A. F. Shestakov

Isotopic effect on thermal mobility of atomic hydrogen in solid xenon

We have studied thermal mobility of atomic hydrogen in solid Xe using decomposition of water molecules as a source for hydrogen atoms. The formation of various isotopomers of HXeH and HXeOH is monitored at temperatures from 37 to 42 K by using infrared absorption spectroscopy, and the activation energy of this diffusion-controlled process is found to be ∼110 meV. Most importantly, the different mobility for hydrogen isotopes is demonstrated, H being faster than D, and the difference between the corresponding activation energies is estimated to be ∼4 meV. The electron paramagnetic resonance measurements of the thermal decay of H atoms and OH radicals show that the formation of HXeH and HXeOH is controlled by hydrogen mobility. The modeling of thermally activated jumps of hydrogen atoms in a relaxed Xe lattice agrees reasonably with experiment with respect to the isotopic effects but it underestimates the jump rate.

Quantum chemical modeling of CO oxidation by the active site of molybdenum CO dehydrogenase.

The catalytic mechanism of molybdenum containing CO dehydrogenase has been studied using hybrid DFT methods with quite large chemical models. The recent high‐resolution X‐ray structure, showing the surprising presence of copper linked to molybdenum, was used as a starting point. A pathway was initially found with a low barrier for CO bond formation and CO2 release. However, this pathway did not include the formation of any SCO2 species, which had been suggested by experiments with an n‐butylisocyanide inhibitor. When these SCO2 structures were studied they were found to lead to deep minima, making CO2 release much more difficult. A large effort was spent, including investigations of other spin states, varying the number of protons and electrons, adding water, etc., until a plausible pathway for SC bond cleavage was found. In this pathway a water molecule is inserted in between molybdenum and the SCO2 group. Full catalytic cycles, including electron and proton transfers, are constructed both with and without SC bond formation. When these pathways are extended to two full catalytic cycles it can be understood why the formation of the SC bond actually makes catalysis faster, even though the individual step of CO2 release becomes much more difficult. These results agree well with experimental findings.

Formation of H2O2 on Au20 and Au19Pd Clusters: Understanding the Structure Effect on the Atomic Level

Supported gold nanoparticles are promising catalysts for production of H2O2 from O2 and H2. Size, structure, and palladium doping effects play the key role in activity and selectivity of a gold catalyst. We performed a study of the influence of Au20 and Au19Pd structure features on the main steps of H2O2 formation on the atomic level, using the DFT/PBE approach with relativistic all electron basis set. The top, edge, and facet atoms of the tetrahedral Au20 cluster as well as a palladium atom of Au19Pd located on the top, edge, and facet of a tetrahedron have been considered as active sites of steps involved in H2O2 synthesis. The thermodynamic and kinetic data including Gibbs free energies and the activation Gibbs free energies were calculated for the steps determining the formation of H2O2 (H(s) + OOH(s) = H2O(2(s)), H2O(2(s)) = H2O(2(g))) and for one step decreasing the selectivity (H2O(2(s)) = OH(s) + OH(s)). Gold tends to have low activity and high selectivity in H2O2 synthesis regardless of the structure of active site. Low coordinated palladium atoms promote H2O2 formation as well as its dissociation. Pd on a facet of a cluster facilitates H2O2 production with high activity and selectivity.

Electroluminescent properties of lanthanide pentafluorophenolates

Lanthanide pentafluorophenolates Ln(OC6F5)3(L)x (Ln = Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb; L = 1,10-phenanthroline, 2,2′-bipyridine; x = 1 and 2) are used as emissive layers in organic light emitting devices (OLEDs). Single-layer ITO/Ln(OC6F5)3(L)x/Yb devices reveal no electroluminescence (EL) with the exception of Tb-derivative-based OLEDs. Bilayer ITO/TPD/Ln(OC6F5)3(L)x/Yb samples exhibit a broad band emission peaked at 580 nm assigned to an electroplex at the TPD/complex interface. Besides, the spectra of the devices based on Pr, Nd, Sm, Eu, Er, Tb and Yb derivatives contain the characteristic narrow bands of f–f transitions. Terbium-based bilayer OLEDs exhibit unusual changes in the EL spectra with increasing the applied voltage. The emission color of the devices tunes from orange towards green. The possible nature of the phenomenon is discussed.

Hydrocarbon Adsorption on Gold Clusters: Experiment and Quantum Chemical Modeling

Heats of adsorption Q of n-alkanes C6–C9 on ZrO2 modified with gold and nickel nanoparticles were determined experimentally. The Q values were found to be higher on average by 7 kJ/mol on the modified samples as compared to the pure support. Density functional theory with the PBE functional and the pseudopotential for gold effectively allowing for relativistic corrections was used to model the adsorption of saturated hydrocarbons on Au and Au + Ni, as exemplified by the interaction of alkanes C1–C3 with Aum, Aum − 1Ni (m = 3, 4, 5) clusters as well as the interaction of C1–C8 with Au20. Based on the calculation results, the probable coordination centers of alkanes on nanoparticle surfaces were found to be vertices and edges, whereas face localization was less probable.

The structures of the dicationic tetranitrosyl iron complex with cysteamine [Fe2S2(CH2CH2NH3)2(NO)4]2+ and its decomposition products in protic media: an experimental and theoretical study

Decomposition products of [Fe2S2(CH2CH2NH3)2(NO)4]SO4·2.5H2O (1′) were studied by electrochemistry and mass spectrometry. The structures of the dicationic tetranitrosyl iron complex with cysteamine of the composition [Fe2S2(CH2CH2NH3)2(NO)4]2+ (1) and possible products of its decomposition and NO replacement by an aqua ligand were studied by quantum chemical methods at the density functional theory level. Taking into account the solvation effects, the replacement of the nitrosyl ligand in dication 1 by an aqua ligand was found to be less favorable in aqueous solution than in the gas phase. The pK value was calculated for the proton abstraction from the NH3 group of compound 1 (7.2), and the removal of NO from the deprotonated form of the complex was found to be much easier. This result is consistent with the experimental data on an increase in the rate of NO formation in aqueous solutions of 1 with increasing pH from 6 to 8 assuming that the increase in pH is accompanied by an increase in the percentage of the less stable deprotonated form of the complex and that OH− does not participate in the elementary step of NO formation. The kinetic curves of NO formation are well described by a two-step scheme of consecutive first-order reactions of the NO formation and consumption. In the gas phase, dication 1 was found to be unstable to decomposition into two mononuclear cationic dinitrosyl iron complexes with cysteamine. This result is consistent with the fact that these cations are observed in the electrospray ionization mass spectrometric experiment. The major peak in the mass spectra is associated with the [Fe2S2(CH2CH2NH3)2(NO)4 − H]+ ion. As follows from the calculations, this is due to the deprotonation of the dication as it gets rid of the hydration shell, because even the dimer of water molecules is more basic than dication 1.

Geometry of the Transition State of Radical Abstraction Reactions Involving Si-H, Ge-H, and Sn-H Bonds

Transition-state interatomic distances in the reactions C˙H3+SiH4, Si˙H3+SiH4, C˙H3+GeH4, and C˙H3+SnH4 are calculated by the B3LYP density functional and intersecting parabolas methods. A semiempirical algorithm is developed for the calculation of the Y...H and C...H distances in the transition state of the radical abstraction reactions R˙+YH involving silanes, germanes, and stannanes and the reverse reactions of silyl, germanyl, and stannyl radicals with hydrocarbons. This algorithm is used to calculate interatomic distances in these reactions. An analysis of the calculated data shows that the Y...H and C...H distances in these reactions depend on the following factors: the enthalpy of reaction, the radius of the Y atom (Y = C, Si, Ge, Sn), and four-electron repulsion during the attack of a radical on the C-H bond adjacent to the double bond. Empirical equations relating the interatomic distances to the enthalpy of reaction and to the Y-R bond length are set up.

Free radical abstraction of H atom from peroxides with concerted dissociation of O-O bond

Quantum chemical calculations of the dissociation energy of the C-H bond in the α-hydroperoxide fragment of Me2CHOOH were carried out. It was shown that abstraction of H atom is accompanied by dissociation of the O-O bond. Density functional calculations of transition states of the reactions of ·CH3, CH3OO·, and HO2· radicals with the C-H bond in the α-hydroperoxide fragment of Me2CHOOH were carried out. It was established that H atom abstraction is accompanied by concerted dissociation of the O-O bond. For 45 peroxides R1R2CHOOH, R1R2CHOOR3, and R1R2CHOOC(O)R3 (R1, R2 = H, Me, Et, Ph, H2C=CH), the enthalpies of H atom abstraction from the C-H bond in the a-hydroperoxide fragment with fragmentation of the peroxides at the O-O bond were calculated. The kinetic parameters for 12 classes of radical abstraction reactions with fragmentation of molecules were calculated from experimental data within the framework of the model of intersecting parabolas. The activation energies and reaction rate constants of H atom abstraction from C-H bonds of a-peroxide fragments involving peroxyl and alkyl radicals were determined for 45 peroxides of different structure.

Formation of {Co(dppe)}2{μ2-η2:η2-η2:η2-[(C60)2]} Dimers Bonded by Single C–C Bonds and Bridging η2-Coordinated Cobalt Atoms

Coordination of two bridging cobalt atoms to fullerenes by the η(2) type in {Co(dppe)}2{μ2-η(2):η(2)-η(2):η(2)-[(C60)2]}·3C6H4Cl2 [1; dppe = 1,2-bis(diphenylphosphino)ethane] triggers fullerene dimerization with the formation of two intercage C-C bonds of 1.571(4) Å length. Coordination-induced fullerene dimerization opens a path to the design of fullerene structures bonded by both covalent C-C bonds and η(2)-coordination-bridged metal atoms.

Structures of bis(1-methyltetrazole-5-thiolato)(tetranitrosyl)diiron and its intermediates in solutions

Single crystals of an iron complex with 1-methyltetrazole-5-thiol of the formula [Fe2(SC2H3N4)2(NO)4] (I) were obtained and examined by X-ray diffraction. According to electrochemical data, tetranitrosyl binuclear complex I rapidly decomposes in protic solvents with elimination of NO. The maximum amount of NO generated by complex I in 1% aqueous DMSO is ∼900 nmol. This amount is reduced by half 15 min after the beginning of the decomposition under anaerobic conditions. The dinitrosyl mononuclear intermediates [Fe(SC2H3N4)2(NO)]− and [Fe(SC2H3N4)2(NO)2]− were detected in solutions and identified by EPR spectroscopy and mass spectrometry. The low number of spins per complex in solutions indicates that the mononuclear complexes undergo further decomposition into NO and the species [Fe(SC2H3N4)3]−, [SC2H3N4]−, and [Fe4S3(NO)7]−. Complex I was found to be substantially more stable in DMSO than in methanol and 1% aqueous DMSO.