Quantum-Chemical Modeling of Nickel Interaction with Acrylic Acid

Abstract

Quantum-chemical modeling of system Niz+ – H2O – HAk (z=+2, +1, 0) using DFT-methods allowed to prove the mechanism of the processes of Ni2+-ions electroreduction in the presence of acrylic acid (HAk). It has been shown that the initial structure is an octahedral complex [Ni2+(H2O)5Ak-], the intermediate is the complex [Ni+(H2O)2Ak-] and the final product exists in three forms: [Ni0(H2O)2Ak], [Ni0(H2O)2(σ-HАk)], [Ni0(H2O)2(π-HАk)]. Due to this, the result of еn– type unsaturated organic compounds effect on nickel electrocrystallization process is not simple and depends on the nature and intensity of the effect of external factors. Nickel electrodeposition is a widespread process in electroplating industry due to a number of important properties of this metal. Electrolytes for nickel electrodeposition are used with various organic additives depending on the intended use of the coating [1]. The substances which contain a double -С=С-bond in their structure are often used as additives in the nickel-plating solutions. Molecules of such organic compounds involve centers with surplus electron density. Due to the fact that the electronic structure of nickel contains a vacant d-orbital, organic molecules tend to form a rather strong π-bond with metal ions and even with atoms [2]. As a result of this interaction the above-mentioned type of organic compounds influences effectively on the metal electrocrystallization process. Acrylic acid (HAk) was chosen as an object of reserch. Quantum-chemical modeling was carried out in WinGAMESS program using nonemperical methods. Cluster systems were calculated using B3LYP DFT method. We used 6-31G** basis set for metal atoms and 6-311G basis set for the atoms of ligands. In the article [3] it was shown that this calculation methodology provides high adequacy of the chousen model to the real processes involving 3d metals. Parameters of the optimized structures are listed in the table I and II. TABLE I. Interatomic distances (Å) between nickel and oxygen atoms in the cluster [Niz +(H2O)5Ak-] Atom number Ligand z = +2 z = +1 z = 0 O1 Н2О 2.046 3.747 3.339 O2 Н2О 2.043 3.646 3.302 O3 Н2О 2.079 3.647 3.557 O4 Н2О 2.058 2.027 2.063 O5 Н2О 2.043 1.978 2.071 O6 СH2=CH-COO- 2.032 1.886 2.103 O7 СH2=CH-COO- 3.343 3.465 2.112 TABLE II. Interatomic distance (Å) between nickel atom and some atoms of ligands in the cluster [Ni0(H2O)5HAk] (distances of more than 4 Å are not given) Atom number Ligand σ-complex π-complex O1 Н2О 3.341 – O2 Н2О 3.446 – O3 Н2О 3.611 – O4 Н2О 2.001 2.009 O5 Н2О 2.049 1.984 O6 СH2=CH-COOН 2.064 2.030 O7 СH2=CH-COOН 2.228 – С3 СH2=CH-COOH – 2.020 For Ni2+ ions the optimized structure is an octahedral complex with almost equivalent Ni–O bonds (Table I). Transition to Ni+ ions is accompanied by significant reorganization of inner coordination sphere of the complex (Table I). The central atom directly bonds only with two from five water molecules. The newly-formed structure [Ni+(H2O)2Аk-] is characterized by irregular allocation of electron density. Bond distance between nickel atom and oxygen atom of carboxyl group decreases from 2.032 Å to 1.886 Å, which indicates that the bond becomes significantly stronger. As a result of complete reduction of nickel ions the central atom in the complex [Ni0(H2O)2Аk-] increases the coordination number from 3 to 4 (Table I). The second oxygen atom of the carboxyl group (O7) is attracted by nickel atom forming a strong bond which is approximately the same as Ni–O6 bond. At the same time, for Ni0complexes the structures with molecular form of HAk are also identified as stable. In these complexes there occurs σ- and π-coordination of HAk with nickel atom (Table II). Based on the analysis of energies of all listed structures we have proposed the most tipical reaction route of the process of Ni2+ions electroreduction in the presence of acrylic acid: +ē +ē [Ni2+(H2O)5Аk-] → [Ni1+(H2O)2Аk-] → [Ni0(H2O)2Аk-] [1] -3H2O +H3O+ [Ni0(H2O)2Аk-] → [Ni0(H2O)2(σ-HАk)] → [Ni0(H2O)2(π-HАk)] [2] It is obvious that external influences, which are realized during nickel electrodeposition, will change the balance of the above-mentioned forms. As a result they will have an effect on the properties of the metal phase which is formed in the presence of е n– type unsaturated organic substances. References G.A. Di Bari Nickel plating // Metal Finishing, p. 209 – 225, Vol. 103, № 6. (2005) E. Fisher. G. Werner. Metal π-Complexes. – p. 264, Elsevier publishing, New York (1968) V.A Seredyuk. and V.F Vargalyuk. Estimation of Reviability of Quantum-Chemical Calculations of Electronic Transitions in Aqua Complexes of Transition Metals // Russian Journal of Electrochemistry. Vol.44, № 10, p. 20 – 27. (2008)