Theoretical calculations of the pressure effect for the β-phase of p-NPNN

Hybrid DFT (HDFT) calculations were carried out for model clusters of paranitrophenyl nitronyl nitroxide (p-NPNN) whose geometrical structures were based on its β-phase crystal structure. The effective exchange integrals between the neighboring p-NPNN molecules were calculated for the qualitative understanding of the pressure-induced transition form ferro- to antiferro-magnetic state. In the high pressure-induced condition, it could be presumed that the p-NPNN molecules in the crystal was gradually shifted along several lines or was rotated around appropriate axes. From these model calculations, it was found that the ferromagnetic exchange interaction between the neighboring dimers changed to antiferromagnetic one by the structure deformations.