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A line width method for determining chemical exchange rates from NMR spectra
Author(s) -
Ramey Kermit C.,
Louick David J.,
Whitehurst Philip W.,
Wise William B.,
Mukherjee Rabindranath,
Moriarty Robert M.
Publication year - 1971
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270030206
Subject(s) - chemistry , amide , spectral line , line (geometry) , chemical shift , mole fraction , analytical chemistry (journal) , mole , formamide , nmr spectra database , nuclear magnetic resonance , physics , organic chemistry , geometry , mathematics , astronomy
The applicability of W ½ , 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 P A = P B . The Gutowsky‐Holm equation, simplified by the assumption of a large T 2 º such that T 2 º−1= 0, was used to produce a family of curves relating W ½ corrected to the rate of exchange for various Δ v AB values. The rate of internal rotation about the CN amide bond has been studied in neat N , N ‐dimethyl‐formamide between 79·5 and 159° by the W ½ method and the results, E a = 24·9 and 24·1 Kcal/mole at 60 and 100 MHz do not agree with those recently reported by Rabinovitz and Pines, E a = 20·5 Kcal/mole obtained by the total line shape method. For neat N , N ‐dimethylacetamide and for solutions of DMA in dimethylsulfoxide‐ d 6 , application of the W ½ method yielded E a , ΔH‡ and ΔS‡ values which are in excellent agreement with those obtained by Neuman and Jonas for DMA‐ d ε using a total line shape analysis. The thermodynamic parameters for neat N , N ‐dimethylcarbamoyl chloride obtained by the total line shape and W ½ methods are also in excellent agreement, 16·4 and 16·8 Kcal/mole for E a and 15·8 and 16·2 Kcal/mole for ΔH‡, respectively. The W ½ method was also used for the determination of the activation parameters for rotation about the carbon‐nitrogen bond in the following thioamides: N , N ‐dimethylthiobenzamide, (E a = 19.8 Kcal/mole), N , N ‐diethylthiobenzamide, (E a = 20·4 Kcal/mole), N , N ‐dimethylphenylthio‐acetamide (E a = 21·4 Kcal/mole). The results obtained for these thioamides are compared with the corresponding amides. Specifically N , N ‐dimethylbenzamide and N , N ‐diethylbenzamide were studied using the W ½ method 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.