Diamagnetic Species Research Articles

Cytochrome c: observation of numerous single-carbon sites of the reduced and oxidized species by means of natural-abundance 13C nuclear magnetic resonance spectroscopy.

Proton-decoupled, natural-abundance (13)C nuclear magnetic resonance spectra (obtained at 15.18 MHz by the Fourier transform method) of aqueous ferrocytochrome c, ferricytochrome c, and mixtures of both species were recorded. The 18 nonprotonated aromatic carbons of amino-acid residues and the 16 nonprotonated aromatic carbons of the heme yielded 22 narrow single-carbon resonances and 6 narrow two-carbon resonances in the spectrum of ferrocytochrome c. Only some of these resonances were detected in the spectrum of ferricytochrome c. Fast electron transfer between ferrocytochrome c and ferricytochrome c produced chemical exchange effects in spectra of mixtures of the two species: 16 nonprotonated aromatic carbons yielded narrow exchange-averaged resonances as a consequence of their small natural linewidths in both redox states and the small changes in their chemical shifts (relative to the reciprocal of the lifetime between electron exchange) when going from the reduced to the oxidized species. These peaks were assigned to carbons situated far from the iron atom. Their fast exchange behavior was used to establish a one-to-one correspondence between resonances in spectra of the two redox states. The other 18 nonprotonated aromatic carbons yielded exchange-broadened resonances as a consequence of large chemical-shift differences between the diamagnetic and paramagnetic species, and/or large paramagnetic broadening of the resonances of ferricytochrome c. We assigned these resonances (only one of which was identified in the spectrum of ferricytochrome c alone) to carbons that are near the iron atom: C(zeta) of Tyr 67, C(gamma) of His 18, and the 16 nonprotonated carbons of the porphyrin ring. Tentative specific assignments for C(zeta) of Tyr 67 (in the spectra of both redox forms) and for C(zeta) of His 18 (in the spectrum of ferrocytochrome c) are also presented.

ESR Study of MnO42− in γ-Irradiated KMnO4

When KMnO4 is γ irradiated at 77°K a single paramagnetic species is detected which is shown to be basically the manganate ion, Mn(VI)O42−, however, the single odd electron interacts at the same time with a second manganese nucleus of a neighboring diamagnetic species. Although the species giving rise to the weaker, superhyperfine 55Mn interaction could not be identified with certainty, it is probably a neighboring MnO4− ion or MnO3+ lattice defect so that the center produced might be written [Mn(VI)O42−··· Mn(VII)On7−2n]. The g tensor and both 55Mn hyperfine splitting tensors have been obtained, and the structure of the radical is discussed. On warming to room temperature a radical not containing manganese, probably O3−, is found.

Preparation and Properties of Iron(II) Complexes with Tetraoxime Analogues

Abstract Three polymeric iron(II)-tetraoxime chelates of the formula CH3C(NOH)C(NOH)-R-C(NOH)C(NOH)-CH3, where R: C6H4-O-C6H4, C6H4-C6H4, and 2,6-pyridyl, are characterized and compared with monomeric α-dioxime analogues. Analytical data indicate that in the solid state these Fe-N4 type chelates do not coordinate an axial base. Axial substitution takes place only upon dissolution into pyridine or similar basic solvents. The resemblance between the substituted polymer chelates and the monomeric dimethylglyoxime compounds, as far as their electronic spectra are concerned, is discussed. While the solid state polymers are all paramagnetic with moments 2.0–2.5 B.M., ESR data show that in solution a conversion into a diamagnetic species takes place. A strong absorption band due to the O–N stretching vibration appears at about 1650 cm−1 upon complexing of the nitrogen atoms. The paramagnetism and the reflectance spectra of the solid state samples are related to their structure.

The Photochemical Decomposition of Ethylamine as Studied by Means of the Electron Spin Resonance

Abstract The photochemical decomposition of ethylamine at low temperatures was studied using ultra-violet light of various wavelengths (120–400 nm), and the photochemical reactions were examined. The initial stage of the decomposition is the release of the hydrogen atom by far ultra-violet light. As the decomposition proceeds, the effective wavelengths for the succeeding reactions become longer. The final products of the decomposition are the methyl radical and other diamagnetic species. The effects of the matrix and the counter ion on the reaction are also discussed.

The structure, redox properties, and reactions of some planar [M II N4] chelate compounds of cobalt, nickel, and copper, and their oxidised products, including paramagnetic cobalt(III) species

An investigation is reported of the structure and some reactions of the cobalt(II), nickel(II), and copper(II) compounds of several tricyclic quadridentate di-anionic Schiff's base ligands derived from o-aminobenzaldehyde and various diamines. The ligands are 1,2-bis-(o-iminobenzylideneamino)ethane (aben), 1,2-bis-(o-iminobenzyl-ideneamino)propane (abpn), 1,3-bis-(o-iminobenzylideneamino)propane (abtn), and 1,2-bis-(o-iminobenzylideneamino)phenylene (abphen). The physicochemical data presented are supplementary to that of two recent papers in proving a four co-planar co-ordination polyhedron in the metal(II) compounds. In addition, a polarographic study of the redox behaviour of these compounds indicates that all should have chemically accessible higher oxidation states. Attempts to prepare the nickel and copper oxidation products of the type [M(L)X](X= Br, I) gave only amorphous solids which have not been characterised. But the crystalline compounds [Co(L)X](X= Cl, Br, I) which were readily isolated, are unique examples of paramagnetic cobalt(III) compounds with nitrogen-donor ligands. A square-pyramidal five-co-ordinate structure is postulated. These compounds react in donor solvents such as pyridine to form six-co-ordinate diamagnetic species of the type [Co(L)(solvent)X]. The crystalline diamagnetic compound [Co(aben)(py)Cl] has been isolated, and evidence is given for an equilibrium in the solutions in donor solvents between the diamagnetic and paramagnetic species.

The Behavior of Radical Anion of Acridine as Revealed from ESR Studies

Abstract Nature of acridine anions produced by the addition of alkali metals to the tetrahydrofuran solutions of acridine has been studied by means of ESR and VU spectra. In excess of acridine (A), a green species G is produced. This has been identified as A−Na+ (or A−K+ etc.). A−Na+ gives under suitable conditions, ESR spectra with hfs consisting more than fifty lines. This species reverts to acridine upon addition of a small quantity of water or ethanol. With the fall of temperature, the intensity of ESR spectra of G decreases remarkably and accompanied with it, UV spectra of G also decline with no change of the spectral shape, logIvs. 1/T plots where I is either the intensity of UV absorption or of the ESR signal are satisfactorily linear; this has been interpreted to be due to a large equilibrium constant of the process, 2A− Na+\ightleftarrows(Na+A−)2 in which (Na+A−)2 is a colorless diamagnetic substance. This interpretation has led to the value of enthalpy of dissociation of (Na+A−)2 as ca. 14 kcal. When sodium is in excess, a diamagnetic violet species is produced. This has been assigned as Na+A2−Na+. The chemical processes pertaining to A−Na+ and Na+A2−Na+ can be summarized as follows: (Remark: Graphics omitted.)

Complexes of 2-methylpropylenediamine with nickel(II) halides

Complexes of nickel(II) halides with 2-methylpropylenediamine (2-mepn) have been prepared and characterised. Three forms of the bis(diamine) chloride have been obtained, viz., a purple paramagnetic trans-diaquo-species, a yellow diamagnetic anhydrous tetragonal species, and a blue paramagnetic (cis-dimeric) anhydrous species. The yellow diamagnetic forms of the bromide and iodide were obtained. Each of the anhydrous complexes, in the solvents water, methanol, and ethanol, gives mixtures of diamagnetic and paramagnetic species, the ratio of which depends on solvent, anion, and temperature. In water spectrophotometric and magnetic studies showed that about 55% of the nickel(II) is present in the triplet state.

Unstable intermediates. Part XLIV. Solvated electrons: further studies of solutions of the alkali metals, including sodium in amines, by use of electron spin resonance and optical spectroscopy

The electron spin resonance absorption of the solvated sodium atom in ammonia–ethylamine mixtures is reported. The atomic characters of the other solvated alkali atoms are compared with that of sodium. The dependence of line-width upon alkali metal nuclear spin configuration is interpreted in terms of an equilibrium between two species, one having a very small hyperfine coupling and the other having a coupling close to the atomic value. Concentration-broadening of the central singlet is interpreted in terms of interactions with the diamagnetic species which account for the majority of the dissolved metal.

Nickel (II) and cobalt (II) complexes of cis - 1,2, bis (diphenylphosphino) ethylene

Nickel(II) and cobalt(II) complexes with 1,2- cis (diphenylphosphino) ethylene are reported. Magnetic susceptibility, infrared and visible spectra, and conductivity studies have been made. The existence of a spin-free spin-paired equilibrium is proposed for the nickel(II) nitrate complex. The magnetic properties and infrared data are discussed in terms of possible structures for the complexes. The magnetic moments of the nickel(II) complexes are intermediate between the values expected from a diamagnetic ana paramagnetic species. The moment for the cobalt(II) complexes are normal from species with one unpaired electron. The infrared spectra of the complexes do not show the presence of coordinate nitrate or per chlorate anions. However, the appearance of the CC stretching absorption, which is infrared inactive for the uncomplexed, symmetrical ligand indicates that some asymmetry exists about the double bond in the solid complexes

Unstable intermediates. Part XXXIV. Solvated electrons: a model for the spin-paired species in liquid ammonia

It is suggested that of the current models representing the diamagnetic species in metal–ammonia solutions, the (e22–)solv. or F′ model is the most satisfactory. Evidence for this postulate is drawn from a wide range of experimental data including optical studies, electron spin resonance and nuclear magnetic resonance spectroscopy, and from results for solutions of europium in liquid ammonia.

Electronic Processes in Solutions of Alkali Metals in Liquid Ammonia. I. A Model Including Two Diamagnetic Species

The various models which have been proposed for metal—ammonia solutions are reviewed in the light of the available experimental data. It is shown that, in order to account quantitatively for the electrical and magnetic properties of these solutions, it is necessary to assume two different species containing paired electrons. A new model postulated for metal—ammonia solutions consists of the following five species in equilibrium: metal ions, polarons or e centers, M centers, M2 centers, and M′ centers. An M′ center is assumed to consist of an electron trapped in the field of an M center. The equilibrium concentrations of the species which are assumed to be present in metal—ammonia solutions are calculated for different metal concentrations. The calculated values agree well with experimental transference numbers and magnetic susceptibilities. The expressions for the equilibrium constants are related to the binding energies of electrons trapped by the various centers, and the relative values of the trapping energies are estimated from the magnetic and transference data. It is concluded that the main contribution to the binding in all centers arises from the polarization of the dielectric. The estimate of the concentration at which a narrow conduction band may begin to form agrees with the experimental temperature coefficient of conductivity which has a sharp maximum at 0.8M.

Nuclear Transverse Relaxation in Electron-Transfer Reactions

The problem of proton magnetic resonance linewidths for molecules that are taking part in one electron transfer reactions is re-examined, and it is concluded that the ``weak-pulse'' limit (aHτp≪1) is more generally applicable than the ``strong-pulse'' limit (aHτp≫1) that was discussed by McConnell and Berger. Density matrix equations are set up using the formalism of Alexander in order to calculate line shapes from the hyperfine splitting constants and the lifetimes of the diamagnetic and paramagnetic species; and it is shown that when more than one absorption line are observed, under certain conditions, the bimolecular rate constant for electron transfer may be determined without a knowledge of the concentration of unpaired electrons. These equations are applied to the electron transfer reaction between NNN′N′-tetramethyl-p-phenylene diamine and its monocation radical in acetonitrile, and we find that the rate constant for electron transfer is (2.0±1.0)×108 liter mole−1 sec−1.

Nuclear Magnetic Resonance Studies of Electron Transfer between Tris (2,2′-dipyridyl) osmium (II) and (III) Ions, between Bis(cyclopentadienyl)iron(II) and (III) Ions, and between Tris(1,10-phenanthroline)iron(II) and (III) Ions

Changes in linewidths and positions of the proton NMR absorption peaks for the diamagnetic species on addition of the corresponding paramagnetic species are interpreted as being due to very rapid electron transfer between the two species. On the assumption that the rates of electron transfer are first-order with respect to each reactant, the following lower limits on the rate constants are obtained: Os(dipy)32+−Os(dipy)33+: k>5×104 mole−1sec−1(6°C and 26°C, D2O,5×10−3 M D+, 0.14 mole Cl−),Fe(C5H5)2−Fe(C5H5)2+: k>1×105 mole−1sec−1(26°C, CD3COCD3),Fe(phen)32+−Fe(phen)33+: k>1×105 mole−1sec−1(26°C, D2O,3 F D2SO4). The reactions could not be studied by the electron spin resonance method because the ESR spectra of the paramagnetic species showed no observable absorption peaks, presumably because the peaks are very broad.

Apparent Molar Volumes of Salts and Metals In Liquid Ammonia at 0°

Apparent molar volumes of some metals and salts in dilute liquid ammonia solutions at 0° have been measured with a direct-mixing dilatometer. There is little volume change involved in dissociation of diamagnetic metal species, but a large volume decrease results from dissociation of ion-pairs in salt solutions. Volumes are estimated for single ions and the solvated electron.

Rates of Paramagnetic Pulse Reactions by Nuclear Magnetic Resonance

Certain fast chemical reactions involving both paramagnetic and diamagnetic molecular species are termed ``paramagnetic pulse'' reactions when each reaction event results in a change in a large hyperfine magnetic field acting at one or more of the molecular nuclei. The frequency distribution of the magnetic pulses is directly related to the reaction rate. The hyperfine intensity and frequency distribution of the paramagnetic pulses affects the nuclear resonance (NMR) line shapes. The Bloch equations are used to establish a quantitative relation between nuclear resonance line shape, nuclear, and electron relaxation times, and the reaction rate. The theory is discussed in terms of the Cu63 NMR work of McConnell and Weaver on electron exchange between Cu++ and Cu+ ions in hydrochloric acid solutions, and the H1 NMR work of Bruce, Norberg, and Weissman on the electron exchange between neutral and positive ion molecules of N,N′-tetramethyl-p-phenylenediamine.

Nuclear Spin Relaxation Studies in Dilute Sodium-Liquid Ammonia Solutions

The nuclear spin relaxation times of the protons in dilute solutions of sodium in liquid ammonia have been measured at 30 Mc/sec over the room temperature-dry ice range. The data can be interpreted on the basis of an associative equilibrium existing between paramagnetic and diamagnetic solute species. This is in agreement with the results of the conventional magnetic susceptibility measurements. The paramagnetic component of the solutions seems to be about one-tenth as effective as a cupric ion in inducing nuclear relaxation.