ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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.
Additional Material:
7 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19860690717
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