GAS-PHASE H-1-NMR STUDIES OF INTERNAL-ROTATION ACTIVATION-ENERGIES AND CONFORMER STABILITIES OF ASYMMETRIC N,N-DISUBSTITUTED FORMAMIDES ANDTRIFLUOROACETAMIDES

Activation parameters and conformational stabilities characterizing th e internal rotation about the peptide bond in a series of N,N-asymmetr ic dialkylformamides (HCONR1R2: R-1 = CH3, R-2 = propyl, butyl, and is opropyl) and N,N-asymmetric dialkyltrifluoroacetamides (F3CCONR1R2: R- 1 = CH3, R-2 = propyl, butyl, and isopropyl) are determined from tempe rature-dependent gas-phase H-1 NMR spectra. Conformer free energy diff erences, Delta G(298)(0)(syn-anti), in cal mol(-1), and activation fre e energies, Delta G(298)(double dagger), in kcal mol(-1), for the form amides are -83(14)/19.4(0.1) for R-2 = Propyl, -80(14)/19.3(0.1) for R -2 = butyl, and -91(13)/19.1(0.1) for R-2 = isopropyl and for the trif luoroacetamides 178(24)/16.8(0.1) for R-2 = propyl, 191(53)/16.6(0.1) for R-2 = butyl, and 218(29)/16.3(0.1) for R-2 = isopropyl. The prefer red conformer in both the gas and Liquid phases has the N-methyl group syn to the carbonyl oxygen in the formamide systems and the N-methyl group anti to the carbonyl oxygen in the trifluoroacetamides. The gas- phase results are compared to liquid-phase values.