A. Yu. Ermilov

Predicting lanthanide cluster properties: a comparison with the observed optical spectra of Ho2

Abstract Ab initio pseudopotential calculations for Ho and Ho 2 have been carried out in order to support an assignment of the bands observed in UV-visible spectra of matrix isolated holmium species. SCF, MCSCF and configuration interaction (CI) procedures have been used with quasirelativistic pseudopotentials to compute the ground and excited state energies of Ho and Ho 2 , together with the dipole transition moments. For Ho 2 , using a Q = 11 pseudopotential (describing the holmium atom in the 4f 10 6s 2 5d 1 electronic state), two transitions from the ground state σ g 2 σ u 2 π u 2 to the states with principal excitations σ u → π g and π u → σ g ∗ are predicted at 499 and 524 nm. These two lines, with predicted close intensities, correlate nicely with the observed features at 498 504 and 558 563 nm in the spectrum of matrix-isolated Ho 2 .

Modeling chemical transformations at the active sites of cholinesterases by quantum-based simulations

The significance of the quantum-mechanical–molecular-mechanical (QM/MM) method in modeling chemical transformations at the active sites of cholinesterases is discussed. Diverse versions of the QM/MM approach are applied to understand the molecular mechanisms of the reactivation reaction of butyrylcholinesterase phosphorylated by the catalytic serine residue.

Formation of mixed d- and f-block metal clusters in inert matrices: comparison of the observed and theoretical optical spectra of AgHo.

The UV visible spectra obtained after simultaneous cocondensation of silver and holmium atoms with argon matrices at 9 K have been studied in the 200-800 nm region. While no new feature can be observed upon deposition, selective irradiation into both silver or holmium atomic absorptions results in growth of a new band at 430 nm, associated with formation of a mixed silver holmium species, tentatively assigned as AgHo. To support the assignment of the observed bands ab initio quantum chemical calculations were carried out for the dinuclear and trinuclear silver and holmium species, using pseudopotential approaches. Results for the electronic excitation energies and corresponding transition dipole moments for the diatomic molecules Ag2, Ho2, AgHo provide evidence that the 430 nm band should be attributed to the mixed cluster AgHo (theoretical band position at 436 nm), while the doublets at 498/504 and 558/563 nm belong to the homonuclear species Ho2 (theoretical values are at 482 and 562 nm). First conclusions are drawn with respect to the formation of the metal trimers Ho3, Ag2Ho, AgHo2.

Using the DFT-D method to describe dispersion interactions in systems of weakly-bonded Xe-aromatic molecules

The optimum version of the DFT-D class of methods (BHHLYP-D2, 6-31G*) is chosen to describe binding in a Xe-phenol system with the aim of subsequent KM/MM calculations for complex Xe-containing protein systems. It is shown that the stability of the Xe-phenol system is due to weak dispersion interactions not described in conventional approaches using the density functional. The MP2 approach using the (aug)-cc-pVTZ basis and Stuttgart pseudopotential, which yield the best reproduction of the characteristics of a Xe2 xenon dimer, is chosen as the reference standard. It is noted that the 2010 DFT-D3 methods underestimate the binding energy by a factor of nearly three, while DFT methods without dispersion corrections do not reproduce the stability of Xe2 and Xe-phenol systems. It is found that in the best version of calculations, BHHLYP-D2, the binding energy in Xe-phenol complex is estimated to be 2.7 kcal/mol versus the 3.1 kcal/mol found using the comparative approach. It is concluded that BHHLYP-D2 adequately reproduces the difference between the two conformers of the Xe-phenol complex and trend toward an increase in binding energy in the series of aromatic amino acids (phenylalanine, tyrosine, and tryptophan). DFT-D can also indicate the existence of excess conformers that are missing in systems according to more precise descriptions (MP2/(aug)-cc-pVTZ).

On existence and properties of plutonium(VIII) derivatives

Abstract The existence of Pu(VIII) was shown in alkaline solutions and in nonpolar solvents (CCl4 and CHCl3) on the ground of such experimental facts like extraction of plutonium species, obtained by means of ozonization of Pu(VI) alkaline solutions into CCl4 and CHCl3; volatility of Pu compounds out of aqueous alkaline solutions, and the mentioned solvents. The product of interaction between PuO3 · 0.8H2O and XeF2 in CCl4 and CHCl3 was of the same chemical form for both cases and identical with the form of the product of volatility of Pu out of these organic solutions. The electronic absorbance spectra of these products in CCl4 and CHCl3 were fairly similar to the ones for OsO4 and RuO4. The band of absorption in these spectra with the maximum at 332 nm corresponds to the charge transfer of metal-ligand type [π(O)→Pu], as the value of molar extinction coefficient ε was found to be higher than 2 · 103 L/mole cm. A satisfactory agreement between the energies for the transitions of PuO4 molecule designed by Xα-SW method and experimental data was observed. A similar accordance was observed for the designed vibration frequency (DFT-method) with the one found as result of analysis of the vibration structure of the experimental spectra.

Tamm states in zigzag carbon nanotubes: Analytic estimates in the Hückel approximation

The frontier molecular orbitals of [Cn]q hydrocarbons, fragments of (n, 0) nanotubes, were considered. Asymptotic estimates for two different sets of frontier molecular orbitals were obtained in the Hückel approximation. The problem was shown to be equivalent to the search for model polyene levels with alternating resonance parameters (β and β′ = γβ). The γ parameter was determined by the symmetry type of molecular orbitals and took on values from 0 to 2. The problem was reduced to an analysis of a regular linear polyene with a modified terminal atom by taking alternant symmetry into account. The states of the system were classified according to quasi-momentum Θ values, which could be complex. Solutions with a complex Θ value were localized at tube ends and could be interpreted as Tamm states. The criterion of the appearance of Tamm solutions for the problem under consideration was formulated, γ < 1 − 1/(q + 2). The conclusion was drawn that these states for an arbitrary [Cn]q fragment lied close to the Fermi level, and their energies were described by the asymptotic equation α ± βρ(q + 1). Delocalized levels were always situated farther from the Fermi level than Tamm states.

Ab Initio Study of the Comparative Stability of Ruthenium(VI) and Osmium(VI) Oxo Hydroxo Complexes with Different Coordination Numbers

The comparative stability of oxohydroxoruthenium(VI) and oxohydroxoosmium(VI) anions with different coordination numbers has been calculated by the relativistic density functional theory method with inclusion of spin-orbit coupling. In line with experimental data, the computations point to the stability of the ruthenium complex RuO3(OH)22− − with CN = 5, whereas osmium is stabilized in a six-coordinate environment (OsO2(OH)42−). The spin state of the stable Ru complex is close to triplet and that of the Os complex, to singlet.

Potential magnetic properties of nanotubes ( n , 0) with Klein and Fujita edges

Analytical solutions for localized states of zigzag-type nanotube (NT) fragments with various combinations of Klein and Fujita borders are considered using the Hückel approach. It is shown that the equations for determining molecular orbitals (MOs) in systems with two Klein edges are similar to equations for systems with two Fujita edges. An analytical formula for the energies of all π MOs is obtained for systems that have a Klein edge on one side and a Fujita edge on the other. It is established that these systems have n orbitals with energy α that are localized on the Fujita and Klein edges in dependence on the MO symmetry. The degeneracy of edge orbitals indicates that there is a tendency toward single occupancy of them and to the appearance of spin (magnetic) properties. In addition, the energies of the states of different multiplicity for NT fragments (8, 0) are calculated using the CASSCF approach. It is shown that the ground state has a multiplicity of 9, as was also indicated by estimates obtained using the density functional method (B3LYP). It is concluded that zigzag-type NTs with asymmetric edges have a tendency to exhibit spin properties. It is noted that the construction of nanoscale magnetic materials based on them is very promising.

The equilibrium structures of the Li[Cn]1 (n = 7–12) complexes and their alternation depending on n

The equilibrium geometric configurations of the Li[Cn]1 (n = 7–12) complexes, where [Cn]1 is a cylindrical hydrocarbon containing the simplest zigzag nanotube fragment, were determined by the density functional theory method with the PBE0 exchange-correlation functional. Analytic molecular orbital (MO) estimates were obtained for isolated [Cn]1 hydrocarbons in the Hückel approximation. The appearance of nonbonding MOs for hydrocarbons with even n was demonstrated. Equilibrium structure types were found to alternate as n increased. This alternation correlated with the behavior of the frontier orbitals of the [Cn]1 hydrocarbon. At odd n, the Li atom was situated near the boundary of the π electron density of the bracelet, and the complex had Cs symmetry. Complexes with even n had the C2v point group, and lithium was situated in the inner cylinder cavity above the center of one of benzene rings.

Modeling the electronic structure of graphite intercalation compounds with lithium by metal complexes with polybenzene systems

The electronic structure of intercalation compounds obtained by inclusion of an alkali metal in graphite is considered using molecular complexes C6H6Li2, (C6H6)2Li, and (C6H6)3Li2 in the lowest energy states as examples. Modeling the electron distributions of the intercalates requires inclusion of metal d-orbitals in the basis set of AOs and rejection of purely ionic models. It was found that exclusion of the lithium d-AO from the basis set significantly increases the ionic character of the distributions.