Homoaromaticity in Aza- and Phosphasemibullvalenes. A Computational Study.
Résumé
The ethalpies of activation ΔH‡ for the Cope rearrangement in several aza- and phosphasemibullvalenes have been investigated by MP4/cc-pVDZ//MP2/cc-pVDZ and CCSD(T)/cc-pVDZ//MP2/cc-pVDZ calculations. One tetraazasemibullvalene and several phosphasemibullvalenes were found to have vanishing ΔH‡ values, which together with calculated large negative Nucleus Independent Chemical Shift (NICS) values and geometrical data shows that these molecules have delocalised and bishomoaromatic minima. Furthermore, three azasemibullvalenes were found to have small ΔH‡ values (≤2 kcal/mol) combined with large negative NICS, suggesting that they could also have bishomoaromatic minima.
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