Oxalyl dihydrazide (left) and ortho-acetamidobenzamide (right). The torsion angles optimized in the crystal energy minimization are labeled (oxalyl dihydrazide: , , , , , , , , and ; ortho-acetamidobenzamide: , , , , , and ). Oxalyl dihydrazide torsions , and ortho-acetamidobenzamide torsion are improper dihedral determining the amine group pyramidalization.
Intramolecular energy penalty for oxalyl dihydrazide (in ) as a function of the rotation of one amide hydrogen and the pyramidalization of the amide group at the level of theory. The surface was computed by constraining the two torsion angles of one amide group and optimizing the rest of the molecular geometry. The circles correspond to the experimentally determined crystal structures: for the polymorphs , , , and , we show two equivalent configurations differing by which amide hydrogen atom is used to define torsion . Polymorph does not lie on an inversion center and hence we show two sets of equivalent configurations. The horizontal black line corresponds to planar amide geometry.
Electrostatic potential maps for oxalyl dihydrazide on the surface [vdW radius for polar hydrogen atoms set to to reflect close distances due to hydrogen bonding; vdW radii for other atoms were taken from Bondi (Ref. 97)] due to the static (left) and induced (right) moments derived from PBE0/aug-cc-pVTZ molecular charge densities for the conformation at the DMAFLEX minimum. The maps were computed from the multipole moments and drawn with ORIENT (Ref. 98) using the same scale for all polymorphs to facilitate comparison.
Electrostatic potential maps on the surface due to the static (top) and induced (bottom) moments for the (left) and (right) polymorphs of ortho-acetamidobenzamide. For clarity both sides of the molecule are shown. The maps were computed as described in Fig. 2.
Optimized crystal structures (right) of the five oxalyl dihydrazide polymorphs at the level of theory with the cell parameters frozen at their experimental values and (left) the corresponding contour map of the difference in electron density between the crystal and a superposition of charge densities of isolated molecules in the bulk geometry. The contour maps are drawn on the plane defined by atoms N1, O1, and N3 in Scheme 1. Continuous, dashed, and dot-dashed lines correspond to accumulation, depletion, and no change in charge density. The range (in ) is for polymorph , for , for , for , and for . Neighboring contour lines differ by
Predicted relative stability of oxalyl dihydrazide polymorphs for (a) relaxed-cell crystal energy minimizations. For methods where there was a significant volume change, the minimizations were repeated with the cell lengths and angles constrained to experimental values and the corresponding relative energies are shown in (b).
Predicted relative stability of ortho-acetamidobenzamide polymorphs for (a) relaxed-cell crystal energy minimizations. For methods where there was a significant volume change, the minimizations were repeated with the cell lengths and angles constrained to experimental values and the corresponding relative energies are shown in (b).
crystal energy minimization of oxalyl dihydrazide and ortho-acetamidobenzamide polymorphs. The effect of intermolecular induction and accurate electrostatics and intramolecular energies on the predicted relative stabilities is studied with single-point lattice and intramolecular energy calculations at the DMAFLEX minimum.
Cell and molecular geometry reproduction of oxalyl dihydrazide polymorphs with periodic electronic structure methods. Experimental geometries were determined at room temperature (Ref. 16).
Cell and molecular geometry reproduction of ortho-acetamidobenzamide polymorphs with periodic electronic structure methods. Experimental geometries were determined at (Ref. 10).
Variation of intermolecular, conformational, and BSSE contributions to crystal energy (in ) with Hamiltonian and basis set for the relaxed-cell quantum mechanical optimizations. was computed as the sum for atom-centered Gaussian basis set and for plane wave calculations. For the dispersion-corrected D-PW91/pw method, the empirical vdW correction contribution is also shown. Vibrational zero point energies are neglected.
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