Persistent Isomeric Disorder in Chlorobenzofuroxan Crystals: Configurational Energetics and Isomerization Pathways from Periodic DFT

Crystalline chlorobenzofuroxan (ClBF) exhibits persistent positional isomeric disorder, with 5-ClBF and 6-ClBF sharing the same crystallographic sites. Periodic density functional theory was used to investigate the associated configurational energetics and solid-state interconversion pathways. All 256 starting arrangements constructed from four local models—5-ClBF, 6-ClBF, and two atom-mapped representations of the same open-dinitroso structural model—over four molecular sites were optimized at fixed experimental lattice parameters. Among the 254 structures retaining the predefined molecular connectivity, the fully 5-ClBF configuration had the lowest electronic energy. Mixed cyclic configurations remained comparatively low in energy and showed non-additive, arrangement-dependent energy penalties. Ring-opened configurations and an alternative ordered model containing four-membered N–O–N–O rings were substantially less favorable. Eight representative one-site transformations yielded non-spin-polarized electronic barrier estimates of 170.1–217.7 kJ mol−1, compared with 745.5 kJ mol−1 for the investigated four-molecule transformation, for the complete four-molecule crystallographic unit cell. Three sequential branches differed in their intermediate barrier estimates but reached the same maximum electronic-energy level within the investigated model. The separation between modest configurational energy differences and substantially larger barrier estimates is consistent with kinetically hindered redistribution after a mixed isomeric population has been incorporated into the crystal, without quantitatively predicting the experimental populations or their origin.