We have investigated the pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole (1a, 2a, 3a, and 4a, 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 1a-4a 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 1a-4a along with the mechanism of each dimerization. Structures 1a-3a show one possibility for HB while 4a represents two possibilities. Structures 1a and 4a display two HH dimers while 2a and 3a show one. Structures 1a-4a undergo HT dimerizations to yield three possible dimers which include trans, cis, and [2+3] isomers. Interestingly, for all 1a-4a, HB dimerization turns out as the most favorable stability pathway for showing no barrier of reaction. Structures 4b and 4c indicate the highest stability with respect to their initial 4a compared to remaining HB dimers 1b-3b. In addition, the 1,2-H shift appears as a possible rearrangement for 1a-4a to yield their corresponding tautomers (1i, 2h, 3h, and 4k, respectively). The reaction profile of this rearrangement indicates that 1a-4a favor HB dimerization pathway more than 1,2-H shift, in terms of kinetic and thermodynamic. Graphical Abstract.
Keywords: Azole; DFT; Dimerization; NHC; Stability; ab-Tetrazole; di-Azole; n-Tetrazole.