Littérature scientifique sur le sujet « Polaron delocalization »

This chapter discusses bonding and molecular shape of electrons. A covalent bond occurs when atoms share a pair of similar electrons. Modern theories on covalent bonding are dominated by molecular orbital theory. Polar covalent bonds occur when different atoms share the electron pair unequally due to electronegativity. With benzene being the most familiar molecule of the bonding, delocalization happens when more than two atoms are involved in the bonding. The valence-shell electron-pair repulsion (VSEPR) theory can help us to visualize the shapes of simple molecules. Additionally, ionic bonding occurs when an atom transfers an electron to another atom and the ions formed a crystal lattice electrostatically.

This chapter reviews some basic aspects of the energetics and kinetics of radical processes and the factors that determine them. It examines the decomposition of a dilute solution of a diacyl peroxide in an unreactive, non-polar medium, such as benzene. It also provides a kinetic analysis of the use of peroxide to initiate a chain reaction in which an alkene is polymerized, and demonstrates how slow propagations may compete with diffusion-controlled termination. The chapter discusses the modest stabilization found with alkyl-substituted methyl radicals that can be attributed in part to hyperconjugation, which is revealed both by theory and by spectroscopic data. The chapter finally explains that stabilization is a thermodynamic property that arise from electron delocalization.

Acyl group transfer processes are plentiful in enzymatic reactions. Examples may be found in ATP-dependent ligation in chapter 11, carbon-carbon bond formation in chapter 14, and fatty acid biosynthesis in chapter 18. In this chapter, we begin by presenting the basic chemistry of acyl group transfer. We then consider four major classes of proteases that catalyze acyl group transfer in the hydrolysis of peptide bonds. Acyl group transfer is so common in organic and biochemistry that the chemistry by which it occurs is often taken for granted. Early studies provided evidence for a mechanism initiated by nucleophilic addition of the acyl group acceptor to the carbonyl group to form a tetrahedral intermediate, analogous to the reversible addition of a nucleophilic molecule to the carbonyl group of an aldehyde or ketone. A mechanism of this type is shown in scheme 6-1 for acyl group transfer from a group :X to a nucleophile :G catalyzed by a general base. This mechanism is drawn from a larger family of possible mechanisms involving specific acid-base, general acid, general base, or concerted general acid-base catalysis of nucleophilic addition to an acyl carbonyl group to form a tetrahedral intermediate, followed by the elimination of :X–H to produce the new acyl compound. In enzymatic reactions the nucleophilic atom G in scheme 6-1 is normally nitrogen, oxygen, sulfur, or a carbanionic species. An acyl carbonyl group is less polar and correspondingly less reactive toward nucleophilic addition than an aldehyde or ketone. The reason is the effect on the heteroatom of nonbonding electrons, which reside in p orbitals that overlap the π orbital of the carbonyl group. The consequent delocalization of electrons stabilizes the carbonyl group and attenuates its reactivity with nucleophiles. Other factors being equal, the order of reactivity is thioester > ester > amide, which is the inverse of the degree of delocalization. Delocalization is least in thioesters because of the high energy of the sulfur p orbitals, which reside in the next higher principal quantum number relative to oxygen in the acyl carbonyl group.

Two-Dimensional Electronic Spectroscopy reveals an ultrafast energy delocalization between 2μm distanced donor/acceptor molecules confined in a microcavity. This mechanism is promoted by the formation of hybrid-polariton states that induces a coupling in the entire system.

The size dependent modifications of the optical and electronic properties of microcrystallites have attracted considerable attention recently[1-4]. As the diameter of the microcrystallite approaches its corresponding exciton Bohr diameter, its electronic and optical properties start to change because of the quantum confinement effect, dielectric effect and the effect of the surface[5]. For microcrystallites in such a small size regime, a large percentage of the atomes is on or near the surfaces. The existence of this vast interface between the microcrystallite and the surrounding medium can have a profound effect on the nonlinear optical properties of the microcrystallites. For the first time, we studied the nonlinear optical properties of translation metal-oxide microcrystallites by coating the surface with a layer of organic polar molecule(DBS etc.), and found that the change of the surface environment could alter the optical properties greatly. For Fe2O3 as example, (1) the absorption incresed toward the high energy side, (2) the laser induced luminescence intensity decreased by 2 orders in magnitude, and on the contrary, the Raman signal of the surface was enhanced greatly, (3) the saturable absorption phenomenon disappeared, (4) larger third order susceptibility and faster excited state relaxation were obtained compared with uncoated Fe2O3 microcrystallite. These phenomena are the results of the change of the electronic structure caused by the quantum confinement effect and the effect of the surface, unlike semiconductor microcrystallites in which the delocalized Wannier excitons can be influenced greatly by the quantum confinement effect (such as PbS microcrystallite). Transition metal oxide microcrystallite has more complicated electronic structure in which localized d electrons influence its electronic and optical properties greatly[6], and the small diameter Frenkel exciton in such material was effected little by the quantum confinement effect, therefore, the exciton structure could not be abserved in the absorption spectrum. But the size of the transition metal oxide microcrystallites influence their electronic structure strongly. For Fe2O3 as example, the energy structure can be quantitatively shown as the Figure (at the end of the paper), in which a is d-d transition, b represents charge transfer, c is orbital promotion and d is interband transitions. As the size of the microcrystallite decreases, the 3d and 4sp state couples increasingly, and the 3d-4sp (orbital promotion) state contribution increases correspondingly. To some extend, the d electrons and the Frenkel exciton will be delocalized, and the excited electron-hole pair can be ionized and scattered to the surface rapidly. In particular, when the surface was coated with a layer of organic polar molecule, the 3d-4sp state interaction was enhanced greatly under the strong polar interaction of the surface, and some 3d-4sp hydride state will exist, thus the d electrons and the Frenkel exciton will became more delocalization, and the laser induced electron-hole pairs interect and scatter to the surface very fast, so the surface delocalization state generate, accumulate and relax very rapidly and the electron-electron coherence effect[7] is enhanced greatly. Such changes not only increased the nonlinear response, but also resulted in shorter lifetime and stronger nonraditive process.