Spin-Hamiltonian axes of some cobalt(II) Schiff base complexes from electron paramagnetic resonance in oriented nematic glass

Hoffman, Brian M.; Basolo, Fred; Diemente, Damon

doi:10.1021/ja00800a081

Cited by 27 publications

(8 citation statements)

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“…This means that a and orbitals cannot be considered as separate. Seventy-four valence orbitals have been considered, producing a molecular orbital arrangement where 41 a electrons are described by 23 a' and 18 a" molecular orbitals and 40 ß electrons by 22 a' and 18 a" molecular orbitals. The analysis of one-electron energy levels has shown (Figure 3) that the splitting produced by the spin polarization process is centered on the a' irreducible representation to which belongs the dz2 metal orbital where the maximum density of spin is localized (Table II).…”

Section: Resultsmentioning

confidence: 99%

Molecular orbital study of a cobalt reversible oxygen carrier

Fantucci¹,

Valenti²

1976

J. Am. Chem. Soc.

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MO-LCAO calculations of ground-state electronic structure of Co(acacen), Co(acacen)NH3, Co(acacen)Oz, and Co(acacen)(NH3)02 have been carried out within the INDO-UHF approximation. The distribution of the unpaired electron obtained from these calculations is in good agreement with experimental spectroscopic data. The most important observations are related to the fact that the addition of an axial ligand, such as NH3, shifts the spin density from the d, , to dZ2 orbital; molecular 0 2 interacts with such a spin density to produce an oxygenated compound where the coordinated 0 2 resembles the dioxygen molecule more than the ionic 0 2 -species. IntroductionThere is considerable interest in cobalt models of respiratory pigments which transport molecular oxygen in the biological A large variety of tetradentate Schiff base complexes of cobalt( 11) undergo reversible binding of molecular oxygen, thus producing cycles of oxygenation-deoxygenation.Despite the large amount of experimental and spectroscopic work 09 the latter complexes, the discussion of the electronic origin of cobalt reactivity with dioxygen has been always rather qualitative.',4-5 As a matter of fact the theoretical description of the ground and excited electronic states of square-planar and pentacoordinated cobalt(I1) complexes with tetradentate Schiff bases have been based, up to now, on calculation models which are not particularly suitable for covalent metal comple~es.~.' The molecular orbital approach would be more suitable for the analysis of the ground state electronic properties of this kind of molecules. However, the large number of atoms and the fact that low spin d7 cobalt(I1) complexes have open shell configurations introduces large computational difficulties for the application of the most sophisticated molecular orbital methods. At the same time the use of more simple empirical methods of the extended Huckel type8 must be considered with great care, particularly because interelectronic repulsions, which are important in open shell configurations, are completely neglected.We have carried out an INDO-UHF c a l c~l a t i o n~~~~ of the ground state of square, planar, pentacoordinated, and oxygenated species related to the complex Co(acacen), where acacenH2 is the Schiff base obtained from the condensation of ethylenediamine with acetylacetone. This method of calculation, although semiempirical, has the great advantage of partly considering interelectronic repulsions and spin polarization effects. Moreover, this is the first example of the application of this calculation method to inorganic molecules of this type.

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Section: Resultsmentioning

confidence: 99%

Molecular orbital study of a cobalt reversible oxygen carrier

Fantucci¹,

Valenti²

1976

J. Am. Chem. Soc.

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“…The majority of low-spin Co(II) complexes have an EPR absorption near the free spin value of g ~2 (Goodman & Raynor, 1970). EPR parameters for several tetracoordinated square-planar low-spin Co(II) complexes have been reported (Walker, 1970(Walker, , 1974Hoffman et al, 1973;Urbach et al, 1974;Chacko & Monoharan, 1974): complexes with symmetry of D2k or less always have one g value ranging from g = 2.6 to g = 3.8 in the plane, with the other two g values near g = 2.0. Fantucci & Valenti (1976) have proposed on theoretical grounds that the unpaired electron in square-planar, low-spin complexes is located mainly in the dyz cobalt orbital.…”

Section: Discussionmentioning

confidence: 99%

Single-crystal EPR studies of photolyzed oxy- and nitric oxide-cobalt myoglobins

Hori

Ikeda‐Saito²,

Leigh

et al. 1982

Biochemistry

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Low-temperature photodissociation of oxygen from oxy-cobalt myoglobin was studied by single-crystal electron paramagnetic resonance (EPR) spectroscopy at 5 K. The photolyzed oxy-cobalt myoglobin exhibited an EPR spectrum consisting of two nonequivalent sets (species I and II) of the principal values and eigenvectors of the g tensors: g1I = 3.55, g2I = 3.47, and g3I = 2.26 for species I, and g1II = 2.04, g2II = 1.93, and g3II = 1.86 for species II, which resembled neither the deoxy nor the oxy form. Possible models of the photodissociated state of oxy-cobalt myoglobin are proposed by comparison with cobalt porphyrin complexes. The photolyzed product of nitric oxide-cobalt myoglobin exhibited new EPR signals at g = 4.3 and a very broad signal at around g = 2. The principal g values have been determined from the single-crystal EPR measurements: g1 = 4.39, g2 = 4.27, and g3 = 4.00. Analysis of another EPR signal around g = 2 was difficult due to its broadness. Magnetic interactions were observed. An isotropic EPR signal at g = 4.3 suggested a weakly spin-coupled system between cobaltous spin (S = 1/2 or 3/2) and nitric oxide spin (S = 1/2).

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“…Two studies (2,5) have used frozen nematic solutions in an attempt to identify the axes but they disagreed as to which molecular axis will be the aligning axis in the liquid crystal. Second, the ebrs did not properly account for the large distortions from axial symmetry found in these complexes and the contribution of low lying excited quartet states to the g and A values.…”

Section: Solutions and Powders In Which It Is Not Possiblementioning

confidence: 99%