*3.5. Energy Frameworks*

The thermodynamic stability profile of each of the polymorphs is a direct implication of the nature and strength of their non-bonding interactions. Energy frameworks were calculated in an attempt to understand the topological dissimilarities of the energy components of the two polymorphs, and subsequently potentially link these characteristics

to their respective packing and thermal behavior [35]. The total energy is divided into Coulomb forces/electrostatic potential forces and dispersion energy.

The negative energy components in form I are distributed along both parallel and nonparallel molecules (Figure 10, top row), hence stabilizing the energy architecture throughout the supramolecular structure in multiple directions. The dispersion forces can mainly be traced along the off-centered parallel stacking (along the axis *a*), as well as between adjacent molecules whose aromatic systems lie along the same plane. The strongest energy contribution (−164 kJ/mol) is attributed to N1–H ... O2 and N2–H ... O4 interactions, located between parallel molecules. The strength of these stabilizing interactions is due to both electrostatic and dispersion forces, the former being the dominant one. The O4– H ... O3 contact contributes towards a lowering of energy (−56.4 kJ/mol), while the combination of O3–H ... N3 and C6–H ... O1 interactions induces a further stabilizing effect of −63.7 kJ/mol.

**Figure 10.** Energy framework diagrams, using the CE-B3LYP model, a scale tube size of 20 and cut-off value of 0 kJ/mol. The top row represents form I (viewed down the *c* axis), and the bottom row is associated with form II (viewed down the *b* axis). Molecules are clustered around the central molecule (pink) with a 3.8 Å radius.

In form II, N1–H ... O1 and N2–H ... N3 are the strongest interactions both of which contribute towards the dominant electrostatic energy (−73.6 kJ/mol) along the planes of aromatic rings. A secondary energy contribution (−45.2 kJ/mol) in the same direction is attributable to the N1–H ... O4 contact. The out-of-plane interactions are mainly a result of dispersion contacts and the O4–H ... O1 electrostatic interaction. However, such a contribution is considerably low (−18.3 kJ/mol) relative to the other energy components along the aromatic systems plane.

The less exhaustive network of intermolecular bonding in form II, in conjunction with the presence of an intramolecular bond, seems to enhance stability within the molecule. In contrast, form I entails a very complex extensive framework of intermolecular contacts, with 16 hydrogen bonds per molecule (and eight different neighboring molecules). Such a multidirectional framework creates a greater stabilizing effect in form I, as illustrated by the relatively thicker cylindrical radius of the total energy tubes (Figure 10). Such an influence, coupled with a more compact packing arrangement, might contribute towards a lower enthalpy in form I relative to form II, which according to L. Yu must be accompanied by a lower entropy in order to conserve the enantiotropic relationship between the two polymorphs [18,43]. With an increase in temperature, in the range of 222 ◦C to 228 ◦C, enough energy is absorbed to break the intermolecular framework in form I, which then transitions to form II (as confirmed by variable temperature X-ray diffraction and differential scanning calorimetry) [15,17,18].

The relatively loose packing and the intermolecular framework in form II seems to be constructed in such a way as to cater for higher energy environments. The energy framework of this polymorph reveals how the stabilizing effect is expanded along planes, whereas much

fewer interactions are observed between layers. This lower extent of intermolecular forces between parallel planes in form II might create an environment that can accommodate higher entropy within the crystal structure, thereby allowing it to be thermodynamically stable at higher temperatures.
