Crystal structure, Hirshfeld surface analysis and DFT studies of 4-methyl-2-({[4-(trifluoromethyl)phenyl]imino}methyl)phenol

The title compound, C15H12F3NO, crystallizes with a single molecule in the asymmetric unit. The phenol ring makes a dihedral angle of 44.77 (3)° with the benzene ring of the trifluoromethyl group. In the crystal, molecules are linked by C—H⋯O interactions, forming polymeric chain along the b-axis direction.


Chemical context
Over the past 25 years, there has been extensive research on the synthesis and use of Schiff base compounds in organic and inorganic chemistry as they have important medicinal and pharmaceutical applications. These compounds show biological activities including antibacterial, antifungal, anticancer and herbicidal activities (Desai et al., 2001;Singh & Dash, 1988;Karia & Parsania, 1999). Schiff bases are also becoming increasingly important in the dye and plastics industries, as well as in liquid-crystal technology and for the mechanistic investigation of drugs used in pharmacology, biochemistry and physiology (Sheikhshoaie & Sharif, 2006). The present work is a part of an ongoing structural study of Schiff bases and their use in the synthesis of new organic, excited-state proton-transfer compounds and fluorescent chemosensors (Faizi et al., 2016Kumar et al., 2018;Mukherjee et al., 2018). We report here on the synthesis and crystal structure as well as the Hirshfeld surface analysis of the new compound, (I). Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) x À 1; y; z.

Figure 2
A view along the b axis of the polymeric chain formed via C-HÁ Á ÁO interactions (see Table 1 for details).

Figure 1
The molecular structure of (I) with the atom-numbering scheme. Displacement ellipsoids are drawn at the 40% probability level. The intramolecular O-HÁ Á ÁN hydrogen bond (Table 1) is shown as a dashed line.

Figure 3
A view of the crystal packing along the a axis.

Supramolecular features
In the crystal of (I), molecules are linked by intermolecular C-HÁ Á ÁO interactions, forming chains extending along the a-axis direction ( Fig. 2 and Table 1). The crystal packing along the a-axis direction is shown in Fig. 3.

Hirshfeld surface analysis and two-dimensional fingerprint plots
In order to visualize the role of weak intermolecular interactions in the crystal, a Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) was carried out along with the associated two-dimensional fingerprint plots (McKinnon et al., 2007) generated using CrystalExplorer17.5 (Turner et al., 2017). The three-dimensional d norm (Fig. 4a) and shape-index ( Fig. 4c) surfaces of (I) are shown with a standard surface resolution and a fixed colour scale of À0.1805 to 1.0413 a.u. The darkest red spots on the Hirshfeld surface indicate contact points with atoms participating in intramolecular C-HÁ Á ÁO (Fig. 4b) interactions that involve C1-H1A and the oxygen atom O1 of the phenol group (Table 1). As illustrated in Fig. 5a, the corresponding fingerprint plots for (I) have characteristic pseudo-symmetrical wings along the d e and d i diagonal axes. The presence of C-HÁ Á ÁO interactions in the crystal is indicated by the pair of characteristic wings in the fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC (Fig. 5b) contacts (29.2% contributions to the Hirshfeld surface). In Fig. 5c, the widely scattered points in the fingerprint plot are related to HÁ Á ÁH contacts, which make a contribution of 28.6% to the Hirshfeld surface. There are also FÁ Á ÁH/HÁ Á ÁF (25.6%; Fig. 5d

DFT calculations
The optimized structure of (I) in the gas phase was generated theoretically via density functional theory (DFT) using the standard B3LYP functional and the 6-311G(d,p) basis-set calculations (Becke, 1993) as implemented in GAUSSIAN 09 (Frisch et al., 2009). The theoretical and experimental results are in good agreement ( Table 2). The C8 N1 bond length is 1.283 (8) Å (experimental) and 1.290 Å (calculated) and the C10-O1 bond length is 1.357 (8) Å (experimental) and 1.342 Å (calculated). The highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) are very important aspects as many electronic, optical and chemical reactivity properties of compounds can be predicted from these frontier molecular orbitals (Tanak, 2019). A molecule with a small HOMO-LUMO bandgap is more polarizable than one with a large gap and is considered a 'soft' molecule because of its high polarizibility while molecules with a large bandgap are considered to be 'hard' molecules. To better understand the nature of (I), the electron affinity (A = ÀE HOMO ), the ionization potential (I = ÀE LUMO ), the HOMO-LUMO energy gap (ÁE), the chemical hardness () and softness (S) (based on the E HOMO and E LUMO energies; Tanak, 2019) were calculated (Table 3). Based on the relatively large ÁE and values, the title compound can be classified as a hard molecule.
The electron distribution of the HOMO and LUMO energy levels is shown in Fig. 6. The DFT study shows that the HOMO and LUMO are localized in a plane extending over the whole 4-methyl-2-[(4-trifluoromethylphenylimino)methyl]phenol unit. From the frontier orbital analysis, it can be concluded that a HOMO-to-LUMO excitation of (I) would be a -* transition that would weaken the imine bond and drive the production of an excited-state keto-amine tautomer from the enol-imine ground state observed in the solid state. The calculated band gap of (I) is 4.076 eV, which is similar to that reported for other Schiff base materials, such as for example   Table 2 Comparison of selected observed (X-ray data) and calculated (DFT) geometric parameters (Å , ) for (I).  Table 3 Interaction energies for (I).

Figure 6
The energy band gap of (I).