ESR spectroscopy of copper(ii) complexes with aza- and thia-containing crown ethers

Six copper(ii) chloride complexes with crown ethers containing besides oxygen also nitrogen or sulfur atoms in 15- or 18-membered cycle were studied by ESR and electron absorption spectroscopies. Theg and HFI tensor components determined by spectral simulation indicate rhombic symmetry and localization of an unpaired electron on the d _xy orbital for all the complexes. The unpaired electron fractions on - and -type metal ion and ligand AO were estimated from ESR and absorption spectra using LCAO MO method. Both - and -type bond covalences were shown to be greater in these complexes compared to only oxygen-containing crown ether complexes. The temperature dependence of g and A components in some complexes may be due to conformational changes.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1938–1944, November, 1994.The present work was carried out with financial support from the Russian Foundation for Basic Research (Project 93-03 04089).     

ESR study of carborane containing organofluorine radicals

It has been shown by ESR that the addition of photochemically generated boroncentered carboranyl radicals to branched fluoroalkenes affords stable spin-adducts. It has been found that the addition of boron-centered radicals with 1-substituted 2-fluoroalkenyl-o-carborane gives stable -carboranyl radicals. Negligible variations in the values of the constants of hyperfine coupling of the unpaired electron with the nucleus of the -¹¹B atom in radicals of various structures imply the stability of the carborane conformation with respect to the 2p _z-orbital of the unpaired electron. This may be caused by hyperconjugation between the boron atom and the unpaired electron as well as steric interaction.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1316–1318, July, 1993.     

What Hinders Electron Transfer Dissociation (ETD) of DNA Cations?

Radical activation methods, such as electron transfer dissociation (ETD), produce structural information complementary to collision-induced dissociation. Herein, electron transfer dissociation of 3-fold protonated DNA hexamers was studied to gain insight into the fragmentation mechanism. The fragmentation patterns of a large set of DNA hexamers confirm cytosine as the primary target of electron transfer. The reported data reveal backbone cleavage by internal electron transfer from the nucleobase to the phosphate linker leading either to a?/w or d/z? ion pairs. This reaction pathway contrasts with previous findings on the dissociation processes after electron capture by DNA cations, suggesting multiple, parallel dissociation channels. However, all these channels merely result in partial fragmentation of the precursor ion because the charge-reduced DNA radical cations are quite stable. Two hypotheses are put forward to explain the low dissociation yield of DNA radical cations: it is either attributed to non-covalent interactions between complementary fragments or to the stabilization of the unpaired electron in stacked nucleobases. MS³ experiments suggest that the charge-reduced species is the intact oligonucleotide. Moreover, introducing abasic sites significantly increases the dissociation yield of DNA cations. Consequently, the stabilization of the unpaired electron by π–π-stacking provides an appropriate rationale for the high intensity of DNA radical cations after electron transfer.

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π-Electron structure of the nitrofuran system

Distribution of the unpaired electron in the first free MO, the partial bond orders created by this electron, and the energy level corresponding to it in anion radicals of the nitrofuran series have been examined on the basis of quantum-chemical calculations by the MOH method of the -electron system of 5-nitrofuran and its 2-substituted derivatives. Distribution of the spin density of the electrons in the anion radicals has also been calculated by McLachlan's method. The parameters of the first free MO obtained by calculation agree satisfactorily with the physicochemical characteristics of the corresponding particulars measured experimentally (HFS constants in the EPR spectra and polarographic half-wave potentials).     

EPR Spectrum of bis(benzonitrile)chromium anion-radical

Conclusions On the basis of the EPR spectra it was shown that in the bis(benzonitrile)chromium anion-radical the unpaired electron occupies an unoccupied orbital that is predominantly represented by the orbital of the ligand.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2392–2394, October, 1982.     

A comparison of the electron paramagnetic resonance spectra of the anion radicals of piazthiole and piazselenole and theirα,Β-dichloro derivatives

It has been shown by a comparison of the EPR spectra of the anion radicals of piazthiole and piazselenole and their, -dichloro derivatives that the density of the unpaired electron in the anion radical of piazthiole is highest at the carbon atom.     

Effect of the heteroatom on the one-electron reduction of α-nitro derivatives of five-membered heterorings

The potentials of the first polarographic half waves of the electrical reduction in dimethylformamide (DMF) of -nitro derivatives of furan, thiophene, selenophene, and their analogs in the benzene series and the hfs constants of the EPR spectra of the electrochemically generated anion radicals of these compounds are presented. It was established that these parameters in the series of examined heterorings regularly follow the electron affinities of the heteroatoms in their s²p²pp valence state. The relative conductivity of the electronic effects of substituents is higher (1.08) in the level of the unpaired electron of the furan ring, while the conductivities in the thiophene (0.95) and selenophene (0.94) ring are lower, than in the case of the benzene ring. The conductivity through the heterorings in the molecular state is higher by a factor of 1.2 than in the level of the unpaired electron.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1606–1612, December, 1980     

Electronic structure and N KVV Auger spectrum of the ammonium radical

A procedure for calculating multiplet-splitting energies in systems with an unpaired electron has been presented. The electronic structure and N KVV Auger spectrum of the ammonium radical have been calculated by the discrete-variation-X method. The results have been compared with experimental data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 10–18, January–February, 1985.     

Unrestricted CNDO-MO calculations. II. MnO 4 2− and CrO 4 3−

The electronic structures of the tetrahedral molecule ions MnO ₄ ^2– and CrO ₄ ^3– have been investigated within an unrestricted CNDO-MO approximation [Theoret. Chim. Acta (Berl.)20, 317 (1971)]. Calculations assuming the unpaired electron occupies the 3a ₁, 2e, and 4t2 molecular orbitals indicate that the 3a ₁ and2e orbitals have similar orbital energies and that the 4t ₂ orbital is at a higher energy. The experimentally indicated2e orbital for the unpaired electron is obtained with expanded O^1– type atomic orbitals for oxygen and valence metal orbitals of the expanded 3d and plus one ion 4p types. The metal 4s orbitals must be held to the neutral atom type. The optimum valence orbitals above with a slightly contracted 4s type metal orbitals yield the minimum total energy and places the unpaired electron in the 3a ₁ orbital. Since the contracted 4s metal orbital produces results that are not in agreement with experimental data, the method used apparently does not adequately take into account the increased electron-electron repulsions that contracted 4s orbitals produce.     

Disulfido iron-manganese carbonyl cluster complexes: Synthesis, structure, bonding and properties of the radical CpFeMn2(CO)7(μ3-S2)2

Two new compounds CpFeMn₂(CO)₇(μ₃-S₂)₂ (2) and Cp₃Fe₃Mn(CO)₄(μ₃-S₂)₂(μ₃-S) (3) were obtained by the treatment of [CpFeMn(CO)₅(μ₃-S₂)]₂ (1) with CO at room temperature in the presence of room light. Compound 2 contains two triply bridging disulfido ligands on opposite sides of an open FeMn₂ triangular cluster. EPR and temperature-dependent magnetic susceptibility measurements show that it is paramagnetic with one unpaired electron per formula equivalent. The electronic structure of 2 was established by DFT and Fenske-Hall (FH) molecular orbital calculations which show that the unpaired electron occupies a low lying antibonding orbital that is located principally on the iron atom. The cyclic voltammogram of 2 exhibits one reversible one-electron oxidation wave at +0.34 V and one irreversible one-electron reduction wave at −0.66 V vs. Ag/AgCl. Compound 3 contains three iron atoms and one manganese atom with two triply bridging disulfido ligands and one triply bridging sulfido ligand and has no unpaired electrons. The molecular structures of compounds 2 and 3 were established by single crystal X-ray diffraction analyses.     

Synthesis,Structure, and Radical Anion of the First Stable p‐Phosphaquinone

Significant localization of the unpaired electron on the phosphorus atom of phosphasemiquinone radical anion 1 was revealed by EPR spectroscopy. This species was generated by reduction of 1 , the first stable p‐phosphaquinone. Employment of the 3,5‐di‐tert‐butyl‐4‐oxocyclohexa‐2,5‐dien‐1‐ylidene moiety was essential for the synthesis as well as the effective kinetic protection of 1 .     

Thermochemical studies of substituted phenylnitrenes: Enthalpies of formation of chlorophenylnitrenes

The gas-phase acidities of chloroanilino-radicals have been measured, and have been combined with the electron affinities of chlorophenylnitrenes to determine the N–H bond dissociation energy of chloroanilino-radicals and the enthalpies of formation for the triplet, singlet, and radical anion states of the isomeric nitrenes. There is little difference found between the bond dissociation energies in the radicals and those in the corresponding anilines, indicating little interaction between the unpaired electrons in the chlorophenylnitrene, as expected. The values obtained are in good agreement with the values obtained from the theoretical calculations.

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Electron Delocalization in 3d Complexes Caused by Dimethylurea Checked by Positron Annihilation

The rate constants, k _CR, of ortho- into para-positronium conversion reactions promoted by paramagnetic 3d complexes are linearly correlated with the electron delocalization, , of unpaired metal electrons caused by ligands, being the ratio between the inter-electronic repulsion parameters in complexes and in the free gaseous ions. By applying a procedure previously described the values of Mn^II, Co^II, Ni^II complexes with dimethylurea were deduced from the mentioned correlations and compared with those of complexes with urea obtained both by the method of Ps reactions here applied and that based on UV-Vis absorption spectroscopy.     

Cyclic voltammetry of tris(2,2′-bipyridine)zinc(ii) diperchlorate detected by electron spin resonance

Electrochemical transformations of the tris(2,2′-bipyridine) complex of zinc(ii) perchlorate were studied by cyclic voltammetry detected by electron spin resonance (DESR CV), which made it possible to indentify the intermediates formed and to monitor the unpaired electron localization in them.     

Interaction of polyphenylacetylene with electron-acceptor molecules

ESR spectra show that the number of paramagnetic centers in polyphenylacetylene (PPA) is linearly related to the oxygen pressure; iodine has an even more marked effect, the limiting concentration (7×10¹⁸ spins per g) being reached for a molar ratio of PPA to iodine of 16. The central part of the ESR line is lorentzian, with gaussian wings. A Lorenz curve fits a large fraction of the spectrum for PPA-iodine. The delocalization region for the unpaired electron corresponds to two units of PPA for oxygen and one for iodine. The results are interpreted in terms of charge-transfer complexes (CTC) between the PPA and electron acceptor (oxygen or iodine). Microwave saturation occurs in pumped PPA; loss of saturation is related to the formation of CTC.     

Dynamic effects in ESR spectra of 4-triphenylmethyl-6-tert-butyl-2-hydroxyphenoxyl and 4-triphenylmethyl-6-phenyl-2-hydroxyphenoxyl

Conclusions In the ESR spectra of 6-triphenylmethyl-4-tert-butyl-2-hydroxyphenoxyl and 6-triphenylmethyl-4-phenyl-2-hydroxyphenoxyl and their deuterated analogs, temperature-dependent dynamic effects have been found; these are explained from the standpoint of a reversible intramolecular transition between different conformations of the radicals, differing in the spin density distribution, owing to direct interaction of the unpaired electron with the orbitals of the phenyl rings of the triphenylmethyl substituent.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1513–1517, July, 1985.     

Aminoxyl Radicals from N-Phenyl-1-(2-oxo-1-azacycloalkyl)-methanesulfonamides

Summary. The synthesis and reaction with two oxidation agents is described for N-phenyl-1-(2-oxo-1-azacycloalkyl)methanesulfonamides. Their oxidation was carried out using radicals and 3-chloroperbenzoic acid. In both cases, the EPR spectra of corresponding aminoxyl radicals were recorded. Their simulation confirmed that the –SO₂– group in the neighbourhood of the – – fragment does not prevent the interaction of the unpaired electron with the methylene protons and the nitrogen atom of the heterocyclic ring.     

Complete Mapping of a Cystine Knot and Nested Disulfides of Recombinant Human Arylsulfatase A by Multi-Enzyme Digestion and LC-MS Analysis Using CID and ETD

Cystine knots or nested disulfides are structurally difficult to characterize, despite current technological advances in peptide mapping with high-resolution liquid chromatography coupled with mass spectrometry (LC-MS). In the case of recombinant human arylsulfatase A (rhASA), there is one cystine knot at the C-terminal, a pair of nested disulfides at the middle, and two out of three unpaired cysteines in the N-terminal region. The statuses of these cysteines are critical structure attributes for rhASA function and stability that requires precise examination. We used a unique approach to determine the status and linkage of each cysteine in rhASA, which was comprised of multi-enzyme digestion strategies (from Lys-C, trypsin, Asp-N, pepsin, and PNGase F) and multi-fragmentation methods in mass spectrometry using electron transfer dissociation (ETD), collision induced dissociation (CID), and CID with MS³ (after ETD). In addition to generating desired lengths of enzymatic peptides for effective fragmentation, the digestion pH was optimized to minimize the disulfide scrambling. The disulfide linkages, including the cystine knot and a pair of nested cysteines, unpaired cysteines, and the post-translational modification of a cysteine to formylglycine, were all determined. In the assignment, the disulfide linkages were Cys138–Cys154, Cys143–Cys150, Cys282–Cys396, Cys470–Cys482, Cys471–Cys484, and Cys475–Cys481. For the unpaired cysteines, Cys20 and Cys276 were free cysteines, and Cys51 was largely converted to formylglycine (>70 %). A successful methodology has been developed, which can be routinely used to determine these difficult-to-resolve disulfide linkages, ensuring drug function and stability.      

EPR spectra and π-electronic structures of anion radicals of a nitro-substituted indoline spiropyran

Electrochemical generation was used to obtain anion radicals of 1,3,3-trimethyl-5-nitrospiro (indoline-2,2-[2H]chromene), and their EPR spectra in acetonitrile, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) were recorded. On the basis of an analysis of the hyperfine structure (hfs) of the EPR spectra it was concluded that, as a consequence of the relatively weak interaction of the -systems of the indoline and benzopyran fragments, cleavage of the bond between the spiro carbon atom and the oxygen atom in the pyran ring does not occur in the formation of the anion radical, and the additional unpaired electron is delocalized in the -system of the indoline fragment of the molecule.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1477–1481, November, 1984.     

Structure of the intermediate particles in the radiolysis of indolinespiropyran

Calculations by the extended Hückel iteration method with self-consistency with respect to the charges were undertaken for the initial molecule, the radicalanion, and the neutral radical formed during protonation of the latter in 1,3,3-trimethylspiro[indoline-2,2-(2H)-chromene]. On the basis of the results from the calculations and analysis of the ESR spectrum of the neutral radical it was concluded that the bond between the spiro carbon atom and the oxygen heteroatom of the pyran ring is not broken during the formation of the radical-anion and its subsequent protonation and that the unpaired electron is delocalized in the system of the benzopyran fragment of the molecule. Protonation in the radicalanion takes place at the carbon atom at position 3.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1620–1623, December, 1986.