Conformation analysis of copper(II) etioporphyrin-II by combined gas electron diffraction/mass-spectrometry methods and DFT calculations

Abstract

Molecular structure of copper(II) etioporphyrin-II (CuEP-II) has been studied by a combined gas-phase electron diffraction and mass spectrometry (GED/MS) method in synchronous mode. The experimental results were complemented by DFT calculations using the B3LYP and PBE functionals with pVTZ basis sets for describing H, C, N atoms and cc-pVTZ for describing Cu atom. The CuEP-II molecule was found to possess quasiplanar geometry of macroheterocycle. DFT calculations predict six conformers for the molecule, which differ by the orientation of the ethyl groups with respect to the macrocycle. Energy differences of five conformers are less, than 0.1kJ/mol (UPBE) or 0.3kJ/mol (UB3LYP). Rotation barriers of single ethyl-group are 14.6 and 19.8kJ/mol (UPBE), 15.7 and 21.6kJ/mol (UB3LYP). Sixth conformer has relatively high energy. Structural parameters of the CuEP-II molecule yielded by DFT calculations are, in general, in a good agreement with the GED data. Structural analysis indicated that the gas-phase electron diffraction method is robust for determination of the macroheterocyclic fragment, but is not enough sensitive to conformational composition of CuEP-II. Substitution effect in pyrrole fragments of CuP, CuOFP, CuOMP and CuEP-II complexes has been analyzed.