Energy transfer Intramolecular mechanisms

In this chapter we shall first outline the basic concepts of the various mechanisms for energy redistribution, followed by a very brief overview of collisional intennoleciilar energy transfer in chemical reaction systems. The main part of this chapter deals with true intramolecular energy transfer in polyatomic molecules, which is a topic of particular current importance. Stress is placed on basic ideas and concepts. It is not the aim of this chapter to review in detail the vast literature on this topic we refer to some of the key reviews and books [U, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32] and the literature cited therein. These cover a variety of aspects of tire topic and fiirther, more detailed references will be given tliroiighoiit this review. We should mention here the energy transfer processes, which are of fiindamental importance but are beyond the scope of this review, such as electronic energy transfer by mechanisms of the Forster type [33, 34] and related processes. 

Biopolymers Boron Hydrides Cohesion Parameters Energy Transfer, Intramolecular Halogen Chemistry Perturbation Theory Protein Structure Quantum Mechanics... 

The process of spin-lattice relaxation involves the transfer of magnetization between the magnetic nuclei (spins) and their environment (the lattice). The rate at which this transfer of energy occurs is the spin-lattice relaxation-rate (/ , in s ). The inverse of this quantity is the spin-lattice relaxation-time (Ti, in s), which is the experimentally determinable parameter. In principle, this energy interchange can be mediated by several different mechanisms, including dipole-dipole interactions, chemical-shift anisotropy, and spin-rotation interactions. For protons, as will be seen later, the dominant relaxation-mechanism for energy transfer is usually the intramolecular dipole-dipole interaction. 

Mechanisms of Intramolecular Electronic Energy Transfer in Bichromophoric Molecular Systems Solution and Supersonic Jet Studies, Chem. Rev. 96, 1953—1976. 

Co-free PAE). In PAE-CoCpl, the fluorescence quantum yield is only 18% of that observed for Co-free PAE, even though the quencher substitutes less than 0.1% of the aryleneethynylene units. The fluorescence in solution disappeared in PAE-CoCp4, where every fifth unit is a cyclobutadiene complex. The mechanism by which this quenching occurs is via the cobalt-centered MLCT states [82,83], conferred onto the polymer by the presence of cyclobutadiene complexes. Even in the solid state the polymers PAE-CoCpl-2 are nonemissive. It was therefore shown that incorporation of CpCo-stabilized cyclobutadiene complexes into PPEs even in small amounts leads to an efficient quenching of fluorescence in solution and in the solid state. Quenching occurs by inter- and intramolecular energy transfer [84]. 

It is often possible to describe the photoexcitation of a molecule in terms of a localized excitation, e.g., excitation of the 0=0 group of a ketone. In most photochemical decompositions the bond that breaks and leads to fragmentation is not the same as the site of localized excitation.165 Therefore, it is necessary to consider the mechanism of intramolecular energy transfer as part of the photochemical reaction. 

An attractive vehicle for study of the detailed mechanism of energy transfer is observation of the intramolecular process. The most straightforward approach involves use of a system such as the following ... 

A few observations of photosubstitution in lanthanide complexes have been reported. Irradiation into the f—f bands of [Pr(thd)3], [Eu(thd)3] and [Ho(thd)3] (thd is the anion of 2,2,6,6-tetramethyl-3,5-heptanedione) results in substitution of thd by solvent.153 The proposed mechanism involves intramolecular energy transfer from an f—f excited state to a reactive IL excited state which is responsible for the observed ligand loss. Photosubstitution has also been observed upon direct excitation into the ligand absorption bands of [Tb(thd)3].154... 

The role of the acid catalyst during the oxidation of epoxides with DMSO has been explored by DFT studies of three acids, namely H30+, Li+, and Mg2+. Stationary points have been obtained at the B3LYP/6-31+- -G(d,p) level of theory and the reaction barriers have been evaluated through tree-energy calculations. The mechanism proceeds in two steps, namely ring opening followed by an intramolecular proton transfer that leads to an a-hydroxy carbonyl compound.88 The epoxidation... 

S. Speiser, Photophysics and mechanisms of intramolecular electronic energy transfer in bichromophoric molecular systems solution and supersonic jet studies, Chem. Rev., 96 (1996) 1953-1976. 

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