The Experts below are selected from a list of 315 Experts worldwide ranked by ideXlab platform
E Ressouche - One of the best experts on this subject based on the ideXlab platform.
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magnetization and spin density in fecp 2 a look back at the magnetic interactions in the chains fecp 2 tcne
Polyhedron, 2001Co-Authors: J Schweizer, S Golhen, E Lelievreberna, L Ouahab, Y Pontillon, Alessandro Bencini, C Carbonera, A J Epstein, Joel S Miller, E RessoucheAbstract:Abstract The discovery of ferromagnetism in the charge transfer salt [FeCp* 2 ] + [TCNE] − has raised a controversy concerning the nature of the magnetic coupling. The group of J.S. Miller invoked a McConnell II mechanism, with configuration interactions. O. Kahn and his coworkers proposed a McConnell I mechanism, implying a positive exchange due to an overlap between spin densities of opposed signs. In the first interpretation there would be a positive spin density located on the carbon of the Cp* rings, but in the second interpretation, this spin density should be negative. To clarify this mechanism, it was decided to independently determine, by polarized neutron diffraction, the spin density of [TCNE] − , associated with a nonmagnetic donor and the magnetization density of [FeCp* 2 ] + , associated with a nonmagnetic acceptor. For [TCNE] − , the measurement was straightforward, but for [FeCp* 2 ] + , we had several failures due to a loss of symmetry on cooling the samples. Finally, we could carry on this measurement on a triclinic crystal where [FeCp* 2 ] + was associated with a polyoxotungstate anion. On [TCNE] − , most of the spin density is located on the central carbons, with a noticeable amount on the terminal nitrogens. On [FeCp* 2 ] + , the Fe atoms carry a moment of 2.0 μ B , a combination of both spin and orbital moments, the carbon of the rings carry −0.005±0.001 μ B and, surprisingly, the methyl carbons carry +0.008±0.001 μ B . The signs of the spin populations support the McConnell I, but their magnitude accounts for a part of the experimental intrachain exchange interaction only. Ab initio DFT calculations predict the very small densities on the carbon atoms of [FeCp* 2 ] + , with the correct signs, but with magnitudes which are three times larger than the experimental ones.
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magnetic interactions and spin densities in molecular compounds an example
Physica B-condensed Matter, 2001Co-Authors: J Schweizer, S Golhen, E Lelievreberna, L Ouahab, Y Pontillon, E RessoucheAbstract:Abstract The discovery of ferromagnetism in the charge transfer salt [Fe(Cp2*)] +[TCNE] − has raised a lot of interest. The nature of this magnetic coupling was controversial. The group of Miller invoked a McConnell II mechanism, with configuration interactions. Another view was given by Kahn and coworkers who proposed a coupling between spin carriers (McConnell I mechanism), implying a positive exchange due to an overlap between spin densities of opposed signs. In the first interpretation, there would be a positive spin density located on the carbon rings of the ferrocene, but in the second interpretation, this spin density should be negative. To clarify this mechanism, it was decided to investigate separately, by polarized neutron diffraction, the spin density of [TCNE] −, associated with a nonmagnetic donor and the magnetization density of [Fe(Cp2*)] +, associated with a nonmagnetic acceptor. For [TCNE] −, the measurement was straightforward: it was found that most of the spin density was located on the central carbon while a noticeable amount was delocalized on the terminal nitrogens. Several attempts to measure the magnetization density of [Fe(Cp2*)] + were unsuccessful due to a loss of symmetry on cooling. To overcome this difficulty, we have finally measured the magnetization density of [Fe(Cp2*)] + in a crystal of space group P 1 , where four of these ions are associated with the polyoxotungstate [SiW12O40]4−. We have found that the Fe atoms carry a moment of 2.0 μB and that the carbons of the rings carry −0.005±0.001 μB. The signs of these carbon spin populations are consistent with the McConnell I mechanism, but their magnitudes are too small to account for the experimental interactions.
Karl K Haase - One of the best experts on this subject based on the ideXlab platform.
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a novel regional right ventricular wall motion abnormality observed in a case of acute pulmonary embolism reverse McConnell sign
Journal of The American Society of Echocardiography, 2005Co-Authors: Dariusch Haghi, Tim Sueselbeck, Tudor C Poerner, Martin Borggrefe, Karl K HaaseAbstract:Among various echocardiographic parameters for diagnosis of pulmonary embolism, an abnormal regional contraction pattern of the right ventricular free wall consisting of normokinesia of the apical segment and akinesia of the midfree wall with persistence of abnormal wall motion at the base has proved to be fairly specific for pulmonary embolism. This echocardiographic abnormality has been termed “McConnell sign.” We describe the case of a patient with acute pulmonary embolism who developed reversible akinesia of the apex and right ventricular midfree wall, a finding we would like to term “reverse McConnell sign.”
J Schweizer - One of the best experts on this subject based on the ideXlab platform.
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magnetization and spin density in fecp 2 a look back at the magnetic interactions in the chains fecp 2 tcne
Polyhedron, 2001Co-Authors: J Schweizer, S Golhen, E Lelievreberna, L Ouahab, Y Pontillon, Alessandro Bencini, C Carbonera, A J Epstein, Joel S Miller, E RessoucheAbstract:Abstract The discovery of ferromagnetism in the charge transfer salt [FeCp* 2 ] + [TCNE] − has raised a controversy concerning the nature of the magnetic coupling. The group of J.S. Miller invoked a McConnell II mechanism, with configuration interactions. O. Kahn and his coworkers proposed a McConnell I mechanism, implying a positive exchange due to an overlap between spin densities of opposed signs. In the first interpretation there would be a positive spin density located on the carbon of the Cp* rings, but in the second interpretation, this spin density should be negative. To clarify this mechanism, it was decided to independently determine, by polarized neutron diffraction, the spin density of [TCNE] − , associated with a nonmagnetic donor and the magnetization density of [FeCp* 2 ] + , associated with a nonmagnetic acceptor. For [TCNE] − , the measurement was straightforward, but for [FeCp* 2 ] + , we had several failures due to a loss of symmetry on cooling the samples. Finally, we could carry on this measurement on a triclinic crystal where [FeCp* 2 ] + was associated with a polyoxotungstate anion. On [TCNE] − , most of the spin density is located on the central carbons, with a noticeable amount on the terminal nitrogens. On [FeCp* 2 ] + , the Fe atoms carry a moment of 2.0 μ B , a combination of both spin and orbital moments, the carbon of the rings carry −0.005±0.001 μ B and, surprisingly, the methyl carbons carry +0.008±0.001 μ B . The signs of the spin populations support the McConnell I, but their magnitude accounts for a part of the experimental intrachain exchange interaction only. Ab initio DFT calculations predict the very small densities on the carbon atoms of [FeCp* 2 ] + , with the correct signs, but with magnitudes which are three times larger than the experimental ones.
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magnetic interactions and spin densities in molecular compounds an example
Physica B-condensed Matter, 2001Co-Authors: J Schweizer, S Golhen, E Lelievreberna, L Ouahab, Y Pontillon, E RessoucheAbstract:Abstract The discovery of ferromagnetism in the charge transfer salt [Fe(Cp2*)] +[TCNE] − has raised a lot of interest. The nature of this magnetic coupling was controversial. The group of Miller invoked a McConnell II mechanism, with configuration interactions. Another view was given by Kahn and coworkers who proposed a coupling between spin carriers (McConnell I mechanism), implying a positive exchange due to an overlap between spin densities of opposed signs. In the first interpretation, there would be a positive spin density located on the carbon rings of the ferrocene, but in the second interpretation, this spin density should be negative. To clarify this mechanism, it was decided to investigate separately, by polarized neutron diffraction, the spin density of [TCNE] −, associated with a nonmagnetic donor and the magnetization density of [Fe(Cp2*)] +, associated with a nonmagnetic acceptor. For [TCNE] −, the measurement was straightforward: it was found that most of the spin density was located on the central carbon while a noticeable amount was delocalized on the terminal nitrogens. Several attempts to measure the magnetization density of [Fe(Cp2*)] + were unsuccessful due to a loss of symmetry on cooling. To overcome this difficulty, we have finally measured the magnetization density of [Fe(Cp2*)] + in a crystal of space group P 1 , where four of these ions are associated with the polyoxotungstate [SiW12O40]4−. We have found that the Fe atoms carry a moment of 2.0 μB and that the carbons of the rings carry −0.005±0.001 μB. The signs of these carbon spin populations are consistent with the McConnell I mechanism, but their magnitudes are too small to account for the experimental interactions.
Michael A Robb - One of the best experts on this subject based on the ideXlab platform.
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does the McConnell i model really work an ab initio study of the magnetic character of some intermolecular contacts
Molecular Crystals and Liquid Crystals, 1999Co-Authors: Juan J Novoa, Merce Deumal, Pilar Lafuente, Michael A RobbAbstract:Abstract The performance of the first model proposed by McConnell to rationalize the nature of the magnetic interactions in purely organic molecular crystals (the McConnell-I model) has been analyzed using ab initio methods. As a first step, we analyzed the form of deriving the basic formula of the McConnell-I model from a general Heisenberg spin Hamiltonian. We reached the conclusion that it is not possible in general, although the two show some resemblance. When the same procedure is applied to the well known example of the [2.2]paracyclophanes, we found that the agreement with the experiment comes from the fortuitous cancellation of terms which is not likely to hold in general. We then studied the high-low spin energy difference for the H2NO dimer and the methyl-allyl radicals for different relative orientations of the two monomers using CASSCF calculations and the 6–31G(d) basis set. It is found that there are regions in which McConnell-I model does not reproduce the ab initio results. So, we can conc...
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on the validity of the McConnell i model of ferromagnetic interactions the 2 2 paracyclophane example
Journal of Physical Chemistry A, 1998Co-Authors: Merce Deumal, Juan J Novoa, Michael J Bearpark, Paolo Celani, And Massimo Olivucci, Michael A RobbAbstract:A formal comparison between a rigorous implementation of a Heisenberg Hamiltonian model in a VB space with the McConnell-I model shows that the validity of the McConnell model rests upon a heuristic one-to-one correspondence between spin density products in the latter with the difference of the spin exchange density matrix elements in the former. Using a rigorous Heisenberg Hamiltonian, a numerical model computation of the singlet/quintet stability for pseudoortho, pseudometa, and pseudopara bis(phenylmethylenyl)[2.2]paracyclophanes (modeled with the correponding singlet/triplet bis(methyl)[2.2]paracyclophanes) shows that the McConnell model makes the correct prediction of low and high spin stability only because the contributions from “closest contact” carbon atoms that are not directly aligned is rather small. In systems where the alignment is not perfect, this cancelation may not hold. The association between spin density products in the McConnell model with the difference of the spin exchange density ...
Dariusch Haghi - One of the best experts on this subject based on the ideXlab platform.
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a novel regional right ventricular wall motion abnormality observed in a case of acute pulmonary embolism reverse McConnell sign
Journal of The American Society of Echocardiography, 2005Co-Authors: Dariusch Haghi, Tim Sueselbeck, Tudor C Poerner, Martin Borggrefe, Karl K HaaseAbstract:Among various echocardiographic parameters for diagnosis of pulmonary embolism, an abnormal regional contraction pattern of the right ventricular free wall consisting of normokinesia of the apical segment and akinesia of the midfree wall with persistence of abnormal wall motion at the base has proved to be fairly specific for pulmonary embolism. This echocardiographic abnormality has been termed “McConnell sign.” We describe the case of a patient with acute pulmonary embolism who developed reversible akinesia of the apex and right ventricular midfree wall, a finding we would like to term “reverse McConnell sign.”