A line width method for determining chemical exchange rates from NMR spectra

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Abstract
The applicability of W½, the line‐width at one‐half height corrected for field inhomogeneity and couplings, for characterization of nuclear magnetic resonance line shapes generated by exchange averaging of chemical shifts has been investigated for the case PA= PB. The Gutowsky‐Holm equation, simplified by the assumption of a large T2º such that T= 0, was used to produce a family of curves relating W½corrected to the rate of exchange for various ΔvABvalues.The rate of internal rotation about the CN amide bond has been studied in neatN,N‐dimethyl‐formamide between 79·5 and 159° by the W½method and the results, Ea= 24·9 and 24·1 Kcal/mole at 60 and 100 MHz do not agree with those recently reported by Rabinovitz and Pines, Ea= 20·5 Kcal/mole obtained by the total line shape method. For neatN,N‐dimethylacetamide and for solutions of DMA in dimethylsulfoxide‐d6, application of the W½method yielded Ea, ΔH‡ and ΔS‡ values which are in excellent agreement with those obtained by Neuman and Jonas for DMA‐dεusing a total line shape analysis. The thermodynamic parameters for neatN,N‐dimethylcarbamoyl chloride obtained by the total line shape and W½methods are also in excellent agreement, 16·4 and 16·8 Kcal/mole for Eaand 15·8 and 16·2 Kcal/mole for ΔH‡, respectively.The W½method was also used for the determination of the activation parameters for rotation about the carbon‐nitrogen bond in the following thioamides:N,N‐dimethylthiobenzamide, (Ea= 19.8 Kcal/mole),N,N‐diethylthiobenzamide, (Ea= 20·4 Kcal/mole),N,N‐dimethylphenylthio‐acetamide (Ea= 21·4 Kcal/mole). The results obtained for these thioamides are compared with the corresponding amides. SpecificallyN,N‐dimethylbenzamide andN,N‐diethylbenzamide were studied using the W½method and activation energies for rotation about the carbon‐nitrogen bond of 17·5 and 15·6 Kcal/mole, respectively, were found. The relative magnitudes of these parameters are discussed on the basis of molecular geometry of the thioamides. Attention is drawn to the fact that for structurally related amides and thioamides, the barrier heights for internal rotation are not greatly different.