Probing the conformational space and molecular structure of 5,10,15,20-tetraphenyl-21h,23h-porphyrin – reference point for tetraphenylporphyrin metal complexes

Abstract

Gas-phase electron diffraction and quantum chemical study of the isolated H2TPP molecule was carried out, which can be considered as a reference point for tetraphenylporphyrin metal complexes. Using Knudsen effusion mass spectrometry, the enthalpy of sublimation of the H2TPP was determined to be 212(4) kJ/mol (535 K). Using DFT/B97D/cc-pVTZ in combination with gas electron diffraction it is shown that the saturated vapor of H2TPP consists of a mixture of conformers. The conformers differ in the mutual orientation of the four phenyl fragments relative to each other and have close energies. Bond lengths and bond angles in conformers were determined. NBO analysis showed a change in the delocalization of the electron density between the phenyl substituents and the macroheterocyclic (MHC) skeleton with a change in the torsion angle. This delocalization is the reason for the nonorthogonal position of phenyl meso-substituents in the conformers of H2TPP and other tetraphenyl substituted porphyrins. Non-covalent interactions between the MHC skeleton and phenyl substituents have been described using the FI-SAPT0 method. The change in geometric and electronic characteristics in the series of molecules H2P, H2TPP, H2FTPP, ZnTPP and PdTPP is considered. Based on Valence Shell Electron Pair Repulsion (VSEPR) theory, an explanation is given for the influence of the substituent on the parameters of the MHC skeleton. Analysis of changes in the energy of frontier orbitals in the indicated series of molecules makes it possible to predict the direction of changes in the red-ox properties of compounds with various modifications of H2TPP.