Spin-Echo NMR Studies of Chemical Exchange. III. Conformational Isomerization of Cyclohexane and d11-Cyclohexane

Abstract

Equations derived previously for NMR chemical-exchange effects have been used in proton spin-echo studies at 26.85 Mc/sec of the chair—chair isomerization of cyclohexane and d11-cyclohexane in 1:1 v/v solutions in CS2. The d11-cyclohexane is particularly suited for such studies because of its very long natural relaxation time T20, and exchange rates of 0.53 to 3.8×104 sec−1 were determined for it in the temperature range between −98° and +25°C. The heteronuclear coupling in d11-cyclohexane does not affect the spin-echo results. However, in cyclohexane the homonuclear coupling and the shorter T20 restricted the temperature range of accurate rate determinations to −60° and −25°C, respectively. The exchange rates are the same within experimental error for both compounds, as are ΔH‡, ΔFcb‡, and ΔScb‡ which were found to be 9.1 and 10.3 kcal/mole and −5.8 eu. The latter are compared with the results of several previous studies, and it is demonstrated that, in most if not all of the studies, there are systematic errors in the exchange rates. These errors have a modest effect upon ΔH‡ but their effect upon ΔS‡ is so large that the values for it are unreliable. Several factors governing the use of spin echoes to study chemical exchange are discussed.