Inversion and tunneling in aziridine and aziridine-N-d. Gas-phase nuclear magnetic resonance band shape analysis and model calculations of the isotope effect

The inversion barriers in aziridine and aziridine-N-d were measured in the gas phase by NMR total band shape analysis. The isotope effect on the inversion barrier may be described by the following parameters (at 650C): ΔΔG2+(= ΔG2+/sub D/ - ΔG2+/sub H/) = 0.57 +- 0.07 kcal/mol, ΔΔH2+ = 2.1 +- 1.2 kcal/mol, ΔΔS2+ = 4.8 +- 3.6 cal/( mol K), and k/sub H//k/sub D/ equals 2.34. Model calculations of the isotope effect, including complete vibrational analyses of the two isotopic species, are able to reproduce the experimental data only if tunneling is taken into account. Reaction coordinate frequencies of parallelν2+/sub H/parallel = 1050 +- 50 cm-1 and parallelν2+/sub D/ parallel = 800 +- 40 cm-1 for the protium and deuterium compounds, respectively, were estimated, and the barrier height, corrected for zero-point energy differences between initial and transition states (and thus comparable with theoretical inversion barriers obtained by molecular orbital methods), was found to be 19.1 kcal/mol