Low Temperature Reaction Rate AnalysisThe low temperature region (33 - 143 K) renders two thermal activated processes kiexp(Ei/kT) with
|k1 = 4.4(1.3) s-1||E1 = 125(15) cm-1|
|k2 = 400(140) s-1 ||E2 = 440(40) cm-1|
The second harmonic of E1 is not observable. The Fit renders a low temperature triplet decay rate KT(T = 0) = kTG(0) + kTR(0) = 13.904(16) s-1 in good agreement with the ODMR value of 13.88(3) s-1 taken at 1.5 K. A constant low temperature reaction rate kTR(0) = k0 is not accessible here. It is obtained from the high pressure data. E1 is in very good agreement with the 127 cm-1 ag-libration found in the Raman spectrum of fluorene. As this motion actually leads to a strong decrease of the H-tunneling distance, the ag-libration around the long molecular axis is identified as the lowest energy intermolecular mode promoting the hydrogen tunneling reaction. The missing of the second harmonic of E1 in the above temperature dependence of the reaction rate is presumably due to phase correlated interference effects with other crystal or intramolecular modes.