Abstract
The nature of the bonding between bridgehead carbon atoms (Ca, Ca′) as well as the ring strain in a family of 10 propellanes formed by three‐, four‐, or five‐member rings: [1.1.1] (I), [2.1.1] (II), [3.1.1] (III), [2.2.1] (IV), [3.2.1] (V), [2.2.2] (VI), [3.3.1] (VII), [3.2.2] (VIII), [3.3.2] (IX), and [3.3.3] (X) are studied by means of the electron localization function (ELF) at the DFT level (B3LYP/cc‐pVTZ). The ELF analysis of smaller propellanes (I, II, and III) reveals the coexistence of two resonance forms: one with a nonbonding electron pair partially delocalized between Ca and Ca′ atoms outside the cage (ionic) and the other with a bridge bond between the same atoms (covalent). The weights of each form are calculated according to the ELF‐basin populations, yielding 94, 88, and 53% for the ionic structure of I, II, and III, respectively, while larger propellanes (IV–X) present only the covalent form. The question of the s‐character of the bridge bond is addressed by dissecting the bridge‐bond ELF basin into the molecular orbital contributions. Finally, σ‐aromaticity associated to surface electron delocalization has been analyzed by means of nucleus‐independent chemical shift (NICS) calculations. The results point out that the stability of the fused ring structure of propellanes I, II, and III, can be assigned to the remarkable σ‐aromaticity of the involved three‐member rings. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007