Crystal structure and DFT computational studies of (E)-2,4-di-tert-butyl-6-{[3-(trifluoromethyl)benzyl]iminomethyl}phenol

The title Schiff base compound was synthesized and its crystal structure characterized by X-ray diffraction. The molecular structure, frontier orbitals and molecular electrostatic potential map were also investigated by DFT methods.

The title compound, C 23 H 28 F 3 NO, is an ortho-hydroxy Schiff base compound, which adopts the enol-imine tautomeric form in the solid state. The molecular structure is not planar and the dihedral angle between the planes of the aromatic rings is 85.52 (10) . The trifluoromethyl group shows rotational disorder over two sites, with occupancies of 0.798 (6) and 0.202 (6). An intramolecular O-HÁ Á ÁN hydrogen bonding generates an S(6) ring motif. The crystal structure is consolidated by C-HÁ Á Á interactions. The molecular structure was optimized via density functional theory (DFT) methods with the B3LYP functional and LanL2DZ basis set. The theoretical structure is in good agreement with the experimental data. The frontier orbitals and molecular electrostatic potential map were also examined by DFT computations.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1(a). The crystal structure is monoclinic and has the spacegroup type P2 1 /c. The CF 3 group exhibits rotational disorder [ Fig. 1(a)]. The site-occupancy factors are 0.798 (6) and 0.202 (6) for F1A/F2A/F3A and F1B/F2B/F3B, respectively. The DFT computations of the title compound were performed with the Gaussian 09W program package (Frisch et al., 2009) using the B3LYP functional and the LanL2DZ basis set. The optimized molecular structure is illustrated in Fig. 1(b). Some selected theoretical bond lengths, bond angles and torsion angles are given in Table 1 along with the experimental values. The molecular structure of the title compound is not planar: the dihedral angle between the 2,4-di-tert-butylphenol and the trifluoromethyl rings is 85.52 (10) . This dihedral angle was calculated to be 65.73 for the B3LYP computationally derived structure. The imino group is nearly coplanar with the 2,4-ditert-butylphenol ring, as indicated by the C1-C14-C15-N1 torsion angle [À3.9 (3) for X-ray and À0.14 for B3LYP]. There is an intramolecular O1-H1Á Á ÁN1 hydrogen bond present ( Fig. 1  indicates double-bond character. In the title compound, the bond lengths and bond angles are within normal ranges and they are comparable with those in related Schiff base structures (Li et al., 2007;Sun et al., 2007;Ç elik et al., 2009;Ş ahin et al., 2009;Kansiz et al., 2018). The C1-O1 and C15 N1 bond lengths confirm the enol-imine form of the title compound (Tanak, 2011;Kaynar et al., 2018).

Supramolecular features
The crystal structure of the title compound is consolidated by C-HÁ Á Á interactions (Fig. 2), details of which are summarized in Table 2. A packing diagram is shown in Fig. 3. The only other interactions are van der Waals contacts.

Molecular electrostatic potential (MEP)
The molecular electrostatic potential (MEP) is a very useful descriptor for classifying and understanding regions that are susceptible to electrophilic versus nucleophilic attack. In order to analyse reactive regions for electrophilic and nucleophilic reactions for the investigated Schiff base molecule, the MEP surface was computed using the B3LYP/LanL2DZ basis set for the optimized geometry. In the MEP surface, the negative potential regions (red areas) are associated with electrophilic reactivity, while the positive potential regions (blue areas) are related to nucleophilic reactivity. The MEP surface of the compound is shown in Fig. 4. The negative MEP regions are mainly over the O1, F1, F2, and F3 atoms and have values of À0.049 a.u., À0.031 a.u., À0.032 a.u. and À0.035 a.u., respectively. The largest maximum positive MEP region is localized on atom H15, and has a value of +0.048 a.u. According to these results, the preferred sites for electrophilic attack are around the oxygen and fluorine atoms, while the preferred region for nucleophilic attack is the imine group C-H atom, H15.

Frontier molecular orbitals
The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are known as frontier molecular orbitals. The electronic, optical and chemical reactivity properties of compounds are predicted by their frontier molecular orbitals (Tanak, 2019). The frontier molecular orbitals of the title compound were obtained using the DFT/B3LYP method with the LanL2DZ basis set. The energy levels and distributions of the frontier molecular orbitals are shown in Fig. 5 The crystal packing of the title compound, viewed along the a axis.

Figure 4
The molecular electrostatic potential map of the title compound. Table 2 Hydrogen-bond geometry (Å , ).

Figure 5
The frontier molecular orbitals.
the chemical reactivity and stability of a molecule. If the molecule has a large HOMO-LUMO gap, the molecule is more stable and less chemically reactive. The term 'hard molecule' is used to describe such cases. The electron affinity (A = -E HOMO ), the ionization potential (I = -E LUMO ), HOMO-LUMO energy gap (ÁE), the chemical hardness () and softness (S) of the title compound were predicted based on the E HOMO and E LUMO energies (Tanak, 2019). For the title compound, I = 5.912 eV, A= 1.807 eV, ÁE = 4.105 eV, = 2.052 eV and S = 0.243 eV. As a result of the large ÁE and values, the title compound can be classified as a hard molecule.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93-0.97 Å and U iso (H) = 1.2-1.5U eq (C). The position of the H1 atom was obtained from a difference map of the electron density in the unit cell and was refined freely.   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.17 e Å −3 Δρ min = −0.14 e Å −3 Special details Experimental. 248 frames, detector distance = 80 mm 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (