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Data with improved accuracy are presented for the low temperature reaction rates k(T) of the H-tunneling reaction between one specific host neighbor molecule in fluorene single crystals and a photoexcited acridine guest. The constant low temperature reaction rate k0 cannot be separated from the precursor intramolecular decay rate. This does not affect the analysis of the temperature dependent reaction rate, which renders the identification of two distinct thermally excited nuclear fluctuation modes promoting the H-transfer reaction. The energies are 125(15) and 440(40) cm−1. They agree well with corresponding lines in the Raman spectra of fluorene crystals. For the first time this result offers the possibility to test quantum mechanical reaction mechanisms without the need of averaging over unspecified distributions of nuclear fluctuation modes.

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