4
54
J Chem Crystallogr (2014) 44:450–458
The Hirshfeld surfaces of the title compounds (I, II) are
illustrated in Fig. 4, showing surfaces that have been
˚
mapped over dnorm (-0.5–1.5 A). It is clear that the large
circular depressions (deep red) visible in the front and back
views of the surfaces are indicating hydrogen- bonding
contacts. The dominant interactions between the C–H (C8
and C5) and carbonyl O (O2) atoms in both the compounds
can be seen in the Hirshfeld surface as red areas. The small
light red colour on the surface, denotes weaker and longer
contact. Some significant p–p interactions are observed in
both compounds. In Fig. 4c, f there are red and blue tri-
angles representing the presence of p–p stacking.
There are three distinct spikes (marked with arrows),
which are appearing in the 2D fingerprint plot (Fig. 5a, e)
for both structures. These spikes indicate different inter-
actions that can occur between two chemically and crys-
tallographically distinct molecules. Complementary
regions are visible in the fingerprint plots where one mol-
ecule acts as donor (d [ d ) and the other as an acceptor
Fig. 2 Packing diagram for I showing C–HꢀꢀꢀO=C/O and C–HꢀꢀꢀN
intermolecular hydrogen bonding motifs in the ac plane and selected
molecular pairs in order of decreasing interaction energy
e
i
compounds I and II. The interaction energy for these
dimers is -7.5/-7.0 kcal/mol for I and -11.5/-10.4 kcal/
mol for II respectively obtained from energy calculations
performed using PIXEL and TURBOMOLE. In addition
the atom, N1 also acts as a bifurcated acceptor interacting
with two hydrogen atoms (involving H11 and H12) for
both the structures, forming a chain motif utilizing the
(d \ d ). The fingerprint plots can be decomposed to
e
i
highlight a particular type of atomic pair contacts. This
decomposition leads the separation of the contributions
towards the total interaction from different interaction
types, which overlap in the full fingerprint. The C–HꢀꢀꢀO
intermolecular interactions appear as one spike in the 2D
fingerprint plots for both molecules. Similar behaviour is
observed for the corresponding C–HꢀꢀꢀN intermolecular
21
-screw axis of symmetry in I and a dimeric motif across
˚
the center of symmetry in II (Figs. 2c and 3d). It is of
interest to note that the C–HꢀꢀꢀO and C–HꢀꢀꢀN dimers (as
indicated by molecular pairs Fig. 3a, d) exist alternately in
the crystal lattice forming a sheet-like structure in II.
Adjacent sheets are held via weak C–HꢀꢀꢀO (involving O4
with H10); (the energy is -4.3/-4.3 kcal/mol) that generates
a dimer in the bc plane (Fig. 3e). In addition, in I, addi-
tional C–H…O–N H-bonds with the nitro group, as
revealed by the presence of molecular pairs Fig. 2d, e
provide additional stability to the crystal packing (the
energies range from 2.2 to 3.0 kcal/mol as obtained from
PIXEL/TURBOMOLE). In II, a weak and directional C–
HꢀꢀꢀO (involving H2B with O3) intermolecular hydrogen
bond generates a dimer in the crystallographic bc plane
interaction. A greater value of d (d = 1.00 A and
i
e
˚
˚
˚
d = 1.30 A for I and d = 1.05 A and d = 1.36 A for II)
i
e
i
indicates the presence of the nitro group and carbonyl
group, which are acting as good acceptors (Fig. 5b, f) of
H-bonds. In case of the C–HꢀꢀꢀN intermolecular interaction,
a greater value of d is indicating the presence of the cyano
i
(CN) group where the N1 atom is acting as a good acceptor
(Fig. 5c, g).
The packing of molecules in II has a greater contribu-
tion of OꢀꢀꢀH intermolecular interaction in comparison to
molecule I by 1.2 %. But the contribution of NꢀꢀꢀH inter-
molecular interaction is slightly higher for I compared to
that in compound II.
In Table 4 we have compared the vibrational frequen-
cies of some selected functional groups involved in non-
covalent interactions [27] in the solid phase with those in
the gaseous phase, obtained from B3LYP/6-31G** calcu-
lations using TURBOMOLE. It is noteworthy that the O
(
Fig. 3c). The characteristic packing feature in both the
molecules consists of p–p stacking interactions [26]
namely p(carbonyl)–p(cyano) (Fig. 2a) and p(nitro)–
p(aromatic) (Fig. 3b), the interaction energies being
-
8.1/-9.9 and -7.8/-9.8 kcal/mol respectively via inputs
atoms of the –NO group are involved in the formation of
2
from PIXEL/TURBOMOLE. p–p stacking in these mole-
cules is responsible for the increased stability of the crystal
packing. These energies are higher in magnitude when
compared to related molecular pairs containing intermo-
lecular hydrogen bonds in the crystal lattice.
intermolecular interactions in the crystal packing in com-
parison to the other functional groups present in the mol-
ecule. The changes in vibrational frequency between the
gas phase and the solid state (for both O–N–O asymmetric
-
1
stretching and symmetric stretching) is 156 and 159 cm
1
23