Next, we examine the simple spiroconjugated molecules 1,6-dimethylspiro [4.4]nonatetraene and 1,4-dimethylspiro[2.2]pentadiene. As with the even [n]cumulenes, metallacumulenes and polyynes, the two orthogonal ⇡-systems may interact and mix. However, only half of the frontier MOs of spiroconjugated molecules become helical when the molecules are appropriately substituted. As the helicality of their MOs might influence optical properties, the quantification of helicality is of interest. The carbon chain is defined as the carbon atoms in the shortest through-bond path between the methyl groups. Note that the distance between the solid circles in Figure (symbolizing carbon atoms) is the bond distance between two adjacent carbons and not the z-coordinate for each carbon.