The gas-phase molecular structure of a thiadiazole-containing expanded heteroazaporphyrinoid (C₄₂H₃₉N₁₅S₃) has been studied by a synchronous gas electron diffraction and mass spectrometric experiment and density functional theory calculations using the B3LYP hybrid method and cc-pVTZ basis sets. The molecule has an equilibrium structure of C_3h symmetry with a planar macrocycle and the thiadiazole rings oriented in such a way that the sulfur atoms point outwards from the inner cavity. The unsubstituted macrocycle (C₃₀H₁₅N₁₅S₃) has been studied by DFT computations. An algorithm for building a complete set of internal coordinates, used in the computation of vibrational corrections, is also described.

The molecular structure of a macrocycle with a 24-membered ring, a thiadiazole-containing expanded heteroazaporphyrinoid, has been, for the first time, directly characterised by a synchronous gas electron diffraction and mass spectrometric experiment and DFT calculations; the molecule has the equilibrium structure of C_3h symmetry with a planar macrocycle.