An experimental study of some NMR methods for the measurement of slow exchange rates

✍ Scribed by P. Baine; J. T. Gerig; A. D. Stock

• Publisher: John Wiley and Sons
• Year: 1981
• Tongue: English
• Weight: 432 KB
• Volume: 17
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270170110

✦ Synopsis

Abstract

The rates of interconversion of the cis and trans rotational isomers of N‐trifluoroacetyl‐N‐methylbenzylamine have been determined by a variety of NMR lineshape experiments and by several double resonance methods. Comparison of the results for the slow exchange region suggests that a Fourier transform version of the transfer of saturation method of Forsen and Hoffman, as well as transfer of magnetization after selective 180° inversion of the resonance of one rotamer, give rate constants nearly as reliable as lineshape methods. Use of the rate of signal recovery after saturation of the resonance of one rotamer gave results which were less accurate. The available data produced the following activation parameters for amide rotation in this compound: Δ__G__^‡^, 18.9 kcal mol^−1^; Δ__H__‡, 22.3 kcal mol^−1^ and Δ__S__‡, 9.8 e.u. when the solute was 2 M in 1,1,2,2‐tetrachloroethane‐d~2~.