Quantum chemical model of solvation for calculation of electrode potentials of redox processes involving ferrocene,cobaltocene, and their ions

Quantum chemical calculations of solvation energy for ferrocene and cobaltocene molecules and their ionic forms in water, acetonitrile, methanol, and acetone are performed in terms of the B3LYP density functional method by taking into account solvation effects and using the polarized continuum model (PCM). Standard electrode potentials of the corresponding redox pairs, the effect of solvent on them, and the overall energy of the transfer of cobaltocene cation and anion between two solvents are calculated. The calculation results well agree with the available experimental data. The present study provides sufficiently reliable grounds for the application of an ion—metallocene molecule redox pair as a pilot system for the comparison of electrode potentials and solvation energies in different solvents.     

Medium effect in the electroreduction of nitromesitylene

One-electron reduction of nitromesitylene to the corresponding radical anion has been studied at a hanging mercury drop electrode in various perchlorate salt solutions in five organic solvents: dimethylsulfoxide, dimethylacetamide, propylene carbonate, hexamethylphosphoramide and N-methylformamide. Standard redox potentials, diffusion coefficients, standard rate constants and transfer coefficients have been evaluated from cyclic voltammetry measurements. The results obtained are compared with the literature data for dimethylformamide and acetonitrile solutions.The standard rate constants were found to depend on the cation of the supporting electrolyte as well as on the solvent. It is shown that the rate constants corrected for both ion-pair formation and the double layer effect cannot be described by classical theories of heterogeneous electron transfer. It is shown that the dynamic dielectric properties of the solvent, described by the dielectric relaxation time, influence the rate of the heterogeneous charge transfer. The greater the dielectric relaxation time of the solvent, the smaller is the reaction rate.     

Decay kinetics of the excited S1 state of the cyanine dye Cy(+)I(-) (thiacarbocyanine iodide): the computation of quantum yields for different pathways

This work explains the unordinary solvent effect which was observed in the photochemical decay kinetics for the cyanine dye thiacarbocyanine iodide (Cy(+)I(-)) in binary solvent mixtures toluene/dimethylsulfoxide. The interpretation is formulated in terms of the probability density F(R) describing the distribution of interionic distances R in the ion pair Cy(+)I(-) and depending on the solvent composition. The proper normalization of this distribution is expressed via the degree of association α for the ion pair in a given solvent mixture. The α values are, in turn, extracted by means of the mass action law from the ionic association constants computed in a separate publication. The detailed kinetic scheme includes the empirical parametrization of the R-dependent kinetic constants for different decay channels. The multiparameter fitting procedure represents, with the reasonable parameter values, the dependence of the observed quantum yields on the solvent composition.     

Dependence of the enthalpies of formation of glycylglycinate complexes of nickel(II) on the composition of a mixed water-dimethylsulfoxide solvent

The heat effects of the complexation reactions of nickel(II) with a glycylglycinate ion in a water-dimethylsulfoxide solvent in a range of compositions of 0.00–0.60 molar parts of dimethylsulfoxide (DMSO) (an ionic strength of 0.1 was maintained using sodium perchlorate) were determined by means of calorimetry at 298.15 K. It is established that the exothermicity of complexation reactions rises by the first two steps and falls upon the addition of a third glycylglycinate anion with an increase in the concentration of DMSO. It is shown that the formation of mono- and bis-glycylglycinate complexes of nickel(II) in a water-DMSO solvent is determined mostly by the enthalpic contribution. It is concluded that the formation of tris-ligand complexes is more associated with the entropic contribution.     

Aminyloxide—XVIII : ESR-spektroskopische untersuchung der tautomerie beim “hydrolyseprodukt des porphyrexids”

The hydrolysis of porphyrexine 2a yields 3a not 1a, as was shown by means of the ¹⁵N-labelled compound 2c. Analysis of the ESR spectra of radicals 4 derived from 3 shows that in dimethylsulfoxide these radicals exist in the tautomeric form B whereas in dioxan they exist in form A. The stabilization of form B in dimethylsulfoxide is shown to be due to a specific effect of the solvent.     

Photostability of the sunscreening agent 4-tert-butyl-4′-methoxydibenzoylmethane (avobenzone) in solvents of different polarity and proticity

The most widely used UVA absorber in broad-spectrum sunscreens is 4-tert-butyl-4′-methoxydibenzoylmethane (avobenzone). However, the photostability of avobenzone is solvent-dependent. The aim of this work was to investigate the photostability of avobenzone in solvents of different polarity and proticity. Four solvents were employed, namely, cyclohexane, ethyl acetate, dimethylsulfoxide and methanol. The cause of the instability of avobenzone in these solvents was determined by means of UV spectroscopy, high performance liquid chromatography, gas chromatography–mass spectrometry and nuclear magnetic resonance spectroscopy. The effect of oxygen on the photo-instability was also determined. Avobenzone was found to lose absorption efficacy as a result of photoisomerisation from the enol to the keto form and/or photodegradation to form photoproducts that absorb principally in the UVC region, depending on the solvent. It was found to be essentially photostable in the polar protic solvent methanol but photoisomerised in the polar aprotic solvent dimethylsulfoxide. In the nonpolar solvent cyclohexane, it photodegraded appreciably. Both photoisomerisation and photodegradation occurred to a similar extent in the moderately polar aprotic solvent ethyl acetate. Photoisomerisation occurred only in the presence of oxygen whereas photodegradation occurred irrespective of oxygen. This knowledge is important in order to achieve the correct formulation for sunscreens incorporating avobenzone.     

Enthalpies of transfer of N-methylformamide,formamide, and N,N-dimethylformamide from methanol to methanol+dimethylsulfoxide mixtures

The enthalpies of transfer of formamide, N-methylformamide and N,N-dimethylformamide from methanol to methanol+dimethylsulfoxide solvent systems have been measured. These data are analysed in terms of a recently developed model of solvation in mixed solvents. The results of the data analyses indicate that preferential solvation of the different functional groups of the amides differs, the carbonyl oxygen being preferentially solvated by methanol and the nitrogen protons are by dimethylsulfoxide.     

Recombination of Photogenerated Lophyl Radicals in Imidazolium‐Based Ionic Liquids

Laser flash photolysis is applied to study the recombination reaction of lophyl radicals in ionic liquids in comparison with dimethylsulfoxide as an example of a traditional organic solvent. The latter exhibits a similar micropolarity as the ionic liquids. The ionic liquids investigated are 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ( 1 ), 1‐hexyl‐3‐methylimidazolium hexafluorophosphate ( 2 ), and 1‐butyl‐3‐methylimidazolium tetafluoroborate ( 3 ). The recombination of the photolytic generated lophyl radicals occur significantly faster in the ionic liquids than expected from their macroscopic viscosities and is a specific effect of these ionic liquids. On the other hand, this reaction can be compared with the macroscopic viscosity in the case of dimethylsulfoxide. Activation parameters obtained for lophyl radical recombination suggest different, anion‐dependent mechanistic effects. Quantum chemical calculations based on density functional theory provide a deeper insight of the molecular properties of the lophyl radical and its precursor. Thus, excitation energies, spin densities, molar volumes, and partial charges are calculated. Calculations show a spread of spin density over the three carbon atoms of the imidazolyl moiety, while only low spin density is calculated for the nitrogens.     

Synthesis of Diaryl Disulfides via Mild Reduction of Arylsulfinates with Hydrazine Monohydrate in DMSO

Arylsulfinates were reduced with hydrazine monohydrate at room temperature in dimethylsulfoxide (DMSO) to afford diaryl disulfides in good yields. A dramatic solvent effect was observed, and DMSO was found to be the best solvent for the reaction.     

Spin transport properties of single metallocene molecules attached to single-walled carbon nanotubes via nickel adatoms

The spin-dependent transport properties of single ferrocene, cobaltocene, and nickelocene molecules attached to the sidewall of a (4,4) armchair single-walled carbon nanotube via a Ni adatom are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with the spin density functional theory. Our calculations show that the Ni adatom not only binds strongly to the sidewall of the nanotube, but also maintains the spin degeneracy and affects little the transmission around the Fermi level. When the Ni adatom further binds to a metallocene molecule, its density of states is modulated by that of the molecule and electron scattering takes place in the nanotube. In particular, we find that for both cobaltocene and nickelocene the transport across the nanotube becomes spin-polarized. This demonstrates that metallocene molecules and carbon nanotubes can become a promising materials platform for applications in molecular spintronics.     

X-ray photoelectron spectroscopy and magnetic studies on the effect of pore size, wall thickness, and wall composition on superparamagnetic cobaltocene mesoporous Nb, Ta, and Ti composites

Recent results in our group demonstrated that mixed oxidation state mesoporous niobium oxide cobaltocene composites display superparamagnetism at certain composition ratios. This was the first report of superparamagnetism in nanoscale molecular ensembles. A series of mesoporous niobium oxide materials were synthesized in order to understand the role of pore size and thickness of the walls in the mesostructure on the magnetic properties. Mesoporous Ti oxide and Ta oxide composites were also synthesized in order to investigate the effect of changing the wall composition on the magnetic properties of this new series of materials. All samples were characterized by X-ray diffraction, nitrogen adsorption, ultraviolet spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and superconducting quantum interference device magnetometry. The results of this study showed that variation of wall thickness or pore size in the Nb system had little effect on the properties and that superparamagnetism most likely arises from mixed oxidation state cobaltocene grains residing in the individual pores and not from the free electrons in the mesostructure or much larger domains. The Langevin function was applied to the isothermal magnetic data from the Nb composites and gave mean superparamagnetic particle sizes of ca. 14 nm in each system. The Co(II) to Co(III) ratios in these materials were approximately 1:1. The Ti and Ta materials showed no sign of superparamagnetism and only very low levels of neutral cobaltocene in the pores. This suggests that a critical amount of cobaltocene is required to bring about superparamagnetic behavior.     

The static electric permittivity of solutions of organic molecules in dipolar solvents

The permittivity of solutions of pyrazine and of 1,4-diazabicyclo-[2,2,2]-octane (TED) in various dipolar solvents has been measured at 10 MHz by means of a sensitive bridge method. The solvents used were water, methanol, formamide, N-methylformamide, N,N-dimethylformamide, and dimethylsulfoxide. The results of the measurements are compared with those of theoretical mixture formulas. It is shown that — depending on structural properties — the permittivity attributed to the solvation shells around the organic solute molecules is changed with respect to the pure solvent value.     

Thermochemistry of the silver(I) complexation with 18-crown-6 ether in binary acetonitrile—dimethylsulfoxide solvents

The effect of acetonitrile-dimethylsulfoxide solvents on the enthalpy of silver(I) complexation with 18-crown-6 ether is studied by means of calorimetry. It is found that an increase in the concentration of dimethylsulfoxide leads to an increase in the reaction exothermicity. It is shown that the determining factor in the change in the reaction enthalpy is the solvation effect of the ligand.     

Properties of DNA + Dipropylsulfoxide or Dibutylsulfoxide + Water Ternary Solutions

Aqueous solutions of DNA in the presence of dipropylsulfoxide (DPSO) and of dibutylsulfoxide (DBSO) have been studied using UV-vis spectroscopy and density measurements. It has been shown that significant thermal denaturation of DNA occurs when DPSO or DBSO are present. The results obtained were compared with those obtained previously with dimethylsulfoxide (DMSO) and diethylsulfoxide (DESO). It is suggested that the lowering of the transition enthalpy with increasing length of the carbon chain of the dialkylsulfoxides is due to significance contribution from hydrophobic interactions.     

Effect of solvent on EPR spectra of Cu(II) complexes

EPR spectra of inner complexes of Cu(II) in various solvents and in glasses were investigated. It was shown that the solvent has an appreciable effect on the electron density distribution between the central ion and the ligands.     

The acid-base equilibrium constants of some azine compounds in various aqueous-organic solvent mixtures

The acid dissociation constants of the protonated form of some azine compounds (acridine, acridine orange and neutral red, BH+) were determined pH-metrically at 25 degrees C and at the constant ionic strength I = 0.1 mol l(-1) (KNO3) in pure water as well as in various aqueous mixtures having different proportions (w/w%) of organic solvents. The organic solvents used are methanol, ethanol (as amphiprotic solvents), N,N-dimethylformamide, dimethylsulfoxide (as dipolar aprotic solvents) and acetonitrile (as a low basic solvent). The results obtained indicated that the pKa values decrease as the content of the organic solvent in the medium is increased. It is deduced that, the major effect responsible for this behaviour is the differences in stabilization of the free base (B) by dispersion forces and of the proton by its interaction with solvent and water molecules in aqueous-organic solvent mixtures (ion-solvent interaction). Moreover, it is concluded that the ability of the solvent to accept hydrogen bond from the protonated form (BH+) contributes significantly to the deprotonation process of the compounds.     

Metal-Containing Initiator Systems. XXIX. Selective Initiation of Vinyl Polymerizations with Cobaltocene

The polymerizations of acrylonitrile (AN), acrolein (AL), butadiene (BD), and isobutyl vinyl ether (EBVE) with cobaltocene were investigated. It was found that both AN and AL could polymerize in dimethyl sulfoxide through a coordination mechanism, but AN and BD polymerized by a radical mechanism in the presence of some organic halides, such as carbon tetrachloride, benzyl bromide, and allyl bromide. The initiator system of cobaltocene and organic halide also induced cationic polymerization of IBVE. On the basis of the results obtained, a mechanism for the selective initiation of polymerization is proposed and discussed.     

2-(2-巯苯基)苯并噁唑分子内质子转移的理论研究

在B3LYP/6-31G(d,p)水平上研究了2-(2-巯苯基)苯并噁唑气态中五种异构体(E1, E2, E3, E4和K)在气态中的稳定性及其在基态下的质子转移, 同时结合极化连续介质模型(PCM)研究了水、二甲亚砜、乙腈、乙醇、苯胺和环己烷等对2-(2-巯苯基)苯并噁唑溶剂化作用的影响. 研究结果表明, 醇式异构体E1为2-(2-巯苯基)苯并噁唑的优势构型; 在E1向K(酮式异构体)转变过程中, 存在一个较小的能垒; 当考虑零点振动能(ZPVE)后, 逆向能垒消失. 在溶液中, 随着溶剂极性的增强, 醇式异构体E1与K之间的反应平衡向K方向移动, 在非极性溶剂环己烷中, E1为优势构型, 而在强极性水溶液中, K为优势构型.     

Heat Effect of the Protonation of Glycine and the Enthalpies of Resolvation of Participating Chemical Species in Water–Dimethylsulfoxide Solvent Mixtures

Enthalpies of the protonation of glycine in water?dimethylsulfoxide (DMSO) mixed solvents are determined calorimetrically in the range of DMSO mole fractions of 0.0 to 0.9, at T = 298.15 K and an ionic strength μ = 0.3 (NaClO₄). It is established that the protonation of glycine becomes more exothermic with an increasing mole fraction of DMSO, and the enthalpies of resolvation of glycine and glycinium ions in water?DMSO solvent mixtures are calculated. It is shown that the small changes in the enthalpy of protonation observed at low mole fractions of DMSO are caused by the contributions from the solvation of proton and protonated glycine cancelling each other out. The enthalpy term of the Gibbs energy of the reaction leading to the formation of glycinium ion is estimated along with the enthalpy of resolvation of the reacting species in the water?DMSO mixed solvent.     

Cationic intermediates in oxidative addition reactions of Cl2 to [PtCl2(cis-1,4-DACH)

Oxidative addition and reductive elimination are fundamental processes in transition-metal chemistry. New interest in this field has been generated by the exploitation of platinum(IV) complexes as antitumor drugs. The two extra ligands can be used to render these species more resistant to attack by biological nucleophiles compared to their platinum(II) counterparts, to anchor additional pharmacologically active moieties, or, finally, to target the drug to specific sites by conferring responsiveness to some type of chemotaxis. On the other hand, platinum(IV) species are considered to be prodrugs and to require reduction to Pt(II) to become active. Thus, reductive elimination promoted by biological reducing agents becomes an important issue and it too could be exploited for targeting purposes. In this paper, we investigated the oxidation step in more detail and shown that, independent of the solvent used, a solvent molecule assists the reaction by entering in a trans position with respect to the attacking oxidant. In the case of bifunctional solvent molecules, such as dimethylsulfoxide, both S- and O-coordinated species are formed, the latter being thermodynamically favored. The substitution of the axially coordinated solvent molecule by a free chloride ion is found to be quite slow in organic solvents, as well as in water. It is also shown that the intermediate solvato-species can be exploited for binding just one molecule of another substrate in the axial position.