Exploring the potential energy surface of E₂P₄ clusters (E=Group 13 element): the quest for inverse carbon-free sandwiches

Theoretical Prediction of Structures, Vibrational Circular Dichroism, and Infrared Spectra of Chiral Be4B8 Cluster at Different Temperatures

Lowest-energy structures, the distribution of isomers, and their molecular properties depend significantly on geometry and temperature. Total energy computations using DFT methodology are typically carried out at a temperature of zero K; thereby, entropic contributions to the total energy are neglected, even though functional materials work at finite temperatures. In the present study, the probability of the occurrence of one particular Be4B8 isomer at temperature T is estimated by employing Gibbs free energy computed within the framework of quantum statistical mechanics and nanothermodynamics. To identify a list of all possible low-energy chiral and achiral structures, an exhaustive and efficient exploration of the potential/free energy surfaces is carried out using a multi-level multistep global genetic algorithm search coupled with DFT. In addition, we discuss the energetic ordering of structures computed at the DFT level against single-point energy calculations at the CCSD(T) level of theory. The total VCD/IR spectra as a function of temperature are computed using each isomer's probability of occurrence in a Boltzmann-weighted superposition of each isomer's spectrum. Additionally, we present chemical bonding analysis using the adaptive natural density partitioning method in the chiral putative global minimum. The transition state structures and the enantiomer-enantiomer and enantiomer-achiral activation energies as a function of temperature evidence that a change from an endergonic to an exergonic type of reaction occurs at a temperature of 739 K.

How do the Hückel and Baird Rules Fade away in Annulenes?

Two of the most popular rules to characterize the aromaticity of molecules are those due to Hückel and Baird, which govern the aromaticity of singlet and triplet states. In this work, we study how these rules fade away as the ring structure increases and an optimal overlap between p orbitals is no longer possible due to geometrical restrictions. To this end, we study the lowest-lying singlet and triplet states of neutral annulenes with an even number of carbon atoms between four and eighteen. First of all, we analyze these rules from the Hückel molecular orbital method and, afterwards, we perform a geometry optimization of the annulenes with several density functional approximations in order to analyze the effect that the distortions from planarity produce on the aromaticity of annulenes. Finally, we analyze the performance of three density functional approximations that employ different percentages of Hartree-Fock exchange (B3LYP, CAM-B3LYP and M06-2X) and Hartree-Fock. Our results reveal that functionals with a low percentage of Hartree-Fock exchange at long ranges suffer from severe delocalization errors that result in wrong geometrical structures and the overestimation of the aromatic character of annulenes.

Li2B24: the simplest combination for a three-ring boron tube

Herein we introduce a strategy employing lithium atoms as a scaffold to stabilize an embryo for boron tubes. The systematical exploration of the potential energy surface via evolutionary algorithms allowed us to find that Li2B24 adopts a tubular structure formed by three stacked rings of eight borons each with two lithium atoms capping the tube. The lithium atoms are essential for stabilization because of the strong electrostatic interaction between the Li cations and the boron framework, and concomitantly, they compensate for the energy cost of distorting a quasi-planar or double ring B24 cluster.

Rotating Iron and Titanium Sandwich Complexes

The origin for the rotational barrier of organometallic versus inorganic sandwich complexes has remained enigmatic for the past decades. Here, we investigate in detail what causes the substantial barrier for titanodecaphosphacene through spin-state consistent density functional theory. Orbital interactions are shown to be the determining factor.

Li2 B12 and Li3 B12 : Prediction of the Smallest Tubular and Cage-like Boron Structures

An intriguing structural transition from the quasi-planar form of B₁₂ cluster upon the interaction with lithium atoms is reported. High-level computations show that the lowest energy structures of LiB₁₂ , Li₂ B₁₂ , and Li₃ B₁₂ have quasi-planar (C_s ), tubular (D_6d ), and cage-like (C_s ) geometries, respectively. The energetic cost of distorting the B₁₂ quasi-planar fragment is overcompensated by an enhanced electrostatic interaction between the Li cations and the tubular or cage-like B₁₂ fragments, which is the main reason of such drastic structural changes, resulting in the smallest tubular (Li₂ B₁₂ ) and cage-like (Li₃ B₁₂ ) boron structures reported to date.

10-π-Electron arenes à la carte: structure and bonding of the [E-(CnHn)-E](n-6) (E = Ca, Sr, Ba; n = 6-8) complexes

In this paper, we provide solid evidence to show that among an overwhelming structural diversity, alkaline earth metals (Ca, Sr, Ba) have the ability to form inverted sandwich compounds with C6H6, C7H7(+), and C8H8(2+) of Dnh symmetry and general formula [E-(CnHn)-E](n-6) (n = 6-8) with planar 10-π-electron aromatic cores by virtue of transferring two electrons per metal atom to the ring. However, the origin of the orbital interaction between the metals and the carbon ring is quite different; while [E-(C6H6)-E] complexes are dominated by δ-interactions, both π- and δ-interactions are important in [E-(C7H7)-E](+) and [E-(C8H8)-E](2+) complexes.

Noble gas supported B3+ cluster: formation of strong covalent noble gas–boron bonds

Ar to Rn atoms formed exceptionally strong bonds with B₃⁺, where the Ng (HOMO) → B₃Ng₂⁺ (LUMO) σ-donation is the key term to stabilize the complexes.

An electronic aromaticity index for large rings

We introduce a new electronic aromaticity index, AV1245, consisting of an average of the 4-center indices along the ring that keep a positional relationship of 1, 2, 4, 5.

The activation strain model and molecular orbital theory

The activation strain model is a powerful tool for understanding reactivity, or inertness, of molecular species. This is done by relating the relative energy of a molecular complex along the reaction energy profile to the structural rigidity of the reactants and the strength of their mutual interactions: ΔE(ζ) = ΔE_strain(ζ) + ΔE_int(ζ). We provide a detailed discussion of the model, and elaborate on its strong connection with molecular orbital theory. Using these approaches, a causal relationship is revealed between the properties of the reactants and their reactivity, e.g., reaction barriers and plausible reaction mechanisms. This methodology may reveal intriguing parallels between completely different types of chemical transformations. Thus, the activation strain model constitutes a unifying framework that furthers the development of cross-disciplinary concepts throughout various fields of chemistry. We illustrate the activation strain model in action with selected examples from literature. These examples demonstrate how the methodology is applied to different research questions, how results are interpreted, and how insights into one chemical phenomenon can lead to an improved understanding of another, seemingly completely different chemical process. WIREs Comput Mol Sci 2015, 5:324-343. doi: 10.1002/wcms.1221.

Structural evolution of small gold clusters doped by one and two boron atoms

The potential energy surfaces (PES) of a series of gold-boron clusters with formula Aun B (n = 1-8) and Aum B2 (m = 1-7) have been explored using a modified stochastic search algorithm. Despite the complexity of the PES of these clusters, there are well-defined growth patterns. The bonding of these clusters is analyzed using the adaptive natural density partitioning and the natural bonding orbital analyses. Reactivity is studied in terms of the molecular electrostatic potential.