Towards Computational Screening for New Energetic Molecules: Calculation of Heat of Formation and Determination of Bond Strengths by Local Mode Analysis

The reliable determination of gas-phase and solid-state heats of formation are important considerations in energetic materials research. Herein, the ability of PM7 to calculate the gas-phase heats of formation for CNHO-only and inorganic compounds has been critically evaluated, and for the former, comparisons drawn with isodesmic equations and atom equivalence methods. Routes to obtain solid-state heats of formation for a range of single-component molecular solids, salts, and co-crystals were also evaluated. Finally, local vibrational mode analysis has been used to calculate bond length/force constant curves for seven different chemical bonds occurring in CHNO-containing molecules, which allow for rapid identification of the weakest bond, opening up great potential to rationalise decomposition pathways. Both metrics are important tools in rationalising the design of new energetic materials through computational screening processes

<i>In situ</i> investigation of controlled polymorphism in mechanochemistry at elevated temperature†

Mechanochemistry routinely provides solid forms (polymorphs) that are difficult to obtain by conventional solution-based methods, making it an exciting tool for crystal engineering. However, we are far from identifying the full scope of mechanochemical strategies available to access new and potentially useful solid forms. Using the model organic cocrystal system of nicotinamide (NA) and pimelic acid (PA), we demonstrate with variable temperature ball milling that ball milling seemingly decreases the temperature needed to induce polymorph conversion. Whereas Form I of the NA:PA cocrystal transforms into Form II at 90 °C under equilibrium conditions, the same transition occurs as low as 65 °C during ball milling: a ca 25 °C reduction of the transition temperature. Our results indicate that mechanical energy provides a powerful control parameter to access new solid forms under more readily accessible conditions. We expect this ‘thermo-mechanical’ approach for driving polymorphic transformations to become an important tool for polymorph screening and manufacturing