Theoretical Study on the Noncovalent Interactions Involving Triplet Diphenylcarbene

The properties of some types of noncovalent interactions formed by triplet diphenylcarbene (DPC3) have been investigated by means of density functional theory (DFT) calculations and quantum theory of atoms in molecules (QTAIM) studies. The DPC3···LA (LA = AlF3, SiF4, PF5, SF2, ClF) complexes have been analyzed from their equilibrium geometries, binding energies, charge transfer and properties of electron density. The triel bond in the DPC3···AlF3 complex exhibits a partially covalent nature, with the binding energy − 65.7kJ/mol. The tetrel bond, pnicogen bond, chalcogen bond and halogen bond in the DPC3···LA (LA = SiF4, PF5, SF2, ClF) complexes show the character of a weak closed-shell noncovalent interaction. Polarization plays an important role in the formation of the studied complexes. The strength of intermolecular interaction decreases in the order LA = AlF3 > ClF > SF2 > SiF4 > PF5. In the process of complexation, the charge transferrs from DPC3 to the antibonding orbital of AlF3/SF2/ClF, the quantity of charge transfer is very small between DPC3 and SiF4/PF5. The electron spin density transferrs from the radical DPC3 to ClF and SF2 in the formation of halogen bond and chalcogen bond, but for the DPC3···AlF3/SiF4/PF5 complexes, the transfer of electron spin density is minimal.

Unexpected electron spin density on the axial methionine ligand in CuA suggests its involvement in electron pathways

The presence of unpaired electron spin density in the axial ligand of the CuA site suggest a new description of the electronic structure of this metal site that supports the feasibility of previously neglected electron transfer pathways.

Unexpected Electron Spin Density on the Axial Methionine Ligand in CuA Suggests Its Involvement in Electron Pathways

<p>The Cu_A center is a paradigm for the study of long-range biological electron transfer. This metal center is an essential cofactor for terminal oxidases like Cytochrome <i>c</i> oxidase, the enzymatic complex responsible for cellular respiration in eukaryotes and in most bacteria. Cu_A acts as an electron hub by transferring electrons from reduced cytochrome <i>c</i> to the catalytic site of the enzyme where dioxygen reduction takes place. Different electron transfer pathways have been proposed involving a weak axial methionine ligand residue, conserved in all Cu_A sites. This hypothesis has been challenged by theoretical calculations indicating the lack of electron spin density in this ligand. Here we report an NMR study with selectively labeled methionine in a native Cu_A. NMR spectroscopy discloses the presence of net electron spin density in the methionine axial ligand in the two alternative ground states of this metal center. Similar spin delocalization observed on two second sphere mutants further supports this evidence. These data provide a novel view of the electronic structure of Cu_A centers and support previously neglected electron transfer pathways. </p>

Unexpected Electron Spin Density on the Axial Methionine Ligand in CuA Suggests Its Involvement in Electron Pathways

<p>The Cu_A center is a paradigm for the study of long-range biological electron transfer. This metal center is an essential cofactor for terminal oxidases like Cytochrome <i>c</i> oxidase, the enzymatic complex responsible for cellular respiration in eukaryotes and in most bacteria. Cu_A acts as an electron hub by transferring electrons from reduced cytochrome <i>c</i> to the catalytic site of the enzyme where dioxygen reduction takes place. Different electron transfer pathways have been proposed involving a weak axial methionine ligand residue, conserved in all Cu_A sites. This hypothesis has been challenged by theoretical calculations indicating the lack of electron spin density in this ligand. Here we report an NMR study with selectively labeled methionine in a native Cu_A. NMR spectroscopy discloses the presence of net electron spin density in the methionine axial ligand in the two alternative ground states of this metal center. Similar spin delocalization observed on two second sphere mutants further supports this evidence. These data provide a novel view of the electronic structure of Cu_A centers and support previously neglected electron transfer pathways. </p>

Electron spin density matching for cross-effect dynamic nuclear polarization

A new design principle for a mixed broad (TEMPO) and narrow (Trityl) line radical to boost the dynamic nuclear polarization efficiency is electron spin density matching, suggesting a polarizing agent of one Trityl tethered to at least two TEMPO moieties.

Insights on spin-polarization via the spin density Source Function

The Source Function (SF) [1-3], enables one to view chemical bonding and other chemical paradigms from a new perspective and using only information from the electron density observable and its derivatives. We show how this tool may be straightforwardly applied to another important observable, the electron spin density, which analogously to the electron density may be locally interpreted in terms of a cause-effect relationship of contributions from the atoms of a molecular or crystalline system. Application of the spin density SF to molecules in vacuo and to slab or crystals, is made possible through an extension (SPINSF code) of our electron density SF code for molecules and through a progress-version of the TOPOND code, respectively. The latter has now been fully integrated in the CRYSTAL-14 code, where it provides, via the keyword TOPO of the properties section of CRYSTAL-14, a complete charge density topological analysis according to the Quantum Theory of Atoms in Molecules. Analysis of the SF for the electron spin density implies the study of its Laplacian scalar field, which may be locally positive or negative even if the two composing densities, ρα and ρβ, have both negative or positive Laplacian densities. When the latter bear the same sign, that of the spin density Laplacian depends on their relative magnitudes, that is on the relative concentration or dilution of ρα and ρβ. Hence, in general, the local source for the spin density, LSs, greatly differs from the analogous function for the density, leading to large differences in their integrated atomic SF contributions. The combined study of LSs and of the spin density neatly reveals which are the molecular or crystal regions that are "ferromagnetically" or "antiferromagnetically" coupled and the local strength of such coupling. Applications to crystals of metal-complexes where the ligands play an innocent or a non-innocent role and to crystals of iron spin-crossover complexes are discussed.