Castro Claire, Chen Zhongfang, Wannere Chaitanya S, Jiao Haijun, Karney William L, Mauksch Michael, Puchta Ralph, Hommes Nico J R van Eikema, Schleyer Paul von Ragué
Department of Chemistry, University of San Francisco, 2130 Fulton Street, San Francisco, California 94117-1080, USA.
J Am Chem Soc. 2005 Mar 2;127(8):2425-32. doi: 10.1021/ja0458165.
The first experimental example of a [4n]annulene derivative with one Mobius twist, 1, was synthesized recently [Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426, 819] and was purported to possess aromatic character. However, critical analysis of the published crystallographic data indicates that the Mobius [16]annulene core of 1 shows large bond alternation (Deltar up to 0.157 A). Delocalization in this core is inhibited by large dihedral angles, which hinders effective pi overlap. This conclusion is supported by computational results (B3LYP/6-311+G) on 1 and several less benzannelated derivatives, based on geometric (Deltar, Deltar(m), Julg A, HOMA) and magnetic (NICS, magnetic susceptibility exaltation) criteria of aromaticity. That benzannelation results in bond localization in the [16]annulene core is shown by additional computations on benzannelated derivatives of other Mobius aromatic species. Additionally, the aromatic stabilization energy (ASE) of 1 has been reinvestigated using two different procedures. Evaluation of uncorrected ISE(II) values of just the polyene bridge portion of 1 and its Huckel counterpart suggests that stabilization of 1 relative to its Huckel isomer is confined to the polyene bridge and is not due to a delocalized pi circuit. Furthermore, application of s-cis/s-trans corrections lowers the ISE(II) value of 1 from 4.0 kcal/mol to 0.6 kcal/mol, suggesting that 1 is nonaromatic.
最近合成了首个具有一个莫比乌斯扭转的[4n]轮烯衍生物1 [阿贾米，D.；厄克勒，O.；西蒙，A.；赫格斯，R.《自然》2003年，426卷，819页]，据称其具有芳香性。然而，对已发表的晶体学数据的批判性分析表明，1的莫比乌斯[16]轮烯核心显示出较大的键长交替（Δr高达0.157 Å）。该核心中的离域作用受到大的二面角的抑制，这阻碍了有效的π重叠。基于芳香性的几何（Δr、Δr(m)、朱尔格A、HOMA）和磁性（NICS、磁化率增强）标准，对1和几种较少苯并稠合的衍生物进行的计算结果（B3LYP/6 - 311 + G）支持了这一结论。对其他莫比乌斯芳香物种的苯并稠合衍生物的额外计算表明，苯并稠合导致[16]轮烯核心中的键定域。此外，已使用两种不同方法重新研究了1的芳香稳定能（ASE）。仅对1的多烯桥部分及其休克尔对应物的未校正ISE(II)值进行评估表明，1相对于其休克尔异构体的稳定性仅限于多烯桥，并非由于离域的π电路。此外，应用s - 顺式/s - 反式校正将1的ISE(II)值从4.0 kcal/mol降至0.6 kcal/mol，这表明1是非芳香性的。
