ESR Spectroscopic Investigations Research Articles

High-Pressure ESR Spectroscopy: On the Rotational Motion of Spin Probes in Pressurized Ionic Liquids.

We report high-pressure (up to 50 MPa) ESR-spectroscopic investigations on the rotational correlation times of the nitroxide radicals 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL), and 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl (ATEMPO) in the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF4), 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), 1-methyl-3-octylimidazolium tetrafluoroborate (omimBF4), and 1-methyl-3-octylimidazolium hexafluorophosphate (omimPF6). The activation volumes (38.5–56.6 Å3) determined from pressure dependent rotational diffusion coefficients agree well with the pressure dependent viscosities of the ionic liquids. Experimentally, the fractional exponent of the generalized Stokes–Einstein–Debye relation is found to be close to one.

Direct photo-oxidation of benzene to phenol over Ti/Si binary oxide prepared by sol-gel method

Titanium-silicon (Ti/Si) binary oxides having a varying Ti content were prepared using the sol-gel method and used as photocatalysts. The photo-oxidation of benzene to phenol was carried out using Ti/Si binary oxide catalysts in the presence of benzene, water and gaseous oxygen. The amounts of benzene used hardly affected the phenol yield. On the other hand, the addition of sulfuric acid into reaction solution led to the improvement of phenol formation. The H2O2 formation rate seemed to be related to the phenol yeilds. The photocatalytic reactivity of Ti/Si binary oxide having different TiO2 contents was investigated and it was found to be dramatically enhanced in the range of lower TiO2 contents. XANES, ESR and XRD spectroscopic investigations of these Ti/Si binary oxide catalysts indicated that Ti species were highly dispersed in SiO2 matrices and existed in a tetrahedral coordination. The photo-irradiation of catalysts having tetrahedral Ti species seemed to effectively lead to the formation of H2O2 as an intermediate.

A “Dendritic Effect” in Homogeneous Catalysis with Carbosilane-Supported Arylnickel(II) Catalysts: Observation of Active-Site Proximity Effects in Atom-Transfer Radical Addition

Transmetalation of polylithiated, carbosilane (CS) dendrimers functionalized with the potentially terdentate ligand [C6H2(CH2NMe2)2-2,6-R-4]- ( = NCN) with NiCl2(PEt3)2 produced a series of nickel-containing dendrimers [G0]-Ni4 (4), [G1]-Ni12 (5), and [G2]-Ni36 (7) in moderate to good yields. The metallodendrimers 4, 5, and 7 are catalytically active in the atom-transfer radical addition (ATRA) reaction (Kharasch addition reaction), viz. the 1:1 addition of CCl4 to methyl methacrylate (MMA). The catalytic data were compared to those obtained for the respective mononuclear compound [NiCl(C6H2{CH2NMe2}2-2,6-SiMe3-4)] (2). This comparison indicates a fast deactivation for the dendrimer catalysts beyond generation [G0]. The deactivation of [G1]-Ni12 (5) and [G2]-Ni36 (7) is caused by irreversible formation of catalytically inactive Ni(III) sites on the periphery of these dendrimers. This hypothesis is supported by results of model studies as well as ESR spectroscopic investigations. Interestingly, the use of two alternative nickelated [G1] dendrimers [G1]*-Ni12 (11) and [G1]-Ni8 (15), respectively, in which the distance between the Ni sites is increased, leads to significantly improved catalytic efficiencies which approximate those of the parent derivative 2 and [G0]-Ni4 (4). Preliminary membrane catalysis experiments with [G0]-Ni4 (4) and [G1]-Ni12 (5) show that 5 can be efficiently retained in a membrane reactor system. The X-ray crystal structure of the Ni(III) complex [NiCl2(C6H2{CH2NMe2}2-2,6-SiMe3-4)] (16), obtained from the reaction of 2 with CCl4, is also reported.

Characterization of Titanium−Silicon Binary Oxide Catalysts Prepared by the Sol−Gel Method and Their Photocatalytic Reactivity for the Liquid-Phase Oxidation of 1-Octanol

Titanium−silicon (Ti/Si) binary oxides having different Ti contents were prepared by the sol−gel method and used as photocatalysts. The photocatalytic reactivity of these catalysts was investigated as a function of the Ti content for the liquid-phase oxidation of 1-octanol to 1-octanal, and it was found to be dramatically enhanced in regions of lower Ti content. In situ photoluminescence, UV−vis reflectance, FT-IR, ESR, XAFS, XRD, and XPS spectroscopic investigations of these Ti/Si binary oxides indicated that the titanium oxide species are highly dispersed in the SiO2 matrixes and exist in a tetrahedral coordination exhibiting a characteristic photoluminescence spectrum due to the radiative decay from the charge-transfer excited state of the tetrahedrally coordinated titanium oxide species. The good parallel relationship between the yield of the photoluminescence and the specific photocatalytic reactivity of the Ti/Si binary oxides as a function of the Ti content clearly indicates that the high photocatalytic reactivity of the Ti/Si binary oxides having a low Ti content is associated with the high reactivity of the charge-transfer excited state of the isolated titanium oxide species in tetrahedral coordination, [Ti3+−O-]*.

Spectrophotometric Study of Catechol Oxidation by Aerial O2 in Alkaline Aqueous Solutions Containing Mg(II) Ions.

Electronic absorption spectroscopy was employed to study the aerial oxidation of catechol (1,2-benzenediol) in alkaline aqueous solution containing an excess of Mg(II) ions. Graphical analysis by the matrix method of UV spectra recorded at regular time intervals gave a good fit for two absorbing species in solution. Based on this result and our earlier ESR spectroscopic investigations we concluded that two main absorbing species in this system are Mg(II)-spin stabilized o-benzosemiquinone anion radical and C-C dimer formed by the nucleophilic attack of catecholate anion on o-benzoquinone. Although the formation of 1,2,4-benzenetriol during the catechol oxidation has been detected in some ESR studies its presence was not indicated by this analysis probably because of the low and/or stable steady state concentration throughout the experiment.

In situ electrochemical ESR investigations of the growth of one- and two-dimensional polypyrrole films

In situ ESR-spectroscopic investigations of the electrochemical growth of polypyrrole (PPy) in 0.1 mol dm −3 toluene sulfonic acid solutions in acetonitrile are presented. The films were deposited potentiostatically and galvanostatically onto gold electrodes. ESR spectra were simultaneously recorded during electrodeposition processes at high and low current densities, respectively. Differences in the paramagnetic properties of the PPy films thus prepared have been observed depending on the applied current densities. Furthermore, the spin concentration in the PPy film is not proportional to the injected charge. Therefore, two different mechanisms of film formation occur depending on the applied current densities.

Decomposition of Alkoxysilanes on the Surface of a Hot Wire‐Study of the Decomposition Mechanism

The decomposition of alkoxymethylsilanes on a hot Mo wire was investigated within the temperature range of 1300–1600 K and at pressures of 12–90 Pa. Within the temperature range 1400–1600 K a rate law of first order was determined. The activation energy of the decomposition of was estimated to be 74.5 kJ mol−1. The rate constants of the ethoxy silane decomposition decreases with increasing as . From these results and the formed products as ethanol and ethene, a surface‐activated decomposition mechanism is proposed assuming a two‐center reaction of the alkoxy groups with the formed surface on the wire. By esr spectroscopic investigations of condensed radicals the formation of radicals as , and ROO• was confirmed.

Proton Acid‐Catalysed Transformations of Estrogen Derivatives: New Results and Some Mechanistic Aspects of the Kober Colour Reaction

AbstractThe 3‐O‐methylated estrogen derivatives 1a‐d and α‐estradiol (1e) underwent sulfuric acid‐catalysed transformations to furnish the steroids 2–6. The processes involved in the reaction sequence are regioselective sulfonation and, above all, the Wagner‐Meerwein rearrangement of the methyl group at C‐13. With the objective of obtaining further information on the course of the Kober colour reaction of estrogens, some UV/VIS and ESR spectroscopic investigations were also carried out.

Addition Polymerization of Aliphatic Dlthlols with Diene Monomers

Abstract The photoinitiated free radical polymerization of triethylene-glycoldithiol and hydrochinonebisallylether or ethyleneglycoldivi-nylether leads to poly (the ethers) having anti-Markovnikov structure M n: 14,000, 12,000 (VPO), respectively. The chain propagation is limited by a cyclic product (nearly 4%). Telechelics may be obtained by the use of nonequivalent molar ratio of the thioVenemonomers. They are useful in preparation of block copol-ymers and networks, respectively. By means of ESR-spectroscopic investigations, it is shown that in dithiole/diene mixtures, which are free of daylight and oxygen, radicals are formed spontaneously. Using the spin-trapping technique, the existence of sulfur and carbon radicals could be proved unambiguously.

ChemInform Abstract: Stabilization of the Open‐Chain Structure of D‐Galacturonic Acid in a Dimeric Complex with Oxovanadium(IV).

AbstractElectronic absorption and ESR spectroscopic investigations establish that the properties of a dimeric oxovanadium complex of D‐galacturonic acid, formed in aqueous solutions, are consistent with a metal‐metal distance of about 5 Å.

ESR Characterizations and Photochemistry of the Free Radical Species from 1,1′-Bi-2-Naphthol and Related Compounds

Abstract The ESR spectroscopic investigations on the formation of neutral and anionic radical species from 1,1′-Bi-2-naphthol and 1,1′-Bi-2-phenol at room temperature are reported. The photogeneration of the neutral radicals is accompanied by an unusual kinetic decay in the presence of light. The nature and the structure of the radicals are discussed.

Untersuchungen zur Maillard-reaktion 18. Mitteilung. radikalische inhibierung der br�unung durch schwefelverbindungen

ESR spectroscopic investigations on the mechanism of Maillard-reactions using the model D-glucose/glycine show radical formation in some of the reaction steps. These are possibly due to competitive reactions against ionic mechanism. The formation of radicals depends on the concentration of reactants and reaches a maximum in thermically induced browning reactions. The addition of L-cysteine leads to a decrease in the radical concentration to about 60%. It can be indirectly proven that the radicals are involved in the formation of browning products. The hyperfine structures of ESR spectra are supposed to be similar to pyrazine cation radicals.

ChemInform Abstract: Reduction of Dirhodium(II) Complexes of the Type [Rh2(O2CCH3)3(L)]+ .

AbstractOn the basis of ESR spectroscopic investigations it is suggested that the first one‐electron reduction process of the title Complexes (I) is ligand‐based, 1 7 1‐: while the second one‐electron reduction is metal‐based.

ChemInform Abstract: ELECTRON‐TRANSFER‐INDUCED CONFORMATIONAL CHANGES. THE EXAMPLE OF 1,8‐DIMETHYL(14)ANNULENE

This communication reports ESR-spectroscopic investigations of the radical anion of 1,8-dimethyl[14]annulene (1) which possesses a flexible molecular framework allowing configurational and conformational mobility. The ESR and ENDOR spectra indicate that at higher temperatures (T > 160 K), exists as a mixture of several distinct isomers. One of them, the sole product at T < 160 K, is found to be energetically preferred. The configuration and the conformation of this species can be determined by interpretation of the hyperfine data in terms of a singly occupied MO of the 14-membered π-perimeter.

ESR-Spectroscopic Investigations on the Radical Cations of p-Phenylene Diamines and Related Compounds. The Determination of the Various Spin Polarisation Parameters

Article ESR-Spectroscopic Investigations on the Radical Cations of p-Phenylene Diamines and Related Compounds. The Determination of the Various Spin Polarisation Parameters was published on February 1, 1984 in the journal Zeitschrift für Physikalische Chemie (volume 141, issue 2).

Einige untersuchungen zum mechanisms der thermolyse von bis(trimethylsilyl)-diimin

To clarify the mechanism of thermolysis, decomposition of nondeuterated and deuterated bis(trimethylsilyl)diimine was examined and studied by ESR-spectroscopic investigations. It followed, that two of the five main products of thermolysis, namely part of tetrakis(trimethylsilyl)hydrazine and all of tetrakis(trimethylsilyl)tetrazene are directly formed from two molecules of bis(trimethylsilyl)diimine by disproportionation and dimerization, respectively. The other products {tris(trimethylsilyl)amine, tris(trimethylsilyl)hydrazine, bis(trimethylsilyl)amine} and a small part of tetrakis(trimethylsilyl)hydrazine originate from radical chain reactions.

ESR spectroscopic investigations of an organic radical having a quartet ground state

ESR spectroscopic investigations on the three-spin system (1) have been carried out. The radical (1) has a quartet ground state and associates to form a diamagnetic dimer. The association constant K 263 = (6.7±0.3) × 10 3 1/mole was determined by susceptibility measurements, using ESR spectroscopy. From the temperature dependence of K, a standard enthalpy of association of Δ H 0 = 0 was obtained. Thus, the dimerisation occurs only as a result of an increase in entropy. The low-temperature ESR spectrum of (1) was interpreted with a quantum-mechanical model. The zero-field parameter D′ = 44.1 G was determined by simulation of the experimental spectrum.