Studies of chemical exchange by nuclear magnetic resonance methods

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Abstract
Several hindered internal rotation barriers have been re-examined using both spin-echo Carr-Purcell measurements and complete fits of the line shapes developed in high resolution N.M.R. spectra. Significant errors in our present work and previous studies are found and their origin is discussed. At the present time, the spin-echo method tends to give rate constants which are too high in the slow exchange region and too low in the fast exchange region compared to complete line shape analysis of high resolution spectra. The activation energies are approximately 10 to 15 per cent too low. The possibility of measuring the relative signs of coupling constants from line shapes when the exchanging nuclei are both coupled to a third nucleus in an intramolecular exchange process is discussed. The molecules studied here include methyl nitrite, N,N-dimethylformamide, N,N-dimethylcarbamyl chloride and cyclohexane. In the case of the measurement of rates of exchange for methyl groups in N,N-dimethylformamide by high resolution methods, a complete four-site rate matrix is used which takes into account the long range cis and trans couplings. In methyl nitrite, the cis/trans ratio is almost independent of temperature leading to a very low value for the enthalpy of isomerization. These results are compared with previous literature which is not consistent. Improvements in spin-echo spectrometers, such as elimination of field drift, may well lead to better rate studies by this method.