New pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole, by DFT.

We have investigated the pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole (1, 2, 3, and 4, respectively), at the M06/6-311++G** level of theory. Optimization and vibrational frequency calculations of ground states (GS) and transition states (TS) are performed to identify Gibbs free energies and nature of stationary points, respectively. Two possible pathways of stability for 1-4 are compared and contrasted which entail dimerization through hydrogen bonding (HB) and covalent bonding (CB). The CB pathway comprises head to head (HH) and head to tail (HT) dimerizations. Plausible reaction profiles are illustrated for 1-4 along with the mechanism of each dimerization. Structures 1-3 show one possibility for HB while 4 represents two possibilities. Structures 1 and 4 display two HH dimers while 2 and 3 show one. Structures 1-4 undergo HT dimerizations to yield three possible dimers which include trans, cis, and [2+3] isomers. Interestingly, for all 1-4, HB dimerization turns out as the most favorable stability pathway for showing no barrier of reaction. Structures 4 and 4 indicate the highest stability with respect to their initial 4 compared to remaining HB dimers 1-3. In addition, the 1,2-H shift appears as a possible rearrangement for 1-4 to yield their corresponding tautomers (1, 2, 3, and 4, respectively). The reaction profile of this rearrangement indicates that 1-4 favor HB dimerization pathway more than 1,2-H shift, in terms of kinetic and thermodynamic. Graphical Abstract.