VT‐NMR Analysis of Rotation‐Inversion of N‐(4‐hydroxybutyl)‐N‐(2,2,2‐trifluoroethyl) tert‐butyl Carbamate: Utilizing the −CH₂CF₃ Appendage as a Reporter on E/Z‐Isomerization

The rotational barrier about the CN carbamate bond of N ‐(4‐hydroxybutyl)‐ N ‐(2,2,2‐trifluoroethyl) tert ‐butyl carbamate 1 was determined by variable temperature (VT) 13 C and 19 F NMR spectroscopy. The −CH 2 CF 3 appendage reports on rotational isomerism and allows for the observation of separate signals for the E ‐ and Z ‐ensembles at low temperature. The activation barrier for E / Z ‐isomerization was quantified using Eyring‐Polanyi theory which requires the measurements of the maximum difference in Larmor frequency Δ ν max and the convergence temperature T c . Both Δ ν max and T c were interpolated by analyzing sigmoidal functions fitted to data describing signal separation and the quality of the superposition of the E ‐ and Z ‐signals, respectively. Methods for generating the quality‐of‐fit parameters for Lorentzian line shape analysis are discussed. Our best experimental value for the rotational barrier Δ G c ≠ ( 1 )=15.65±0.13 kcal/mol is compared to results of a higher level ab initio study of the model N ‐ethyl‐ N ‐(2,2,2‐trifluoroethyl) methyl carbamate.