The usefulness of aromaticity/antiaromaticity concepts to foresee structural stability patterns and salient features of the electronic structure of small inorganic and all-metal rings has been put forward. A critical revision of the advances made in the theoretical methods to assess the aromaticity/antiaromaticity of these compounds has also been made. In particular, the performance of local versus non-local indices has been reviewed. Finally, the passivation of these rings has been put forward as a key issue in order to prevent them from collapsing into larger aggregates and to provide them protection against the environment.

Time-dependent density-functional theory (TDDFT) is used to study the excitation energies of the global minima of small Zn(i)O(i) clusters, i = 1-15. The relativistic compact effective core potentials and shared-exponent basis set of Stevens, Krauss, Basch, and Jasien (SKBJ), systematically enlarged with extra functions, were used throughout this work. In general, the calculated excitations occur from the nonbonding p orbitals of oxygen. These orbitals are perpendicular to the molecular plane in the case of the rings and normal to the spheroid surface for 3D clusters. The calculated excitation energies are larger for ringlike clusters as compared to 3D clusters, with the excitation energies of the latter structures lying close to the visible spectrum. The difference between Kohn-Sham eigenvalues of the orbitals involved in the electronic excitations studied have also been compared to the TDDFT results of the corresponding excitations for two approximate density functionals, that is, MPW1PW91 and B3LYP, the latter being more accurate. Moreover, they approach the TDDFT value as the cluster size increases. Therefore, this might be a practical method for estimating excitation energies of large Zn(i)O(i) clusters.

The water-promoted hydrolysis of a highly twisted amide is studied using density functional theory in conjunction with a continuum dielectric method to introduce bulk solvent effects. The aim of these studies is to reveal how the twisting of the C-N bond affects the neutral hydrolysis of amides. To do so, both concerted and stepwise mechanisms are studied and the results compared to the ones from the hydrolysis of an undistorted amide used as reference. In addition, an extra explicit water molecule that assists in the required proton-transfer processes is taken into account. Our results predict important rate accelerations of the neutral hydrolysis of amides when the C-N bond is highly twisted, the corresponding barrier relaxation depending on the specific reaction pathway and transition state involved. Moreover, our calculations strongly suggest a change in reaction mechanism with degree of amide bond twist, and clearly point to a concerted mechanism at neutral pH for the hydrolysis of highly twisted amides. In addition, the twisting of the amide bond also provokes a higher dependence on an auxiliary water molecule for the concerted mechanism, due to the orthogonality of the lone pair of the nitrogen and the carbonyl pi orbital. There is a direct implication of these findings for biological catalytic mechanism of peptide cleavage reactions that undergoes ground-state destabilization of the peptide.

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We present an ab initio study of the pK a of a series of twisted amides in aqueous solution. The shift of the pK a with respect to the known value for trimethylamine (TMA) is used in the evaluation. Calculations were made with density functional theory using the B3PW91 functional. Aqueous solvent effect is introduced by use of the polarizable continuum model. The protocol is tested on a series of tertiary amines and twisted amides, and a good agreement with the available experimental data is found. Finally, mechanistic implications of the change in basicity of the nitrogen as a function of the twist of the amide bond for the activation of C-N bond hydrolysis are discussed.