Barriers by Thermal Racemization

The corresponding barrier, i.e., the free enthalpy of activation AG%, is calculated from ke and the temperature according to the Eyring equation. 

In cases of higher temperatures of racemization, the thermal stability was occasionally checked by an independent experiment in which the racemate was heated in the same solvent, or the deuterated solvent, at the same temperature and for the same period oftime as in the kinetic experiments. H-NMRspectra did not show any evidence of decomposition. 

All racemizations were monitored in the absence of external light. A test experiment46 showed that the irradiation by the polarimeter lamp is too weak to cause significant photoracemization. Therefore, the decrease of the rotation angle is effected thermally. 

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