Abstract
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-D3-homocub-4-ylidene, and bis-CS-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 1H and 13C 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-D3-homocub-4-ylidene and bis-CS-homocub-8-ylidene retain their planarity due to β-CC-hyperconjugation and intramolecular dispersion attractions. Received: 26.12.2019Revised: 17.01.2020Accepted: 27.02.2020
Highlights
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 oneelectron π-bonding and is close to that of the prototypical ethylene radical cation (29°)
We found that density functional theory (DFT) reproduced the experimental ionization potentials of large saturated hydrocarbons well [15 – 19]
As the noncovalent CHHC contacts across the C=C bond are close to the optimal value of 2.5 Å and are positioned in the attractive part of the vdW potential, such twisting must derive from antibonding interactions in the D2-symmetric helical highest occupied molecular orbital (HOMO)-1 of 4b
Summary
Alkenes 1 and 2 in the neutral and ionized states were studied using various DFT levels, as well as the MP2 ab initio method (Fig. 3). We probed the computational reproducibility of the experimental adiabatic potentials [28, 29] of 1 and 2 (Table) Both B3LYP and M06-2X results agree well with the experimental values and accounting for dispersion has almost no effect; again, the M06-2X is slightly more accurate. Since α-hyperconjugation affects mostly the structures of highly twisted forms, accounting dispersions has only little influence on the geometry of 3b+. Computations at our levels of theory reproduce satisfactorily the experimental X-ray crystal structure geometries of neutral hydrocarbons [33]. As the noncovalent CHHC contacts across the C=C bond are close to the optimal value of 2.5 Å and are positioned in the attractive part of the vdW potential, such twisting must derive from antibonding interactions in the D2-symmetric helical HOMO-1 of 4b. MS, m/z (I, %): 288 (71) [M]+, 223 (20) [M–C5H5]+, 222 (68) [M–C5H6]+, 209 (39) [M–C6H7]+, 165 (22), 156 (63), 155 (45), 144 (12), 143 (79) [M–C11H13]+, 142 (31), 141 (45), 129 (35), 128 (55), 115 (53), 91 (40) [C7H7]+, 80 (10), 79 (100) [C6H7]+, 78 (24) [C6H6]+, 77 (62) [C6H5]+, 66 (25) [C5H6]+, 65 (24) [C5H5]+
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