Phonon-assisted reactions

Next, we discuss the concept of phonon-assisted reactions. In relation to thermal reactions, they can be assisted by phonon-mode softening leading to large-amplitude overdamped oscillations. In the case of a photochemical reaction, a strong electron-phonon coupling can assist in polymerization. Then some non-linear spectroscopic studies are discussed which illuminate on the dynamics of photopolymerization process. Then follows a discussion of results on reaction in a different kind of molecular assembly, the Langmuir-Blodgett films. Finally, some gas-solid interface reactions which produce polymers in a doped state are discussed. 

Recently, we have Introduced a new concept of phonon-assisted reaction (2). In other words, a reaction In the solid state can be assisted by these low frequency cooperative molecular motions. The concept of the phonon-assisted chemical transformation bears a direct analogy to that of a phonon-assisted physical transformation In solid. Important phonon Interactions can be divided Into two categories (1) Anharmonic phonon-phonon Interactions and (11) electron-phonon Interactions. 

This paper presents studies of solid state polymerization aimed towards formulating a dynamic model of reactivity in the condensed phase. Phonon spectroscopy is successfully used to elucidate the mechanism of lattice control of the reaction. Novel concepts of phonon-assisted thermal and photochemical reactions are introduced, supported by experimental data. Non-linear laser spectroscopy is used to find the importance of biexcitonic processes in photopolymerization. Also, spectroscopic studies of reactions in Langmuir-Blodgett films and at gas-solid interface which produce ordered polymers are presented. 

An example of the phonon assisted thermal reaction is the solid state thermal rearrangement of methyl-p-dimethyl amlnobenzene sulfonate (MSE) which can be illustrated as follows (1) ... 

An example of phonon-assisted photochemical reaction is provided by the photodimerIzatlon of 2,6-dimethyl benzoqulnone in the solid state. Our Infrared spectral study of the reactant and the product dimer suggests that the following reaction occurs(8) ... 

However it turned out that the structural, chemical and dynamical details are essential for complex descriptions of long-range proton transport. These parameters appear to be distinctly different for different families of compounds, preventing proton conduction processes from being described by a single model or concept as is the case for electron transfer reactions in solutions (described within Marcus theory [23]) or hydrogen diffusion in metals (incoherent phonon assisted tunneling [24]). 

Therefore, it is modulated by the intermolecular displacement occurring in a phonon motion. Photochemical aggregation reactions, such as dimerization or polymerization reactions, can be assisted by the occurrence of strong electron-phonon interaction in the reactive electronic state. This strong electron-phonon interaction creates a local lattice-deformation in the reactive (excited) electronic state. The deformation traps the electronic excitation and, at the same time, it may provide a local preformation of the product lattice if the distortion is along the reaction co-ordinate. Both these features assist a photochemical aggregation reaction. 

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