Pancake π-stacking produces shorter contacts than van der Waals bonding but it has strongly preferred configurations. By high-level multireference average quadratic coupled cluster theory for the singlet and triplet, we identify the specific orbital component and the nonspecific vdW contributions in the prototypical pancake-bonded dimer of phenalenyl thereby explaining the configurational preferences.

The concept of a double-bonded pancake bonding mechanism is introduced to explain the extremely short π–π stacking contacts in dimers of dithiatriazines. While ordinary single pancake bonds occur between radicals and already display significantly shorter interatomic distances in comparison to van der Waals (vdW) contacts, the double-bonded pancake dimer is based on diradicaloid or antiaromatic molecules and exhibits even shorter and stronger intermolecular bonds that breach into the range of extremely stretched single bonds in terms of bond distances and binding energies. These properties give rise to promising possibilities in the design of new materials with high electrical conductivity and for the field of spintronics. The analysis of the double pancake bond is based on cutting edge electron correlation theory combining multireference (nondynamical) effects and dispersion (dynamical) contributions in a balanced way providing accurate interaction energies and distributions of unpaired spins. It is also shown that the present examples do not stand isolated but that similar mechanisms operate in several analogous nonradical molecular systems to form double-bonded π-stacking pancake dimers. We report on the amazing properties of a new type of stacking interaction mechanism between π conjugated molecules in the form of a “double pancake bond” which breaks the record for short intermolecular distances and provides formidable strength for some π–π stacking interactions.

The π-bonded tetracyanoethylene anion dimer (TCNE2(2-)) and the neutral K2TCNE2 system have been investigated to obtain new insights into the unique features of two-electron multicenter (2e-mc) π-pancake bonding. The inter-radical interaction leads to a significant diradicaloid character described by two singly occupied molecular orbitals (SOMOs) of the monomers. A highly correlated approach, the multireference averaged quadratic coupled-cluster (MR-AQCC) method, has been used to achieve a balanced description of the different types of electron correlation that occur in this system. The analysis of the interaction energies for the two systems in the singlet and the lowest triplet states and of the unpaired electron densities demonstrate the importance of diradical π bonding in addition to the conventional van der Waals interactions that occur in intermolecular interactions. In this analysis, the separation of the repulsive Coulomb interaction energies from the remaining terms turned out to be a crucial prerequisite to achieve consistent results. Our calculations also confirm that the driving force behind the energetic stability of the pancake bonds predominantly derives from the overlap of the SOMO-SOMO bonding interaction.

Quantum-chemical calculations at the CCSD(T)/cc-pVTZ level of theory show that beryllium subfluoride, Be2 F2 , has a bond dissociation energy of De =76.9 kcal mol(-1) , which sets a record for the strongest Be-Be bond. The synthesis of this molecule should thus be possible in a low-temperature matrix. The discus-shaped species Be2 B8 and Be2 B7 (-) possess the shortest Be-Be distance for a molecule in the electronic ground state, but there is no Be-Be bond. The cyclic species Be2 B8 and Be2 B7 (-) exhibit double aromaticity with 6σ and 6π electrons, which strongly bind the Be2 fragment to the boron atoms. The very short interatomic distance between the beryllium atoms is due to the Be-B σ and π bonds, which operate like spokes in a wheel pressing the beryllium atoms together. The formation of the Be-B bonds has effectively removed the electronic charge of the valence space between the beryllium atoms. Along the Be-Be axis, there are two cage critical points adjacent to a ring critical point at the midpoint, but there is no bond critical point and no bond path.

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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.

Among the fascinating planar tetracoordinate carbon (ptC) species, pentaatomic molecules belong to the smallest class, well-known as "pptC". It has been generally accepted that the planarity of pptC structure is realized via the "delocalization" of the p(z) lone pair at the central carbon and the ligand-ligand bonding interaction. Although "localization" is as key driving force in organic chemistry as "delocalization", the "localization" concept has not been applied to the design of pptC molecules, to the best of our knowledge. In this paper, we apply the "localization" strategy to design computationally a series of new pptC. It is shown that the central carbon atom and one "electronegative" ligand atom X (compared to the Al ligand) effectively form a highly localized C-X multiple bond, converting the lone pair at the central carbon to a two-center two-electron π-bond. At the aug-cc-pVTZ-B3LYP, MP2 and CCSD(T) levels, the designed 18-valence-electron pptC species [XCAl(3)](q); [(X,q) = (B,-2), (C,-1), (N,0)] are found to each possess a stable ptC structure bearing a C-X double bond, indicated by the structural, molecular orbital, Wiberg bonding, potential energy surface and Born-Oppenheimer molecular dynamics (BOMD) analysis. Moreover, our OVGF calculations showed that the presently disclosed (yet previously unconsidered) pptC structure of [C(2)Al(3)](-) could well account for the observed photoelectron spectrum (previously only ascribed to a close-energy fan-like structure). Therefore, [C(2)Al(3)](-) could be the first pptC that bears the highly localized C-X double bond that has been experimentally generated. Notably, the pptC structure is the respective global minimum point for [BCAl(3)](2-) and [NCAl(3)], and the counterion(s) would further stabilize [BCAl(3)](2-) and [C(2)Al(3)](-). Thus, these newly designed pptC species with interesting bonding structure should be viable for future experimental characterization. The presently applied "localization" approach complements well the previous "delocalization" one, indicating that the general "localization vs. delocalization" concept in organic chemistry can be effectively transplanted to exotic pptC chemistry.

Herein we introduce a strategy employing lithium atoms as a scaffold to stabilize an embryo for boron tubes.