One of the most elementary consequences of the wave nature of matter is the ability of particles to penetrate a potential barrier. Such so called tunneling is observable in a series of physical processes and is always pronounced when the de Broglie wavelength of the particles is comparable to the potential width. As this wavelength depends on the mass, tunneling is primarily observed for light particles like electrons and protons. In the last years such a tunneling mechanism could be uniquely identified in some chemical reaction systems and evidence was found that tunneling is also important in biological systems [11, 12, 26].

The most convincing evidence for a tunneling mechanism in chemical reaction systems is found in the temperature dependence of the reaction rate, which differs characteristically from a classical Arrhenius behavior. In general (i) a strong isotope effect, (ii) a non Arrhenius reaction rate behavior together with (iii) the transition of this rate into a constant level at low temperatures are regarded as strong indications for a tunneling mechanism. The photochemical solid state H-transfer reaction in acridine doped single crystals of fluorene presented here is one of the few system where all these three characteristics of a tunneling mechanism could be observed in the reaction rate by temperature and pressure dependent measurements.
From the modern theories of tunneling in chemical reactions [22, 28, 29, 36, 40] it is generally accepted, that the temperature dependence of the reaction rate is evoked by low energy nuclear fluctuation modes, which induce an effective decrease of the tunneling distance. At low temperatures in the transition region between a constant and a temperature dependent reaction rate at least the lowest energy reaction promoting modes should be identifiable in the reaction rate temperature dependence. Although the knowledge of these modes is of great importance to test the theoretical models, an identification of these modes has so far not been achieved yet.

In this presentation reaction rate data are given, which together with Raman data of the fluorene crystal for the first time allow an identification of the lowest energy reaction promoting modes in the acridine/fluorene system. With these data various tests of theoretical tunneling models have been preformed [45 - 47, 58, 60, 63, 65 - 68, 70 - 83]. More tests are undergoing.


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