Homoconjugation in the Adamantane Cage:  DFT/IGLO Studies of the 1,3-Dehydro-5-adamantyl Cation, Its Isoelectronic Boron Analogue 1,3-Dehydro-5-Boraadamantane, and Related Systems^1,

Abstract

The structure of 1,3-dehydro-5-adamantyl cation and its isoelectronic boron analogue 1,3-dehydro-5-boraadamantane as well as some related adamantane systems, hitherto unknown as persistent long-lived species, were investigated by using the density functional theory (DFT) method. (13)C and (11)B NMR chemical shifts of the compounds were also calculated using the IGLO method. Stabilization of the systems due to homoconjugation is discussed.

Electronic structure of adamantane

“…The calculated structural parameters of adamantane are very close to the experimental ones [8]. The trends in the bond lengths for the series of the structures I and IV-VI are the same as obtained in [6] from DFT calculations.…”

“…The sharp decrease in the bridgehead interatomic distances is generally regarded as the manifestation of the trishomoaromatic [9,10] nature of these systems [6].…”

“…In Ref. [6] the nature of such bonding is discussed on the base of the concept of trishomoaromatic interactions. The analysis of the molecular orbitals obtained from our calculations shows that for each of structures IV-VI it is possible to single out a molecular orbital (usually the HOMO) that is responsible for this bonding.…”

“…As in Refs. [5][6][7][8][9], the energy difference for LUMO and HOMO is rather high (15 eV). The HOMO consists mainly of boron atomic orbitals, thus being responsible for the boron-boron interactions.…”

“…The problem of relative stability of different derivatives of adamantane was the subject of the works of von Schleyer et al [3,4] and Schaefer et al [5]. In the work of Loah et al [6] the 1,3-dehydro-5,7-adamantanediyl dication C 10 H 12 2ϩ and some other cations were studied by the density functional theory (DFT) method. It was shown qualitatively that this dication can be described as a three-dimensional (3D) aromatic system in which valent p-orbitals of all four bridgehead carbon atoms overlap inward in cell in tetrahedral fashion involving two electrons, unlike its neutral analogue.…”