (c direction). The directions, [110] (mol-
ꢀ
ecular chains) and [110] (crosslinked chains), are at 908.
With asb, it might be expected that the chains in each
direction are not equivalent. Such an expectation (which
would hold for a closely similar orthorhombic lattice) is
incorrect, as is apparent from Fig. 5(a) and (b). These show a
portion of the H2amben lattice, with the structural motif
extended to a 3!3!3 model. Comparison of Fig. 5(a),
representing the molecular chains in the [110] direction,
with Fig. 5(b), representing the crosslinked chains within
ꢀ
the crystal lattice in the [110] direction, reveals that the
chains propagating in each direction are equivalent. For
comparison, views of H2salen [10] in the same directions
are shown in Fig. 6(a) and (b). The diagrams show
the remarkable similarity of the H2amben and H2salen
Fig. 3. Molecular chains (intermolecular hydrogen bonding) within
H2amben crystal lattice.

S. Gakias et al. / Journal of Molecular Structure 737 (2005) 69–74
73
Fig. 6. A 3!3!3 structural motif of the H2salen crystal lattice: (a)
molecular units (propagating in [110] direction) and (b) molecular units
ꢀ
(propagating in [110] direction).
Fig. 5. A 3!3!3 structural motif of the H2amben crystal lattice: (a)
molecular chains (propagating in [110] direction) and (b) crosslinked chains
ꢀ
(propagating in [110] direction).
presence of tosyl moieties (used to replace the hydrogen atom
on the aromatic amine), but also to the strong intramolecular
forces which are thought to minimize the steric hindrance of
the tosyl groups [13]. Similar intramolecular forces are
observed for H2amben also; however, what is unique to this
system is the presence of additional intermolecular hydrogen
bonds. Molecules within the crystal lattice of H2salen are
held together only by van der Waals forces [10], whilst the
presence of the intermolecular hydrogen bonds within
the crystal lattice of H2amben is expected to influence
significantly the properties of the molecule.
lattices, which arises from the similarity of the crystal
packing requirements. However, it is noteworthy that the
presence of additional intermolecular hydrogen bonding in
H2amben is accommodated through a moderate degree of
rehybridization of the amino-nitrogen atoms, N(1) and N(4),
as discussed earlier.
It is well accepted in the literature that strong intramo-
lecular forces govern the properties exhibited by Schiff
bases. In the case of H2salen, similar strong intramolecular
interactions are observed. Likewise, the structural behaviour
of tosylamben derivatives is attributed not only to the

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S. Gakias et al. / Journal of Molecular Structure 737 (2005) 69–74
4. Conclusion
References
[1] S. Yamada, Coord. Chem. Rev. 190-192 (1999) 537.
[2] M.D. Hobday, T.D. Smith, Coord. Chem. Rev. 9 (1973) 311.
[3] R.H. Holm, G.W. Everett Jr., A. Chakravorty, Prog. Inorg. Chem. 7
(1966) 83.
The anomalously higher melting point of H2amben
(176–178 8C) compared to H2salen (126–128 8C)—which
has remained unexplained for more than 60 years—can now
be attributed to the network of intermolecular hydrogen
bonds present within the crystal lattice. The lower rate of
dissolution of H2amben, compared to H2salen, in a variety
of solvents is consistent with the extensive intermolecular
hydrogen bonding present in the solid state. In addition, the
manner in which molecules crystallize from solution is
dictated strongly by hydrogen bonds formed within the
molecule and the crystal lattice. In the case of H2amben, the
isolation of suitable crystals for X-ray analysis proved to be
a challenging task, and numerous solvents and solvent
mixtures were explored under varying conditions in an
attempt to isolate a crystalline material suitable for X-ray
analysis. Frequently, the H2amben molecules grew prefer-
entially in a ‘pancake’ arrangement, forming a sequence of
thin, stacked platelets, unsuitable for analysis. This problem
was solved eventually by using very slow evaporation in the
chosen solvent mixture, which yielded crystalline plates of
more uniform dimensions, appropriate for X-ray analysis.
Based on the crystallographic data obtained in this work, it
can be seen why the molecules stack in the a and b
directions of the unit cell rather than the c direction (which
would produce needle or rod shaped crystals), as inter-
molecular hydrogen bonding within the unit cell occurs in
the a and b directions only. In addition, the spacing in
H2amben (P21/n) is less than that in H2salen (P21/c), which
accounts for the thinness of the platelets arising from rapid
growth in the hydrogen-bonded directions.
[4] A. Filarowski, T. Glowiaka, A.J. Koll, Mol. Struct. 484 (1999) 75.
´
[5] P.M. Dominiak, E. Grech, G. Barr, S. Teat, P. Mallinson, K. Wozniak,
Chem. Eur. J. 9 (2003) 963.
´
[6] G. Wojciechowski, P. Przybylski, W. Schilf, B. Kamienski,
B. Brzezinski, J. Mol. Struct. 649 (2003) 197.
ˇ
´
´
´
´
[7] Z. Popovic, G. Pavlovic, D. Matkovic-Calogovic, V. Roje, I. Leban,
J. Mol. Struct. 615 (2002) 23.
[8] E. Hadjoudis, M. Vittorakis, I. Moustakali-Mavridis, Tetrahedron 43
(1987) 1345.
[9] B. Brzezinski, Z. Rozwadowski, T. Dziembowska, G.J. Zundel, Mol.
Struct. 440 (1998) 739.
[10] B. Brescian-Pahor, M. Calligaris, G. Nardin, Randaccio L, Acta.
Crystallogr. Sect. B 34 (1978) 1360.
[11] M.S. Somma, C.J. Medforth, N.Y. Nelson, M.M. Olmstead,
R.G. Khoury, K.M. Smith, Chem. Commun. 13 (1999) 1221.
[12] P.G. Owston, R. Peters, E. Ramsammy, P. Tasker, J.J. Trotter, Chem.
Soc. Chem. Commun. 24 (1980) 1218.
´ ´ ´
[13] J. Mahıa, M.A. Maestro, M. Vazquez, M.R. Bermejo, J. Sanmartın,
M. Maneiro, Acta Crystallogr. Sect. C 55 (1999) 1545.
´ ´
[14] J. Mahıa, M.A. Maestro, M. Vazquez, M.R. Bermejo, A.M. Gonzalez,
´
M. Maneiro, Acta Crystallogr. Sect. C 56 (2000) 347.
´
´
´
[15] J. Mahıa, M.A. Maestro, M. Vazquez, M.R. Bermejo, A.M. Gonzalez,
M. Maneiro, Acta Crystallogr. Sect. C 56 (2000) 492.
[16] P. Pfeiffer, Th. Hesse, H. Pfitzner, W. Scholl, H. Thielert, J. Prakt.
Chem. 149 (1937) 217.
[17] M. Green, P.A. Tasker, J. Chem. Soc. A 15 (1970) 2531.
[18] Bruker, SHELXTL (Version 6.12), Bruker AXS, Inc., Madison, WI,
2001.
¨
[19] G.M. Sheldrick, SHELXL97, University of Gottingen, Germany,
1997.
[20] L. Zsolnai, ORTEP, University of Heidelberg, Germany, 1997.
[21] Cambridge, Mercury (Version 1.1.2), Cambridge Crystallographic
Data Centre, University of Cambridge, London, 2001.
[22] M. Green, J. Smith, P.A. Tasker, Discuss, Faraday Soc. 47 (1969) 172.
[23] R. Karlsson, L.M. Engelhardt, M. Green, J. Chem. Soc. Dalton Trans.
22 (1972) 2463.
Finally, it should be noted that the crystal structures for
the nickel- and copper-amben complexes [30] as well as
those for substituted amben analogues of Co(II) [23], Ni(II)
[31] and Cu(II) [32], indicate the flexibility of the
CH2–CH(X) (XaH, CH3) bridge, which allows the
trans-configured ligand to readily adopt the cis-
stereochemistry necessary to bind to a metal ion in a
tetradentate manner.
[24] F.H. Allen, O. Kennard, D.G. Watson, J. Chem. Soc., Perkin Trans. 2
12 (1987) S1.
[25] H. Nazir, M. Yildiz, H. Yilmaz, M.N. Tahir, D. Ulku¨, J Mol. Struct.
524 (2000) 241.
[26] M. Kabak, J. Mol. Struct. 655 (2003) 135.
[27] K.V.A. Gowda, M.K. Kokila,
Puttaraja, M.V. Kulkarni,
N.C. Schivaprakash, Indian J. Phys. A 55 (2000) 441.
[28] T.J. Brett, S. Liu, P. Coppens, J. Stezowski, J. Chem. Commun. 6
(1999) 551.
[29] N. Brescian-Pahor, M. Calligaris, P. Delise, G. Dodic, G. Nardin,
L. Randaccio, J. Chem. Soc. Dalton Trans. 23 (1976) 2478.
[30] G. Brewer, J. Jasinski, W. Mahany, L. May, S. Prytkov, Inorg. Chim.
Acta 232 (1995) 183.
Acknowledgements
[31] N.A. Bailey, E.D. McKenzie, Inorg. Chim. Acta 43 (1980) 205.
[32] G. Brewer, C.T. Brewer, P. Kamaras, S. Prytkov, M. Shang,
W.R. Scheidt, Inorg. Chim. Acta 321 (2001) 175.
We thank Associate Professor Malcolm Hobday for
initiating this research project.