ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2014, Vol. 88, No. 3, pp. 471–477. © Pleiades Publishing, Ltd., 2014. Original Russian Text © L.V. Serebrennikov, A.V. Golovkin, D.I. Davlyatshin, A.L. Serebrennikova, 2014, published in Zhurnal Fizicheskoi Khimii, 2014, Vol. 88, No. 3, pp. 473–479. 471 INTRODUCTION Studying chemical reactions that involve small clusters of transition metals is of unquestionable inter- est, particularly for understanding and modeling mechanisms of catalysis on metal surfaces [1]. How- ever, data on both nickel clusters and their reactions with elementary molecules are rather sparse. Small clusters of Ni 3 and Ni 4 were studied earlier by spectro- scopic and quantum chemical methods [2, 3]; their reactions with methane and water molecules were investigated in [4, 5]. This work presents the results of quantum chemical studies on reactions between acet- ylene and atoms and small clusters of nickel. No sys- tematic computational studies of the Ni n + C 2 H 2 sys- tems had been performed earlier. In [6, 7], Ni and Ni 2 + C 2 H 2 were investigated using IR spectra and argon matrices. Bands at 1600–1700 cm –1 were detected and attributed to the C=C vibrations in NiC 2 H 2 , NiCCH 2 , Ni(C 2 H 2 ) 2 , and Ni 2 C 2 H 2 com- plexes. The structure of the reaction products was dis- cussed. It was assumed that all of the complexes except for NiCCH 2 (observed during photolysis) were formed by nickel atoms coordinating acetylene molecules during the formation of matrices. Two C–H bonds were retained in all of the reaction products, with the frequency of vibrations at 1600–1700 cm –1 corre- sponding to the carbon–carbon double bond. Mean- while, experimental and computational studies of the Ni 3 + CH 4 system [4] showed that the HNi 3 CH 3 struc- ture corresponds to the ground state. The Ni 3 CH 4 complex lies 3.4 kcal/mol higher (according to its total energy) and is separated from the global minimum by an 8 kcal/mol barrier. The characteristic bands belonging to bridge hydrogen atoms coordinated on Ni–Ni bonds were observed in the IR spectra. The objectives of this study were therefore to determine the possibility of the formation of such structures as HNi n CCH, and to study their energies with respect to Ni n C 2 H 2 complexes along with the activation energies of isomerization in Ni n + C 2 H 2 systems. METHODS OF COMPUTATION High-level nonempirical calculations for systems with several nickel atoms are currently unavailable. At the same time, it has been demonstrated [8, 9] that the density functional theory allows us to attain quite ade- quate results. The GAUSSIAN 09W software package was used in this work to calculate the electron energy values, equilibrium geometric structures, and vibra- tion frequencies of nickel clusters with acetylene mol- ecules [10]. Problems associated with SCF conver- gence are known to frequently arise for systems con- taining nickel when performing full-electron calculations with the 6-31++G(d, p) basis or higher. We therefore used different models (including pseudo- potential calculations) in order to find all of the extrema on the potential energy surface (PES). It turned out that convergence problems also arose if the pseudopotential was specified for nickel atoms only, while the full-electron form was used for the rest of the atoms. As a result, several versions of the B3LYP den- sity functional theory were used for calculations: with the bases 6-311++G(d, p) (I) and cc-pVTZ (II) for all the atoms; the pseudorelativistic pseudopotential ECP10MDF [11, 12] and the built-in consistent basis 8s7p6d2f 1g for nickel atoms and 6-311++G(d, p) for carbon and hydrogen atoms (III); the pseudorelativis- tic pseudopotential ECP10MDF and the basis 8s7p6d2f 1g for nickel atoms, ECP2MWB [13] for car- bon atoms, and 6-311++G(d, p) for hydrogen atoms (IV). In addition, some structures were calculated using the M06 density functional [14] in the 6- 311++G(d, p) basis (V). If not specified in the text, all Quantum Chemical Calculations and the Structure of Ni n (C 2 H 2 ) Complexes (n = 1–4) L. V. Serebrennikov a , A. V. Golovkin, D. I. Davlyatshin, and A. L. Serebrennikova Department of Chemistry, Moscow State University, Moscow, 119991 Russia e-mail: lvs@phys.chem.msu.ru Received April 9, 2013 Abstract—Complexes of nickel atoms and small clusters with acetylene molecules are studied within the density functional theory. A trend toward the predominant formation of structures with bridge hydrogen atoms is observed in reactions between Ni n and acetylene with rising n. Keywords: nickel, clusters, matrix isolation, density functional theory. DOI: 10.1134/S0036024414020228 STRUCTURE OF MATTER AND QUANTUM CHEMISTRY