Reduction of nitro-aromatic compounds (NACs) proceeds through intermediates with a partial electron transfer into the nitro group from a reducing agent. To estimate the extent of such a transfer and, therefore, the activity of various model ferrous-containing reductants toward NAC degradation, the unrestricted density functional theory (DFT) in the basis of paired Lowdin-Amos-Hall orbitals has been applied to complexes of nitrobenzene (NB) and model Fe(II) hydroxides including cationic [FeOH]⁺, then neutral Fe(OH)₂, and finally anionic [Fe(OH)₃]^-. Electron transfer is considered to be a process of unpairing electrons (without the change of total spin projection Sz) that reveals itself in a substantial spin contamination of the unrestricted solution. The unrestricted orbitals are transformed into localized paired orbitals to determine the orbital channels for a particular electron-transfer state and the weights of idealized charge-transfer and covalent electron structures. This approach allows insight into the electronic structure and bonding of the {Fe(PhNO₂)}⁶ unit (according to Enemark and Feltham notation) to be gained using model nitrobenzene complexes. The electronic structure of this unit can be expressed in terms of π-type covalent bonding [Fe⁺²(d⁶, S = 2) - PhNO₂(S = 0)] or charge-transfer configuration [Fe⁺³(d⁵, S = 5/2) - {PhNO₂}^-((π*)¹, S = 1/2)].

DENSITY FUNCTIONAL THEORY STUDIES OF NITROUS OXIDE ADSORPTION AND DECOMPOSITION ON Ga-ZSM-5

V.N. Solkan*, G.M. Zhidomirov, V.B. Kazansky*

(*Zelinsky Institute of Organic Chemistry, Moscow, Russia)

Int. J. Quantum Chem., 107(13) (2007) pp. 2417-2425.

In this study, density functional theory (DFT/B3LYP) was used to assess a possible reaction pathway for the catalytic dissociation of N₂O. The active centers were taken to be mononuclear [Ga]⁺ and [Ga=O]⁺, and the surrounding portion of the zeolite was represented by a 3T cluster, namely (AlSi₂O₄H₈). The first step of N₂O decomposition involves the formation of [GaO]⁺ and the release of N₂. The metaloxo species produced in this step then react with N₂O again, to release N₂ and form GaO₂. The calculated activation energies for N₂O dissociation in Ga-ZSM-5 and GaO-ZSM-5 at B3LYP/6-31+G* level are 22.2 and 24.9 kcal/mol, respectively. The calculated energy of the molecular oxygen elimination from 3T-(GaO₂) cluster is ΔH (298 K) = +46.5 kcal/mol and ΔG (298 K) = 35.9 kcal/mol.

SWIFT HOPPING GALLIUM [AlO₄]^-TETRAHEDRA IN Ga/ZSM - A DFT STUDY

I.V. Kusmin*, G.M. Zhidomirov, V.N. Solkan*

(*Zelinsky Institute of Organic Chemistry, Moscow, Russia)

Int. J. Quantum Chem., 107(13) (2007) pp. 2434-2441.

Density functional theory calculations were carried out to investigate gallium species (Ga⁺, [GaH₂]⁺, and [GaO]⁺) stabilization in Ga-exchanged HZSM-5, using cluster modeling approach. Three isomeric gallium positions over [AlO₄]^- zeolite fragment at T12 position were found. These isomers are turning into each over with low activation energy barrier and gallium fragment revolves around [AlO₄]tetrahedron by hopping between cationic positions. Activation energies of gallium fragment hopping were computed and compared for different gallium containing cations. Those barriers were found to be times less than the activation energies of catalytic processes on gallium-exchanged zeolite.

MODELS OF ACTIVE SITES IN SUPPORTED Cu METAL CATALYSTS IN 1,2-DICHLOROETHANE DECHLORINATION. DFT ANALYSIS

V.I. Avdeev, V.I. Kovalchuk*, G.M. Zhidomirov, J.L. d’Itri* (*University of Pittsburgh, Pittsburgh, USA)

J. Struct. Chem., supply_48 (2007) pp. S160-S170.

It is suggested that a set of discrete Cu nanoclusters satisfying the conditions of structural and electron stability should be used as models of active sites on supported metal catalysts. The close-packed Cu₂₀ tetrahedral nanocluster that satisfies these two conditions was taken as a base model of active sites on supported copper catalysts. Theoretical analysis of two possible mechanisms of C-Cl bond dissociation of 1,2-dichloroethane on copper catalysts was performed by the density functional theory method. The first mechanism involves sequential splitting of C-Cl bonds in the molecule in three stages with further stabilization of chloroalkyl intermediates (stepwise mechanism). All these stages are activated. The limiting stage is the one that corresponds to dissociation of the first C-Cl bond with an activation energy of E^# =34.3 kcal/mol. The second mechanism corresponds to the simultaneous elimination of two chlorine atoms from 1,2-dichloroethane with liberation of ethylene in the gas phase; this is a one-stage process with an activation energy of E^#=26.1 kcal/mol (direct mechanism). A comparison of the two reaction routes shows that the direct mechanism is most probable on copper catalysts.

DFT ANALYSIS OF THE MECHANISM OF 1,2-DICHLOROETHANE DECHLORINATION ON SUPPORTED Cu-Pt BIMETALLIC CATALYSTS

V.I. Avdeev, V.I. Kovalchuk*, G.M. Zhidomirov, J.L. d’Itri * (*University of Pittsburgh, Pittsburgh, USA)

J. Struct. Chem., supply_48 (2007) pp. S171-S183.

The reaction routes of 1,2-dichloroethane dechlorination to ethylene on discrete nanoclusters that served as models of the active sites of supported Cu-Pt catalysts were calculated. Two reaction pathways were predicted. The first route corresponds to sequential elimination of the chlorine atoms from 1,2-dichloroethane; this is a three-stage reaction that occurs via two stable intermediates (stepwise mechanism). The limiting stage is the stage that corresponds to the dissociation of the first C-Cl bond. The second channel corresponds to a simultaneous one-stage elimination of two chlorine atoms (direct mechanism). Both reaction routes are thermodynamically possible, but the stepwise process is more probable, in contrast to the process on monometallic Cu catalysts. For the stepwise process, the vibrational spectra of stable intermediates were calculated for identification of the latter. A set of spectral data characteristic for the stepwise mechanism were determined. The three-step molecular mechanism suggested for 1,2-dichloroethane dechlorination to ethylene is compared with several kinetic schemes known from the literature. Possible modifications of the reaction route that forms ethane and monochloroethane are analyzed.

DENSITY FUNCTIONAL THEORY MOLECULAR CLUSTER STUDY OF COPPER INTERACTION WITH NITRIC OXIDE DIMER IN Cu-ZSM-5 CATALYSTS

I.I. Zakharov,Z.R. Ismagilov, S.Ph. Ruzankin, V.F. Anufrienko, S.A. Yashnik, O.I. Zakharova*

(*East Ukrainian National University, Severodonetsk, Ukraine)

J. Phys. Chem. C, 111(7) (2007) pp. 3080-3089.

The various quantum chemical models of catalytic active site in Cu-ZSM-5 zeolites are analyzed. The density functional theory (DFT) is used to calculate the electronic structure of molecular cluster (HO)₃Al-O-Cu-O-Cu modeling the catalytic active site in Cu-ZSM-5 zeolites and study the interaction and decomposition of NO. It is assumed that the rate-determining stage of the low-temperature selective catalytic reduction of NO is the formation of the π-radical (N₂O₂)^-on electron donor sites of Cu-ZSM-5 catalysts. This is in good agreement with the high electron affinity of the molecular dimer ONNO (E_a = -1.5 eV) and is confirmed by the experimental data on the formation of surface anion π-radical (N₂O₂)^- on electron donor sites of supported organo-zirconium surface complex. The DFT calculated electronic structure and excitation energy spectra for the model system (HO)₃Al-O-Cu-O-Cu show that it is a satisfactory model for description of experimental UV-vis spectra of Cu-ZSM-5, containing (-O-Cu-O-Cu-) chain structures in the zeolite channels. The calculated reaction energy profile of ONNO adsorption and decomposition on the model catalytic active site shows the possibility of the low-temperature decomposition of dimer (NO)₂ with low activation energy and the important role of copper oxide chains (-O-Cu-O-Cu-) in the channels of Cu-ZSM-5 zeolite during selective reduction of NO.

DFT QUANTUM-CHEMICAL CALCULATIONS OF NITROGEN OXIDE CHEMISORPTION AND REACTIVITY ON THE Cu(100) SURFACE

I.I. Zakharov, A.V. Suvorin*, A.I. Kolbasin*, O.I. Zakharova* (*East Ukrainian National University, Severodonetsk, Ukraine)

J. Struct. Chem., supply_48 (2007) pp. S147-S159.

A DFT quantum-chemical study of NO adsorption and reactivity on the Cu₂₀ and Cu₁₆ metal clusters showed that only the molecular form of NO is stabilized on the copper surface. The heat of monomolecular adsorption was calculated to be ΔH_m = −49.9 kJ/mol, while dissociative adsorption of NO is energetically unfavorable, ΔH_d = +15.7 kJ/mol, and dissociation demands a very high activation energy, E_a = +125.4 kJ/mol. Because of the absence of NO dissociation on the copper surface, the formation mechanism of the reduction products, N₂ and N₂O, is debatable since the surface reaction ultimately leads to N-O bond cleavage. As the reaction occurs with a very low activation energy, E_a = 7.3 kJ/mol, interpretation of the NO direct reduction mechanism is both an important and intriguing problem because the binding energy in the NO molecule is high (630 kJ/mol) and the experimental studies revealed only physically adsorbed forms on the copper surface. It was found that the formation mechanism of the N₂ and N₂O reduction products involves formation (on the copper surface) of the (O_adN-NO_ad) dimer intermediate that is chemisorbed via the oxygen atoms and characterized by a stable N-N bond (r_N-N ~1.3 Å). The N-N binding between the adsorbed NO molecules occurs through electron-accepting interaction between the oxygen atoms in NO and the metal atoms on the “defective” copper surface. The electronic structure of the (O_adN-NO_ad) surface dimer is characterized by excess electron density (ON-NO)^δ− and high reactivity in N-O_ad bond dissociation. The calculated activation energy of the destruction of the chemisorbed intermediate (O_ad N-NO_ad) is very low (E_a = 5–10 kJ/mol), which shows that it is kinetically unstable against the instantaneous release of the N₂ and N₂O reduction products into the gas phase and cannot be identified by modern experimental methods of metal surface studies. At the same time, on the MgO surface and in the individual (Ph₃P)₂Pt(O₂N₂) complex, a stable (O_adN-NO_ad) dimer was revealed experimentally.

QUANTUM CHEMICAL EXAMINATION OF INTERACTION OF CYTOSTATIC FLUOROURACIL WITH DEOXYRIBONUCLEIC ACIDS

G. Yuldasheva*, G.M. Zhidomirov (*Biochemistry, National Centre of Examination of Medical Products, Almaty, Kazakhstan)

Int. J. Quantum Chem., 107(13) (2007) pp. 2384-2388.

Within the framework of semiempirical method of quantum chemical PM3, the possibility of formation of paired stack structures under interaction of fluorouracil with pyrimidine and purine nitrogenous bases of nucleotides has been examined. Possible mechanism of transformation of 2-deoxyuridine-5-monophosphate into metabolite-5fluorin-2-deoxyuridine-5-monophosphate has been given. The calculations that were made allow to suppose that biotransformation of 5-FU in 5-fluorin-2deoxyuridine-5-monophosphate, most likely, is carried out not in free nucleotides, but in the structure of DNA in two nucleotide triplets UUC and UGU, including the case when directly two nucleotides of deoxyuridine monophosphate, are transformed into 5-fluorin-2-deoxyuridine-5-monophosphate.

Cytostatic ability of 5-FU is increased by its capacity to be selectively embedded into nucleotide triplets creating new chemical compounds that violate matrix RNA formation and accordingly violate protein synthesis.

CHEMICAL BONDING AND ELECTRONIC STRUCTURE OF LaMnO₃ AND La_0.75MnO₃ ORTHORHOMBIC CRYSTALS

V.M. Tapilin

J. Struct. Chem., 48(2) (2007) pp. 212-218.

The electronic structure of the LaMnO₃ orthorhombic crystal of a stoichiometric composition and of La_0.75MnO₃ crystals with a La vacancy in the unit cell is calculated in the LSDA+U approximation of density functional theory. The calculations showed that LaMnO₃ is an insulator with a forbidden gap of 0.5 eV and with antiferromagnetic ordering of magnetic moments. The magnetic moment on the manganese ions is 3.78 BM. The La atom has ionic bonds in the lattice, while the bond between oxygen and manganese is covalent. After lanthanum has been removed, geometry optimization of the unit cell leads to La_0.75MnO₃ stable structures. In one of the structures, which is lower in energy, the states of manganese may be attributed to Mn⁴⁺ ions. In both structures with removed lanthanum, the oxygen ions have reduced effective charge, so that one can speak about O- ions appearing along with O^2- in the structure. The oxygen, as well as lanthanum and manganese, ions are nonequivalent in these structures; their nonequivalence is primarily reflected by the local densities of states. This leads to charge and magnetic nonequivalence of ions. In La_0.75MnO₃ crystals, the degree of bond covalence between manganese and oxygen decreases.

THEORETICAL AND EXPERIMENTAL DAPS STUDY OF CLEAN, OXYGEN, AND HYDROGEN COVERED Pt(100) SINGLE CRYSTAL SURFACE

A.R. Cholach, V.M. Tapilin

Phys. Chem.: An Indian J., 2(1) (2007) 6 pages.

Local density of states is calculated for the three-fold Pt(100) slab surface covered with various coverage of hydrogen and oxygen. The formation of new surface states region below the platinum conduction band is responsible for covalent contribution to the bond formation between adsorbed species and surface Pt atoms. Resonant states around the substrate Fermi level give an ionic constituent to the adsorbate-surface chemical bond due to considerable DOS difference between adsorbed and surface platinum atoms. Calculated and experimental extended disappearance potential spectra related to different H and O coverage are in a good agreement in spite of baffling complexity of spectral structures. Remarkable similarity between theoretical and experimental results evidences for reliability of obtained data on the LDOS structure of adsorbed system considered.

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