US EPA

8702928 

Journal Article 

Spin-echo NMR studies of chemical exchange. III. conformational isomerization of cyclohexane and d11-Cyclohexane 

Alleehand, A; Chen, FUM; Gutowsky, HS 

1965 

Yes 

Journal of Chemical Physics
ISSN: 0021-9606
EISSN: 1089-7690 

42 

9 

3040-3047 

English 

10.1063/1.1696376 

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 T 2°, 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 d11cyclohexane does not affect the spin-echo results. However, in cyclohexane the homonuclear coupling and the shorter T2° 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‡ , ΔFeb‡, and ΔSeb‡ 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.