Density functional theory study of the Jahn-Teller effect in cobaltocene

A detailed discussion of the potential energy surface of bis(cyclopentadienyl)cobalt(II), cobaltocene, is given. Vibronic coupling coefficients are calculated using density functional theory (DFT). Results are in good agreement with experimental findings. On the basis of our calculation there is no second-order Jahn-Teller (JT) effect as predicted by group theory. The JT distortion can be expressed as a linear combination of all totally symmetric normal modes of the low-symmetry, minimum-energy conformation. The out-of-plane ring deformation is the most important mode. The JT distortion is analyzed by seeking the path of minimal energy of the adiabatic potential energy surfac

(Strept)avidin as host for biotinylated coordination complexes: stability, chiral discrimination, and cooperativity

Incorporation of a biotinylated ruthenium tris(bipyridine) [Ru(bpy)₂(Biot-bpy)]²⁺ (1) in either avidin or streptavidin-(strept)avidin-can be conveniently followed by circular dichroism spectroscopy. To determine the stepwise association constants, cooperativity, and chiral discrimination properties, diastereopure (Λ and δ)-1 species were synthesized and incorporated in tetrameric (strept)avidin to afford (δ-[Ru(bpy)₂(Biot-bpy)]²⁺)x⊂avidin, (Λ- [Ru(bpy)₂(Biot-bpy)]²⁺)x⊂avidin, (δ-[Ru(bpy)₂(Biot- bpy)]²⁺)x⊂streptavidin, and (Λ-[Ru(bpy)₂(Biot-bpy)]²⁺) x⊂streptavidin (x = 1-4) For these four systems, the overall stability constants are log β₄ = 28.6, 30.3, 36.2, and 36.4, respectively. Critical analysis of the CD titrations data suggests a strong cooperativity between the first and the second binding event (x = 1, 2) and a pronounced difference in affinity between avidin and streptavidin for the dicationic guest 1 as well as modest enantiodiscrimination properties with avidin as host

A study of the photophysical and photochemical properties of metal complexes using density functional theory

La chimie des complexes des métaux de transition est un domaine qui a considérablement évolué depuis ses débuts. Dans divers domaines de la chimie moderne, les composés de coordination sont sujets à de nombreuses applications et porteurs de nouveaux espoirs. Nombre de ces applications font intervenir une interaction entre la molécule et la lumière: télécommunication, développement de l’optique en informatique, conversion de l’énergie solaire en énergie électrique, … Parallèlement, dans les domaines de la chimie computationnelle et de la chimie théorique, la théorie de la fonctionnelle de densité (TFD) s’affirme de plus en plus comme méthode de choix dans la modélisation de certains phénomènes chimiques. Dans cette méthode qui est décrite au chapitre 2, l’énergie d’un système électronique est déterminée, de façon univoque, par sa densité électronique. Regroupant les deux thèmes précédemment cités, le but premier de cette thèse est le calcul des énergies ainsi que des propriétés des états excités de complexes métalliques à l’aide de la TFD. La méthodologie est décrite au chapitre 3. Les différentes méthodes ont été appliquées à trois différents systèmes chimiques. Le premier complexe métallique étudié est le tris(2,2’-bipyridine)ruthénium(II). Le chapitre 4 de cette thèse présente une étude des états excités du [Ru(bpy)3]2+, un accent particulier ayant été mis sur les états excités de transfert de charge ainsi que sur les états excités centrés sur le métal et la photodissociation que leur population engendre. Cette molécule a suscité un grand intérêt parmi les chimistes depuis quelques décennies déjà en raison d’un comportement photochimique remarquable. Le [Ru(bpy)3]2+ ainsi que d’autres molécules de la même famille sont en effet potentiellement applicables à la conversion de l’énergie solaire en énergie électrique (photolyse de l’eau). Malgré cet intérêt et le grand nombre d’études sur la molécule, le mécanisme exact de la photochimie du [Ru(bpy)3]2+ n’est pas encore exactement connu. Le mécanisme que nous proposons ici implique une réévaluation complète du rôle des états excités centrés sur le métal lors de la dissociation photochimique de la liaison métal-ligand. L’étape clé de ce mécanisme implique une élongation de la liaison Ru-N lorsque le [Ru(bpy)3]2+ se trouve dans sa configuration excitée de plus basse énergie. Par cette élongation, le caractère de l’orbitale σ* métallique est transféré sur une orbitale de type σ* plus basse en énergie. Il en résulte un abaissement de l’énergie de la transition électronique centrée sur le métal, transition responsable de la photochimie du [Ru(bpy)3]2+. Dans un deuxième temps, nous nous sommes intéressés à une autre molécule ayant suscité un grand intérêt chez les chimistes, le nitroprusside. Du fait de cet intérêt, de nombreux détails sur la nature de l’état fondamental du nitroprusside ainsi que sur la nature de ses deux états métastables ont été publiés. Malgré cela, un modèle clair des chemins réactionnels reliant l’état fondamental et les deux états métastables étaient toujours manquant. En étudiant les états excités de ces trois minima appartenant à la surface de potentiel de l’état fondamental, nous sommes arrivés à établir un modèle expliquant la photochimie et la photophysique reliant les trois différents minima entre eux. Dans le dernier chapitre de la thèse, nous nous sommes intéressés à certains composés chimiques présentant des propriétés d’optique non-linéaire. La principale classe de composés présentant un intérêt en optique non-linéaire possède un groupe donneur d’électron et un groupe accepteur reliés entre eux. L’exemple typique est la paranitroaniline. En utilisant la théorie de la fonctionnelle de densité dépendante du temps (TFDDT) dans notre modèle, nous avons obtenus de bonnes estimations des propriétés d’optique non-linéaire dans le cas de molécules de la famille de la para-nitroaniline. Dans une deuxième partie du chapitre 6, nous avons étendu la méthode à des composés organométalliques susceptibles de présenter d’intéressantes propriétés pour l’optique non-linéaire.The first objective of this thesis is the calculation of excited state energies and properties of transition metal complexes using Density Functional Theory (DFT). To explore this wide topic, we did choose three different chemical systems. The first one, presented in the chapter 4, is about the photodissociation of the tris(2,2’- bipyridine)ruthenium(II) ([Ru(bpy)3]2+) ion. Over the last two decades, there has been a vivid interest in the literature for this complex ion because of its remarkable photochemical behavior. [Ru(bpy)3]2+ and complexes belonging to the same family are actually good candidates for solar energy conversion. Even if the electronic structure of the complex begins to be well known, the understanding of the photosubstitution and photoracemization is still an unresolved problem. In very recent years, the photochemistry and photophysics of complexes with low-lying metal-to-ligand charge transfer (MLCT) states have attracted considerable interest. With the help of those studies, a mechanism of the photochemistry of [Ru(bpy)3]2+ is emerging and is presented in this work. This mechanism implies a complete reassessment of the role of metal centered excited states in the photochemical dissociation of metal-ligands bonds. The key step in this mechanism involves an elongation of the Ru-N bond length, which is possible since the lowest MLCT states have long life times. This leads to a transfer of the metal σ* character to one of the lowest unoccupied σ* orbital. The result is an energy lowering of the MC transition, which is a fully dissociative state. As a second chemical system, we were also interested, in chapter 5, by a molecule that attracted the chemist’s interest for quite a while, i.e. nitroprusside. Due to this interest, many details on the nature of nitroprusside’s ground state and its two metastable states were known. However, a clear picture of the reaction pathways between the three minima on the ground state potential energy curve was still missing. By studying the excited states corresponding to all three minima, we could setup, in this work, a model explaining the photochemistry and photophysics responsible for the population of the three different states on the ground state potential energy curve. Last but not least, the third kind of transition metal complexes we were interested in are compounds showing nonlinear optical properties. The most important class of nonlinear optical (NLO) compounds are molecules possessing both electron donating and accepting groups which are electronically coupled. In this work, a computational chemistry approach is used to model some nonlinear optical parameters. Using time dependent DFT, we obtained good estimates for the NLO parameters for molecules of the family of the para-nitroaniline. Organometallic donating and accepting groups also exhibit interesting features which enables a tuning or even a switching of the NLO properties. In a second part of chapter 6, we thus extended our study to the computation of dipolar bimetallic sandwich-like complexes composed of sesquifulvalene and metal-ligand fragments

Polytopal rearrangement of [Ni(acac)₂(py)]: A new square pyramid ⇄ trigonal bipyramid twist mechanism

The interconversion mechanisms between three idealized polytopal forms (a square pyramid and two trigonal bipyramids) of [M(bidentate)₂(unidentate)] were investigated by an original combination of molecular mechanics (MM) and density functional theory (DFT) approaches. MM was used to model the mechanistic rearrangement path, and DFT to study selected points along this path. The test case was a five-coordinate [Ni(acac)₂(py)] species. In the case of [Ni(acac)₂(py)] it was confirmed (both by MM and by DFT) that the three polytopal forms do indeed represent shallow local minima, of which the square pyramid (SQP) is more stable than the other two. Small energy barriers that separate the three minima prevent spontaneous rearrangement among the polytopal forms in geometry-optimization simulations. The driving force for MM simulation of the polytopal rearrangements was supplied through the L-M-L angle bending terms. MM results for relative energies and geometries are fully supported by DFT. Finally, the implication of the present results to explain some racemization mechanisms of octahedral complexes (namely, the intramolecular bond rupture of tris(chelate) species, and intermolecular dissociation of bis(bidentate) species) is briefly discussed

Density functional theory for the study of the multimode Jahn-Teller effect

The Jahn-Teller (JT) theorem states that in a molecule with a degenerate electronic state, a structural distortion must occur that lowers the symmetry, removes the degeneracy and lowers the energy. The multideterminental-DFT method performed to calculate the JT parameters for JT active molecules is described. Within the harmonic approximation the JT distortion can be analyzed as a linear combination of all totally symmetric normal modes in any of the low symmetry minimum energy conformation, which allows the intrinsic distortion path (IDP) to be calculated, exactly from the high symmetry point to the low symmetry configuration. Results obtained by the approach described here give direct insight into the coupling of electronic structure and nuclear movements

DFT Study of the Jahn-Teller Effect in Cu(II) Chelate Complexes

Density Functional Theory (DFT) in conjuction with the Intrinsic Distortion Path (IDP) is employed to study the Jahn-Teller (JT) effect in all four diastereoisomers of tris(ethylenediamine)copper(II) ([Cu(en)₃]²⁺) and tris(ethyleneglycol)copper(II) ([Cu(eg)₃]²⁺) complexes. As a consequence of the JT effect all the isomers tetragonally elongate to the C₂ configurations. Although there are energy differences between the isomers of [Cu(en)₃]²⁺, almost equal JT parameters suggest that chelate ring conformation does not have affect on the JT distortion. In a case of [Cu(eg)₃]²⁺ JT effect causes additional hydrogen bond formation and these two effects define the overall geometry of isomers

Intrinsic distortion path in the analysis of the Jahn–Teller effect

The multideterminental-DFT approach was performed in order to calculate the Jahn–Teller (JT) parameters for the JT active molecules. Within the harmonic approximation the JT distortion can be analysed as a linear combination of all totally symmetric normal modes in any of the low symmetry minimum energy conformation, which allows to calculate the Intrinsic Distortion Path (IDP), exactly from the high symmetry point to the low symmetry configuration. Results obtained by both methods are consistent and give direct insight into the coupling of electronic structure and nuclear movements. As examples, the results for Cu₃ cluster, cobaltocene and manganocene are reported

Density functional study of nitroprusside: Mechanism of the photochemical formation and deactivation of the metastable states

In this article, we present a theoretical investigation about the mechanism of the photochemical formation and deactivation of the metastable states observed in nitroprusside ions. The quantum chemical calculations are based on density functional theory. The peculiar photochemical and photophysical behavior of this molecule has attracted chemists' interest for a while. Due to this interest, many details on the nature of nitroprusside's ground state and its two metastable states were known. However, a clear picture of the reaction pathways between the three minima on the ground-state potential energy curve was still missing. By studying the excited states corresponding to all three minima, we could set up, in this work, a model explaining the photochemistry and photophysics responsible for the population of the three different states on the ground-state potential energy curve