Crystal structure, PIXEL calculations of intermolecular interaction energies and solid-state characterization of the herbicide isoxaflutole

The packing of isoxaflutole molecules can be rationalized in terms of a hierarchy of interaction energies within columnar and layer arrangements, in each case dominated by the dispersion energy term.


PIXEL calculations
Energy contributions from the twelve most important molecule/molecule interactions in the crystal structure of isoxaflutole are listed in Table S2 and visualized in Fig. S2. The total PIXEL energy of the lattice is, Etot,Cry = 140.3 kJ mol 1 , can be partitioned into contributions from Coulombic (ECol = -56.6 kJ mol -1 ), polarization (Epol = -20.7 kJ mol -1 ), dispersion (Edis = -151.2 kJ mol -1 ) and repulsion (Erep = 88.2 kJ mol -1 ) terms. To evaluate the relative contribution from certain structure motifs, such as a column of molecules along the b axis or a molecular layer parallel the bc plane which results from the stacking of such columns along the c axis via inversion operations, sums of total energy contributions from all involved interactions  i Etot were calculated and compared to the sum of all total energy contributions in the crystal Etot, = 144.8 kJ mol 1 . (There is small difference between Etot,Cry and Etot, as PIXEL energies are not mutually additive.) Table S2. Pairwise interaction energies (kJ mol 1 ) for (i = index of an interaction, d = distance between the centroids of two molecules, ECol = Coulombic energy, Epol = polarization energy, Edis = dispersion energy, Erep = repulsion energy, Etot = total PIXEL energy,  i Etot = sum of the fist i total PIXEL energies and  i Etot / Etot. is their relative contribution to the sum of all total PIXEL energies).
i  S2. Visualization of total energy contributions from the first 12 molecule/molecule interactions in the crystal structure of isoxaflutole. Interaction energy contributions are represented as red (intra-columnar), green (intercolumnar along the c axis) or gray (stacking of bc planes along the axis). ∑ is the sum of total PIXEL energies of the first i interactions and Etot, is the sum of the total PIXEL energies.

Hot-stage microscopy (HSM)
The prismatic crystals obtained from acetonitrile (

Differential scanning calorimetry (DSC ) and thermogravimetric analysis (TGA)
DSC recordings of isoxaflutole show only one sharp melting endotherm at 140.3 °C (Fig. S5) with an enthalpy of fusion of 34.2 kJ mol -1 . The TGA starts to slightly decrease from about 120 °C which indicates sublimation and is consistent with thermomicroscopic observations. Above 150 °C the mass loss rate due to evaporation increases quickly.
Thermochemical data obtained for isoxaflutole are collected in Table S3. A melting point of 140 °C has been reported previously, 1 which is consistent with the results of this study.  Experimental: Differential scanning calorimetry was performed with a DSC 7 (PerkinElmer, Norwalk, Ct., USA). Approximately 1 to 5 ± 0.0005 mg sample (using a UM3 ultramicrobalance, Mettler, Greifensee, CH) were weighed into Al-Pans (30 µL) and sealed with a cover. Dry nitrogen was used as the purge gas (purge: 20 mL min -1 ). The instrument was calibrated for temperature with pure benzophenone (mp 48.0 °C) and caffeine (236.2 °C), and the energy calibration was performed with indium (mp 156.6 °C, heat of fusion 28.45 J g −1 ). The errors on the stated temperatures (extrapolated onset temperatures) and enthalpy values were calculated at the 95 % confidence interval (CI) and are based on three measurements. Thermogravimetric analysis was carried out with a TGA7 system (Perkin-Elmer) using a sample amount of roughly 5 mg and a platinum sample pan. Temperature calibration was performed with ferromagnetic materials (Alumel and Ni, Curie-point standards, Perkin-Elmer). A heating rate of 5 °C min −1 was applied and dry nitrogen was used as a purge gas (sample purge: 20 mL min −1 , balance purge: 40 mL min −1 ).

ATR-FTIR spectroscopy
The ATR-FTIR spectra of isoxaflutole (Fig. S6) show a characteristic band at 1667 cm -1 (ν C=O) and at 1663 cm -1 (ν C=O) for the crystalline and amorphous state, respectively. The obtained peak positions of the crystalline substance match with the bands listed in literature 2 (Table S4).

Raman spectroscopy
The Raman spectra (Fig. S7) of the crystalline and amorphous substance are shown and the Raman bands are compared in Table S5. Experimental: Raman spectra were recorded with a Bruker BRAVO hand-held spectrometer (Bruker GmbH, Karlsruhe, D) in the spectral range from 3200 to 300 cm -1 with 32 scans and an integration time of 1000 ms per spectrum. The solid samples were prepared on aluminum sample holders.

Powder X-ray diffraction (PXRD)
The comparison of the simulated and experimental PXRD patterns shown in Fig. S8 confirms that the phase of the investigated bulk material is the same as that of the reported single structure determination. Experimental: The PXRD patterns were obtained with an X'Pert PRO diffractometer (PANalytical, Almelo, The Netherlands) equipped with a θ/θ coupled goniometer in transmission geometry, programmable XYZ stage with well plate holder, Cu-Kα1,2 radiation source with a focusing mirror, a 0.5° divergence slit, a 0.02° soller slit collimator and a 0.5° anti-scattering slit on the incident beam side, a 2 mm anti-scattering slit, a 0.02° soller slit collimator, a Ni-filter and a solid state PIXcel 1D detector. The patterns were recorded at a tube voltage of 40 kV, tube current of 40 mA, applying a step-size of 0.013° 2θ with 40 s per step in the angular range of 2° to 40°.