Spiro-polyenes, compounds with two perpendicular π-systems joined by a common spiro-atom, exhibit through-space spiroconjugation. One important representative of spiroconjugated systems is spiro[4.4]nonatetraene (3). The molecular structures of spiro[4.4]nona-1,3,7-triene (2) and (3) have been determined by X-ray diffraction and compared with the redetermined structure of cyclopentadiene (1) in order to detect the slight distortions expected from spiroconjugation. Comparison of bond lengths and angles reveals a slight shortening of the double bonds and a small elongation of the single bonds connecting the spiro atom in (3). The same effect is also found from ab initio calculations at Hartree—Fock level 6–31G^*, although to a minor extent. This is shown by applying a difference analysis of structural parameters in order to cancel out the systematic errors and differences of both methods.

中文翻译：

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螺共轭的结构效应：螺[4.4]壬酸酯和螺[4.4]壬娜-1,3,7-三烯的晶体结构

螺多烯是具有两个垂直π系统并由一个常见的螺原子连接的化合物，具有贯穿空间的螺共轭作用。螺共轭体系的一个重要代表是螺[4.4]壬酸酯（3）。通过X射线衍射确定了螺[4.4] nona-1,3,7-三烯（2）和（3）的分子结构，并与重新确定的环戊二烯（1）的结构进行了比较，以检测微小的畸变预期螺旋结合。键长和键角的比较显示（3中连接螺原子的双键略有缩短，单键较小）。在Hartree-Fock级别6–31G ^*中从头算也可以发现相同的效果，尽管程度很小。通过对结构参数进行差异分析以抵消两种方法的系统误差和差异，可以看出这一点。