Combined gas-phase electron diffraction/mass spectrometry and DFT study of the molecular structure of zinc(II) etioporphyrin-II

Abstract

The molecular structure and conformational variety of the zinc(II) etioporphyrin have been studied by the combination of gas-phase electron diffraction (GED), mass spectrometry and quantum chemical calculations. The DFT (B3LYP and PBE) calculations predict six conformers, which differ in the orientation of the ethyl groups relative to the porphyrin skeleton. The energy differences between conformers I–V do not exceed 0.3 kJ mol−1; the conformer VI has relatively high energy. Our investigation has shown that GED is not able to solve the conformational problem of the isolated zinc(II) etioporphyrin molecules. At the same time, this method allows to obtain reliable structural parameters of the macrocycle, as well as to determine the position of the ethyl groups with respect to the porphyrin core. Selected experimental parameters are as follows: rh1(Zn–N) = 2.042 (5) A, rh1(N–Cα) = 1.374 (5) A, rh1(Cα–Cβ) = 1.453 (4) A, rh1(Cα–Cm) = 1.396 (4) A, rh1(Cβ–Cβ) = 1.376(4) A, ∠ZnNCα = 126.8 (2)°, ∠CαNCα = 106.3 (3)°, ∠CαCmCα = 127.0 (4)°.