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The theoretical investigation of tetramethylammonium (TMA)−imidazole and TMA−furan complexes has been performed to justify the preferred structure of the cation−aromatic complexes predicated by the...

A detailed theoretical investigation of the tetramethylammonium(TMA)-benzene and TMA-pyrrole complexes has been performed to obtain the interaction properties of TMA with aromatics. Diffuse functions have been found to be important in the computational studies of these noncovalent complexes. Adding diffuse functions to the basis set decreases the binding energy by about 10% for the TMA-aromatic systems. Dispersion interactions in the TMA-aromatic systems are very important. They enhance the binding interactions between the TMA and the aromatic ring systems by about 0.5 kcal·mol-1 per interacting atomic pair, which is in agreement with the estimates of Rappe and Bernstein.1 Also, for the TMA-pyrrole complex, the presence of the dispersion interaction leads to a dramatic change in the optimized structure. Because B3LYP cannot handle properly the dispersion in the calculation, use of the Moller−Plesset second-order perturbation or other sophisticated methods should be considered in computational studies of c...

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.

DFT/B3LYP calculations were carried out on complexes formed by NH4+ with aromatics, viz. benzene, phenol, pyrrole, imidazole, pyridine, indole, furane, and thiophene, to characterize the forces involved in such interactions and to gain further insight into the nature and diversity of cation−aromatic interactions. Such calculations may provide valuable information for understanding molecular recognition in biological systems and for force-field development. B3LYP/6-31G** optimization on 35 initial structures resulted in 11 different finally optimized geometries, which could be divided into three types: NH4+−π complexes, protonated heterocyclic−NH3 hydrogen bond complexes, and heterocyclic−NH4+ hydrogen bond complexes. For NH4+−π complexes, NH4+ always tilts toward the carbon−carbon bond rather than toward the heteroatom or the carbon−heteroatom bond. The calculated CHelpG charges suggest that the charge distribution of a free heterocyclic may be used to predict the geometry of its complex. Charge populati...

Tetraethylammonium (TEA) and other quaternary ammonium (QA) ions are potent blockers of potassium channels. In order to shed light on the blockade mechanism of QA ions, we have carried out a series of computations on the phenol–ammonium model with density functional theory (DFT) and Moller–Plesset second order perturbation (MP2) methods at levels of 6-31G* and 6-31G** basis sets. NH–aromatic π interaction and NH–OH hydrogen bond interaction, which are important in biological systems, are responsible for the binding of NH4+ to phenol. From analysis of structures, energies, charge populations and transition state features, both the cation–π interaction and hydrogen bond or electrostatic interaction between QAs and the key amino acid residues at the entryways of K+ channels are seen to be significant in the blockade mechanism of QA ions.