Tautomerism and Thermal Decomposition of Tetrazole: High-Level ab Initio Study

Abstract

The mutual interconversion and decomposition reactions of four tetrazole isomers (1H-TZ, 2H-TZ, 5H-TZ, and an N-heterocyclic carbene 14H) have been studied theoretically using the W1 high-level procedure. Computations allowed resolution of the existing discrepancies in the mechanism and key intermediates of TZ thermolysis. The tautomeric equilibria between 1H-TZ, 2H-TZ, and 14H turned out to play a very important role in the mechanism of thermal decomposition. Although the barriers of monomolecular tautomeric transformations were found to be high (∼50-70 kcal/mol), the concerted double H atom transfer reactions in the H-bonded complexes of TZ tautomers have profoundly lower barriers (∼18-28 kcal/mol). These reactions lead to fast interconversion between 1H-TZ, 2H-TZ, and 14H. The carbene 14H has never been considered before; however, it was predicted to be a key intermediate in the mechanism of thermal decomposition of TZ. For all species considered, the unimolecular reactions of N(2) elimination were predicted to dominate over the elimination of hydrazoic acid. In agreement with existing experimental data, the effective activation energy of thermolysis was calculated to be 36.2 kcal/mol.

Activation of Dinitrogen as A Dipolarophile in 1,3-Dipolar Cycloadditions: A Theoretical Study Using Nitrile Imines as “Octet” 1,3-Dipoles

“…3. For the ring opening of 2,5-dimethyltetrazol, we find barriers of 167.4 (Δ G  = 142.5 + 24.9) and 171 kJ·mol –1 (Δ H  = 93.9 + 77.2), values that compare well with those reported for the non-substituted derivative, 153.6 (Δ G ) and 159.4 kJ·mol –1 (Δ H ) 6 .…”

“…Using a simplified model (R 2  = R 5  = H), the ring-opening reaction was studied theoretically by Kiselev et al . 6 which reported barriers of 153.6 (Δ G ) and 159.4 kJ·mol –1 (Δ H ).

Figure 1Ring opening of N 2 -substituted tetrazoles.

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UV-induced proton transfer in 3-amino-1,2,4-triazole

“…Such tautomerism involves the hydrogen migration between different nitrogen atoms within the ring. Knowledge of the tautomeric preferences in organic molecules, especially in tetrazoles and triazoles, is of importance since their functionality or physicochemical activity may be tautomer sensitive [14][15][16][17]. A number of papers have been published concerning different triazole tautomers, solvent effects on the equilibria between them [18][19][20][21] as well as the proton transfer phenomena in azole-based coordination polymers [22].…”

A combined experimental and theoretical study of the tautomeric and conformational properties of (5-phenyl-tetrazol-2-yl)-acetic acid methyl ester

“…Experimental techniques [17][18][19] and theoretical approaches [20][21][22] were applied to the study of this equilibrium. Further 1-and 2-substitution of the tetrazole hydrogen by organic group is also feasible, giving access to a variety of 1-and 2-substituted tetrazole species.…”