LI-7 NMR-STUDY OF THE EXCHANGE KINETICS OF THE LITHIUM ION WITH CRYPTAND C221 IN METHANOL SOLUTION - TEMPERATURE-DEPENDENCE OF THE EXCHANGEMECHANISM

The exchange kinetics of Li(+)C221 cryptate in methanol solution was s tudied by lithium-7 NMR line-shape analysis at a wide range of tempera ture. Depending on the temperature, two different exchange mechanisms were found to coexist to different extents. A dissociative pathway dom inates at high temperatures (from 5 to 25 degrees), while a bimolecula r mechanism prevails at lower temperatures (from -30 to -60 degrees C) . At the temperature range of -30 to 5 degrees C, both the dissociativ e and bimolecular mechanisms contribute to the metal exchange. For the dissociative pathway, the activation parameters were calculated as: E -a = 22 +/- 2 kJ mol(-1); Delta H double dagger = 20 +/- 2 kJ mol(-1); Delta S double dagger =-144 +/- 8 J mol(-1) K-1. While for the bimole cular mechanism they are: E-a = 9.9 +/- 0.4 kJ mol(-1); Delta H double dagger = 8.0 +/- 0.4 kJ mol(-1); Delta S double dagger = - 146 +/- 2 J mol(-1) K-1. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.