STEREODYNAMICS OF N-ETHYL-N-METHYL-2-AMINOPROPANE - H-1 AND C-13(H-1) DNMR STUDIES - MOLECULAR MECHANICS CALCULATIONS

N-Ethyl-N-methyl-2-aminopropane (EMAP) is one of the simplest tertiary aliphatic amines that has a chiral center at nitrogen. Racemization occurs by inversion-rotation at the pyramidal nitrogen. For each EMAP enantiomer, additional conformational interconversions occur via isolated rotation about carbon-nitrogen bonds. The H-1 and C-13(H-1) dynamic NMR (DNMR) spectra of EMAP and a selectively deuterated derivative decoalesce into four subspectra at 95 K. The spectrum at 95 K is best rationalized in terms of five equilibrium conformations present at concentrations high enough to be NMR-detectable including two conformations that interchange rapidly even at 95 K. A two-letter designation is used to name the various conformations. The first letter defines the orientation of the ethyl methyl group (G denotes gauche to the lone pair and to the N-methyl group; G' denotes gauche to the lone pair and to the isopropyl group; A denotes anti to the lone pair). The second letter defines the orientation of the isopropyl methine proton (G denotes gauche to the lone pair and to the N-methyl group; G' denotes gauche to the lone pair and to the ethyl group; A denotes anti to the lone pair). The major subspectrum at 95 K is assigned to a family of G'G' and GG' conformations (59%) that interconvert rapidly at 95 K. The other three subspectra are assigned to the GG (34%), AA (5%), and GA (2%) conformations. Simulations of the DNMR spectra reveal a barrier to inversion-rotation at nitrogen (Delta G(double dagger) = 7.5 kcal/mol at 160 K) that is higher than the barriers for a number of observable isolated rotations about the N-CH2 and N-CH bonds (Delta Ge-double dagger = 4.7-6.4 kcal/mol). A 5000-point optimized energy surface computed as a function of two dihedral angles by using Allinger's MM2(87) computer program, MM2(87) energy calculations for all the optimized equilibrium conformations of EMAP, and MM2(87)-calculated isolated rotation barriers show excellent agreement with the DNMR data.