The role of weak intermolecular C-H. F interactions in supramolecular assembly: Structural investigations on 3,5- Dibenzylidene-piperidin-4-one and database analysis

Article abstract of DOI:10.1007/s12039-011-0091-6

The fluorinated and non-fluorinated dibenzylidene-4-piperidones were synthesized and their structures examined using X-ray crystallography. Interestingly, the para-fluorosubstituted dibenzylidene compound, in contrast to other analogs, is characterized by

Full text of DOI:10.1007/s12039-011-0091-6

c
J. Chem. Sci. Vol. 123, No. 4, July 2011, pp. 403–409. ꢀ Indian Academy of Sciences.
The role of weak intermolecular C–H. . . F interactions
in supramolecular assembly: Structural investigations
on 3,5- dibenzylidene-piperidin-4-one and database analysis
R S RATHOREa,∗, N S KARTHIKEYANb, Y ALEKHYAa, K SATHIYANARAYANANb and
P G ARAVINDANc
aBioinformatics Infrastructure Facility, Department of Biotechnology, School of Life Sciences,
University of Hyderabad, Hyderabad 500046, India
bChemistry Division, School of Advanced Sciences, VIT University, Vellore 632014, India
cPhysics Division, School of Advanced Sciences, VIT University, Vellore 632014, India
e-mail: ravindranath_rathore@yahoo.com
MS received 23 July 2010; revised 25 March 2011; accepted 11 April 2011
Abstract. The fluorinated and non-fluorinated dibenzylidene-4-piperidones were synthesized and their struc-
tures examined using X-ray crystallography. Interestingly, the para-fluorosubstituted dibenzylidene compound,
in contrast to other analogs, is characterized by C–H. . . F bonded one-dimensional packing motif. To evaluate
the ability of hydrogen bond donors and acceptors for forming interactions, in general and competitive situ-
ation, we have defined statistical descriptors. Analysis of Cambridge Structural Database using these newly
defined parameters reveals high propensity of C–H. . . F interactions in organic crystals. The present structural
study suggests much larger role of fluorine driven intermolecular interactions that are even though weak, but
possess significant ability to direct and alter the packing.
Keywords. Dibenzylidene-4-piperidones; C–H. . . F interaction; fluorine interactions; weak intermolecular
interactions; crystal structure; X-ray diffraction.
1. Introduction
interactions in the database,1,2 is not necessarily true.
Several structures having significant role of C–H. . . F
and C–F. . . π interactions have been observed. Even
though these interactions involving fluorine are much
weaker than conventional N/O–H. . . O hydrogen bonds,
their role in determining the modes of molecular pack-
ing can not be ignored.3,5,6 Indeed, there are reported
examples that clearly demonstrate that weak fluorine
interactions certainly alter the packing modes.7 Guru
Row and co-workers have extensively studied the struc-
tural property of fluorine and they have presented sev-
eral elegant examples of fluorine-directed crystal pack-
ing.7–10 In recent years, fluorine has been shown to
influence non-bonded intermolecular association and
ligand-receptor binding via different modes of inter-
actions such as steroelectronic effects, as observed in
many drug-receptor binding e.g., Prozac, Ciprofloxacin
and Atorvastatin.11,12
Fluorine is known for its odd behaviour in non-bonded
interactions.1 Fluorine atom is the most electronega-
tive element and approximately isosteric to oxygen.
While inorganic fluoride ion is the most powerful
proton-acceptor (its strength of hydrogen bond is
40 kcal mol−1),2 in staggering contrast the covalently
bound fluorine is a very weak intermolecular hydrogen-
bonding acceptor (strength of X–F. . . H–Y hydrogen
bond is 2–3.2 kcal mol−1 as compared to 5–10 kcal
mol−1 for H. . . O hydrogen bond, X and Y are cova-
lently attached atoms).3 Indeed, fluorine is considered
to hardly ever form hydrogen bond. This anomalous
behaviour of fluorine has been attributed to many elec-
trostatic and steric factors such as its low polarizability
and tightly contracted lone pairs.1–4
In recent years, a large number of structures con-
taining fluorine have been reported. Upon examination,
it turns out that the role assigned to fluorine as ‘odd
one out,’ based-on analysis of N–H. . . F and O–H. . . F
It was our purpose to examine the structure directing
role of fluorine in organic molecules, we have selected
3,5-dibenzylidene-piperidin-4-one class of molecules.
We have recently standardized its synthetic procedure
to prepare diverse set of compounds. Some of the fluo-
∗For correspondence
403

404
R S Rathore et al.
pounds (figure 1) are: (3E,5E)-1-benzyl-3,5-bis(3-fluo-
robenzylidene)piperidin-4-one (I), (3E,5E)-1-benzyl-
3,5-bis(4-fluorobenzylidene)piperidin-4-one (II), (3E,
5E)-1-benzyl-3,5-bis(4-methoxybenzylidene)piperidin-4-
one (III). The results are correlated with database surv-
ey on intermolecular interactions involving fluorine.
2. Experimental
2.1 Synthesis of dibenzylidene-4-piperidones
A mixture of 1-benzyl-4-piperidone (0.01 mol) and res-
pective benzaldehyde (0.02 mol) was added to a warm
solution of ammonium acetate (0.01 mol) in absolute
ethanol (15 ml). The mixture was gradually warmed on
a water bath until the yellow colour changed to orange.
The mixture was kept aside overnight at room tempera-
Figure 1. Chemical scheme.
rinated and non-fluorinated (3E,5E)-3,5-dibenzylidene- ture. Reactions were monitored with TLC for complete-
4-piperidones (D4P) compounds were prepared and we ness. The solid obtained was separated and the crude
could crystallize few of them. The investigated com- compound was purified using silica gel column chro-
Table 1. Crystal data.
(I)
(II)
(III)
Crystal Data
Empirical formula
Molecular weight
Morphology
Crystal size (mm)
Cell Parameters
a(Å)
C26H21F2NO
401.4
yellow, block
0.38 × 0.28 × 0.22
C26H21F2NO
401.4
yellow, block
0.32 × 0.30 × 0.24
C28H27NO3
425.5
yellow, block
0.30 × 0.16 × 0.16
6.6254(1)
10.5486(2)
15.7097(4)
103.352(1), 98.846(1), 103.249(1)
17.9058(6)
6.3548(2)
18.6239(6)
90.0, 96.958(1), 90.0
2103.56(12)
Monoclinic
P21/n
4/1
1.268
0.089
multi-scan
840
12.970(5)
9.168(5)
19.529(5)
90.0, 105.848(5), 90.0
2233.9(16)
Monoclinic
P21/c
4/1
1.265
0.082
multi-scan
904
b(Å)
c(Å)
a, β, y(◦)
V (Å3)
1014.75(4)
Crystal system
Space group
Z/Zꢁ
Triclinic
¯
P1
2/1
Dx(cal.) (g/cm3)
1.314
0.092
multi-scan
420
μ (mm−1
)
Absorption correction
F(000)
Data Collection
θ-range (◦)
1.37 – 28.31
φ and ω scans
4741
1.49 – 27.41
φ and ω scans
4781
1.63 – 28.24
φ and ω scans
5505
Scan type
Independent reflections
Observed [I > 2σ(I)]
Refinement
3596
2424
3563
Final R[F2 > 2(F2)]
0.0449
0.1526
0.0496
0.1665
0.0432
0.1443
wR(F2)_all
Goodness-of-fit (S)
ꢀρmax and ꢀρmin (e Å−3
0.941
0.989
0.136 and −0.184
1.040
0.226 and −0.196
)
0.224 and −0.268
ꢀꢁ
ꢂ
ꢃ
ꢄ
ꢂ
ꢃꢀ
w = 1 σ2 F02 + (aP)2 + bP , where P = Fo2 + 2Fc2 3P = (Fo2+2Fc2)/3, parameters a and b are: 0.1000, 0.1667 (I),
0.0855, 0.0000 (II), 0.0740, 0.2385 (III), respectively.

The role of weak intermolecular C–H. . . F interactions in supramolecular assembly
405
matography with hexane and ethyl acetate as elutant. isfied the Jeffrey criterion as described earlier. VISTA
Final yields: 89.84%(I), 93.87%(II), and 83.20%(III); (Version 2.0) included in the CSD software suite was
m.p. 136–140◦C (I), 146–150◦C (II), and 150–154◦C used for statistical analysis and calculating average
(III). Suitable single crystals for (I–III) were grown values.
by heating in a mixture of ethanol and tetrahydrofuran,
and cooling them to room temperature. Crystals were
filtered after two days.
3. Results and discussions
3.1 Molecular structure
2.2 X-ray crystallography
The stereochemistry of (I–III) is illustrated in figure 2
and supplementary figures S1 and S2 respectively.
Piperidinone ring (also called piperidone; atoms
N1/C2-C6) adopts characteristic sofa conformation as
observed in analogous structures16–18,21 with N1 atom
shifted out of the base plane (C2/C3/C4/C5/C6) by
−0.740(1) Å, −0.689(2) Å and −0.776(1) Å in (I–III),
respectively. The N1-substitent (1-benzyl group) is in
equatorial position of piperidinone ring, except it occu-
pies axial position (endo-skeleton with respect to the
ring) in 3,5-bis[(E)-2-chlorobenzylidene]-1-[(R)-1-phe-
nylethyl]piperidin-4-one.16 The C3,C5 diene moieties
possess E-configuration. Details of hydrogen-bonding
and short-contact geometry are described in table 2.
The conformation of 1-benzyl group is character-
ized by two torsion angles about N1-C7 and C7-C8
bonds. To describe the conformation, only the lowest
value of the torsion angles have been used, since the
corresponding aromatic proton is only able to form
intra-molecular interaction. In (I–III), and previously
analogous structures,16–18,21 the average torsion angles
have been observed to be |69(6)|◦ and |43(15)|◦, respec-
tively. In (I–III) the torsion angles are following: (I),
C2–N1–C7–C8 = −74.15(16)◦, N1—C7—C8—C13 =
−56.95(18)◦; (II), C2—N1—C7—C8 = −62.6(2)◦,
N1—C7—C8—C13 = −49.4(3)◦; (III), C6—N1—
X-ray data were collected on a single-crystal Bruker
SMART CCD area-detector diffractometer.13 Empiri-
cal absorption correction was applied to the data using
SADABS.13 The structure was solved by applying the
direct phase-determination technique using SHELXS-
97, and anisotropically refined by full-matrix least-
square on F2 using SHELXL-97.14 Structure calcula-
tions were performed with WinGX suit of programs
(version 1.70.01). Hydrogens were placed in the geo-
metrically expected positions and refined with the rid-
ing options. The torsion angles for the methyl group
hydrogens were set with reference to a local differ-
ence Fourier calculation. The calculated distances with
hydrogen atoms are: C(sp2)–H = 0.93 Å, C(methyl)–
H = 0.96 Å, C(methylene)–H = 0.97 Å, and Uiso
=
1.2 Ueq(parent), or 1.5 Ueq(Cmethyl). For identification
of hydrogen bonds, following Jeffrey criterion, recom-
mended by IUCr was used: H. . . A distance < (r1 +
r2 − 0.12) Å and D–H. . . A angle was within 100–180◦,
where r1 and r2 are the van der Waals radii of hydro-
gen and the acceptor atoms, respectively. Essential crys-
tal data are listed in table 1. Crystallographic data have
been deposited at Cambridge Crystallographic Data
Center with entry numbers: CCDC 759527 (I), CCDC
759526 (II), and CCDC 759525 (III).
2.3 CSD database analysis
C10
C9
C11
Search in Cambridge Structural Database (Version 5.31,
updated)15 was performed using Conquest (Version
1.12). While selecting structures for their involvement
in hydrogen-bonding, stringent criteria were used to
assure quality extraction of data and to maximize exclu-
sion of short van der Waals contacts. All the intra-
molecular contacts were discarded. Single crystal struc-
tures of organics having R values less than 0.10 with
their three-dimensional coordinates determined, pos-
sessing no ions, polymeric structure, disorder or errors,
were extracted. Hydrogen bond distances were nor-
malized to their neutron diffraction values. The possi-
ble intermolecular interaction was assumed, if it sat-
C8
C12
C13
C7
N1
C19
C20
C26
C27
C2
C6
C5
C18
F1
C3 C15
C14
C22
C25
F2
C4
O1
C21
C17
C16
C23
C24
Figure 2. A view of (a) (I) with non-H atoms shown as
probability ellipsoids at 30% levels.

406
R S Rathore et al.
Table 2. Hydrogen bond and short-contact geometry in (I–III).
D–H. . . A
D–H (Å)
H. . . A (Å)
D. . . A (Å)
D–H. . . A (◦)
(I)
C16—H16. . . O1i
0.93
0.97
0.93
0.93
0.93
0.93
0.93
0.96
2.59
2.63
2.83
2.51
2.55
2.59
2.82
2.95
3.330(2)
3.521(2)
3.638(2)
3.236(4)
3.400(2)
2.899(2)
3.643(3)
3.865(3)
137
153
146
135
152
100
148
159
C7—H7A. . . πii (Cg2)
C12—H12. . . πiii (Cg4)
C10—H10. . . F1iv
(II)
C21—H21. . . O1v
(III)
C13—H13. . . N1
C9—H9. . . πvi (Cg3)
C28–H28B. . . πvii (Cg4)
ii
v
i1−x,1−y,−z; −x,1−y,1−z; iii1−x,1−y,1−z; iv1−x,−y,−z; 2−x,2−y,−z; vix,1/2−y,1/2 + z; vii1 + x, y, z. Cg2, Cg3
and Cg4 refer to the centers of (C8-C13), (C15-C19) and (C22-C27) rings, respectively. Significant π–π interactions were
observed in (III). Cg2 makes a stacking interaction with Cg2viii [symmetry code (viii): −x, −y, 2−z] with a centrioid-to-
centroid distance of 4.021(2)Å, a perpendicular distance of 3.5037(7)Å, and a slippage of 1.973Å.
C7—C8 = 72.45(16)◦, N1—C7—C8—C13 = 26.4 dimer. Additionally, C7–H7A... πii (Cg2) and C12–
(2)◦. The intra-molecular C—H. . . N interaction is H12... πiii (Cg4) interactions give rise to an adjacent
possible when torsion angles about N1–C7 and C7– dimer. Symmetry codes and ring-details are described
C8 bonds are around |90|◦ and 0◦, respectively. The in table 2. The crystal packing is shown in supple-
intra-molecular C–H. . . N interaction is observed in mentary figure S3. Similarly, in ortho-F and ortho-Cl
(III), and previous analogs (3E,5E)-3,5-dibenzylidene- substituted analogs, namely, (3E,5E)-1-benzyl-3,5-bis
1-phenyl-piperidin-4-one, (3E,5E)-1-Benzyl-3,5-bis(4- (2-fluorobenzyl-idene)piperidin-4-one17 and 3,5-bis[(E)-
allyloxybenzylidene)piperidin-4-one, and (3E,5E)-1- 2-chlorobenzylidene]-1-[(R)-1-phenylethyl]piperidin-4-one,16
benzyl-3,5-bis[(2-fluorophenyl)methylidene]piperidin- the halogen fails to participate in any non-bonded
4-one.17,18,21 Compounds (I–III) possess Cs point interaction scheme.
group symmetry with mirror plane passing through
N1 and C4. However, molecular symmetry is not con-
3.3 C—H. . . F and C—H. . . O Directed packing motif
served in any of the crystals. A database study suggests
that the molecular symmetry is retained only in 26%
of molecules possessing Cs symmetry.22,23 The non-
retention of molecular symmetry in the crystal could
be attributed to steric reasons and packing forces, as
evident from the fact that protons of 1-benzyl moiety
are involved in C–H. . . F/N/π interactions (table 2).
The notable interaction in (II) is an intermolecular
C10–H10. . . F1iv interaction (table 2), leading to a di-
mer. Another dimer is formed via intermolecular C21–
H21. . . O1v hydrogen bond. The dimeric intermolec-
ular C–H. . . O and C–H. . . F interactions in tandem,
give rise to a linear chain (figure 3). The one-
dimensional motif is approximately parallel to [2 1 0]-
direction.
3.2 Crystal packing
Interestingly, the structure of (III), with ether in the
Fluorine of meta-fluorobenzylidene in (I) does not par- para position fails to form hydrogen bond. Ether, simi-
ticipate in any intermolecular interaction. The pack- lar to fluorine, is known to be a weak hydrogen-bond-
ing is governed by C16–H16. . . O1i, resulting in a ing acceptor.19 Similarly, in (3E,5E)-3,5-bis(4-ally-
H10
C21
H21
C10
v
iv
F1
O1
Figure 3. Packing motif (linear chain) in (II) directed by C–H. . . F and C–
H. . . O interactions. Symmetry codes are with reference to table 2.

The role of weak intermolecular C–H. . . F interactions in supramolecular assembly
407
loxybenzylidene)-1-benzylpiperidin-4-one and 3,5-bis interaction scheme.18 Two C9—H9... πvi (Cg3) and
[(E)-4-chlorobenzylidene]-1-[(R)-1-phenylethyl]piper- C28–H28B. .. πvii (Cg4) interactions in (III) (table 2)
idin-4-one, the ether oxygen or chlorine at the para- make a herringbone-like arrangement as shown in the
position does not participate in any intermolecular supplementary figure S4.
Table 3. Statistics for global (absolute) and relative competitiveness24 of fluorine for forming intermolecular interactions.
Donor* Acceptor Fragments
Fragments
involved in
intermolecular
interactions,
N2§
Fragments
exclusively
involved in
intermolecular
interactions,
N3§
ASC$ RSC$ ESC$ Mean H. . . A
in database,
N1
(%)
(%)
(%) distance (A◦)/
D-H. . . A
angle(◦)
Overall competitiveness (effectiveness)
X–H. . . F
X–H. . . Cl
X–H. . . Br
X–H. . . I
C–H
N–H
O–H
Z–H
C–H
N–H
O–H
Z–H
C–H
N–H
O–H
Z–H
C–H
N–H
O–H
Z–H
N–H
O–H
Z–H
F
F
F
7789
2115
1633
7915
14163
3818
2966
14215
7929
1581
1736
7947
2217
341
4291
213
94
4441
4201
154
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
55
10
6
56
30
4
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
2.45/145
2.27/143
2.29/133
2.44/145
2.75/148
2.60/150
2.59/139
2.75/148
2.85/149
2.66/150
2.64/142
2.84/148
2.99/150
2.86/150
2.77/144
2.98/150
2.03/156
1.90/158
2.29/151
F
Cl
Cl
Cl
Cl
Br
Br
Br
Br
I
I
I
I
O
O
O
98
3
4341
2065
58
67
2129
411
30
12
441
31
26
4
4
27
19
9
360
3
2230
32112
36714
115670
20
66
63
76
X–H. . . O
21117
23104
87491
Relative competitiveness between acceptor-1 and acceptor-2
F and O
F and Cl
F and Br
F and I
Z–H
Z–H
Z–H
Z–H
Z–H
Z–H
F
O
F
Cl
F
Br
F
I
F
S
F
π
5906
5906
752
752
311
311
248
248
1640
1640
5948
5948
3156
4231
359
178
165
58
154
28
817
237
3476
2396
671
1746
247
66
128
21
131
5
676
96
53
72
48
24
53
19
62
11
50
14
58
40
42
58
67
33
74
26
85
15
78
22
59
41
11
30
33
9
41
7
53
2
41
6
2.44/145
2.30/150
2.43/144
2.74/149
2.43/145
2.86/147
2.43/145
2.99/145
2.45/145
2.74/152
2.44/145
2.81/142
F and S
F and π
1976
896
33
15
N2
N1
$percentage figures rounded to the nearest integers, Formulae – ASC=100 ×
for RSC - in a data set (containing N1 frag-
ments) having HB groups A, B, C etc., at first their ASC values are calculated [ASC(A) = N2(A)/N1, ASC(B) = N2(B)/N1
and so on, where N2(A), N2(B) ... are the numbers of hydrogen bonding refcodes for HB groups A, B ... respectively. RSC
ASC (i)
N3 (i)
N1
ꢅ
is then expressed as RSC (i) = 100 ×
, ESC (i) = 100 ×
, where i = HB groups namely, A, B, C etc., N3(i)
ASC (i)
i
represents cases when HB group (i) forms hydrogen bond and rest of the groups (under consideration) fail to form it.
∗Z refers to any atom other than H.
§In RSC figures: the N2 (Column 4th) refers to cases wherein Acceptor-1 was found hydrogen bonded and it does not mat-
ter if competing Acceptor-2 is hydrogen bonded or not. In ESC figures: The N3 (Column 5th) refers to fragments where
Acceptor-1 is hydrogen bonded and Acceptor-2 does not participate in any kind of hydrogen bonding

408
R S Rathore et al.
ity to form H. . . F intermolecular interactions. It is also
3.4 High occurrence of C–H. . . F interactions
in organic molecules
interesting to note that there are still 11% cases, when
it is able to form exclusive interactions. Fluorine has
high competitiveness with respect to other weak (such
as π) acceptors. It would be worthwhile to compare the
effectiveness of fluorine with respect to other halogens
(for C–H. . . halogen interactions see31–33). In compari-
son to other halogens, fluorine has the highest effective-
ness (as expected) with all the three statistical parame-
ters are higher for fluorine in all the cases. The order of
effectiveness for halogen is as follows F > C l> Br > I.
The ability of hydrogen bonding donors and acceptors
(hereafter referred to as HB groups) to form interac-
tions and direct packing in the presence of multiple such
groups is determined by many factors.20,31 We call the
ability of an HB group to form interactions as effec-
tiveness (or competitiveness). To quantify this effec-
tiveness of HB groups, we have recently formulated
a new concept of statistical competitiveness, defined
in terms of three numerical parameters, referred to as
ASC (absolute statistical competitiveness), RSC (rela-
tive statistical competitiveness) and ESC (exclusive sta-
tistical competitiveness) (see table 3 for the definition
of parameters).24 ASC indicates the global effective-
ness (or propensity) of an HB group to form interac-
tions. The RSC and ESC signify the probability of an
HB group to form hydrogen bond in a co-existing sit-
uation. ESC, unlike RSC suggests the ability to form
interactions exclusively. Few attempts in this direction
were earlier also made, to rank the effectiveness of
HB groups and the quantities were referred by various
names such as probabilities, relative success, propensity
or competition function.25–30
To investigate the efficacy of fluorine for mak-
ing interactions, these parameters were calculated
using Cambridge Structural Database (CSD) (Ver-
sion 5.31).15 All intra-molecular short-contacts were
excluded. The results are summarized in table 3. An
interesting fact that emerges from the study is the pre-
dominant global effectiveness of C–H. . . F interactions
in organic crystals. An overwhelming 55% of cases
are reported to have C–H. . . F interactions (56% for Z–
H. . . F interactions, Z represents any atom other than
H). The propensity of C–H. . . F intermolecular interac-
tions is at par with some of the common intermolec-
ular interactions. The ASC values for N–H. . . O, O–
H. . . O and Z–H. . . O interactions are 66%, 63% and
76%, respectively. On the other hand, the ASC value for
intermolecular N–H. . . F and O–H. . . F interactions are
barely 10% and 6%, respectively. The ‘odd’ behaviour
of fluorine, attributed in previous database studies2
is largely due to its low propensity (effectiveness) of
forming interactions with these strong NH/OH donors.
The present statistics reveals considerable role of weak
interactions involving fluorine in a large number of
reported examples.
4. Conclusion
Three dibenzylidene-4-piperidones derivatives were
synthesized and structures examined. In para-
fluorosubstituted dibenzylidene compound, fluorine
participates in C–H. . . F interaction, leading to a one-
dimensional packing motif. Rest of the investigated
compounds are governed by C–H. . . O and C–H. . . π
interactions. The database analysis suggests significant
role of fluorine-mediated intermolecular interactions.
Based on three statistical descriptors that quantify the
effectiveness of HB groups to form intermolecular
interactions, it turns out that the global effectiveness
(or propensity) for intermolecular C–H. . . F interac-
tion is substantially higher, which is comparable to
conventional strong intermolecular N–H. . . O and O–
H. . . O hydrogen bonds. Moreover, fluorine possesses
significant ability to form exclusive intermolecular
interactions as well, when in competition with oxygen
acceptor.
As illustrated in the present examples, and revealed
in the database study, the intermolecular interactions
involving fluorine are even though weak, but do have
significant ability to direct the molecular packing, sug-
gesting much larger role of fluorine in crystal pack-
ing than considered in earlier studies. The frequently
associated term ‘hardly ever accepts hydrogen bond’ is
gradually being replaced by ‘it can also play structure
directing role’. The present investigation adds to this
theoretical framework.
Supplementary information
For supplementary information figures S1, S2, S3 and
S4 see www.ias.ac.in/chemsci website.
We also compared relative competitiveness of flu-
orine with respect to other acceptors. The RSC and
ESC parameters suggest that fluorine, when in compe-
tition with oxygen acceptors, retains significant abil-
Acknowledgements
RSR thanks Council of Scientific and Industrial
Research, New Delhi, for funding under the scientist’s

The role of weak intermolecular C–H. . . F interactions in supramolecular assembly
409
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