NUCLEAR MAGNETIC RESONANCE STUDIES OF INTERNAL ROTATION IN ALIPHATIC TERTIARY AMIDES

The temperature dependence of the nmr line shapes of N-methyl-N-n-butyltrimethylacetamide, N-acetyl-2-methylpiperidine, N,N-dimethyltrimethylacetamide (TMA), and N,N-diethyltrimethylacetamide show that the peaks due to the N-methyl protons are coalesced at 35° but gradually separate into two nmr signals at low temperatures. Therefore, at 35°, these highly substituted amides are rotating faster about the central C-N bond than simple amides such as dimethylacetamide. The activation energy for rotation of TMA in 10 mol % methylene chloride solution was found to be 11.5 ± 0.3 kcal/mol; log A, 12.3 ± 0.3; and the standard free energy of activation, 12.2 kcal/mol. The method of total line shape analysis was used to calculate the activation parameters.