Barriers to rotation about the N—CO bond in N-vinyl amides; a new two-site approximation method

Abstract

The barriers to rotation about the N—CO bonds in a series of N-vinyl amides have been determined by a study of their proton spectra at variable temperatures. A complete LACN 3 analysis of compound 1 gave the parameters used in a complete line shape (CLS) analysis to obtain ΔG≠. A new approximate method was shown to give a value for ΔG that agreed within 0.1 kcal/mol of that obtained by CLS. The approximate method was employed to determine ΔG≠ for other members of the series. The barrier in the N-vinyl amide 1 was found to be 2.5 kcal/mol higher than that in its saturated analog. Variations within the series of N-vinyl amides amounted to only 1.2 kcal/mol. A steric effect is proposed for the increase in barrier height. The new approximation formula was tested for the general case of unequal singlet coalescence and found to give k's accurate to within 4% for Δν > 10 and to within 10% at lower values of Δν.