Why Is (SiO)4 Calculated To Be Tetrahedral, Whereas (CO)4 Is Square Planar? A Molecular Orbital Analysis

The inversion of tetrahedral p-block element compounds: general trends and the relation to the second-order Jahn-Teller effect

The tetrahedron is the primary structural motif among the p-block elements and determines the architecture of our bio- and geosphere. However, a broad understanding of the configurational inversion of tetrahedral compounds is missing. Here, we report over 250 energies (DLPNO-CCSD(T)) for square planar inversion of third- and fourth-period element species of groups 13, 14, and 15. Surprisingly low inversion barriers are identified for compounds of industrial relevance (e.g., ≈100 kJ mol-1 for Al(OH)4 -). More fundamentally, the second-order Jahn-Teller theorem is disclosed as suitable to rationalize substituent and central element effects. Bond analysis tools give further insights into the preference of eight valence electron systems with four substituents to be tetrahedral. Hence, this study develops a model to understand, memorize, and predict the angular flexibility of tetrahedral species. Perceiving the tetrahedron not as forcingly rigid but as a dynamic structural entity might leverage new approaches and visions for adaptive matter.

Steric and electric field driven distortions in aromatic molecules: spontaneous and non-spontaneous symmetry breaking

The present calculations provide a unified picture of molecular distortions, both spontaneous and external field driven.

B2N2O4: Prediction of a Magnetic Ground State for a Light Main-Group Molecule

Cyclobutanetetrone, (CO)4, has a triplet ground state. Here we predict, based on electronic structure calculations, that the B2N2O4 molecule also has a triplet ground state and is therefore paramagnetic; the structure is an analogue of (CO)4 in which the carbon ring is replaced by a (BN)2 ring. Similar to (CO)4, the triplet ground-state structure of B2N2O4 is also thermodynamically unstable. Besides analysis of the molecular orbitals, we found that the partial atomic charges are good indicators for predicting magnetic ground states.