Chemical bonding and aromaticity analyses of petroporphyrins with vanadium or nickel

V- or Ni-containing petroporphyrins were studied by quantum chemical calculations. A series of experimentally reported structures were chosen for understanding their electronic structure and chemical bonding characteristics. Different alkyl substituents produce changes in the chemical bonds between metal center and nitrogen atoms within the porphyrins. Aromaticity studies were conducted to measure the changes in the rings due to the alkyl substituents and metal. Important changes were obtained due to the addition of the metal in the center of porphyrins, specifically for interchanging NH for NM species, alkyl substituents also modify the aromatic character in the rings. Deeper studies in the NM chemical bonds were conducted by means of the Quantum Theory of Atoms in Molecules (QTAIM). More covalency was found in the NNi bonds, whereas V-containing molecules produce higher localized chemical bonds. Finally, the Laplacian Bond Orders explained the recalcitrant nature of these chemical compounds in the crude oils. These studies are intended to contribute for a better understanding of the chemical bonds involved in such complexes within petroleum matrix.