Intramolecular conversions over low barriers. VII. The aziridine inversion—Intrinsically non-RRKM

Abstract

The rate of bond inversion at the nitrogen atom in aziridine (dimethylene-imine) was measured in the gas phase for a range of pressures and temperatures. 1H NMR spectra were recorded of samples at 5–1100 Torr, and from 298 to 388 K. Rate constants for inversion were derived by complete line shape analysis of the NMR spectra. Their temperature dependence at the high pressure limit (1100 Torr) gave Ea =15.8±0.4 kcal mol−1; ΔH‡ =15.2±0.4 kcal mol−1; ΔS‡=−5.5±3 eu; and ΔG‡298=16.8±0.5 kcal mol−1. These values are in excellent agreement with previously reported mangitudes obtained for solutions and in the gas phase at high pressures. However, at all temperatures we found no dependence of the first order rate constants on pressure down to 5 Torr, in sharp contrast to the fall-off curves calculated with standard RRKM codes, which indicated that for pressures below 100 Torr this system should be in the bimolecular regime. A less extended set of measurements was made with 2-methyl-aziridine which does show a small but clear fall off in kuni. These results can be rationalized in terms of the regional phase space model, previously proposed for methylnitrite. In systems such as aziridine, where the density of states at the barrier height is sparse (≈10/cm−1) the rate of inversion is limited by the relaxation time for intramolecular vibrational energy redistribution. Our results indicate that many nanoseconds are required to attain relaxation rather than picoseconds, estimated from the Marcus formula for k23(E)=N‡(E+)/hρ(E). In the methyl-substituted analog the barrier height is approximately 3 kcal mol−1 higher, and the density of states at this barrier is higher than in aziridine. Relaxation data for conformational changes in C6H12, H3CONO, SF4, and C5H10O were reanalyzed in terms of the regional phase-space model. At low pressures these also show departures from RRKM predictions.