Self-diffusion and electron spin exchange of hydrophobic probes in dilute solution

Electron spin exchange rate constants have been measured by ESR spectroscopy for a nitroxide spin probe in a number of solvents, including water. The apparent collision rate constants (k _c ^′ ) calculated from the spin exchange rate constants showed marked deviations from the Smoluchowsky equation (k _c ^′ η=const), which were greatest in solvents of lowest viscosity. These effects are attributed to inefficiency of the spin exchange process. Self-diffusion coefficients (D) were measured for diamagnetic analogs of the nitroxide spin probe in similar solvent systems by pulsed field gradient NMR spectroscopy. TheD values gave reasonable agreement when corrected for viscosity (Dη=const). Collision rate constants calculated fromD were in good agreement with those measured by ESR in solvents of high viscosity. Thek _c ^′ value for the spin probe in water was significantly lower than that in isoviscous organic solvents. This effect is discussed in terms of a hydrophobic hydration shell for the spin probe which acts as an additional barrier to collision.     

Isolation,properties and association phenomena of alkaline salts and their crown complexes of a radical anion

The obtention of (tetradecachloro-4-oxidotriphenylmethyl)^–. M⁺ (M=Li, Na, K, n-Bu₄N) salts in ethereal solution and the isolation of some alkaline complexed salts (M=Li-12C4, Na-18C6, K-18C6, K(THF)₁–(H₂O)_3–4, n-Bu₄N) are described and discussed. The association phenomena of these salts has been studied by electronic spectroscopy, osmometry and electron spin resonance. Linear correlations between radii counterions and the position maxima of the electronic spectra bands permit the study of the species present in solution (free ions, ion pairs and quadrupolar aggregates).     

Angular dependent linewidths of ESR spin probes in oriented smectic systems. An application to oriented lecithin—cholesterol multibilayers

A theory, based on the diffusion model of molecular reorientation extended with an orienting potential, is developed for the angular dependence of the ESR linewidths of symmetrical top radicals in oriented smectic liquid crystals. The theoretical results have been compared with linewidth data obtained with the cholestane spin label in oriented multibilayers of equimolar quantities of dipalmitoyl lecithin and cholesterol. For the determiination of the linewidths, the ESR spectra were fitted to a line shape function, which includes unresolved proton hyperfine interactions.From the comparison of theory and experiment the rotational diffusion tensor and order parameter S can be found, giving D_⊥ = 3.4 × 10⁶ s^?1, D|/D_⊥ = 40 and S = 0.86 at 49°C. The high value of D|/D_⊥ indicates a strong anisotropic motion of the cholestane spin label. The order parameter found is in good agreement with the order parameter calculated from the angular dependence of the ESR line positions.     

Ligand exchange reactions between copper(II)- and nickel(II)- chelates of different sulphur- and selenium-containing ligandsIII: Single-crystal ESR study on the mixed-ligand chelate (n-Bu4N) [63Cu(mnt) (et2dsc)] and crystal and molecular structure of (n-Bu4N) [Ni(mnt) (et2dsc)]

A single-crystal ESR study of the mixed-ligand complex tetra - n - butylammonium - (maleonitriledithiolato)(diethyl - di - selenocarbamato)cuprate(II) diamagnetically diluted by the corresponding Ni(II) complex, and the crystal and molecular structure of the host complex determined by a single-crystal X-ray diffraction study, is reported. (n-Bu₄N)[Ni(mnt)(et₂dsc)] is monoclinic, space group C2/c, with eight molecules in the unit cell; a = 19.932(9) Å, b = 9.597(3) Å, c = 37.580(13) Å and β = 101.32(3)°. Based on the structure the spin Hamiltonian parameters have been calculated from the results of extended Hückel molecular orbital calculations and compared with the experimentally obtained values. The principal axes of the g? and the metal hyperfine tensor do not coincide reflecting the large influence of the et₂dsc^? ligand for which this behaviour was found to be typical. A comparison between the ESR parameters and the Mulliken spin densities of the mixed-ligand chelate and the corresponding parent complexes shows that in the former one spin density appears to be transfered from the S atoms to the Se atoms of the et₂dsc^? part of the complex resulting in an increased CuSe covalency. Back-bonding effects were considered in discussing the bonding properties.     

Molecular orbital calculations on transition metal complexes. Part IX

Potential energy curves for the ground and some low energy excited states of a number of complexes with a 3d ⁵ electronic configuration have been computed from INDO type SCF MO calculations. The results agree extremely well with the known ground states of the complex ions MnF ₆ ^4? , FeF ₆ ^3? , CoF ₆ ^2? , and Fe(CN) ₆ ^3? , in particular the crossover from high to low spin being obtained for changes in both central metal ion oxidation state and ligand. The calculated contraction in metal ligand distance on passing from the high spin to the low spin state is ～ 0.05 Å for each complex in very good agreement with the value indicated by pressure dependent magnetic measurements. Computed electronic transition energies involving bothd-d type and charge-transfer excitations compare favourably with observed spectroscopic values.     

Spin-flop and spin-Peierls transition in doped CuGeO3

Ni⁺⁺ ions doped inorganic CuGeO₃ sample has been studied by using electron spin resonance (ESR) technique in the temperature range of 3–300 K. The ESR spectrum of Cu⁺⁺ ion has been observed to be strongly temperature dependent for inorganic spin-Peierls (SP) Cu_0.96Ni_0.04GeO₃ samples. The ESR line width and ESR amplitude exponentially vanishes below a critical temperature, T_sp = 14 K. The one-dimensional (1D) antiferromagnetic (AF) spin chain formed of Cu⁺⁺ is broken by Ni⁺⁺ (spin-1) ion, giving uncoupled spins at the end of the chains that give extra contribution to the spectra at lower temperature and stabilizes a Néel state. The g-factor is much smaller than the expected value for isolated Cu⁺⁺ and Ni⁺⁺ ions and is much more anisotropic than for undoped samples [O. Yalçın, B. Aktaş, J. Magn. Magn. Mater. 258/259 (2003) 137 (reference therein)]. It is shown that the ground state of dimerized spins is singlet. The spin-flop (SF) phenomenon is obtained from AF state mixed condition and then ferromagnetic (FM) state. The spin-flop field slightly increases when increasing temperature in the temperature range 100–300 K. The SF transition is showed almost AF order for Cu_0.96Ni_0.04GeO₃.     

Modulation effects in the electron spin echo of trans bis-(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals

Electron spin echo modulation effects have been investigated in trans bis(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals involving ¹⁴N or ¹⁵N nuclei in different positions. The modulation frequencies are shown to be the nitrogen nuclear transition frequencies in radical fragments at m_S = 0 of a triplet electronic spin. Quadrupole splitting parameters for these nitrogen nuclei have been determined.     

Free radical copolymerization in the presence of lewis acids. I. Alternating copolymerization of vinyl acetate with acrylic acid in the presence of GeCl4 and BCl3

Alternating copolymers of vinyl acetate (VAC) and acrylic acid (AA) were obtained by free radical polymerization in the presence of GeCl₄ and BCl₃. For the GeCl₄ system, the reaction rate was proportional to [initiator]^1/2. Optimum rate was obtained when the molar ratio of the monomers was 1:1. The chain transfer agent CCl₄ had no effect on the reaction. By means of ultra-violet spectra analysis, it was concluded that both VAC and AA formed complexes with GeCl₄. ESR analysis gave us the information that salt complexed acrylic acid radical had greater cationic characteristics than uncomplexed radical. Thus the nature of alternation may be due to both complexed AA radical and activated monomer complexes.     

On optically detected ESR spectra from radical cations of liquid hydrocarbon solvents

The optically detected electron spin resonance (OD ESR) method has been employed to study the origin of radical-cation ESR signals in some saturated hydrocarbons with small amounts of 2.5-diphenyloxazol or p-terphenyl under radiolysis. In cyclohexane, the ESR, signal with resolved hyperfine structure was ascribed to c-C₆H₁₀⁺/PPO^? radical-ion pairs produced from primary c-C₆H₁₂⁺/PPO^? ones by monomolecular decay, of cyclohexane radical cations to cyclohexene radical cations. Cis- and trans-decalin under radiolysis accumulate 9,10-octalin which captures solvent holes and form 9,10-octalin radical cations giving a resolved OD ESR spectrum. 9,10-octalin is present in non-irradiated commercial decalin as an impurity. The OD ESR technique has been shown to be very sensitive to some impurities in hydrocarbon solvents.     

Substituent effects in π-(tricarbonylchromium)arenes: V. Thermodynamic dissociation constants of some substituted π-(tricarbonylchromium)benzoic acids

The thermodynamic dissociation constants of a series of 38 substituted π-(tricarbonylchromium)benzoic acids in 50% aqueous ethanol at 25°C have been determined. The results require revision of some literature values.The pK_a^*-values of the π-(tricarbonylchromium)benzoic acids were correlated with the electronic substituent parameters in terms of the Yukawa-Tsuno equation. The reaction constant (ρ) decreases from 1.4 for the benzoic acids to 0.8 for the π-(tricarbonylchromium)benzoic acids, reflecting the decreased ability of the complexed aromatic system to transmit electronic substituent effects. For the alkylsubstituted π-(tricarbonylchromium)benzoic acids, conformational effects of the Cr(CO)₃ group can account for some of the anomalies observed. The substituent parameters, σ_meta and σ_para, of the π-(Cr(CO)₃)phenyl group as a substituent were derived from the dissociation constants of the complexed phenylbenzoic acids.     

Stabilizing and suicide-peroxide protecting effect of Ni2+ on horseradish peroxidase

The present research discusses the structure stabilizing and protecting effects of Ni²⁺ against suicide-peroxide inactivation of horseradish peroxidase (HRP). Suicide inactivation of HRP by hydrogen peroxide (3 mM) was monitored by measuring change in the absorbance of the colored product (tetraguaiacol) of the catalytic reaction cycle at 470 nm. Progress curves of the catalytic reaction cycle were obtained at 27 °C, phosphate buffer (5 mM), pH 7.0. The corresponding kinetic parameters (e.g., initial enzyme activity (α_o) and the apparent rate constant (k_i) of suicide inactivation of HRP by peroxide) were evaluated using a kinetic equation derived in this study. Comparative activatory and inhibitory effects of Ni²⁺ on the kinetics of suicide-peroxide inactivation of HRP are discussed.     

Characterization and physical properties of Li2O-CaF2-P2O5 glass ceramics with Cr2O3 as a nucleating agent—Physical properties

In this paper, studies on various physical properties, viz., dielectric properties (dielectric constant, loss tan δ, a.c. conductivity σ) over a wide range of frequency and temperature, optical absorption, ESR at liquid nitrogen temperature and magnetic susceptibility at room temperature of Li₂O-CaF₂-P₂O₅: Cr₂O₃ glass ceramics, have been reported. The optical absorption, ESR and magnetic susceptibility studies indicate that the chromium ions exist in Cr⁵⁺, Cr⁴⁺ and Cr⁶⁺ states in addition to Cr³⁺ state in these samples. The dielectric constant and loss variation with the concentration of Cr₂O₃ have been explained on the basis of space charge polarization mechanism. The dielectric relaxation effects exhibited by these samples have been analysed by a graphical method and the spreading of dielectric relaxation has been established. The a.c. conductivity in the high-temperature region seems to be connected both with electronic and ionic movements.     

Spectroscopic and thermal investigations of complexes of Cr<Superscript>III</Superscript>, Mn<Superscript>II</Superscript>, Zn<Superscript>II</Superscript> &; Cd<Superscript>II</Superscript> - carboxamide

Complexes of Cr^III, Mn^II, Zn^II & Cd^II with the polydentate carboxamide ligandN′, N′′-bis(3-carboxy-1-oxoprop-2-enyl) 2-Amino-N-arylbenzamidine (H₂L) have been synthesized and characterized by elemental analyses, spectroscopic studies (Vibrational, electronic, ESR and ¹H-NMR), magnetic susceptibility measurements, thermal studies and powder diffraction studies. The vibrational spectral data are in agreement with coordination of amide and carboxylate oxygen of the ligands with the metal ions. The electronic spectra indicates octahedral or tetrahedral geometry around the metal ions, has been supported by magnetic susceptibility measurements. The results of electron spin resonance & ¹H-NMR spectra have supported the results of other spectral techniques. Kinetic and thermodynamic parameters were computed from the thermal data using Coats and Redfern method, which confirm first order kinetics. Powder diffraction determines the cell parameters of the complexes.     

Spin polarization in the lowest triplet state of chlorophyll

Chlorophyll-b in glassy solution has a spin-polarized lowest triplet state at and above 77 K. The magnitude of the effect is different for MTHF and ethanol as solvents, in contrast to what is found for the porphin free base. Chlorophyll-a does not exhibit spin-polarization under identical conditions as for chlorophyll-b. Zero-field parameters are found to be:chlorophyll-a (MTHF) D = (281 ± 6) × 10^?4 cm^?1; E = (39 ± 3) × 10^?4 cm^?1;chlorophyll-b (MTHF) D = (289 ± 4) × 10^?4 cm^?1; E = (49 ± 3) × 10^?4 cm^?1,From ESR signal kinetics it follows that for chlorophyll-b, population and depopulation mainly involve the spin level y?, describing a spin moving in a plane perpendicular to the molecular plane:P_y ? P_x ? P_z; k_x = 240 ± 40 s^?1; k_y = 600 ± 120 s^?1; k_z ? 75 s^?1,where P_i and k_i denote populating and decay rates. Thus, the kinetic scheme for the chlorophyll triplet is different from that of porphyrins with heavier metal ions, but very similar to that of the porphin free base. The spin-lattice relaxation time is found to be anisotropic and shorter than the decay rates of individual spin levels. Nevertheless, spin polarization can be observed, essentially because the ESR signal amplitude depends on population differences.     

Copper(II) complexes of thiosemicarbazones derived from 2-acetylpyridine and its n-oxide

A series of Cu(II) complexes of the thiosemicarbazone, 3-azabicyclo[3.2.2]-nonene-3-thiocarboxylic acid 2-[1-(2-pyridinyl)ethylidene]hydrazide(HL) and the corresponding N-oxide (HLO) have been prepared and characterized. Both ligands undergo deprotonation and appear to coordinate via the thione sulfur, the imine nitrogen and the pyridyl nitrogen (or N-oxide oxygen). A single anionic ligand such as Cl^?, Br^?, NCS^? and N^?₃ completes the bonding to the Cu(II) center of these 4-coordinate complexes. When the complexes are prepared using Cu(II) perchlorate, the solids isolated contain a neutral thiosemicarbazone ligand as well as the deprotonated ligand. The solids are primarily characterized by IR, electronic and electron spin resonance spectroscopy. In addition, electronic and ESR spectra of their chloroform solutions were recorded. Most of the solids (except the nitrates) were unaltered upon dissolution. Simulation of the solution ESR spectra was used to estimate the coupling constants of the various coordinated nuclei.     

Intermolecular nuclear relaxation in paramagnetic solutions: from free radicals to rare earths

The principles of the intermolecular relaxation of a nuclear spin by its fluctuating magnetic dipolar interactions with the electronic spins of the paramagnetic surrounding species in solution are briefly recalled. It is shown that a very high dynamic nuclear polarization (DNP) of solvent protons is obtained by saturating allowed transitions of free radicals with a hyperfine structure, and that this effect can be used in efficient Earth field magnetometers. Recent work on trivalent lanthanide Ln³⁺ aqua complexes in heavy water solutions is discussed, including paramagnetic shift and relaxation rate measurements of the ¹H NMR lines of probe solutes. This allows a determination of the effective electronic magnetic moments of the various Ln³⁺ ions in these complexes, and an estimation of their longitudinal and transverse electronic relaxation times T_1e and T_2e. Particular attention is given to Gd(III) hydrated chelates which can serve as contrast agents in magnetic resonance imaging (MRI). The full experimental electronic paramagnetic resonance (EPR) spectra of these complexes can be interpreted within the Redfield relaxation theory. Monte-Carlo simulations are used to explore situations beyond the validity of the Redfield approximation. For each Gd(III) complex, the EPR study leads to an accurate prediction of T_1e, which can be also derived from an independent relaxation dispersion study of the protons of the probe solutes.     

Esr spectra of semidiones derived from indandione-1,3

The formation of free radical anions of 18 various indandione-1,3 derivatives and their aza analogues, I–III, during the process of electrochemical reduction on a mercury electrode in N,N-dimethylformamide has been proved by ESR spectroscopy.In the case of 2-substituted indandione-1,3 derivatives of non-fixed diketo structure, ESR spectra of the one-electron reduction products have been registered, while in the case of 2,2-substituted compounds, ESR spectra of both one- and three-electron reduction products have been obtained. Unpaired electron distribution in free radical anions and the relative electrophilic values in the corresponding positions of the phthaloyl ring have been evaluated on the basis of hyperfine structure (HFS) of the ESR spectral obtained.In the case of compounds I (X ≠ NO₂), the unpaired spin is mainly localized (~ 0.8 of an electron) on the carbonyls, but a fairly high unpaired spin density is created in the 5- and 6-positions of the phthaloyl ring whilst that in the 4- and 7-positions is insignificant. A high unpaired electron density is created on the nitrogen atom of compound III, whilst the unpaired electron density on the hetero-atom in compound II is insignificant.Free radical products have also been found in the reduction process by alkaline metals in dimethoxyethane. However, in this case a mixture of radical ions is formed, their composition being difficult to identify from the HFS of the ESR spectra.     

Distortion of the coordination polyhedron in mononuclear copper(II) complexes. Optical absorption and electron spin resonance studies on some five-coordinate copper(II) complexes

The preparation and spectral properties are reported for a series of ten mixed-ligand copper(II) complexes of the form [Cu(A)(B)_n](X), where A is acetylacetonate anion, B represents a mono- or bidentate ligand such as morpholine, piperidine, isonicotinic acid hydrazide, 2,2′-bipyridine, 1,10-phenanthroline and X is variously NO^?₃ or ClO^?₄ anion and n = 1 or 2. The coordination of the anions and ligands has been demonstrated by infrared and electronic spectral methods. Electron spin resonance spectral data show the square-pyramidal five-coordinated arrangement around copper(II) in these complexes. Parameters such as g₆, g_⊥, A₆, A_⊥, 〈g〉 and 〈A〉 calculated from electron spin resonance data in solid and solution state at room temperature as well as frozen solution indicate the presence of the unpaired electron in the d_x2_?y2 or d_z2 orbital. These results are in good agreement with electronic and photoacoustic spectral studies.     

Cyclopericodiol and new chlorinated melleins isolated from Periconia macrospinosa KT3863

Cyclopericodiol (1) and two new chlorinated melleins (2 and 3) were isolated from Periconia macrospinosa KT3863. The results of density functional theory (DFT)-based calculations while estimating the values of the ¹³C chemical shifts and the spin coupling constants allowed to determine the relative configurations of 1 and 2. The absolute configurations of these compounds were established by comparing the experimental electronic circular dichroism (ECD) spectra with those obtained while performing time-dependent DFT calculations. Interestingly, the methyl and hydroxy groups at C3 and C4, respectively, of 2 assume an 1,2-anti-diaxial conformation due to absence of 1,3-diaxial repulsion. This conclusion was supported by the observation that 2 afforded an ECD spectrum that was almost the mirror image of that of 3. Furthermore, the theoretical spin coupling constants ³J_H-3/C1, ²J_H-3/C4, ³J_H-3/C4a, and ²J_H-3/C9 were in agreement with the heteronuclear multiple-bond correlation (HMBC) intensities of H-3 with C1, C4, C4a, and C9.     

Saturation ESR spectroscopy of PA12 polyamide

This work presents the results of our EPR study of polyamide 12 (PA12) -[NH-(CH₂)₁₁-CO]_n-NH-(CH₂)₁₁-CO-. In this study the spin probe method with “conventional” or “continuous saturation” at low temperatures (4-260 K) was used. The aim of our investigation was to determine if it possible to obtain the saturation effects and establish experimental conditions by using commercial ESR spectrometers. We also wanted to know what information about the relaxation processes in polymeric materials could be obtained. We have found that the information about the relaxation processes (below frequency ≈10⁴ Hz) at low-temperatures in polymers could be obtained. The process, which could be associated with mobility of the spin probe is probably connected with the process called γ (in PA12 as yet unrecorded by using others methods). This process in other systems is usually connected with mobility of the methylene sequences and could be used to study the amorphous regions of PA12.