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The inversion barriers in aziridine and aziridine-N-d have been 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 65OC): AAG*(= AG*D -A&) = 0.57 f 0.07 kcal/mol, AAH: = 2.1 f 1.2 kcal/mol, AAS: = 4.8 f 3.6 cal/(mol K), and kH/kD = 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 Iv*H~ = 1050 f 50 cm-l and l u * d = 800 f 40 cm-I for the protium and deuterium compounds, respectively, have been 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), is found to be 19.1 kcal/mol. The barrier to nitrogen inversion in aziridines has received a great deal of attention from both e~p e r i m e n t a l~.~ and t h e o r e t i~a l~.~ points of view during the past 20 years. The inversion barrier in aziridine (ethylenimine) itself, the "mother" compound, had until recently6-8 eluded experimental efforts at its determination, except for two "lowerlimit" microwave estimation^.^^^^ On the other hand, theoretical calculations of this barrier,5 ranging in sophistication and complexity from extended HUckelSe and valence force fieldSa to ab initi0,~~-~3' have not been lacking. Our own interest in this molecule arises from two equally intriguing sources: first, we wished to establish a reliable gasphase value for the inversion barrier from complete band shape analysis of Fourier transform (FT) NMR spectra, for comparison with the theoretical data; and second, we wished to determine the kinetic deuterium isotope effect on the barrier in the gas phase, and to carry out a detailed theoretical calculation of the isotope effect using the method of Wolfsberg and Stern,' and including consideration of tunneling. In the present paper we report the results of our efforts along these two lines of investigation, and we make an attempt to use the results to aid in the elucidation of the details of the inversion process, in particular the shape and dimensions of the potential barrier, and the structure of the transition state. Experimental SectionMaterials. The aziridine used in this work was a commercial product, which was stored over KOH in a refrigerator. Aziridine-N-d was prepared by exchange with D20, after which the resulting solution was made basic with KOD (from repeated exchange of KOH with &0), and the aziridine-N-d was distilled off into a receiver containing KOD. The product was redistilled in vacuo onto fresh KOD and stored under vacuum in a refrigerator. The degree of deuteration was roughly estimatedI2 to be about 86% by means of an N M R spectrum recorded at +15OoC, at which temperature part of the signal due to the protium compound appeared at the base of the singlet from aziridine-A'-d. T...

Calculated isotope effects on the two-hydron transfer within the formic acid dimer calculated at different levels of theory including perturbational electron correlation correction (MP2) together with normal mode analysis form a basis for the interpretation of isotope effects in two-hydron transfer reactions.

Ab initio calculations at the VDZ and VDZ + P levels of theory of the two-proton transfer reaction within an isolated formamidine dimer give transition states with low symmetry, and with 6-31G" basis sets indicate stepwise two-proton transfers even in the absence of solvent.