Identification of Cu2(N2) and Cu2(N2)2 Complexes: Matrix Isolation and Density Functional Studies

Metal Cluster Models for Heterogeneous Catalysis: A Matrix-Isolation Perspective

Metal cluster models are of high relevance for establishing new mechanistic concepts for heterogeneous catalysis. The high reactivity and particular selectivity of metal clusters is caused by the wealth of low-lying electronically excited states that are often thermally populated. Thereby the metal clusters are flexible with regard to their electronic structure and can adjust their states to be appropriate for the reaction with a particular substrate. The matrix isolation technique is ideally suited for studying excited state reactivity. The low matrix temperatures (generally 4-40 K) of the noble gas matrix host guarantee that all clusters are in their electronic ground-state (with only a very few exceptions). Electronically excited states can then be selectively populated and their reactivity probed. Unfortunately, a systematic research in this direction has not been made up to date. The purpose of this review is to provide the grounds for a directed approach to understand cluster reactivity through matrix-isolation studies combined with quantum chemical calculations.

Reactivity of Tantalum Carbide Cluster Anions TaC n- ( n = 1-4) with Dinitrogen

Dinitrogen activation/fixation is one of the most important and challenging subjects in synthetic as well as theoretical chemistry. In this study, the adsorption reactions of N₂ onto TaC _n^- ( n = 1-4) cluster anions have been investigated by means of mass spectrometry in conjunction with density functional theory calculations. Following the experimental results that only TaC₄^- was observed to adsorb N₂, theoretical calculations predicted that the TaC₄^- reaction system (TaC₄^- + N₂ → TaC₄N₂^-) has a negligible barrier on the approach of N₂ molecule while insurmountable barriers are located on the reaction pathways of TaC_1-3^-/N₂ reaction systems. The natural bond orbital and molecular orbital analysis indicated that the more positive charge on the metal center of TaC_1-4^- would facilitate the initial approach of the nonpolar N₂ molecule, and the appropriate frontier orbital of TaC_1-4^- with proper symmetry (π-type 5d orbital) which can match up well with the π* antibonding orbital of the N₂ molecule with less σ repulsion and more possibility for π back-donation would be helpful for the formation of the stable encounter complexes. This study reveals the fundamental rules and key factors governing the N₂ adsorption.

CO-promoted N(2) adsorption on copper atoms

Reactions of laser-ablated copper atoms with carbon monoxide and dinitrogen in excess argon have been investigated by matrix-isolation infrared spectroscopy. The NNCuCO and (NN)(2)CuCO complexes, as well as copper carbonyls, are formed as reaction products during sample deposition and further annealing whereas no copper dinitrogen complexes were observed. These carbonylcopper dinitrogen complexes are characterized on the basis of the results of the isotopic substitution and the reagent concentration change. Density functional theory calculations of the geometry structures, vibrational frequencies, relative absorption intensities, and isotopic shifts strongly support the experimental assignments. The experimental results reveal promoted adsorption of N(2) on Cu atoms by preadsorbed CO. The theoretical results for reaction characteristics between the coadsorbed CO and N(2) agree with the experimental findings. The joint investigations provide insights with regard to CO and N(2) cooperative adsorption effects and consequent reaction mechanisms.

Assessment of the CCSD and CCSD(T) Coupled-Cluster Methods in Calculating Heats of Formation for Cu Complexes

Heats of formation for nine complexes of the form CuX(n) (X = Cu, H, O, OH, S, F, F(2), Cl, Cl(2)) were calculated using the CCSD and CCSD(T) coupled cluster methods with the 6-31G** and TZVP basis sets as well as the LANL2DZ basis set/pseudopotential on Cu with both the 6-31G** and TZVP basis sets applied to the nonmetal atoms. These values were compared with literature heat of formation values. A second order Douglas-Kroll-Hess relativistic correction was applied at the CCSD/TZVP and CCSD(T)/TZVP levels of theory. Overall, the CCSD(T)/TZVP level of theory with the relativistic correction was most suited for the heat of formation calculations possessing low absolute average error and RMSD and the ability to analyze each copper complex, except for the problematic case of copper(II) fluoride. Finally, experimental geometric parameters were compared with the calculated structures in such cases where these data were available. None of the investigated levels of theory predicted bond lengths consistently better than other methods, and it was determined that the most accurate bond length does not necessarily result in the most accurate calculated heat of formation value for a given complex.

Infrared Spectra and Structures of the OSc(N2), OScNN, and OScNN+ Complexes in Solid Argon

Scandium monoxide-dinitrogen complexes-OSc(N2), OScNN, and OScNN+-have been prepared by the reactions of laser-evaporated scandium monoxide with N2 or scandium atoms with N2O in solid argon. The ground-state scandium monoxide molecule reacted with N2 to form the side-bonded OSc(N2) complex spontaneously on annealing. This complex rearranged to the end-on bonded OScNN complex upon UV irradiation. Both the OSc(N2) and OScNN complexes in solid argon can be assigned to have 2A' ' electronic ground state with Cs symmetry arising from the 2Delta first excited-state ScO. The neutral complexes can also be photoionized to the OScNN+ cation complex upon UV irradiation.

Formation and Characterization of the (η2-N2)Sc(H)OH and (η-NN)xSc(H)OH (x = 1, 2) Complexes in Solid Argon

Hydrido scandium hydroxide dinitrogen complexes, (eta2-N2)Sc(H)OH and (eta1-NN)xSc(H)OH (x = 1, 2), have been prepared by the reactions of laser-ablated scandium atoms with H2O/N2 mixtures in solid argon. The end-on bonded (eta1-NN)xSc(H)OH (x = 1, 2) complexes were formed spontaneously on annealing, whereas the side-bonded (eta2-N2)Sc(H)OH complex was generated on broad-band irradiation. These complexes were characterized by infrared absorption spectroscopy as well as density functional theoretical calculations.