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A DFT calculation was performed at the B3LYP/6-31G* level on the complexes formed by NH4+ and aromatic nitrogen heterocyclics, viz. pyrrole, imidazole, pyridine and indole, in order to investigate the mechanism and complexity of the interaction between the ammonium group and the aromatic heterocyclic in biomacromolecules. The optimized geometries suggested that there are two different types of complexes: one is a cation–π complex and the other is a hydrogen bond complex. A cation–π complex will be formed if the heteroatom has no localized lone-pair electrons. A hydrogen bond complex will be formed by proton transfer from NH4+ to the heteroatom if the heteroatom has localized lone-pair electrons. In the case of the cation–π complex, the predicted geometries, atomic charges and thermodynamic parameters revealed that ammonium binds more strongly to heterocyclics than it binds to benzene. The calculated orbital coefficient and the optimized structures implied that NH4+ interacts with the π electrons of the CC bond of heterocyclics to form a cation–π complex mainly through one hydrogen atom. Regarding the hydrogen bond complex, although the calculated binding strength is similar to that for the cation–π complex, the ΔH of the whole reaction process suggested that the formation of the hydrogen bond complex is favorable to the stability of the whole system. Calculated IR spectra showed that three groups of new bands appear when NH4+ binds to heterocyclics. Normal mode analysis showed that these new bands are all related to the relative motion of the two parts in the formed complexes. All these results suggest that the NH4+–heterocyclic system is a better model for studying the nature and complexity of the interaction between the ammonium group and the aromatic ring structure in biomolecules.

Quantum chemical ab initio (U)HF/4-31G investigation on buckminsterfullerene and some proposed intermediates in its formation is carried out in this study with a view to better understanding how small carbon species carry out self-assembly to form fullerenes. The calculations on 19 carefully designed fullerene intermediates reveal that the core of an intermediate, rather than the number of its dangling bonds or abutting pentagon rings, has an intrinsic effect on its energy. The computational results show that hexagonal-core structures have lower energies than pentagonal-core structures. In addition, the pentagonal core enclosed completely by hexagonal rings has the highest energy. The UHF/4-31G results also suggest that some intermediates such as C18, C21 and C30 with hexagonal cores have unusually low energies in comparison with their isomers or neighbours. Based on these calculated results, we outline the possible pathways from precursor to intermediates to fullerenes, subject to synthesis conditions and raw materials. These pathways support some existing proposals, such as medium monocyclic ring stacking and small ring polymerization mechanisms. However, our results do not suggest that the numbers of dangling bonds or abutting pentagonal rings have the highest impact on fullerene formation. The calculated thermodynamic parameters of the dimerization and addition reactions between two bowl-shaped intermediates suggest that these reactions are favorable to fullerene formation, and that the concentration of bowl-shaped fullerene intermediates should be very low in all detectable carbon species.