The barrier to the internal rotation around the C-N bond in N-trifluoroacetylpyrrolidine (TFAPYR) was characterized using dynamic NMR spectroscopy. Kinetic parameters, DeltaG*(298) = 71.4(1.2) kJ mol(-1), DeltaH* = 67.9(1.4) kJ mol(-1), and DeltaS* = -11.8(4.0) J mol(-1)), were found to be lower in the gas phase than in solution. The pyrrolidine ring substituent in TFAPYR reduces the bulkiness around the C-N amide bond, stabilizing the ground state and increasing the value of the internal rotation barrier, DeltaG*(298), compared to NN-dimethyl- and N,N-diethyltrifluoroacetamide. The magnitude of DeltaG*(298) in TFAPYR is also comparable to that found in primary amide bonds in peptides, emphasizing the importance of having a relatively small energy difference between cis and trans conformations in proline's role as a switch in protein signaling.