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_a, 2_a, 3_a, and 4_a, 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_a-4_a 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_a-4_a along with the mechanism of each dimerization. Structures 1_a-3_a show one possibility for HB while 4_a represents two possibilities. Structures 1_a and 4_a display two HH dimers while 2_a and 3_a show one. Structures 1_a-4_a undergo HT dimerizations to yield three possible dimers which include trans, cis, and [2+3] isomers. Interestingly, for all 1_a-4_a, HB dimerization turns out as the most favorable stability pathway for showing no barrier of reaction. Structures 4_b and 4_c indicate the highest stability with respect to their initial 4_a compared to remaining HB dimers 1_b-3_b. In addition, the 1,2-H shift appears as a possible rearrangement for 1_a-4_a to yield their corresponding tautomers (1_i, 2_h, 3_h, and 4_k, respectively). The reaction profile of this rearrangement indicates that 1_a-4_a favor HB dimerization pathway more than 1,2-H shift, in terms of kinetic and thermodynamic. Graphical Abstract.