V. I. Pergushov

Rotational and Translational Diffusion of Spin Probes in Room-Temperature Ionic Liquids

We have studied the rotational and translational diffusion of the spin probe 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL) in five imidazolium-based room-temperature ionic liquids (RTILs) and glycerol by means of X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times and rate constants of intermolecular spin exchange have been determined by analysis of the EPR line shape at various temperatures and spin probe concentrations. The model of isotropic rotational diffusion cannot account for all spectral features of TEMPOL in all RTILs. In highly viscous RTILs, the rotational mobility of TEMPOL differs for different molecular axes. The translational diffusion coefficients have been calculated from spin exchange rate constants. To this end, line shape contributions stemming from Heisenberg exchange and from the electron-electron dipolar interaction have been separated based on their distinct temperature dependences. While the Debye-Stokes-Einstein law is found to apply for the rotational correlation times in all solvents studied, the dependence of the translational diffusion coefficients on the Stokes parameter T/η is nonlinear; i.e., deviations from the Stokes-Einstein law are observed. The effective activation energies of rotational diffusion are significantly larger than the corresponding values for translational motion. Effects of the identity of the RTIL cations and anions on the activation energies are discussed.

Copper(II) organic compounds as intermediates of photochemical transformations of quaternary ammonium tetrachlorocuprates

171 Stable organometallic compounds with a Cu(II)–C σ bond have not been described [1]. Pulse radiolysis and photolysis of divalent copper salts in aqueous solutions [2, 3] and in n -donor solvents [4] gave short-lived copper complexes with organic radicals in which the copper oxidation state was presumably two. Dissociation [2] CuR + Cu + + R • and bimolecular processes [3] 2CuR + 2Cu + + R 2 , CuR + + H 3 O + Cu 2+ + RH + H 2 O, were considered as the key routes for the destruction of these complexes. Copper(II) Organic Compounds as Intermediates of Photochemical Transformations of Quaternary Ammonium Tetrachlorocuprates E. N. Golubeva a, A. V. Lobanov a , V. I. Pergushov a , N. A. Chumakova a, and A. I. Kokorin b Presented by Academician A.L. Buchachenko February 5, 2008

Influence of the nuclearity of copper(II) chloride complexes on their activity in catalytic C-Cl bond metathesis

Catalytic systems containing mainly mononuclear (CuCl42−) or dinuclear (Cu2Cl62−) tributylbenzylammonium chlorocuprates are prepared by adsorption on silica surfaces of different structures (Aerosil and Silokhrom). Using ESR, XANES, and electronic spectroscopy, the CuCl42− ions are shown to be reduced rapidly under conditions of C-Cl bond metathesis, whereas the dinuclear chlorocuprates are relatively stable. A correlation between the number of copper ions in the chlorocuprate anion and its catalytic activity is established: the mononuclear complexes are several times more active than their dinuclear counterparts.

CYCLOPROPANE RING OPENING IN THE REACTION OF METHYLENECYCLOPROPANE WITH SILYL RADICALS STABILIZED ON AN ACTIVATED AEROSIL SURFACE

Alkyl type radicals stable at room temperature and incorporating a double bond not conjugated with the free valence, ≡Si−C(=CH2)−CH2−CH2, are formed in the reaction of methylenecyclopropane with silyl radicals (≡SiO)3Si on an activated Aerosil surface.

Phototransformations of methylsubstituted oxiranes’ radical cations

It has been established that reversible photoinduced transformations of 2,3-dimethyloxirane and methyloxirane radical cations (RCs), observed in freonic matrices at 77 K, are related to the conversion between the open and cyclic forms of the RCs. For the trimethyloxirane RC the action of light on the trans-isomer of the open form results in its photoinduced transformation into a C-centered radical with low quantum efficiency (≈4 × 10−3). Upon the X-ray irradiation of 2,2-dimethyloxirane in freonic matrices at 77 K, a cyclic form of the RC is stabilized (presumably, as part of a complex with matrix molecules) which transforms into a distonic C-centered RC under the action of light with the quantum yield of ≈10−3. Tetramethyloxirane RC, stabilized in its open form, is resistant to the action of light. Probable causes of the observed effects are discussed.

Photochemistry of Aliphatic Nitroxide Radicals Grafted onto Activated Aerosil Surface

The mechanism of photoinduced transformations of nitroxide radicals ≡SiON(O·)CH3 (I) and ≡SiOCH2N(O·)CH3 (II) obtained on the activated aerosil surface was studied. The nitroxides were photolyzed with 436-nm light at 77 K. It was shown that the action of light in the long-wavelength absorption band corresponding to the n–π* transition resulted in the dissociation of the O–N or C–N bond in radicals I or II, respectively. The quantum yields of these reactions were found to be ≈0.6 and ≈0.002, respectively.

Molecular dynamics of alkyl radicals grafted onto silica surface

Correlation times for≡SiOC·X2 radicals grafted onto activated silica surface were estimated to be 1.3·10−8s (X=H) and 2.5·10−8s (X=Me) at room temperature.

Phototransformations of azetidine radical cations stabilized in freonic matrices

It has been established that transformations of azetidine radical cations observed in freonic matrices under the action of light with λ = 436 nm (T = 77 K) are associated with C-N bond cleavage which corresponds to the cyclic form yielding a mixture of open distonic C-centered radical cations of the following structure: ·CH2CH2CH=NH2+

Photochemistry of trimethylene oxide and trimethylene sulfide radical cations in freonic matrices at 77 K

It was shown that trimethylene oxide (oxetane) radical cations were converted at 77 K into either distonic radical cations ·CH2CH2CH=OH+ or 2-oxetanyl radicals, depending on the freonic matrix used, by the action of light at λ = 546 nm and trimethylene sulfide radical cations transformed into distonic radical cations CH2CHSH+CH2· under 436-nm irradiation. The quantum yields of the photochemical reactions were determined. Quantum-chemical calculations on the structure and HFC constants of the radical cations and possible paramagnetic products of their transformation were performed. The reasons behind the observed difference in reactivity between the radical cations under the action of light are discussed.

Photochemical transformations of 1,3,5-trioxane radical cations in freonic matrices at 77 K

It was found that the principal photochemical reaction of 1,3,5-trioxane radical cations in freonic matrices at 77 K is their cycle-opening dissociation yielding the distonic radical cation in which the unpaired electron is preferentially localized on the oxygen atom. The dissociation of the trioxane radical cations at 77 K is characterized by high quantum yields, which vary from 0.24 to 0.36 in different matrices. The distonic radical cations produced during photolysis are unstable at 77 K and undergo further transformations, which occur at different rates in freonic matrices. The structure of the intermediates produced and a possible mechanism of the processes are discussed with the use of quantum-chemical calculation data.