Brown Eric C, Henze Daven K, Borden Weston Thatcher
Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195-1700, USA.
J Am Chem Soc. 2002 Dec 18;124(50):14977-82. doi: 10.1021/ja027392p.
Unrestricted density functional theory (UB3LYP), CASSCF, and CASPT2 calculations have been employed to compute the relative energies of the C(s) and C(2v) geometries of several 1,5-disubstituted semibullvalenes. Substitution at these positions with R = F, -CH(2)-, or -O- affords semibullvalenes that are predicted to have C(2v) equilibrium geometries. Calculated singlet-triplet energy splittings and the energies of isodesmic reactions are used to assess the amount of bishomoaromatic character at these geometries. The results of these calculations show that employing strain to destabilize the C(s) geometries of semibullvalenes can lead to a significant decrease in the amount of bishomoaromatic stabilization of the C(2v) geometries, due to reduced through-space interaction between the two allyl groups. However, the C(2v) equilibrium geometries of the 1,5-disubstituted semibullvalenes with R = F and -RR- = -O- do benefit from stabilizing through-bond interactions between the two allyl groups. These interactions involve mixing of the bisallyl HOMO with the low-lying C-F or C-O sigma orbital combinations of the same symmetry. In contrast, for -RR- = -CH(2)-, through-bond interactions destabilize the bisallyl HOMO and are predicted to make the ground state of this semibullvalene a triplet.
采用无限制密度泛函理论（UB3LYP）、完全活性空间自洽场（CASSCF）和完全活性空间微扰理论二级近似（CASPT2）计算方法，计算了几种1,5 - 二取代半环戊二烯的C(s)和C(2v)几何构型的相对能量。在这些位置用R = F、-CH(2)-或-O-进行取代，得到的半环戊二烯预计具有C(2v)平衡几何构型。通过计算单重态 - 三重态能量分裂和等键反应的能量，来评估这些几何构型下双同芳香性的程度。这些计算结果表明，利用应变使半环戊二烯的C(s)几何构型失稳，会导致C(2v)几何构型的双同芳香稳定化程度显著降低，这是由于两个烯丙基之间的空间相互作用减弱所致。然而，R = F和-RR- = -O-的1,5 - 二取代半环戊二烯的C(2v)平衡几何构型确实受益于两个烯丙基之间的成键稳定相互作用。这些相互作用涉及双烯丙基最高占据分子轨道（HOMO）与具有相同对称性的低能级C - F或C - O σ轨道组合的混合。相比之下，对于-RR- = -CH(2)-，成键相互作用会使双烯丙基HOMO失稳，并预计使该半环戊二烯的基态为三重态。
