Triazene: An ab initio Molecular‐Orbital Study of Structure, Properties, and Hydrogen‐Transfer Reaction Pathways
Authors: Nguyen Minh‐Tho , Hoesch Lienhard 
Published: December 2004

Ab initio calculations of structure, properties, and tautomerization reactions of triazene (1) at the HF/3‐21G//3‐21G, HF/6‐31G*//6‐31G*, HF/6‐31G**//6‐31G*, and MP2/6‐31G*//6‐31G* levels led to the following conclusions and predictions: (a) Calculations of the ground‐state structure of (E)‐ and (Z)‐triazene (1a and 1b, respectively) at various levels of theory show for both isomers C1 geometry with a rather flat pyramidal configuration at N(3), and small energy differences (0.2–7.2 kJ/mol) between C1 and Cs geometry, i.e. inversion at N(3) is a quasi‐free process. With all levels of calculations, 1a is found to be of lower energy than 1b by 23‐30 kJ/mol. (b) Comparison of vibrational frequencies of (E)‐diazene (3) calculated at the HF/3‐21G level with experimental values reveals that HF/3‐21G calculations are reliable for the prediction of vibrational frequencies of polyaza compounds, if corrected by a factor of 0.91. On this basis, the harmonic vibrational frequencies of 1a and 1b were predicted. (c) For the rotation around the N(2)—N(3) bond of 1a two conceivable transition states, 5a (syn) and 5b (anti) were located (HF/3‐21G). The energy differences between 5a or 5b, and 1a are in the order of magnitude of 50‐56kJ/mol and show a slight preference for the anti‐mode, i.e. energy barriers for the N(2)—N(3) rotation are obtained comparable to those observed experimentally with substituted (E)‐triazenes (4). (d) Protonation of 1a at N(1), N(2), or N(3) leads to 6a, 6b, and 6c, respectively –the last one resembling an intermediate of formation of 1 from hydrogendiazonium ion (7) and ammonia (8). Energetically, the conjugate acids of 1a follow the sequence 6a < 6c < 6b. (e) The preference of N(1) protonation of 1a is also reflected in the relatively high gain of energy in the formation of H‐bonded dimers of 1a with H‐bonds from N(3)—H to N(1). Calculations of three different H‐bonded dimers 9a–c of 1a with the 3‐21G basis show that an eight‐membered cyclic dimer 9c with two H‐bonds from N(3)H to N(1) is energetically most favoured (67.5 kJ/mol below two separate molecules of 1a). This dimer might well be the starting situation of double intermolecular H‐transfer leading to an automeric dimer 9c via an energetically low‐lying transition state 12, thus offering a low‐energy pathway for the known easy tautomerization of mono‐ or disubstituted (E)‐triazenes. For 9c⇄9c, the activation energy including correction for polarization and correlation effects as well as for vibration zero‐point energy is estimated to be ca. 54kJ/mol. (f) A six‐membered cyclic dimer 9b of 1a with two H‐bonds from N(3)H to N(2) might be involved for double H‐transfer via a transition state 11 to a dimer 10 of (E, Z)‐azimine (2). This process, however, turns out to be energetically highly disfavoured (estimated energy barrier for 9b→10: 232 kJ/mol) in contrast to the reverse reaction (10→9b via 11: 4 kJ/mol). This leads to the prediction that azimines bearing an H‐atom at N(2) might be kinetically too instable for isolation, being, instead, easily tautomerized to triazenes by bimolecular H‐transfer.