Piperonylic anhydride: Isolation and conformational analysis by X-ray crystallography and density functional theory calculations

The title compound (1), a symmetrically substituted acyclic anhydride, is shown to crystallise in the monoclinic space group P21/n with a = 4.7032(1) Å, b = 17.8748(6) Å, c = 15.6869(5) Å, β = 98.3980(17)° and V = 1304.64(7) Å3. There are four molecules in the unit cell. Bond lengths and angles of 1 are comparable to the limited number of benzoic anhydride crystal structures that have been previously disclosed. Compound 1 adapts a syn-conformation in the solid-state with a pseudo-torsion angle for the OC⋯CO groupings of approximately 25.1°. This structure is compared to other benzoic anhydride (BA) materials that have been structurally elucidated either via single crystal X-ray diffraction methods and/or examined from a theoretical perspective. Calculations at the DFT level (B3LYP 6-311G**) for a theoretical gas phase molecule of 1 reveals a similar overall global structure to that observed in the solid-state; these results suggest that crystal packing effects are negligible with respect to the observed ground state syn-conformation in the solid phase. Compound 1 also represents the first structurally characterised di-substituted benzoic anhydride.