Is Delocalization a Driving Force in Chemistry? First- and Second-Row Heteroannulenes

Conjugated squares and hexagons X_n (n = 4, 6; X = CH, N, SiH, P) are investigated computationally in the context of the title question. These species exhibit contrasting distortive behaviors with respect to a localizing distortion to bond-alternated structures. In contrast to the case for benzene, P₆ and Si₆H₆ are found to resist only weakly such a distortion, and the square rings Si₄H₄ and P₄ are indifferent to a distortion toward a rectangular geometry, at variance with the case for C₄H₄ and N₄. These trends are rationalized on the basis of valence bond (VB) correlation diagrams, leading to the following general features: (i) the adequate unified description of all conjugated molecules is in terms of a competition between symmetrizing σ frames and distortive π; (ii) the π singlet-triplet energy gap of the corresponding localized two-electron-two-center bonds is the very parameter that correlates with the π distortion energy; (iii) all molecules possess a sizable quantum-mechanical resonance energy (QMRE), but there is a large quantitative difference between “aromatic” and “antiaromatic” compounds. It follows that the Hückel rule cannot be a safe base for predicting geometries of molecules other than hydrocarbons. On the other hand, the VB approach allows an understanding of why the rule is successful for hydrocarbons and when contradictions to it should be expected. In such cases, new structural behaviors emerge that are potentially interesting for the synthesis of molecules with new properties.