Molecular and crystal structure, lattice energy and DFT calculations of two 2′-(nitrobenzoyloxy)acetophenone isomers

The two isomers 2′-(4-nitrobenzoyloxy)acetophenone with para and ortho positions of the nitro substituent have been crystallized and studied. It is evident that the variation in the position of the nitro group causes a significant difference in the molecular conformations.

The two isomers 2 0 -(4-nitrobenzoyloxy)acetophenone (systematic name: 2-acetylphenyl 4-nitrobenzoate) (I) and 2 0 -(2-nitrobenzoyloxy)acetophenone (systematic name: 2-acetylphenyl 2-nitrobenzoate) (II), both C 15 H 11 NO 5 , with para and ortho positions of the nitro substituent have been crystallized and studied. It is evident that the variation in the position of the nitro group causes a significant difference in the molecular conformations: the dihedral angle between the aromatic fragments in the molecule of I is 84.80 (4) , while that in the molecule of II is 6.12 (7) . Diffraction analysis revealed the presence of a small amount of water in the crystal of I. DFT calculations of the molecular energy demonstrate that the ortho substituent causes a higher energy for isomer II, while crystal lattice energy calculations show that the values are almost equal for two isomers.

Structural commentary
The corresponding bond lengths and bond angles in isomers I and II are very similar in the two molecules and are close to the standard values. The only unexpected value is angle O1-C14-C7, which is 111.42 (10) (I) and 111.15 (9) (II) for steric reasons, which is quite common for a bridging geometry in molecules with the same molecular core, such as phenyl benzoate and fluorinated phenyl benzoates (Dey & Chopra, 2017).
In the molecule of I, the dihedral angle between the phenyl rings is 84.84 (6) (i.e. rings are almost perpendicular to each other), while in the molecule of II the phenyl rings are almost parallel, the dihedral angle between them being 6.11 (4) . It is possible that the significant difference in the molecular conformations of the isomers is caused by the different positions of the nitro substituents.

DFT calculations
DFT calculations of isomers I and II at the B3LYP/6-311G(d,p) level of theory were carried out using GAUSSIAN 16 software (Frisch et al., 2016). The geometrical parameters of the two isomers were optimized starting from the molecular geometry in the crystal. No significant differences between the experimental and optimized bond lengths and angles were found. As mentioned above, the observed O1-C14-C7 angles are smaller than the standard value, and the calculated values are also smaller [111.41 (I) and 110.04 (II), which are very close to experimental values of 111.42 (10) (I) and 111.15 (9) (II)]. A comparison of the conformational characteristics of isomers I and II according to X-ray data and quantum chemical DFT calculations is presented in Table 1. This shows that the deviations of the nitro and acetyl groups from the planes of the corresponding aromatic rings are small and almost the same according to the X-ray and DFT data for isomer I. The data for isomer II indicate that the sterically stressed ortho position of the nitro group leads to larger  Table 1 Comparison of conformational characteristics ( ) of isomers I and II from diffraction (X-ray) and computational (DFT) data.

Conformational parameters
Isomer I -X-ray Isomer I -DFT Isomer II -X-ray Isomer II -DFT

Figure 1
Views of the formula units of (a) isomer I and (b) isomer II with the atomlabeling schemes. Displacement ellipsoids are shown with 50% probability. H atoms are shown as fixed spheres of radius 0.15 Å .

Figure 2
Structure of the dimeric associate in the crystal of I with the molecules connected by a 0.074 (2) occupancy bridging water molecule.
differences between the molecular conformation in the crystal and that calculated for an isolated molecule. Hence, the deviations of the nitro and acetyl groups from the planes of aromatic rings are larger, as well as from the bridging plane, which is different in the isolated molecule of II.

Supramolecular features
As a result of the presence in isomers I and II of oxygen atoms of the carbonyl, nitro, and ether groups, the title molecules are capable of forming C-HÁ Á ÁO hydrogen bonds (Tables 2 and  3). In the crystal of I, a low-occupancy [0.074 (2)] partial water molecule forms a bridge between two molecules of I (Fig. 2). The O2Á Á ÁO6 distance of 2.912 (6) Å indicates that this interaction is weak (Brown, 1976). In addition,interactions between phenyl rings are observed in both structures.
In I, theinteractions lead to the formation of dimers (Fig. 3a), while in II they lead to the formation of ladder-like chains along the [1 16 1] direction (Fig. 3b). The crystal packing is shown in Figs. 4 and 5. Despite the differences in the packing in the crystals of the two isomers, their lattice energies are very similar (see below).

Lattice energy calculations
The crystal lattice energies ( Molecular associates connected byinteractions in the crystals of I (dimer) and II (ladder-like chain). In the dimer (I), the distance between parallel phenyl rings is given. In the chain (II), several short contacts between carbon atoms are indicated.

Figure 4
Molecular packing in the crystal of isomer I. Molecules of water with 0.074 (2) occupancy are shown.

I I I
Cell volume, Å 2694.5 (10) 1294.2 (9) Density, g cm À3 program package (version 3.0, available from http://www.angelogavezzotti.it; Gavezzotti, 2011). The hydrogen-atom positions for the lattice energy calculations were assigned by the software. In structure II, which has a higher packing coefficient, the repulsive and Coulombic components are larger than in the structure of I, which has a lower packing coefficient, although the dispersion energy is lower in I. The total contribution of all the components results in the lattice energy for the crystals of the two isomers being almost equal.
As the amount of water in I is low (the water molecule has 0.074 occupancy, see Refinement section), it was difficult to evaluate the effect of water on the total lattice energy. However, it is clear that the presence of water makes the structure of I less densely packed.

Database survey
A search of the Cambridge Crystallographic Database (CSD version 5.40, update of September 2019; Groom et al., 2016) for the molecules with the same structure as isomers I and II gave no entries. Three entries were found for the same core structure as in the title molecules. (Adams & Morsi, 1976;Dey & Chopra, 2017;Shibakami & Sekiya, 1995). One is an inclusion compound of phenylbenzoate with Ni complexes with isonicotinic acid and thiocyanato coordination bridges (Sekiya et al., 2004). Several methyl-substituted phenylbenzoates have been described by Gowda and co-workers, in particular the 2,3-, 2,4-and 2,5-isomers (Gowda, Foro et al., 2008;Gowda et al., 2009). Compounds with the same core and nitro-group substituents are rare and are mostly limited to halogen-substituted phenylbenzoates. The dihedral angles between the two aromatic rings vary. The methyl-substituted compounds tend to have a near-perpendicular geometry with dihedral angles ranging from 73.04 (8) to 87.43 (5) (Gowda, Tokarcík et al., 2008;Gowda et al., 2009), while pure phenylbenzoate and many of its fluorinated derivatives have angles in the range 52.66 (7) to 62.76 (4) (Adams & Morsi, 1976;Dey & Chopra, 2017). The number of entries in the database for nitro-substituted phenylbenzoates is limited and is not sufficient for drawing final conclusions on the role of the nitrogroup position on the molecular geometry (Saha & Desiraju, 2017). Finally, the presence of phenylbenzoate in inclusion compounds seems to have a 'flattening' effect on the molecule, lowering the dihedral angle; such a compound was described by Sekiya et al. (2004) with a dihedral angle between the aromatic rings of 20.9 (19) . Careful analysis of substituted phenyl benzoate derivatives (415 entries in the CSD) presented by Saha & Desiraju (2017) has shown a strong preference for Ar-Ar torsion angles of between 40 and 90 (91% of entries).

Synthesis and crystallization
The synthesis of isomers I and II was performed according to Barros & Silva (2006

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. For both structures, the C-bound hydrogen atoms were freely refined. A large residual electron density peak was found for I. It was modelled as a partial water molecule. The O6 atom of the water molecule occupies a site on a crystallographic C 2 axis (Fig. 2). The water molecule was freely refined with a resulting occupation factor of 0.074 (2). The water H atoms were added geometrically taking into account the direction of potential hydrogen bonds in the structure of I.

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.