Intramolecular energy transfer molecules

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. 

Figure A3.13.1. Schematic energy level diagram and relationship between mtemiolecular (collisional or radiative) and intramolecular energy transfer between states of isolated molecules. The fat horizontal bars indicate diin energy shells of nearly degenerate states.

A 3.13.4 INTRAMOLECULAR ENERGY TRANSFER STUDIES IN POLYATOMIC MOLECULES... 

Gerber, R.B., Buch, V., Ratner, M.A. Time-dependent self-consistent field approximation for intramolecular energy transfer. I. Formulation and application to dissociation of van der Waals molecules. J. Chem. Phys. 77 (1982) 3022-3030. 

Extrusion or elimination of small stable species such as CO2 Intramolecular energy transfer Quenching involving translational or vibrational excitation of another molecule... 

Both RRGM and SLP have been used to compute various transition amplitudes with high efficiency and accuracy. Their applications, which have been reviewed in the literature,56 57 59 include laser-molecule interaction,43 44 99 correlation functions,45 104 absorption and emission spectra,100 103 105-107 intramolecular energy transfer,108-115 vibrational assignment,103 116 117 and reaction dynamics. ... 

Figure 6.15 Structure of some flexible and rigid molecules which show intramolecular energy transfer.

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. 

In the theory of Mies and Kraus, and also in the theory of Peters, a reaction coordinate is identified but its coupling to other modes of motion is not studied. Also, in the theories mentioned it is supposed that the molecule can be prepared in the state that will decompose, thereby avoiding discussion of intramolecular energy transfer. Thus, in the theory of Mies and Kraus, it is supposed that the true molecular state describing the... 

It is clear that the analysis outlined is only the first step in developing a satisfactory theory of photochemical reactions. For example, the energy spectrum studied is not typical of all photodecomposable molecules, it is possible that direct photoexcitation of the bond that breaks is most important in some reactions, and that intramolecular energy transfer is important in others, etc. Much more work will be required before we have complete understanding of even the simplest photodissociation reactions. 

Vibrational Predissociation, in this section we discuss the case of a transition from a predissociative state to the photofragment state that occurs on a single adiabatic pes. Such processes cannot occur for diatomic molecules, but they can be observed for polyatomic systems. The transition is caused by intramolecular energy transfer, that is, by internal redistribution of vibrational energy. 

The situation is different from that with the intramolecular energy transfer catalyzed by solute encounters. If an active molecule with two excited states, A and B, say, singlet and triplet, meets quencher C considered as structureless but carrying heavy atom (halogen substituent), then the latter catalyzes the... 

The efficiency of intramolecular energy transfer has been studied in certain molecules which consist of two independent systems of conjugated double bonds separated by up to three saturated carbon atoms.4882,94 Only the lowest states of any multiplicity were found to emit. Thus although the absorption spectrum of such a species consists of a superposition of characteristic absorption due to the separate conjugated groups, fluorescence and phosphorescence from only one group are observed even when the other group is excited. Further studies... 

A type of donor-fulgimide-acceptor (D-F-A) molecule 69e (see Scheme 22) has been used to study intramolecular energy transfer. In this molecule, the fulgimide unit is a switch for energy transfer. By controlling the E or C form of the fulgimide molecule, an intramolecular energy transfer is possible, but the transfer mechanism cannot be determined definitely.91... 

The transfer of electronic energy (and excitation) between two different molecules (intermolecular energy transfer pathway iv of Fig. 1) or between two different electronic states (intramolecular energy transfer pathway v) plays a central role in photochemistry. The equivalent processes involving electrons, in which charge is transferred to a different molecule, or is moved in position in one molecule, may also be promoted photochemically. These processes are explored in the present section. 

It is instructive to examine a zeroth-order calculation of intramolecular energy transfer in a model molecule in which the energies of the vibrational and rotational modes are conserved separately, in which the initial excitation of the van der Waals stretching mode is zero (n = 0), and in which the initial values of... 

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