Molecular Structure of Nickel Octamethylporphyrin-Rare Experimental Evidence of a Ruffling Effect in Gas Phase

Abstract

The structure of a free nickel (II) octamethylporphyrin (NiOMP) molecule was determined for the first time through a combined gas-phase electron diffraction (GED) and mass spectrometry (MS) experiment, as well as through quantum chemical (QC) calculations. Density functional theory (DFT) calculations do not provide an unambiguous answer about the planarity or non-planar distortion of the NiOMP skeleton. The GED refinement in such cases is non-trivial. Several approaches to the inverse problem solution were used. The obtained results allow us to argue that the ruffling effect is manifested in the NiOMP molecule. The minimal critical distance between the central atom of the metal and nitrogen atoms of the coordination cavity that provokes ruffling distortion in metal porphyrins is about 1.96 Å.

Keywords: electron diffraction; molecular structure; nickel; porphyrin; quantum chemistry; ruffling distortion.

Figure 1

Possible types of porphyrin skeleton distortions: (a)—ruffling, (b)—dome shaped, (c)—saddle shaped, (d)—wave shaped, (e)—propeller shaped. A top view perpendicular to the macrocycle plane shows the positions of atoms relative to the plane of the porphyrin core: the light circle denotes an atom located above the plane, the dark circle denotes an atom located below the plane and the absence of a circle denotes the location of the atom in the plane of the porphyrin core.

Figure 2

Molecular structure of NiOMP with atom numbering. The red dashed line shows an angle χ_ruf = χ(C_α-N∙∙∙N-C_α) quantifying the degree of ruffling distortion.

Figure 3

Experimental (cycles) and theoretical (black solid lines) molecular scattering intensities sM(s) and the difference curves ΔsM(s) (red solid lines) for NiOMP: disagreement factor R_f = 4.24%.

Figure 4

Experimental (cycles) and theoretical (black solid lines) radial distribution curves and the difference curve 3×Δf(r) (red solid lines) for NiOMP: disagreement factor R_f = 4.24%.

Figure 5

Molecular orbital diagrams for NiOMP according to different DFT methods. The energy of HOMO is chosen as zero level.

Figure 6

Molecular orbitals of NiOMP.

Figure 7

PES scans along χ_ruf = χ(C_α-N∙∙∙N-C_α) angle for NiOMP (squares), CuOMP (circles), ZnOMP (triangles) according to PBE/cc-pVTZ and B3LYP/cc-pVTZ.

Figure 8

Internuclear distances N-C_α, C_α-C_m, C_α-C_β, C_β-C_β and relative energies of the structures vs. the internuclear distance of Ni-N in the NiP molecule from the B3LYP calculations with the pVTZ (H, C, N) and cc-pVTZ (Ni) basis sets. Red circles—for ruffling distorted structures, black squares—for flat structures. Red italics numbers indicate values of χ_ruf = χ(C_α-N∙∙∙N-C_α) for ruffling-distorted structures.

Figure 9

The ratio of disagreement factors R_f/R_{f min} as a function of the torsion angle C_α-N∙∙∙N-C_α, responsible for ruffling distortion: (a)–using ω_ruf = 28 cm⁻¹ and ω_sad = 19 cm⁻¹, (b)—ω_ruf = 70 cm⁻¹ and ω_sad = 19 cm⁻¹. R_f–factor of disagreement between experimental and theoretical molecular scattering intensities sM(s); R_{f min}–minimal value of disagreement factor obtained in the LS analysis; R_Ham–uncertainty according to Hamilton’s statistical criterion [53] at significance level 0.05.