Nuclear magnetic resonance studies of internal rotation—III Rotational barriers in META‐ and PARA‐substituted N, N‐dimethylcinnamamides

Abstract

Abstract-The rotr?tional barriers about the C-Pi bond of eight in-and p-substituted N,N-dimethyl cinnaiiiamides have been determined by the iterative total line shape WMR method. The AG$8,,-values have beer. correlated with the substituent constants 0, on and o+. By comparison of the results with literature data, some conc!usions about the accuracy of the barrier determination as well as the transmittance of po!ar effects in conjugated amides have been drawn. I N T R O D U C T I O NSINCE 1955: much attention has been devoted to the determination of the rotational barriers about the C-N bond in amides and related compounds by the dynamic NMR method, and most of the results have been summarized in recent review^.^,^.^ According to the general conclusio!i reached at present by many authors, most of the AH* and AS*-values obtained so far are of little significance, owing to the large systematic errors introduced during the experiment and/or by the use of approximate methods of calculation. It is generally agreed now that the total line shape (TLS) analysis is the only reliable method by which accurate kinetic NMR data may be obtained. Even in such cases, discussion of the results usually coficeiitrates on the AC+-values, which seem to be much less sensitive to experimental errors and their accuracy is often believed to be within 50.1 kcal/mole or better. Unfortunately, for symmetrically N,N-disubstituted amides the real accuracy of the results cannot be checked by the independent method of equilibration.in order to use barrier height-structure correlations, study of larger series of closely related compounds under the same conditions would be particularly desirable. For this purpose, we decided to examine by the iterative TLS method, the rotational barriers in a series of N,N-dimethyl-trans-cinnamamides (1) having various polar sutstituents in the m-andp-position of the ring.'N-CH 3

A Small Change in Structure, a Big Change in Flexibility

“…Recently, ab initio studies [20] of an internal rotation barrier around a C-N bond in N,N-dimethylcinnamamides, which were previously investigated by dynamic NMR spectroscopy [21,22], have been published. Scientific interest in cinnamamides and, in particular, coumaric amides has increased in recent years due to their potential antioxidant activity [23,24].…”

Dynamic NMR: A new intensity ratio for evaluation of lifetime in paripartite A ⇌ B systems

“…The results thus obtained are averaged over the whole frequency range of the spectrum to give rough estimates for the two unknowns. 5 Assuming…”

“…Assuming that the two points vi and vi are fixed, and making some transformations and certain approximations in Eqns (1) to (4) (see the Appendix), one obtains a formula for evaluation of the lifetime (5) …”