Noncovalent interactions in crowded olefinic radical cations

Aim. To study the effect of electronic (α- and β-hyperconjugations) and steric (noncovalent interactions) factors on the structures of olefinic radical cations. Results and discussion. The effect of intramolecular dispersion interactions on the structures of crowded alkenes in the neutral and ionized forms has been studied at the density functional theory (DFT) level with and without dispersion corrections included, as well as at the MP2 theory level with medium size basis sets. The results obtained are compared to the available experimental data. An excellent agreement has been found between the experimental and MP2/DFT-computed geometries of sesquihomoadamantene, adamantylidene adamantane, bis -2,2,5,5-tetramethylcyclopentylidene, bis -D 3 -homocub-4-ylidene, and bis -C S -homocub-8-ylidene in the neutral and ionized forms. The experimental ionization potentials are better reproduced with the DFT-methods. Experimental part. The structure and composition of compounds were proved by the methods of 1 H and 13 C NMR-spectroscopy, and GC-MS-analysis. Elemental analysis was performed for the compounds obtained. Conclusions. The twisting of the olefinic moieties in the sesquihomoadamantene and adamantylidene adamantane radical cations is determined by the balance between the σ-π-hyperconjugation and residual one-electron π-bonding and is close to that of the prototypical ethylene radical cation (29°). The twisting reaches 55° for the bis -2,2,5,5-tetramethylcyclopentylidene radical cation due to substantial steric repulsions between methyl groups. At the same time, the ionized states of bis -D 3 -homocub-4-ylidene and bis -C S -homocub-8-ylidene retain their planarity due to β-CC-hyperconjugation and intramolecular dispersion attractions. Received: 26.12.2019 Revised: 17.01.2020 Accepted: 27.02.2020