E3 M3+ (E=C-Pb, M=Li-Cs) Clusters: The Smallest Molecular Stars.

Extensive potential energy surface explorations of twenty-five clusters with the formula E3 M3+ (E=Group 14 element and M=Group 1 element) through density functional theory and high-level ab initio computations reveal that the lowest-energy isomer for all these systems corresponds to a non-classical D3h star-like structure in the singlet state, where three M atoms interact electrostatically with the triangular E3 core, occupying three bridging positions around it. More than 18 200 calculations were done in the search for the minima structures, starting with a first phase at the PBE0/LANL2DZ level and ending with an analysis of the most representative clusters at the CCSD(T)/def2-TZVP//PBE0/def2-TZVP level. The title clusters represent the smallest molecular stars with three planar tetracoordinate E atoms (E=Group 14 element). All these E3 M3+ clusters behave like superalkali cations with small vertical electron affinities (smaller than Cs), large vertical electron detachment energies, and HOMO-LUMO energy gaps. Their energetics, bonding, and electron delocalization are discussed in detail. The high stability of these clusters is reflected from the large dissociation energy needed for different dissociation channels. The electron delocalization is confirmed by the presence of two delocalized π electrons over the E3 core and strong diatropic responses.

C5Li7(+) and O2Li5(+) as noble-gas-trapping agents.

The noble-gas-trapping ability of the star-shaped C(5)Li(7)(+) cluster and O(2)Li(5)(+) super-alkali cluster is studied by using ab initio and density functional theory (DFT) at the MP2 and M05-2X levels with 6-311+G(d,p) and 6-311+G(d) basis sets. These clusters are shown to be effective noble-gas-trapping agents. The stability of noble-gas-loaded clusters is analyzed in terms of dissociation energies, reaction enthalpies, and conceptual DFT-based reactivity descriptors. The presence of an external electric field improves the dissociation energy.

Isomerization energy decomposition analysis for highly ionic systems: case study of starlike E5Li7(+) clusters.

The most stable forms of E(5)Li(7)(+) (E = Ge, Sn, and Pb) have been explored by means of a stochastic search of their potential-energy surfaces by using the gradient embedded genetic algorithm (GEGA). The preferred isomer of the Ge(5)Li(7)(+) ion is a slightly distorted analogue of the D(5h) three-dimensional seven-pointed starlike structure adopted by the lighter C(5)Li(7)(+) and Si(5)Li(7)(+) clusters. In contrast, the preferred structures for Sn(5)Li(7)(+) and Pb(5)Li(7)(+) are quite different. By starting from the starlike arrangement, corresponding lowest-energy structures are generated by migration of one of the E atoms out of the plane with the a corresponding rearrangement of the Li atoms. To understand these structural preferences, we propose a new energy decomposition analysis based on isomerizations (isomerization energy decomposition analysis (IEDA)), which enable us to extract energetic information from isomerization between structures, mainly from highly charged fragments.

Planar tetracoordinate carbon versus planar tetracoordinate boron: the case of CB4 and its cation.

In this study, we analyzed CB(4) and its cation, CB(4)(+). Using CCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVTZ quantum-chemical calculations, we found that the neutral molecule is in accord with the results of Boldyrev and Wang, having a C(s) global minimum with a planar tricoordinate carbon structure, contradicting previous studies. In contrast, CB(4)(+), which was reported by an early mass spectroscopic study, has a planar tetracoordinate carbon (ptC) atom, demonstrating that a modification of the charge can promote the stabilization of a ptC structure.

Starlike aluminum-carbon aromatic species.

Is it possible to achieve molecules with starlike structures by replacing the H atoms in (CH)(n)(q) aromatic hydrocarbons with aluminum atoms in bridging positions? Although D(4h) C(4)Al(4)(2-) and D(2) C(6)Al(6) are not good prospects for experimental realization, a very extensive computational survey of fifty C(5)Al(5)(-) isomers identified the starlike D(5h) global minimum with five planar tetracoordinate carbon atoms to be a promising candidate for detection by photoelectron detachment spectroscopy. BOMD (Born-Oppenheimer molecular dynamics) simulations and high-level theoretical computations verified this conclusion. The combination of favorable electronic and geometric structural features (including aromaticity and optimum C-Al-C bridge bonding) stabilizes the C(5)Al(5)(-) star preferentially.

Stabilizing carbon-lithium stars.

We have explored in silico the potential energy surfaces of the C(5)Li(n)(n-6) (n = 5, 6, and 7) clusters using the Gradient Embedded Genetic Algorithm (GEGA) and other computational strategies. The most stable forms of C(5)Li(5)(-) and C(5)Li(6) are two carbon chains linked by two lithium atoms in a persistent seven membered ring capped by two Li atoms. The other Li atoms are arrayed on the edge of the seven membered ring. In contrast, the global minimum structure for C(5)Li(7)(+) is a bicapped star of D(5h) symmetry. The molecular orbital analysis and computed magnetic field data suggest that electron delocalization, as well as the saturation of the apical positions of the five-membered carbon ring with lithium atoms in C(5)Li(7)(+) plays a key role in the stabilization of the carbon-lithium star. In fact, the planar star sub-structure for the carbon ring are unstable without the apical caps. This is also what has been found for the Si analogues. The split of the B(ind)(z) in its σ- and π-contribution indicates that C(5)Li(7)(+) is a π-aromatic and σ-nonaromatic system.