Octamethylporphyrin copper, C28H28N4Cu – A first experimental structure determination of porphyrins in gas phase

A first example of experimental study of molecular structure of free porphyrino metal complexes is given. Gas-phase electron diffraction was applied for the molecular structure determination of octamethylporphyrin copper(II), C28H28N4Cu, at an effusion cell temperature of 674(10) K. The molecule was found to possess a geometric configuration of D4h symmetry with the following main internuclear distances and valence angles: rh1(Cu–N) = 2.013(5) Å, rh1(C(N)–N) = 1.378(4) Å, rh1(C(N)–C) = 1.454(3) Å, rh1(C–C(Me)) = 1.503(3) Å, rh1(C(Me)–C(Me)) = 1.372(3) Å, rh1(C(N)–C(α)) = 1.390(3) Å, ∠∠Cu–N–C(N) = 127.1(1)°, ∠∠C(N)–N–C(N) = 105.7(2)°, ∠∠N–C(N)–C(Me) = 110.8(2), ∠∠C(N)–C(Me)–C(Me) = 106.4(2)°, ∠∠C(N)–Cα–C(N) = 125.5(3)°, ∠∠C(H)–C(Me)–C(Me) = 128.1(4)°. Quantum chemical calculations, DFT/B3LYP with 6-31G* and TZV (d,p) basis sets, reproduce the experimental bond distances with accuracy within 0.02 Å. NBO-analysis of electron density distribution was carried out, the energy of donor–acceptor interaction was estimated, and it was shown that the back donation makes a minor contribution to this interaction.