Crystal structure, Hirshfeld surface analysis and DFT study of (2Z)-2-(4-fluorobenzylidene)-4-(prop-2-yn-1-yl)-3,4-dihydro-2H-1,4-benzothiazin-3-one

In the title compound, the heterocyclic portion of the dihydrobenzothiazine unit adopts a shallow boat conformation. The propynyl substituent is nearly perpendicular to the plane formed by the rails of the boat. In the crystal, inversion dimers are formed by weak C—H⋯F hydrogen bonds with the dimers forming oblique stacks along the a-axis direction.

The title compound, C 18 H 12 FNOS, is built up from a 4-fluorobenzylidene moiety and a dihydrobenzothiazine unit with a propynyl substituent, with the heterocyclic portion of the dihydrobenzothiazine unit adopting a shallow boat conformation with the propynyl substituent nearly perpendicular to it. The two benzene rings are oriented at a dihedral angle of 43.02 (6) . In the crystal, C-H Flurphen Á Á ÁF Flurphen (Flurphen = fluorophenyl) hydrogen bonds link the molecules into inversion dimers, enclosing R 2 2 (8) ring motifs, with the dimers forming oblique stacks along the a-axis direction. Hirshfeld surface analysis of the crystal structure indicates that the most important contributions to the crystal packing are from HÁ Á ÁH (33.9%), HÁ Á ÁC/CÁ Á ÁH (26.7%), HÁ Á ÁF/FÁ Á ÁH (10.9%) and CÁ Á ÁC (10.6%) interactions. Hydrogen bonding and van der Waals interactions are the dominant interactions in the crystal packing. Density functional theory (DFT) optimized structures at the B3LYP/6-311 G(d,p) level are compared with the experimentally determined molecular structure in the solid state. The HOMO-LUMO behaviour was elucidated to determine the energy gap.

Figure 2
A partial packing diagram viewed along the a-axis direction. The intermolecular C-H Flurphen Á Á ÁF Flurphen (Flurphen = fluorophenyl) hydrogen bonds are shown as dashed lines.

Figure 1
The molecular structure of the title compound with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

Hirshfeld surface analysis
In order to visualize the intermolecular interactions in the crystal of the title compound, a Hirshfeld surface (HS) analysis (Hirshfeld, 1977;Spackman & Jayatilaka, 2009) was carried out by using CrystalExplorer17.5 (Turner et al., 2017). In the HS plotted over d norm (Fig. 4), the white surface indicates contacts with distances equal to the sum of van der Waals radii, and the red and blue colours indicate distances shorter (in close contact) or longer (distinct contact) than the van der Waals radii, respectively (Venkatesan et al., 2016). The brightred spots indicate their roles as the respective donors and/or acceptors; they also appear as blue and red regions corresponding to positive and negative potentials on the HS mapped over electrostatic potential (Spackman et al., 2008;Jayatilaka et al., 2005) as shown in Fig   View of the three-dimensional Hirshfeld surface of the title compound plotted over d norm in the range À0.0943 to 1.2826 a.u.

Figure 5
View of the three-dimensional Hirshfeld surface of the title compound plotted over electrostatic potential energy in the range À0.0500 to 0.0500 a.u. using the STO-3 G basis set at the Hartree-Fock level of theory. Hydrogen-bond donors and acceptors are shown as blue and red regions around the atoms corresponding to positive and negative potentials, respectively.

Figure 6
Hirshfeld surface of the title compound plotted over shape-index. reflected in Fig. 7b as widely scattered points of high density due to the large hydrogen content of the molecule. In the absence of C-HÁ Á Á interactions, the pair of scattered wings in the fingerprint plot delineated into HÁ Á ÁC/CÁ Á ÁH contacts (26.7% contribution to the HS) have a nearly symmetrical distribution of points, Fig. 7c, with the thick edges at d e + d i $2.70 Å . The pair of characteristic wings in the fingerprint plot delineated into HÁ Á ÁF/FÁ Á ÁH contacts (Fig. 7d, the 10.9% contribution to the HS) arises from the C-HÁ Á ÁF hydrogen bonds (Table 1) as well as from the HÁ Á ÁF/FÁ Á ÁH contacts (Table 2) and is shown as a pair of spikes with the tips at d e + d i = 2.52 Å . The CÁ Á ÁC contacts (Fig. 7e, 10.6% contribution to the HS) have an arrow-shaped distribution of points with the tip at d e = d i $1.68 Å . The pair of characteristic wings in the fingerprint plot delineated into HÁ Á ÁO/OÁ Á ÁH contacts (Fig. 7f Table 2 Selected interatomic distances (Å ).

Figure 7
The full two-dimensional fingerprint plots for the title compound  The Hirshfeld surface analysis confirms the importance of H-atom contacts in establishing the packing. The large number of HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH interactions suggest that van der Waals interactions and hydrogen bonding play the major roles in the crystal packing (Hathwar et al., 2015).

DFT calculations
The optimized structure of the title compound, (I), in the gas phase was generated theoretically via density functional theory (DFT) using standard B3LYP functional and 6-311G(d,p) basis-set calculations (Becke, 1993) as implemented in GAUSSIAN 09 (Frisch et al., 2009). The theoretical and experimental results were in good agreement. The highestoccupied molecular orbital (HOMO), acting as an electron donor, and the lowest-unoccupied molecular orbital (LUMO), acting as an electron acceptor, are very important parameters for quantum chemistry. When the energy gap is small, the molecule is highly polarizable and has high chemical reactivity. The electron transition from the HOMO to the LUMO energy level is shown in Fig. 9. The HOMO and LUMO are localized in the plane extending from the whole (Z)-2-(4-fluorobenzylidene)-4-(prop-2-ynyl)-2H-1,4-benzothiazin-3(4H)-one ring. The energy band gap [ÁE = E LUMO -E HOMO ] of the molecule was about 3.92 eV, and the frontier molecular orbital energies, E HOMO and E LUMO were À5.85 and À1.93 eV, respectively.

Database survey
Using the search fragment II (R 1 = Ph, R 2 = C) in the Cambridge Crystallographic Database (Groom et al., 2016;updated to Nov. 2018  , CH 2 C CH IIe (Sebbar, Zerzouf et al., 2014). In addition there are examples with R 1 = 4-ClC 6 H 4 and R 2 = CH 2 Ph2  IIf and R 1 = 2-ClC 6 H 4 , R 2 = CH 2 C CH (Sebbar et al., 2017c). In the majority of these, the heterocyclic ring is quite non-planar with the dihedral angle between the plane defined by the benzene ring plus the nitrogen and sulfur atoms and that defined by nitrogen and sulfur and the other two carbon atoms separating them ranging from ca. 29 (IIe) to 36 (IId). The other three (IIa, IIc, IIf) have the benzothiazine unit nearly planar with the corresponding dihedral angle of ca 3-4 . In the case of IIa, the displacement ellipsoid for the sulfur atom shows a considerable elongation perpendicular to the mean plane of the heterocyclic ring, suggesting disorder, and a greater degree of non-planarity but for the other two, there is no obvious source for the near planarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were located in a difference-Fourier map and freely refined.

Funding information
The support of NSF-MRI grant No. 1228232 for the purchase of the diffractometer and Tulane University for support of the The energy band gap of the title compound.

(2Z)-2-(4-Fluorobenzylidene)-4-(prop-2-yn-1-yl)-3,4-dihydro-2H-1,4-benzothiazin-3-one
Crystal data 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. 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.