G. M. Zhidomirov

DFT study of oxygen-bridged Zn2+ ion pairs in Zn/ZSM-5 zeolites

A cluster model for Zn/ZSM-5 zeolite is proposed, which consists of an oxygen-bridged [Zn–O–Zn]2+ moiety attached to two framework aluminum ions of two adjacent ZSM-5 5-rings. Its stability and catalytic activity in ethane dehydrogenation were considered using the DFT method and compared with those for single Zn2+ ions in the same rings. It is shown that the oxygen-bridged Zn2+ pair is rather reactive towards ethane dissociation and that the rate-limiting step is release of hydrogen.

Cluster Model DFT Study of the Intermediates of Benzene to Phenol Oxidation by N2O on FeZSM-5 Zeolites

An Fe(II) ion at an α-cation exchange position of ZSM-5 zeolite (Fe/Z) was taken as a model for the active site in the nitrous oxide decomposition and in the selective oxidation of phenol with nitrous oxide. The oxygen deposited by decomposition of N2O is commonly referred to as α-oxygen (OFe/Z). Cluster model DFT calculations show that the interaction of the OFe/Z center with benzene resulted easily in arene oxide formation. The results indicate a rather low activation energy for this step. Possible transformations of the adsorbed arene oxide are considered and the experimental evidence for the absence of the kinetic H/D isotope effect in phenol formation is discussed. It is concluded that the rate-limiting step for the in situ oxidation of benzene to phenol is the desorption of the product.

Effect of cisplatin binding on guanine in nucleic acid: an ab initio study

Abstract To investigate the molecular mechanism of the antitumour activity of cisplatin, ab initio Hartree-Fock pseudo potential calculations have been performed for some Pt(II) complexes with the guanine nucleic base. We propose that cisplatin binding to guanine in DNA leads to a point mutation as the latter is the only change that the nucleic acid could “remember” after replication. Two ways of producing the point mutation are examined. The first is shifting the keto-enol tautomerization of guanine towards the enol form (rare in DNA) in the presence of cisplatin. The second is the action of cisplatin on the guaninecytosine pairing in DNA through binding to the O6 site of guanine which is involved in one of three hydrogen bonds between these nucleic bases. For this purpose the complex of cis-Pt(NH3)32+ and the hydrogen-bonded formamide-formamide pair has been calculated. The results obtained suggest, first, that the keto-enol tautomerization is not subject to crucial change in the presence of cisplatin. Second, it is found that the oxygen of the formamide coordinated by Pt(II) is no longer available for hydrogen bonding with the corresponding part of the complementary molecule. Hence, one can predict that binding of cisplatin to the O6 site of guanine breaks the corresponding hydrogen bond between guanine and cytosine which, in turn, can cause the point mutation.

A DFT quantum‐chemical study on the structures and active sites of polymethylaluminoxane

DFT quantum-chemical calculations have been performed to elucidate the geometrical and electronic structure of methylalumoxanes (-Al(Me)0-)n with different size (n=6,8,12). The three-dimensional oxo-bridged (cage) structures of methylalumoxane (MAO) have been analyzed.

A density functional study of CO adsorption on three- and five-coordinate Al in oxide systems

Conventional cluster models of strong and medium strength Lewis acid sites in alumina and zeolites, three-coordinate [Al(OH)3] and five-coordinate [Al(OH)3(OH2)2] respectively, are studied with the help of a density functional method. A constraint space orbital variation analysis reveals that the charge transfer from probe CO molecules adsorbed on cationic Al centre and the CO polarization comprise essential contributions to the adsorption energyDe. An analysis of the adsorption-induced C-O frequency shiftδΩ(C-O) is also provided. Structural modifications of the Lewis acid sites are considered with respect to their influence onDe andδΩ(C-O). A comparison of the measured and calculated C-O frequency shifts supports a hypothesis on the existence of Lewis acid sites in oxides in the form of four-coordinate Al cations.

Study of high-temperature oxygen states on the silver surface by XPS and UPS

XPS and UPS methods have been used to study the nature of the oxygen states on silver. Three oxygen states with high Eb(O1s)=530.5 eV(Oepox), 530.9 eV (Oβ), 532.0 eV (Ores)can be reliably attributed to different adsorbed species of quasimolecular oxygen. These states are characterized by an unusually high thermostability(T>800 K) and a subsurface location.

Non-empirical cluster model calculations of the adsorption of H2O on Ni(111)

H2O adsorption on a Ni(111) surface is calculated by an ab initio method taking into account four valence GVB-pairs of water molecules. Ni(111) is simulated by the surface cluster Ni10, calculated with a modified effective potential (MEP). The calculated structure of an adsorbed H2O monomer with an inclined molecular axis (α ≈ 35°-60°) and an adsorption energy Eads ≈ 50–70 kJ/mol is in agreement with the experimental results. The critical effect of the d-orbitals of the oxygen atom on the structure of the adsorbed H2O monomer, based on these calculations, is recognized. Molecular complexes of H2O and H2S with Li are also calculated. For the first time a quasi-tetrahedral structure is obtained for these complexes.

Quantum chemical study of the electronic structure of the NiMoS2 hydrodesulfurization catalysts

Abstract The role of the electronic state of the Ni atoms in the sulfide catalysts is studied by means of ab initio molecular orbital calculations. It is shown that the Ni ion with d8 electron configuration in the square-planar sulfur surrounding is not active in the hydrodesulfurization (HDS) process. The d8-state can be transformed to the d6-state after adsorption of the H2S molecule and formation of the square-pyramidal surrounding for the nickel ion. A square-pyramidal structure of the H2S adsorption complex on Ni MoS 2 catalysts with Ni(d6) is calculated as the active center. The HDS catalytic cycle, for which the H2S adsorption complex is the initial and final state, is proposed.

Comparative energies of Zn(II) cation localization as a function of the distance between two forming cation position aluminium ions in high-silica zeolites

Periodical calculations of Zn(II) metal cation stabilization in cationic positions with distantly placed aluminium ions has been performed for high-silica ferrierite. It was found that decrease of the stabilization energy at large distances between Al ions (more than 10 Å) is about of 2 eV in comparison with nearest possible position of two Al ions in the zeolite lattice and weekly depended on following increase of the Al-Al distance. Main changes in stabilization energy occured within a 3-Å interval of these distances. Only for the localizations of both Al ions in one zeolite ring zinc cationic form is more stable than hydrogen form.

Quasimolecular stable forms of oxygen on silver surface. Theoretical analysis by the density functional theory method

Abstract A quantum chemical investigation of possible associative oxygen forms on defective silver surface has been performed with the use of gradient-corrected density functional theory (DFT) in the cluster approximation. A surface defect was simulated by a cation vacancy (V). Cluster models (M1: Ag12 and M2: O3Ag12) of an active adsorption site (AS) with a vacancy were chosen on the basis of the structure of high-temperature layered silver oxide. The adsorbed layer on the defective surface was simulated by three oxygen atoms. For the atomic and associative (molecular) forms, the computations gave two stable structures of the adsorbed layer with C 3V and C 2V symmetry. The quasimolecular C 2V structure –Ag–O–O ep –O–Ag– resembling metal ozonides is by 44 kcal/mol more stable than the atomic C 3V structure. The C 2V structure is singlet and has an electrophilic epoxidizing oxygen atom O ep . A theoretical estimation of the density of states (DOS) in the quasimolecular oxygen form revealed a complex structure below the 4d band. Additional DOS peaks in this region are due to associative O–O bonds.