STEREODYNAMICS OF ISOPROPYLDIMETHYLAMINE - C-13(H-1) AND H-1 DYNAMIC NMR-STUDIES - MOLECULAR MECHANICS CALCULATIONS

Since it is the simplest tertiary aliphatic amine that has one isopropyl group, isopropyldimethylamine (IDMA) is an important member of a group of simple aliphatic amines. Both the H-1 and C-13{H-1} NMR spectra of IDMA are decoalesced into two subspectra at about 94 K due to slowing isolated rotation about the methine carbon-nitrogen bond. The major subspectrum is assigned to the enantiomeric conformations that have one isopropyl methyl group anti (A) and the other gauche (G) to the nitrogen lone pair (AG and GA forms). The minor subspectrum is assigned to the GG conformation that has both isopropyl methyl groups gauche to the lone pair. At about 94 K, there is a free energy preference for the AG (or GA) conformation over the GG equal to 0.070 +/- 0.020 kcal/mol. Simulations of the exchange-broadened NMR spectra indicate that the AG (or GA) to GG conformational exchange occurs at a faster rate than the AG to GA process. The latter process is sufficiently slower that it contributes little to exchange broadening of the NMR line shape. The free energy of activation (DELTAG(double dagger)) for the faster NMR-visible AG (or GA) to GG conversion is 4.5 +/- 0.1 kcal/mol at 95 K. This barrier is among the lowest measured by using the NMR method. The lower limit for DELTAG(double dagger) associated with the AG to GA process is 5.2 kcal/mol at 95 K. Molecular mechanics calculations of conformational energies and rotation barriers agree well with the experimental data. The molecular mechanics calculations indicate a small enthalpy preference for the GG conformation (0.19 kcal/mol) and a barrier for the AG to GA conversion (DELTAH(double dagger) = 7.84 kcal/mol) that is higher than that for the AG (or GA) to GG process (5.37 kcal/mol).