Multi and Single-Reference Methods for the Analysis of Multi-State Peroxidation of Enolates

In spite of being spin-forbidden, some enzymes are capable of catalyzing the incorporation of O₂ (³Σ⁻_g) to<br>organic substrates without needing any cofactor. It has been established that the process followed by these<br>enzymes starts with the deprotonation of the substrate forming an enolate. In a second stage, the peroxidation<br>of the enolate formation occurs, a process in which the system changes its spin multiplicity from a triplet state<br>to a singlet state. In this article, we study the addition of O₂ to enolates using state-of-the-art multi-reference<br>and single-reference methods. Our results confirm that intersystem crossing is promoted by stabilization of<br>the singlet state along the reaction path. When multi-reference methods are used, large active spaces are<br>required, and in this situation, Semistochastic Heat-Bath Configuration Interaction (SHCI) emerges as a<br>powerful method to study these multi-configurational systems and is in good agreement with LCCSD(T)<br>when the system is well-represented by a single-configuration.<br><br>

Multi and Single-Reference Methods for the Analysis of Multi-State Peroxidation of Enolates

In spite of being spin-forbidden, some enzymes are capable of catalyzing the incorporation of O₂ (³Σ⁻_g) to<br>organic substrates without needing any cofactor. It has been established that the process followed by these<br>enzymes starts with the deprotonation of the substrate forming an enolate. In a second stage, the peroxidation<br>of the enolate formation occurs, a process in which the system changes its spin multiplicity from a triplet state<br>to a singlet state. In this article, we study the addition of O₂ to enolates using state-of-the-art multi-reference<br>and single-reference methods. Our results confirm that intersystem crossing is promoted by stabilization of<br>the singlet state along the reaction path. When multi-reference methods are used, large active spaces are<br>required, and in this situation, Semistochastic Heat-Bath Configuration Interaction (SHCI) emerges as a<br>powerful method to study these multi-configurational systems and is in good agreement with LCCSD(T)<br>when the system is well-represented by a single-configuration.<br><br>

Density Functional Theory Reactivity Studies on X3N@C80 (X = Sc, Gd, Lu) Fullerenes

Density functional theory studies have been performed to reveal the reactivity of the sites in Sc3N@C80, Gd3N@C80 and Lu3N@C80 endohedral fullerenes. The condensed Fukui functions have been calculated using Mulliken atomic charges. The calculations show that the carbon atom sites are in direct contact with the endohedral cluster favourable nucleophilic attack. Similarly, the carbon atoms which are away from the direct bonding with the cluster are favourable for the electrophilic attack. This is also confirmed from the charge transfer analysis. It is noted that the spin multiplicity decides the reactivity sites and stability of the Gd3N@C80 system. The HOMO-LUMO gap value indicates that Gd3N@C80 with S = 7 is stable than the S = 21 system. Finally, present studies indicate that the charge transfer between the C80 cage and X3N plays a major role to determine the reactivity of the sites in the C80 cage.

Theoretical study of OH− nucleophilic attack on cobalt(II)-verdoheme complex

OH[Formula: see text] attack on a four, five and six-coordinated cobalt(II)-verdoheme-coordinated with imidazole (1 M) while the axial ligand was studied using B3LYP method. Different spin multiplicities were considered, namely doublet, quartet, and sextet. Results show the most positive charge to be concentrated at the cobalt and carbon atoms adjacent to the oxygen in the cobalt(II)-verdoheme complex. Data obtained show that a stable intermediate was formed by a nucleophilic attack on one of the latter carbon atoms. The intermediate is initially formed by a nucleophilic attack on one of the aforementioned carbon atoms. This intermediate is then directly converted to a helical open-ring complex by passing it through a transition state. It is specified that if every nucleophilic attack is considered separately, then the OH[Formula: see text] attack on cobalt(II)-verdoheme occurred at all spin multiplicity and coordination states, from a thermodynamics and kinetics point of view, all except an OH[Formula: see text] attack on five-coordinated cobalt(II)-verdoheme at quartet state. However, comparison of reaction paths with different spin in the same coordination show that such a nucleophilic attack is not proceeded, while the reactant is a four and six-coordinated cobalt(II)-verdoheme because the latter reaction spin state is not conserved. It is clear that OH[Formula: see text] attack on five-inatedcoordinated Co(II)-verdoheme at doublet spin multiplicity is the most stable reaction path. Moreover, these findings were confirmed by NBO analysis and molecular orbital calculations.

Short-lived metal-centered excited state initiates iron-methionine photodissociation in ferrous cytochrome c

The dynamics of photodissociation and recombination in heme proteins represent an archetypical photochemical reaction widely used to understand the interplay between chemical dynamics and reaction environment. We report a study of the photodissociation mechanism for the Fe(II)-S bond between the heme iron and methionine sulfur of ferrous cytochrome c. This bond dissociation is an essential step in the conversion of cytochrome c from an electron transfer protein to a peroxidase enzyme. We use ultrafast X-ray solution scattering to follow the dynamics of Fe(II)-S bond dissociation and 1s3p (Kβ) X-ray emission spectroscopy to follow the dynamics of the iron charge and spin multiplicity during bond dissociation. From these measurements, we conclude that the formation of a triplet metal-centered excited state with anti-bonding Fe(II)-S interactions triggers the bond dissociation and precedes the formation of the metastable Fe high-spin quintet state.

Computer Simulation of Small Magnetic Clusters of 3d-Transition Metals of the Iron Subgroup Using the Hybrid Density Functional Method

This paper presents the results of s study focused on the stability of small 3d-transition-metal magnetic clusters (metals of an iron subgroup) in spin-polarized states using the hybrid density functional method. Computer modeling and full variational optimization of geometric structures of clusters were performed for various values of the spin multiplicity of electronic states. The binding energies, the bond lengths, and the frequencies of atomic zero-point vibrations in small clusters with a nuclearity of n = 2, 3, 4, 5, 6 were calculated depending on the metal (Fe, Co, Ni) and spin multiplicity M in the zero-charge state. The calculations were carried out using the hybrid density functional B3LYP method in the def2-TZVP basis set of the ORCA package algorithms. A comparison of the calculated results with the available experimental data is presented. It is shown that the calculated data obtained by the hybrid density functional method are in satisfactory agreement with the experimental data for “naked” clusters in inert media both for the spin multiplicity of the ground state and for the energy of atomic shock dissociation of clusters in inert gas flows.