Density functional theory study of dipicolinic acid isomers and crystalline polytypes

Six isomers of dipicolinic acid (DPA) in the gas phase are predicted based on a detailed density functional theory study. Energetics include the binding energy, ionization potential, and electron affinity. Dipole moments, rotational constants, atomic charges, and Fukui functions are also calculated. Several isomerization reactions are shown as possible, with calculated reaction path and transition barriers in the range 6–14 kcal/mol. Prediction of five DPA dimers and three crystalline DPA polytypes are also reported. Vibrational analysis for monomers and dimers is calculated and compared to existing experimental results. The electronic band structure of three crystalline polytypes shows that DPA crystals are insulators. The three stable polytypes display distinct hydrogen-bonding formations that originate sheets of DPA molecules that are further stacked to form layered crystals that are energetically nearly degenerate.