Hydrogen-bonding interactions involving the Imidazol-2-ylidene and its Heavy-atom analogues

The hydrogen-bonded complexes formed between the imidazol-2-ylidene and its heavier congeners C2H4N2T: (T = C, Si, Ge, Sn, Pb) and the three selected electron acceptors (HF, HCN and C2H2) have been optimized at the aug-cc-pVTZ level, and the nature of these complexes has been investigated by natural bond orbital (NBO), atoms in molecules (AIM) and energy decomposition analyses. The complexes can be classified into type-A structure and type-B structure according to the location of the electron acceptors. The electron acceptors are around T atoms of C2H4N2T: for the type-A complexes, and the electron acceptors are above the five-membered ring (n-system) of C2H4N2T: for the type-B complexes. The electron-donating ability of T atoms becomes weaker, and that of n-systems becomes stronger when T atoms become heavier for C2H4N2T:, which is consistent with the molecular electrostatic potentials (MEP) maps of C2H4N2T:. For a given C2H4N2T:, the Eint values of the type-A complexes go in the order HF > HCN > C2H2, and the Eint values of the type-B complexes go in the order HF approximate to HCN > C2H2. The contribution of electrostatic energy to the interaction energy of complexes becomes smaller for the type-A complexes when the T atoms become heavier. The interaction energies are dominated by the elec-trostatic energies for all the type-B complexes involving HF, and the interaction energies are dominated by the dispersion energies for most type-B complexes involving HCN and C2H2. The relaxed potential energy surface scans were performed for some selected complexes involving HF and HCN to examine the difference in binding behavior between HF and HCN.