Octamethylporphyrin copper, C sub(28)H sub(28)N sub(4)Cu - 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), C sub(28)H sub(28)N sub(4)Cu, at an effusion cell temperature of 674(10) K. The molecule was found to possess a geometric configuration of D sub(4h) symmetry with the following main internuclear distances and valence angles: r sub(h1)(Cu-N) = 2.013(5) Aa, r sub(h1)(C(N)-N) = 1.378(4) Aa, r sub(h1)(C(N)-C) = 1.454(3) Aa, r sub(h1)(C-C(Me)) = 1.503(3) Aa, r sub(h1)(C(Me)-C(Me)) = 1.372(3) Aa, r sub(h1)(C(N)-C(a)) = 1.390(3) Aa, [angle]Cu-N-C(N) = 127.1(1), [angle]C(N)-N-C(N) = 105.7(2), [angle]N-C(N)-C(Me) = 110.8(2), [angle]C(N)-C(Me)-C(Me) = 106.4(2), [angle]C(N)-C sub(a)-C(N) = 125.5(3), [angle]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 Aa. 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.