Solid state conformational preferences of a flexible molecular backbone derived from acetone: Dependence on electron donating/withdrawing ability of substitutions

Article abstract of DOI:10.1021/cg100021f

Conformational preferences, crystal packing, and intermolecular interactions can play a pivotal role in solid state reactions, and in particular for bis(phenyl)acetones, these factors are known to determine the rate of the photodecarbonylation process. In

Full text of DOI:10.1021/cg100021f

DOI: 10.1021/cg100021f
Solid State Conformational Preferences of a Flexible Molecular
Backbone Derived from Acetone: Dependence on Electron
Donating/Withdrawing Ability of Substitutions
2010, Vol. 10
2298–2305
Sunil Varughese† and Sylvia M. Draper*
School of Chemistry, Trinity College Dublin, College Green, D2, Ireland. †Current address:
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India.
Received January 6, 2010; Revised Manuscript Received March 18, 2010
ABSTRACT: Conformational preferences, crystal packing, and intermolecular interactions can play a pivotal role in solid state
reactions, and in particular for bis(phenyl)acetones, these factors are known to determine the rate of the photodecarbonylation
process. In the present work, conformational pliability and the supramolecular synthons present in a series of variably
substituted bis(phenyl)acetones have been assessed in terms of the electron donating/withdrawing ability of the substituents.
While an exo-exo conformation has been observed in the case of electron donating substituents, electron withdrawing
analogues preferred an endo-endo arrangement. A linear correlation has been observed between the angular shift and the
Hammett constants of the functional groups. The shift in the IR spectra with respect to that of acetone has been used to ascertain
the spectral and structural analogy. SEM analysis of the nano-/microparticles of the compounds reveals the direct dependence of
the hydrophobic/hydrophilic character of the functional groups on the nature and morphology of the particles.
Introduction
In the area of crystal engineering, urea has been a target
molecule in achieving networks for inclusion compounds,
nonlinear optical materials and modular assemblies stabilized
by hydrogen bonds.7 N,N0-Diaryl ureas (Scheme 1a) predo-
minantly exhibit a W-conformation with the formation of an
Studies pertaining to conformationally flexible molecules
are interesting, as they can provide insights on the intricacies
involved in various complex molecules and recognition pro-
cesses.1,3 The crystallographic information obtained from
simple systems can be utilized in understanding various
structural motifs present in complex assemblies. For example,
as universal models of protein folding and the interactions
involved in the process, foldamers are well studied.2 In such
cases, various oligoimides are designed, synthesized, and
characterized using X-ray diffraction, NMR, and CD analysis.
Recently, Fujita and co-workers reported the conformational
preferences of a series of short peptide fragments in the hydro-
phobic constrained cavities of a porphyrin containing coordi-
nation cage.3a
R-network, a chain of bifurcated N-H O hydrogen bonds.8
3 3 3
Nangia and Etter have reported strategies for effectively
designing assemblies with urea, non-urea, and urea-solvent
hydrogen bonding motifs by the appropriate functionaliza-
tion of the aryl rings.9 Further, diphenylcarbonates and their
conformational preferences have been an interesting topic
over several years.10 Unlike the cases of urea derivatives, ab
initio calculations on a series of carbonates (Scheme 1b)
demonstrated that they can exist as cis-cis, cis-trans, and
trans-trans conformations.11
An isoelectronic compound, 1,3-bis(phenyl)acetone
(Scheme 1c), and its derivatives have been target molecules
for studying the photodecarbonylation reactions in the solid
state.12 In a series of substituted bis(phenyl)acetones, the rate
of triplet R-cleavage has been reported to be in the order
-OCH3>-CH3>-H. Wong and co-workers did fit the rate
of decarbonylation reaction to Hammett plots.13 It has been
observed that the triplet photochemistry of the bis(phenyl)
acetone is in fact controlled by a conformational “switch”.14
Resendiz et al. reported that, in crystalline p,p0-disubstituted
bis(phenyl)acetones, the relative quantum yields and chemical
efficiencies of photodecarbonylation reactions are a function
of the Hammett constants of the substituents. The authors
further stated that “... reactions in crystals can be enginee-
red from known molecular structure parameters”.15 In 2007,
Coppens and co-workers demonstrated that intermolecular
hydrogen bonding can induce the quenching of photodecar-
bonylation reactions in the monoketones.16 Though the solid
state photochemical processes of bis(phenyl)acetones are depen-
dent on the conformations of the molecules, surprisingly, a
systematic evaluation of their conformational preferences in
their crystal structures has not been done to date. Herein, we
In evaluating the properties of a flexible molecule and in
determining its conformational preferences, in addition to the
hydrogen bonds, various factors such as electronic effect,
steric factors, hydrophobicity, etc. can contribute significan-
tly.4 Molecular conformation data available from crystal
structures are usually affected by crystal packing forces and
cause impedance in computational modeling. Although crystal
structures generally give a good indication of the conforma-
tional preferences of a given molecule, intermolecular inter-
actions with sufficient strength can presumably lead to a
strained arrangement.5 In 2008, Allen and co-workers stated
that “...conformational diversity increases with an increasing
number of different crystal environments and with an increasing
number of flexible torsion angles. Overall molecules with one or
more acyclic flexible torsion angle are observed to exist in more
than one conformation in ca. 40% of cases.”6 In this context,
crystallographic analysis of a series of chemically related
flexible molecules is engrossing and can provide insight on
the conformational preferences in their crystal lattice.
*Telephone: þ353-1-8962026. E-mail: smdraper@tcd.ie.
r
pubs.acs.org/crystal
Published on Web 03/31/2010
2010 American Chemical Society

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2299
analyze the conformational flexibility of a series of substituted
1,3-bis(phenyl)acetones as a function of the electron donating/
withdrawing ability of the substituents. For an analogy, the
structures have been compared with the dimorphic forms of
acetone.
Acetone. Two distinct phases of acetone are known in the
literature.17 The room temperature high pressure form is
composed of molecules stacked along the [010] direction. In
the crystal lattice, each molecule acts as a four-donor four-
acceptor hydrogen bond forming species, as shown in
Figure 1a. The low temperature phase consists of molecules
forming corrugated tapes running in antiparallel directions.
Individual tapes consist of molecules stabilized by bifurcated
X-ray diffraction (Scheme 2). The variations in the conforma-
tional arrangements and synthons have been evaluated as a
function of substituents. Further, to correlate the structural
and spectral information, infrared spectral data were ana-
lyzed. Organic nano-/microparticles, prepared using a pre-
cipitation method, were analyzed using scanning electron
microscopy.
1,3-Bis(phenyl)acetone, 1. 1,3-Bis(phenyl)acetone, 1, crys-
tallizes in the P21/c space group with a molecule in the
asymmetric unit and exhibits considerable variations in the
intermolecular interactions with respect to the dimorphs of
acetone (Figure 2). The molecule adopts an endo-exo con-
formation, with two arms of the compound having opposite
orientations with respect to the central carbonyl group.
Unlike the cases of acetone dimorphs, in which the methyl
groups make intermolecular interactions, the molecules of 1
C-H O hydrogen bonds (Figure 1b).
3 3 3
Results and Discussion
form linear chains through bifurcated (R12(6)) C-H
O
(H O, 2.71 and 2.99 A) hydrogen bonds involving aromatic
The crystal structures of a series of symmetrically substi-
tuted bis(phenyl)acetones were studied using single crystal
3 3 3
˚
3 3 3
as well as a methylene hydrogen atoms.18 Adjacent molecular
˚
chains are stabilized by C-H O (H O, 2.48 A) hydrogen
3 3 3
3 3 3
bonds, making use of the methylene bridges.19
Effect of Electron Donating Substituents. The influence of
the electron donating substituents on the conformational
flexibility was studied using -CH3, -C(CH3)3, and -OCH3
substituted monoketones as the representative examples.
1,3-Bis(4-methylphenyl)acetone, 2, was prepared by a
phase-transfer catalysis reaction of R-bromo-p-xylene with
Fe(CO)5. It has been noted that the two phenyl substituents
are in an exo-exo conformation with respect to the carbo-
nyl group. This observation shows a clear distinction from
the theoretically calculated model, which predicted a bent
Figure 1. Intermolecular interactions present in the dimorphs of
acetone.
Scheme 1. Isoelectronic Biphenyl Derivatives of Urea, Carbonate, and Acetone
Scheme 2. Bis(phenyl)acetone Derivatives Considered for the Present Study

2300 Crystal Growth & Design, Vol. 10, No. 5, 2010
Varughese and Draper
The -OMe substituted compound, 4, was redetermined,
as the structure was reported by Coppens and co-workers as
a part of the photochemical studies on a series of ketones.16
The monoketone, 4, also formed a similar assembly, stabi-
˚
lized by C-H O (H O, 2.53 A) hydrogen bonds. Ad-
3 3 3
3 3 3
jacent linear chains are held together by centrosymmetric
2
˚
C-H O (H O, 2.64 A) hydrogen bonds (R2(6)) formed
3 3 3
3 3 3
by the methoxy substituent (Figure 3c).
In the case of compounds 2-4 with electron donating
substituents, the molecules prefer an exo-exo conformation
with the synthon formation exclusively through aromatic H-
atoms and the central carbonyl group. Hence, there exist clear
variations in conformations and recognition patterns with
respect to the acetone dimorphs and also with the unsub-
stituted compound, 1. This hints at a possible electronic
communication between the central carbonyl functionality
and the substituents on the periphery of the molecule. To
further analyze this effect, a series of molecules with electron
withdrawing substituents were studied.
Figure 2. Intermolecular interactions present in the crystal struc-
ture of 1. The distances are in angstroms.
Effect of Electron Withdrawing Substituents. The influence
of the electron withdrawing substituents was studied using
-Br, -COOCH3, and -NO2 substituted bis(phenyl)acetones
as representative examples.
The -Br substituted compound, 5, was synthesized from
4-bromobenzyl bromide and recrystallized from a metha-
nol-CH2Cl2 mixture. The compound crystallizes in the P21/
c (Z0 = 2) space group and exhibits an endo-endo confor-
mation (Table 1). In the crystal, symmetry independent
molecules are stacked alternately and interact through bi-
furcated (R12(6)) (H O, 2.69 and 2.73 A) and linear
˚
3 3 3
˚
C-H O (H O, 2.58 A) hydrogen bonds established
3 3 3
3 3 3
between methylene H-atoms and carbonyl groups
(Figure 4a). Adjacent linear chains are stabilized by weak
˚
Type-II Br Br interactions (Br Br, 3.70 and 3.82 A).
A similar endo-endo conformation is observed in the ester
(-COOCH3) compound, 6. But, unlike the case of the -Br
20
3 3 3
3 3 3
derivative, the molecules are stabilized by bifurcated
˚
C-H O (H O, 2.47 and 2.74 A) hydrogen bonds
3 3 3
3 3 3
comprising the methylene H-atoms and the carbonyl group
of the ester functionality. This variation may be brought
about by a better hydrogen bond forming environment of the
ester functionality compared to the sterically demanding
neighborhood of the central carbonyl group.
The nitro substituted compound, 7, crystallizes in a mono-
clinic crystal system (Z0 =2), and the molecules exhibit an
endo-endo conformation as in 5 and 6. The molecules are
stabilized by bifurcated (R12(6)) C-H O (H O, 2.47,
2.55 and 2.38, 2.62 A) hydrogen bonds, and the recognition
pattern is similar to that observed in the case of the acetone
dimorphs. The two symmetry independent linear chains are
Figure 3. Intermolecular C-H O hydrogen bonds existing in the
3 3 3
3 3 3
3 3 3
crystal lattices of (a) 2, (b) 3, and (c) 4. The distances are in angstroms.
˚
U-conformer.14 The molecules make linear chains through
˚
bifurcated C-H O (H O, 2.51 A) hydrogen bonds
arranged alternately and form C-H O hydrogen bonds
3 3 3
3 3 3
3 3 3
exclusively through aromatic H-atoms (Figure 3a). Thus,
unlike the case of 1, the introduction of a methyl (-CH3)
substituent led to an increase in the number of atoms
(R12(10)) involved in the recognition pattern.
using -NO2 functionality.21
Thus, compounds 5-7, with electron withdrawing sub-
stituents, prefer an endo-endo conformation. In the assem-
blies, the molecules make bifurcated C-H O hydrogen
3 3 3
The 4-tert-butyl substituted monoketone, 3, crystallizes
in the P1h space group with two molecules in the asymmetric
unit (Z0 = 2). There exists a slight conformational difference
between the two symmetry independent molecules brought
about by the flexibility of the central -CH2COCH2- bridge.
The molecules exhibit an exo-exo conformation and
bonds through methylene H-atoms, unlike in 2-4, with
electron donating substituents, where they are through aro-
matic H-atoms. This is possibly due to a lower electron
density available in the aromatic ring as a consequence of
electron withdrawing groups. A CSD analysis and a struc-
tural comparison revealed that the -F derivative also ex-
hibits an endo-endo conformation and is in good agreement
with the observations made in the structural analysis of the
monoketones.22
make a linear assembly, stabilized by bifurcated (R12(10))
˚
C-H O (H O, 2.75 and 2.80 A) hydrogen bonds, as
3 3 3
observed in 2.
3 3 3

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2301
Table 1. Crystallographic Information Table for 1-7
complex
1
2
3
4
5
6
7
formula
C15H14
O
C17H18
O
C23H30
CCDC 735820
322.47
triclinic
P1
6.205(4)
18.353(3)
19.187(3)
69.920(1)
89.150(2)
89.080(2)
2051.8(14)
4
1.044
123(2)
0.062
50.18
O
C17H18O3
CCDC 735819
270.31
monoclinic
C2/c
27.805(4)
5.854(1)
9.059(1)
90
104.349(3)
90
1428.5(4)
4
1.257
123(2)
0.085
49.98
7202
1251
1097
93
1.050
0.0379; 0.1061
C15H12Br2O
CCDC 735814
368.07
triclinic
P1
9.039(1)
10.061(1)
16.451(2)
75.367(3)
76.554(3)
84.260(3)
1406.5(3)
4
1.738
123(2)
5.748
50.06
7638
4776
2493
325
C19H18O5
CCDC 735815
326.33
monoclinic
C2/c
16.480(1)
8.385(2)
12.303(1)
90
93.624(2)
90
1696.7(4)
4
1.278
123(2)
0.092
49.98
8721
1487
1290
110
1.571
0.0496; 0.1831
C15H12N2O5
CCDC 735818
300.27
monoclinic
P2/c
23.489(5)
4.679(1)
34.741(5)
90
131.997(9)
90
2837.6(10)
8
1.406
123(2)
0.108
50.04
28450
5018
3089
397
1.029
0.0531; 0.1355
CCDC no.
formula wt
crystal system
space group
CCDC 735816
210.26
monoclinic
P21/c
9.471(1)
5.684(1)
21.341(2)
90
96.525(2)
90
1141.4(3)
4
1.224
123(2)
0.075
50.00
5756
2010
1801
145
0.947
0.0352; 0.0985
CCDC 735817
238.31
monoclinic
C2/c
27.038(1)
6.079(2)
8.646(3)
90
97.990(7)
90
1407.3(7)
4
1.125
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
Dcalc (g cm-3
T (K)
)
123(2)
0.068
50.24
3565
1247
μ (mm-1
)
2θ range (deg)
total reflns
unique reflns, R(int)
reflns used
no. of parameters
GOF on F2
final R1, wR2
15924
6990
5046
433
1.056
0.0615; 0.1845
962
83
1.095
0.0500; 0.1575
0.848
0.0493; 0.1186
Figure 5. Various
conformations
present
in
1,3-bis-
(phenyl)acetones (1-7).
the crystal lattice depending upon the nature of substituents
(Figure 5). The extent of variations in the angles between the
two arms of the compounds is a function of the electron
donating and withdrawing abilities of the functional groups.
These angles when plotted against the corresponding Ham-
mett constants of the functional groups exhibit a linear
correlation, as shown in Figure 6b and Table 2. In the case
of electron donating substituents, in order to bring about a
comparative analysis, instead of θED, 180 - θED has been
plotted. The unsubstituted monoketone, 1, exhibited a clear
deviation from the linear correlation observed in the case of
substituted compounds, and the reason for the observed shift
is still elusive.
Synthon Evaluation
In the bis(phenyl)acetone series, considerable variations in
the recognition pattern, as a function of the substitution, have
been observed (Scheme 3). While a cyclic six-membered
(R12(6)) pattern (synthon I) involving aromatic and methylene
H-atoms is present in the unsubstituted bis(phenyl)acetone 1,
the molecules with electron donating substituents (2-4) make
hydrogen bonds (R12(10), synthon II) exclusively through
aromatic hydrogen atoms. For bis(phenyl)acetones with elec-
tron withdrawing substituents (5-7), the observed recogni-
tion patterns are found to be substitution dependent. In the
case of the -Br substituted compound, 5, the assembly is
stabilized by two distinct synthons. The symmetry related
Figure 4. Hydrogen bond patterns observed in the cases of (a) 5,
(b) 6, and (c) 7. The distances are in angstroms.
molecules are held together by bifurcated C-H O hydro-
3 3 3
gen bonds (synthon III), while the symmetry independent
molecules make linear interactions (synthon IV). In both the
cases, the hydrogen bond is realized exclusively through the
methylene hydrogen atoms. As against the central carbonyl
Conformational Analysis
The monoketones exhibit three distinct conformational
arrangements;endo-exo, exo-exo, and endo-endo;in

2302 Crystal Growth & Design, Vol. 10, No. 5, 2010
Varughese and Draper
Scheme 3. Synthons Present in Bis(phenyl)acetones
known in the literature that with the hydrogen bond formation
the >CdO bond is lengthened compared to the case of the
free molecule.24 In the seminal work on weak hydrogen bonds
by Desiraju and Steiner, the utility of IR analysis in quantify-
ing various interactions in crystals has been discussed.25 While
Badger and Bauer made correlations between spectral para-
meters and energies of strong hydrogen bonds, it was observed
that such correlations are difficult for weak interactions,
presumably due to the diffusive nature of the interactions
and the deformable nature of the bond geometries.26
Figure 6. (a) Schematic representation of the arms of 1,3-bis(phenyl)
acetones and the angles between the arms. (θED and θEW are the
angle between the two wings in the case of electron donating and
electron withdrawing substituents, respectively. In the case of electron
donating derivatives, 180 - θED has been considered for graphical
representation). (b) Correlation of the Hammett constants of the
functional groups with the angle between the two arms of the
monoketones.
Table 2. Hammett Constants of the Functional Groups and the Corre-
sponding Angles Existing between the Two Wings of the Substituted
Biphenylacetones
The infrared spectral analysis was carried out to analyze the
C-H O hydrogen bonds present in bis(phenyl)acetones
3 3 3
angle (deg)
and the effect of these interactions in the vibrational peak
shifts (Table 3). In the case of compounds 1-7, depending
upon the substitutions and interactions present, the molecules
exhibited different carbonyl stretching frequencies. For the
unsubstituted bis(phenyl)acetone 1 and the ester-derivative 6,
a
assembly substitution Hammett constant (θEW and 180 - θED
)
1
2
3
4
5
6
7
-H
0
102.07
151.55
153.65; 153.33
158.84
127.51; 120.96
117.71
113.09; 109.12
-CH3
-0.17
-0.197
-0.268
0.232
0.45
-tert-butyl
-OCH3
-Br
-COOCH3
-NO2
the >CdO stretching frequencies are 1710 and 1717 cm-1
,
respectively, and are more toward the values corresponding to
υCdO of liquid acetone (1715 cm-1).27 The methyl (2) and
methoxy (4) derivatives exhibited >CdO frequencies corre-
sponding to higher energy (1738 and 1739 cm-1, respectively).
The tert-butyl substituted compound, 3, has exhibited a lower
stretching frequency (1681 cm-1) which correlates well with
0.778
a In the case of electron donating substituents, instead of θ, (180 - θ)°
has been considered.
functionality in the other bis(phenyl)acetones, the bifurcated
˚
the longer C-O (1.227 A) bond length. It is interesting to note
C-H O hydrogen bonds (synthon V) in 6 are establis-
3 3 3
that one of the acetone crystalline forms also exhibits a similar
C-O bond length. In the case of the -Br, 5, and the -NO2, 7,
derivatives, two frequencies were observed;one at lower
energy (1718 and 1721 cm-1, respectively) and one at higher
energy (1738 cm-1). Further, the C-H stretching frequen-
cies of the monoketones are comparable to that observed
in the case of acetone (1360 cm-1) except for the case of the
nitro derivative (1344 cm-1). The C-CO-C stretching and
bending bands of bis(phenyl)acetones are in agreement with
those of acetone except for the cases of 3 (1288 cm-1) and 6
hed through the peripheral ester group and the methylene
hydrogen atoms. The molecules of the nitro compound, 7,
make hydrogen bond patterns (R12(6)) similar to those for
synthon III.
IR Spectral Analysis
As there exists a strong correlation between υCdO and
carbon-oxygen bond distances, IR spectroscopy is a very
useful analytical tool for studying hydrogen bonds.23 It is

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2303
Table 3. Infrared Spectral Information of the Monoketones
no.
group
CdO distance
IR(CdO), cm-1
IR(-CH-), cm-1
IR(C-CO-C), cm-1
liquid acetone
1715
1360
1422
1213
1
2
3
-H
1.207
1.195
1.227
1.232
1.205
1.204
1.204
1.190
1.202
1.211
1710
1738
1681
1218
1217
1288
-Me
1371
-tert-butyl
4
5
-OMe
-Br
1739
1738
1718
1717
1738
1721
1366
1366
1217
1217
6
7
-ester
-nitro
1277
1217
1344
morphologies.29 Yao and co-workers have reported that
substituents can in fact determine the shape of nanopartic-
les of some stilbazolium-like dyes.30 In this connection, we
have investigated the aforesaid ability of substituents in bis-
(phenyl)acetones. The aggregation of molecules was induced
by injecting 50 μL of concentrated dimethylformamide (DMF)
solution (1 ꢀ 10-3 M) of 1-7 into 20 mL of water under
ultrasonication. A homogenized transparent solution was
obtained upon the dispersion of the compounds in water
(a poor solvent). The resulting solutions were digested for
24 h at 45 °C and were kept undisturbed for a fortnight. The
experiments were repeated by varying the concentration of
the stock solution. It was assumed that the hydrophilic/
hydrophobic properties of functional groups can regulate
the interactions established by the compounds with the
water-DMF solvent system and thereby the morphologies
of the particles. Diverse morphologies were obtained, depend-
ing on the substituents, as indicated by the scanning electron
microscopy (SEM) images (Figure 7). The samples derived
from -Br, -NO2, and -OMe substituted compounds consist
of nano-/microcrystals with well-defined faces, while those of
-Me and -COOMe derivatives produced amorphous sphe-
rical particles. The unsubstituted compound, 1, failed to
produce nanoparticles from a range of solvent mixtures, as
it is soluble in almost all solvents and solvent mixtures.
The methyl substituted bis(phenyl)acetone, 2, forms amor-
phous spherical particles with an average size of 200 nm, and
the product from 3 consists of crystalline materials embedded
in an amorphous matrix. The formation of the matrix is
independent of the concentration, as 10-4 M, 10-3 M, and
10-2 M stock solutions yielded particles with similar nature
and morphology. Microplates with well-defined faces are
obtained in the case of the methoxy (-OMe) substituted
compound, 4. In addition, the surfaces of the microplates
are clean and smooth. Crystalline microparticles with an
average size of 3 ꢀ 4 μm2 have been obtained from the bromo
derivative, 5, and are found to be rich in defects. The hydro-
phobic nature of the ester (-COOMe) substitution led to the
generation of small amorphous spherical nanoparticles, as in
the case of the methyl substituent. Microparticles of the
monocot leaflike morphology, with an average dimension of
30ꢀ3 μm2, are obtained for the nitro derivative. Thus, the pre-
sent series makes an account of the role of the hydrophobic/
hydrophilic nature of substituents in regulating the interac-
tions established by the molecules with the solvent system and
thereby the morphology of nano-/microparticles.
Figure 7. SEM images of nanoparticles of (a) 2, (b) 3, (c) 4, (d) 5, (e)
6, and (f ) 7.
(1277 cm-1). This may be attributed to the bulky functional-
ities attached to the periphery of the aromatic rings and also
due to the intermolecular interactions existing in the ester
derivative. Thus, even in simple molecules such as bis(phenyl)-
acetone, due to the weak nature of the interactions, the
spectral complexity makes it difficult to make a proper inter-
pretation or a clear correlation between the structural and
spectral information (Experimental details, ORTEP dia-
grams, IR data, and SEM images are provided in the Support-
ing Information.) and further underscores the statement of
Desiraju and Steiner, “ ...the effects of weak hydrogen bonds on
vibrational spectra are not always as clear as for strong bonds,
and can be quite dissimilar for different kinds of weak hydrogen
bond.”25
Organic Nano-/Microparticle Analysis
Although there have been extensive studies on the size-
dependent properties of organic nano-/microparticles, the
ability to predict and control the nature and morphology
of these nanoscale materials is limited.28 The different nuclea-
tion and growth processes involved are known to influence
the formation of the nano-/microparticles with different
Conclusions
The flexible backbone of the 1,3-bis(phenyl)acetones ex-
hibits exo-endo, exo-exo, and endo-endo conformations in

2304 Crystal Growth & Design, Vol. 10, No. 5, 2010
Varughese and Draper
the solid state, and the conformational preferences are found
to be a function of the electron donating/withdrawing ability
of the substitution. While the compounds with electron
donating substituents prefer the exo-exo conformation, an
endo-endo conformation has been observed in the case of
bis(phenyl)acetones with electron withdrawing groups.
Further, there existsa linearcorrelation between the Hammett
constants and the angles between the two arms of the mono-
ketones. The weak nature of the interactions existing in the
assemblies makes it difficult to formulate a proper interpreta-
tion or a clear correlation between the structural and IR
spectral information. The hydrophobic/hydrophilic nature
of the substituents does have a significant role in determining
the nature and morphology of the particles, as revealed by
SEM analysis of the nano-/microparticles. This is presumably
due to the variation in the nature of interactions established
between the molecules and the solvent system. Thus, the
present study addresses hydrogen bond forming ability,
electron donating/withdrawing effect, and the hydrophilic/
hydrophobic nature of functional groups in determining the
conformational preferences of a series of acetone derivatives
in crystal packing and also the nature and morphology of
nano-/microparticles.
calculated positions using appropriate AFIX commands and were
refined isotropically. Intermolecular interactions were computed
using the PLATON program.35
Nanoparticle Preparation and Analysis. The nanoparticles were
prepared by the rapid injection of 50 μL of 10-3 M compound in
dimethylformamide (DMF) (filtered through a nanoporous alumi-
na membrane (Whatman, Anodisc 13)) into 20 mL of Millipore
water under ultrasonication. The solution was kept under isother-
mal conditions (45 °C) for 24 h and was filtered through anodizc
(20 nm). The morphology of the nanoparticles was analyzed using
a TESCAN scanning electron microscope using a beam voltage of
5 kV.
Acknowledgment. This material is based upon works sup-
ported by Science Foundation Ireland [05PICAI819]. S.V.
thanks Dr. S. Philip Anthony for fruitful discussions and
SEM analysis.
Supporting Information Available: ORTEP diagrams of 1-7,
table of hydrogen bond distances, preparation methods for 2-7,
experimental methods, IR spectra of 1-7, and SEM images of 2-7.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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