Solvent effect on the crystal morphology of 2,6-diamino-3,5-dinitropyridine-1-oxide: A molecular dynamics simulation study

• Crystal morphology of ANPyO in vacuum and solvent was predicted by attachment energy.
• Crystal shape becomes very close to a flake in trifluoroacetic acid solvent.
• Interaction energies include hydrogen bond, Coulomb and Van der Waals forces.
• Analysis of solvent self-diffusion coefficient shows stirring affects crystal habit.

The attachment energy (AE) calculations were performed to predict the growth morphology of 2,6-diamino-3,5-dinitropyridine-1-oxide (ANPyO) in vacuum. The molecular dynamics (MD) method was applied to simulate the interaction of trifluoroacetic acid solvent with the habit faces and the corrected AE model was adopted to predict the growth habit of ANPyO in the solvent. The results indicate that the growth morphology of ANPyO in vacuum is dominated by (1 1 0), (1 0 0), (1 0 −1) and (1 1 −2) faces. The corrected AE energies change in the order of (1 1 0) > (1 0 −1) > (1 1 −2) > (1 0 0), which causes the crystal morphology to become very close to a flake in trifluoroacetic acid solvent and accords well with the results obtained from experiments. The radial distribution function analysis shows that the solvent molecules adsorb on the ANPyO faces mainly via the solvent–crystal face interactions of hydrogen bonds, Coulomb and Van der Waals forces. In addition to the above results, the analysis of diffusion coefficient of trifluoroacetic acid molecules on the crystal growth faces shows that the growth habit is also affected by the diffusion capacity of trifluoroacetic acid molecules. These suggestions may be useful for the formulation design of ANPyO.

The equilibrium configuration of trifluoroacetic acid adsorbing on (1 1 0) crystal face of ANPyO at 288 K. The MD simulation result of trifluoroacetic acid interacting with ANPyO (1 1 0) face indicates that the solvent molecules have contacted closely with ANPyO face, including extensive interactions (hydrogen bonds, Coulomb and Van der Waals forces) between them. Consequently, the solvent molecules have to be removed from crystal faces before they can grow. The process requires an expenditure of energy and the energy increases with the strength of interaction. Therefore, the interaction reduces the growth rate of certain crystal faces. And finally the solvent modifies the crystal shape of ANPyO by its different interaction with different crystal faces.Figure optionsDownload high-quality image (170 K)Download as PowerPoint slide