Total Line Shape Analysis Research Articles

Activation parameters for chair–twist equilibration in 3,3,6,6-tetramethyl-1,2,4,5-tetrathian by direct thermal stereomutation and total nuclear magnetic resonance line-shape analysis

Activation parameters (Ea, ΔG‡, ΔH‡, ΔS‡) for the chair–twist process in 3,3,6,6-tetramethyl-1,2,4,5-tetrathian have been obtained from the direct equilibration of the pure twist conformer and from matching experimental n.m.r. spectra with theoretical spectra over an appropriate temperature range.

A nuclear magnetic resonance total line-shape treatment of internal rotation in dimethylformamide

We have shown1 that dimethylformamide (DMF) molecules exhibit strong polar interactions in solution. These interactions are of the order of 6 Kcal/mole and must have an appreciable influence on the process of internal rotation in these molecules. In order to discuss the nature and extent of any such perturbations. we clearly require an accurate method for examining the internal rotation in this system and determining the magnitude and reliability of the thermodynamic parameters associated with it’ . Here, we describe preliminary results of a total nmr line-shape analysis of the hindered Internal rotation in DMF; this method seems to be the only one meeting the above requirements. A computer was available and it was a simple matter to introduce the N-methylformy: proton coupling into the treatment, as well as the temperature dependence of the limiting X-methyl proton chemical shift3. However, the main advantage of this method is that it furnishes a direct check on the values of the parameters derived, by examination of the “closeness-of-fit” between the experimental and computer-generated theoretical nmr curves. Linearity of Arrhenius plots alone does not seem to be a sufficient criterion, as in many cases linear plots have been reported for the case of DMF, each leading to widely different values for the activation energy and frequency factor4.