Abstract
The portions of the N~3~H~3~ singlet potential energy surface corresponding to triaziridines (1), azimines (2) and triazenes (3) have been calculated by ab initio SCF using 3‐21G, 6‐31G, and 6‐31G** basis sets. Minima and transition states were located by force gradient geometry optimization. The most important computation results are: (1) Triaziridines (1): The configuration at the 3 N‐atoms is pyramidal. There are 2 stereoisomers, 1a and 1b. The c,t‐isomer 1a has less energy than the c,c‐isomer 1b. The 2 stereoisomerizations by N‐inversion hve rather high activation energies. The N,N bonds in 1 are longer and weaker (STO‐3G estimation) than in hydrazine. The N‐homocycle 1 exhibits less ring strain than the C‐homocycle cyclopropane or three‐membered heterocycles. (2) Azimine (2): All 6 Atoms are in the same plane. There are 3 stereoisomers, 2a, 2b, and 2c. The order of ground state energies is (Z,Z) < (E,Z) ≫ (E,E). The 2 N,N bond lengths correspond to multiplicity 1½. The electronic structure of 2 corresponds to a 1,3‐dipole with almost equal delocalization of the 4 π‐electrons over all 3 N‐atoms. The negative net charge at the central N‐atom is much less than that at the terminal N‐atoms. Azimines should behave as π‐donors in complexation with transition metals (3) Triazene (3): All 6 atoms are in the same plane. There are 2 stereoisomers, 3a and 3b. The order of ground‐state energies is (E) < (Z). The stereoisomerization proceeds as pure N‐inversion. N‐Inversion has a high energy barrier inversion at N(1) is faster than at N(2). One of the N,N bond lengths is typical for a double, the other for a single bond. The electronic structure of triazene 3 entails rather localized π‐ and p‐electron pairs at N(1),N(2) and at N(3). Triazenes should behave as p‐donors in complexation with transition metals. (4) ‐N~3~H~3~‐Isomers: The order of ground‐state energies is 3 < 2 < 1. The energy differences between these constitutional isomers are much larger than between the stereoisomers of each. The [1,2]‐H shifts for conversions of 2 to 3 and the [1,3]‐H shift for tautomerization of 3 have relatively high activation energies; both shifts can be excluded as modes of thermal, unimolecular transformations.