Andrey M. Kuznetsov

Spin Crossover in Tetranuclear Cyanide-Bridged Iron(II) Square Complexes: A Theoretical Study

Spin crossover in a series of six cyanide-bridged iron(II) tetranuclear square complexes was analyzed using density functional theory (DFT) methods. As the spin crossover between the low-spin (LS) and high-spin (HS) states can occur only for two of four iron ions, we characterized energetically and structurally the [LS-LS], [HS-LS], and [HS-HS] spin-state isomers. For all studied complexes, the energy of the mixed [HS-LS] spin state does not deviate essentially from the halfway point between the energies of homogeneous spin states, thereby satisfying the conditions for an one-step transition between the [LS-LS] and [HS-HS]. This fact reflects the weak elastic coupling between the environments of transiting centers. The two-step spin transition observed in one complex can appear only due to the crystal packing effects. We also evaluated the strength of exchange coupling between the paramagnetic ions in the [HS-HS] state.

Redox potential of the Rieske iron-sulfur protein Quantum-chemical and electrostatic study

Quantum-chemical study of structures, energies, and effective partial charge distribution for several models of the Rieske protein redox center is performed in terms of the B3LYP density functional method in combination with the broken symmetry approach using three different atomic basis sets. The structure of the redox complex optimized in vacuum differs markedly from that inside the protein. This means that the protein matrix imposes some stress on the active site resulting in distortion of its structure. The redox potentials calculated for the real active site structure are in a substantially better agreement with the experiment than those calculated for the idealized structure. This shows an important role of the active site distortion in tuning its redox potential. The reference absolute electrode potential of the standard hydrogen electrode is used that accounts for the correction caused by the water surface potential. Electrostatic calculations are performed in the framework of the polarizable solute model. Two dielectric permittivities of the protein are employed: the optical permittivity for calculation of the intraprotein electric field, and the static permittivity for calculation of the dielectric response energy. Only this approach results in a reasonable agreement of the calculated and experimental redox potentials.

Formation thermodynamics of cucurbit[6]uril macrocycle molecules: a theory study.

Macrocyclical molecules are very important molecular cavitands for the supramolecular chemistry in view of host-guest complexation which is a basis for design of molecular devices. One striking example of macrocycles is the family of cucurbit[ n]uril molecules (CB[n]). For effective application of the cavitands of this family it is needed to elucidate the stabilization mechanism of different homologues CB[n]. In this study we have carried out a thermodynamical analysis of macromolecule cyclization from a monomer to a pentamer and CB6. It was found that water molecules, which are formed as one of the products of the reaction of glicoluril with formaldehyde, construct dimer and oligomer CB(m) so that an angle between adjacent building units corresponds to the stable homologue CB[6]. In the framework of the density functional theory (DFT), the structures of hydrated oligomers were optimized. Calculated thermodynamical functions were used for a description of cyclization mechanism of oligomers up to formation of CB[6]. It is assumed that the proposed model can be extended to the formation of higher homologues in diluted water-acid solutions.

Formation of Pb(III) Intermediates in the Electrochemically Controlled Pb(II)/PbO2 System

The formation of lead dioxide PbO(2), an important corrosion product in drinking water distribution systems with lead-bearing plumbing materials, has been hypothesized to involve Pb(III) intermediates, but their nature and formation mechanisms remain unexplored. This study employed the electrochemical (EC) method of rotating ring disk electrode (RRDE) and quantum chemical (QC) simulations to examine the generation of intermediates produced during the oxidation of Pb(II) to PbO(2). RRDE data demonstrate that PbO(2) deposition and reduction involves at least two intermediates. One of them is a soluble Pb(III) species that undergoes further transformations to yield immobilized PbO(2) nanoparticles. The formation of this intermediate in EC system is mediated by hydroxyl radicals (OH(•)), as was evidenced by the suppression of intermediates formation in the presence of the OH(•) scavenger para-chlorobenzoic acid. QC simulations confirmed that the oxidation of Pb(II) by OH(•) proceeds via Pb(III) species. These results show that Pb(III) intermediates play an important role in the reactions determining transitions between Pb(II) and Pb(IV) species and could impact lead release in drinking water.

Rotating Ring-Disk Electrode and Quantum-Chemical Study of the Electrochemical Reduction of Monoiodoacetic Acid and Iodoform

This study examined the electrochemical (EC) reduction of monoiodoacetic acid (MIAA) and iodoform (CHI3), which are typical iodine-containing disinfection byproducts (I-DBPs). Experiments carried out using the method of a rotating ring-disk electrode (RRDE) with a gold working electrode showed that the reduction of CHI3 and MIAA is diffusion-controlled. The MIAA diffusion coefficient was determined to be (1.86 ± 0.24)·10(-5) cm(2) s(-1). The yield of the iodide ion formed as a result of MIAA or CHI3 reduction was affected by the presence of dissolved organic matter (DOM) and resorcinol. Increasing concentrations of DOM or resorcinol did not affect the EC reduction of the examined I-DBPs, but the formation of iodide was suppressed. This indicated that free iodine, ·I, was formed as a result of the first step in the EC reduction of MIAA and CHI3. This also indicated that the pathway of the EC reduction of MIAA and CHI3 was different from that typical for the reduction of Br- and Cl-containing DBPs, in which case Br(-) or Cl(-) tend to be formed as a result of the electron transfer. Quantum-chemical (QC) calculations confirmed the thermodynamic likelihood of and possible preference to the formation of free iodine species as a result of the EC reduction of MIAA, CHI3, and other I-DBPs.

Quantum-chemical simulations of the hydration of Pb(II) ion: structure, hydration energies, and p K a 1 value

AbstractThermodynamic and structural aspects of the hydration of Pb(II) ions were explored based on DFT calculations combined with the supermolecular/continuum solvent model. Hydration of Pb(II) was considered as the formation of Pb(H2O)n2+ aqua complexes (n=6−9) from the gas phase Pb(II) ion. Hexa- and hepta-aqua Pb(II) complexes were shown to exhibit the hemidirected symmetry, while those containing eight and nine water molecules are characterized by the holodirected symmetry. The calculations showed that because Pb(H2O)n2+ complexes with six to nine water molecules have comparable thermodynamic stabilities, such complexes are likely to coexist in aqueous solutions. The deprotonation of Pb(H2O)n2+ complexes was shown to result in the formation of the mono-hydroxo complex [Pb(H2O)4OH]+. The pKa1 value determined for this reaction (7.58 for Pb(H2O)62+) was close to the experimental value of 7.61 used in recent models of aquatic equilibria. The density functional method ω-B97X(PCM-UAO) in combination with the atomic basis set 6-311++G(d,p) for O and H and the small-core electron effective pseudopotential (ECP) with the aug-cc-pvdz-PP basis set for Pb can be recommended for such calculations. Graphical abstractStructures of Pb(II) ions with varying numbers of water molecules in the inner hydration shell

Water structuring inside the cavities of cucurbit[n]urils (n = 5–8): a quantum-chemical forecast

In this work we report findings of the quantum-chemical examination of water structuring in the cavities of cucurbit[n]urils (CB[n]), n = 5–8 obtained within the density functional theory. The thermodynamically most stable structures of inclusion compounds (H2O)m@CB[n] were determined for different numbers m of H2O molecules inside the cavities. From the viewpoint of thermodynamics, the most probable numbers m of water molecules in the CB[n] homologues are the following: m = 2 for CB[5], m = 4 for CB[6], m = 8 for CB[7] and m = 10 for CB[8]. For the case of CB[6] synthesized in aqueous solution, we compared its experimental IR spectrum with that calculated quantum-chemically for the model inclusion systems (H2O)m@CB[6] where m ranges from 1 to 6. The best agreement between the experimental and theoretical spectra was observed for (H2O)4@CB[6], in complete agreement with the conclusion made based on the thermodynamic estimations. Our results are also in good agreement with other available estimates of the most probable number of water molecules in CB[n].

Silver mediated duplex-type complexes of pyrimidinophanes and their acyclic counterparts

Complexation of metal cations with acyclic and macrocyclic nucleobase derivatives containing uracil and 2-thiocytosine units linked by polymethylene spacers was studied. The investigated compounds have demonstrated high extraction selectivity for the Ag+ ion over a large number of competitive cations (Na+, Ca2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+). The structure and composition of Ag+ complexes have been investigated by a variety of physical-chemical methods. The crystal structures of three Ag+ complexes with 2-thiocytosine have been established by X-ray analysis. To refine the composition of the complexes formed in solution, the self-diffusion coefficients have been determined by NMR spectroscopy. For the estimation of the most reliable complex structures, the combined analysis of NMR data and DFT calculations was successfully applied. It was shown that the tendency of 2-thiocytosine moieties to form N1-nitrogen coordinated complexes and the presence of a polymethylene spacer between amine groups in the pyrimidinophanes provides the formation of Ag+ mediated duplex-type multi-component complexes.