Quantitative analysis of weak non-covalent interactions in (Z)-3-(4-chlorophenyl)-2-phenylacrylonitrile: insights from PIXEL and Hirshfeld surface analysis

The crystal and molecular structures of (Z)-3-(4-chlorophenyl)-2-phenylacrylonitrile are reported and the weak non-covalent interactions present in the crystal structure have been investigated.

In this work, we report the synthesis and the crystal and molecular structures of an acrylonitrile derivative, namely (Z)-3-(4-chlorophenyl)-2-phenylacrylonitrile (I). We also report herein a detailed analysis of the intermolecular inter- ISSN 2056-9890 actions for different molecular pairs observed in I using the PIXEL method (Gavezzotti, 2002(Gavezzotti, , 2011. Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) was also performed to visualize the short contacts in the crystal of I and to determine the relative contributions of the various non-covalent interactions present in the crystal structure using two-dimensional (2D) fingerprint plots (Spackman & McKinnon, 2002;McKinnon et al., 2007). We also highlight the importance of the weak halogen bonds observed in the crystal structure.

Computational details
Structural optimization was carried out using GAUSSIAN09 (Frisch et al., 2013) with the M06-2X/cc-pVTZ level of theory followed by vibrational frequency calculations. The lattice and intermolecular interaction energies were calculated using the CLP-PIXEL program (Version 3.0; Gavezzotti, 2002Gavezzotti, , 2011. For the intermolecular interaction energy calculations, the crystal structure geometry along with normalized C-H bond lengths to their respective neutron values (Allen, 1986) was used and the electron density has been obtained at the MP2/6-31G(d,p) level of theory using GAUSSIAN09.

Structural commentary
The molecular structure of compound I is shown in Fig. 1. The whole molecule is disordered over two orientations with a refined occupancy ratio of 0.86 (2):0.14 (2). Only the major component is considered for further analysis and discussion. The bond lengths in I clearly indicate the presence of electron delocalization throughout the molecule. The geometrical features of the molecule were further analyzed using the MOGUL geometry check utility available in Mercury (Macrae et al., 2008). The result suggests that the torsion angles C8-C7-C15-N2 [À166.6 (2) ] and C1-C7-C15-N2 [10.5 (2) ] are unusual. The molecule adopts a twisted conformation and the dihedral angle between the planes of the phenyl (C1-C6) and 4-chlorophenyl (C9-C14) rings is 51.91 (8) . When the unsubstituted phenyl ring in I was Structural overlay of the X-ray (grey) and optimized (green) structures. Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
A view along the b axis of the crystal packing of compound I, showing the nitrile stacking in the purple rectangles.  replaced by a pyridine ring (Venkatesan et al., 2018), the molecular twist was reduced by at least 50%, and in pyridine containing compounds, the dihedral angles between the two rings are in a range of ca 1-27 (Cambridge Structural Database; Groom et al., 2016).
To understand the conformational flexibility of I, we performed a structural optimization using the GAUSSIAN09 program (Frisch et al., 2013), without any constraints. The vibrational frequency calculation confirmed that the optimized structure is found to be the true energy minima on the potential energy surface, since there were no negative frequencies observed for the optimized geometries. The X-ray and optimized structures superimpose well, with an r.m.s. deviation of 0.13 Å (Fig. 2).

Lattice and intermolecular interaction energies
The lattice energy calculations reveal that the crystal packing is predominantly stabilized through dispersion energy (71%) and the electrostatic (Coulombic + polarization) energy contributes 29% towards the stabilization of the crystal structure. The total lattice energy (À28.9 kcal mol À1 ) is the sum of the Coulombic (À10.5 kcal mol À1 ), polarization (À4.7 kcal mol À1 ), dispersion (À36.6 kcal mol À1 ) and repulsion (22.9 kcal mol À1 ) terms. Furthermore, different motifs formed in the major component of I and their energetics are discussed below (Table 2).
Inversion-related molecules form the strongest dimer (motif M1) which is held by intermolecular C-HÁ Á Á interactions with an interaction energy of À9.5 kcal mol À1 . As expected, the dispersion contribution (70%) is more significant towards the stabilization of this dimer. Further, this dimer is flanked on both sides by other molecules. As shown in Fig. 4(a), these molecules interact with the central dimer (motif M1) through two C-HÁ Á Á interactions (motif M3; interaction energy = À7.3 kcal mol À1 ). It is to be noted that the motif M3 is more dispersive in nature (78%) than motif M1. The nitrile group of one molecule stacks with the nitrile group of an inversion-related molecule (motif M2; interaction energy = À8.7 kcal mol À1 and 71% dispersion contribution). The shortest distance observed between two C15 atoms is 3.274 (4) Å and the motif M2 is also flanked on both sides by motif M3. These motifs act together to link the molecules into a chain which runs parallel to the b axis (Fig. 4b).
502 Udayakumar et al. The molecular sheet assembled by intermolecular C-HÁ Á ÁN and C-HÁ Á ÁCl interactions. Table 2 Intermolecular interaction energies (in kcal mol À1 ) for different molecular pairs observed in the major component of the title compound; CD is the centroid-to-centroid distance. Motif M4 (interaction energy = À5.9 kcal mol À1 ) is stabilized by three-centred intermolecular C-HÁ Á ÁN interactions in which the nitrile N atom acts as an acceptor and the vinylic proton (H9) and one of the protons (H10) of chlorophenyl ring are involved as donors (Fig. 5). These three-centred interactions link the molecules into a chain which runs parallel to the a axis. 53% of the electrostatic and 47% of the dispersion energy contribute towards stabilization of motif M4.
The energetically least-stable dimers (motifs M5 and M6) are formed by intermolecular C-HÁ Á ÁCl interactions (Fig. 5). These two interactions help to link adjacent columns in the crystal, as mentioned above. The molecules form an R 2 2 (8) loop in the case of motif M5, with an interaction energy of À2.8 kcal mol À1 . We note that the dispersion energy (67%) contributes nearly double that of the electrostatic energy (33%) for the stabilization of this motif. Further, a molecular chain is related to motif M6 (interaction energy = À1.6 kcal mol À1 ) propagating along the c axis direction. This dimer is more dispersive in nature and 75% of the dispersion energy contributes towards the stabilization. Motifs M4-M6 combine to form sheets parallel to the ac plane (Fig. 6).

Hirshfeld surface analysis and 2D fingerprint plots
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated 2D fingerprint plots (McKinnon et al., 2007) were performed with CrystalExplorer17 (Turner et al., 2017) for both the major and the minor disordered components. For each component, the occupancies of all atoms were made equal to 1. Hirshfeld surface (HS) analysis was carried out in order to gain more insight into the nature and extent of the intermolecular interactions and to quantify the relative contributions of the different non-covalent interactions that exist in the crystal. The HS surface was mapped over d norm and the diagram reveals that motifs M2 and M4 are visible as red spots on the HS (Fig. 7) in the major disordered component. It is to be noted that a pale-red spot is noticed for motif M3 when compared to the other two motifs. As mentioned above, motif M4 has two intermolecular C-HÁ Á ÁN interactions and one of them is found to be a close contact (C8-H8Á Á ÁN2).
2D fingerprint plots for the major and the minor components are illustrated in Figs. 7 and 8. For the major component of I, it is found that the contributions for the HÁ Á ÁC (33.6%) and HÁ Á ÁH (28.6%) contacts are relatively high in comparison to other non-covalent interactions (Fig. 7). It is of interest to note that the HÁ Á ÁCl contacts also contribute substantially (17.9%) to the crystal packing. As noted above, neighbouring columns are interlinked in the crystal via intermolecular HÁ Á ÁCl contacts. The intermolecular HÁ Á ÁN contacts contribute 10.6% towards the crystal packing. The other contacts, such as CÁ Á ÁC (4.1%) and CÁ Á ÁN (3.8%), also supplement the overall crystal packing. The former contact represents the motifs M2 and M3, while the latter contact is mainly due to the stacking of the nitrile groups.
In the case of the minor component, the relative contributions of some of the intermolecular contacts are very similar to those for the major component, as shown in Fig. 8. However, the HÁ Á ÁCl contacts are reduced by 4.9%. This difference clearly indicates the importance of halogen interactions in the major component of the title compound.  2D fingerprint plots for different intermolecular contacts and the Hirshfeld surface mapped over d norm to hightlight the short intermolecular contacts for the major disordered component of I.

Synthesis and crystallization
A mixture of phenylacetonitrile (0.53 ml, 4.6 mmol) and 4-chlorobenzaldehyde (4.6 mmol, 0.65 g) was stirred at room temperature for 10 min. Subsequently, the temperature was increased gradually to 403 K and maintained at that temperature for 39 h. Initially, the mixture was colourless and then became viscous and dark. This viscous solution was cooled, treated with hexane and finally filtered. The filtrate contained small colourless crystals. Further purification of the title compound (yield 83%, m.p. 368-370 K) was carried out by recrystallization from hexane. Colourless plate-like crystals, suitable for X-ray diffraction analysis, were obtained by slow evaporation of a solution of I in ethanol at 277 K after a period of 7 d.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The whole molecule was disordered and the major and minor components of the disorder refined to 0.86 (2) and 0.14 (2), respectively. All H atoms were placed in calculated positions and treated as riding, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C).      (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.