The potential energy surfaces of the allene and spiropentadiene radical cations also show this ambiguity relating to the frontier MOs. In the D 2d structure their respective SOMOs are not well-defined. However, it is clear that the JT distortion must happen along internal coordinates that lead to torsion into D 2 symmetry, bond-length alternation into C 2v symmetry, or a combination of both. The D 2 structure of the allene cation is 0.36 eV (8.3 kcal/mol) more stable than the C 2v structure (Figure ). JT distortion along the torsion coordinate thus provides more stabilization than either bond-length alternation or a combination of the two. Given the symmetry is reduced to D 2 , the frontier MOs of the optimized allene radical cation are helical. Despite their structural differences, the JT distortion of the spiropentadiene radical cation is remarkably similar to that of allene (Figure ). Again, JT distortion along the torsion coordinate is favored by 0.12 eV (2.8 kcal/mol) over alternation of equivalent bond lengths. What is driving the torsion over the bond-length alternation in allene and spiropentadiene? In this article we examine the relation between geometry and electronic structure of the π-systems of allene and spiropentadiene radical cations. First, we reexamine a simple relation between the structure and number of π-electrons in linear molecules and discuss its application. Using density functional theory (DFT), we assess the potential energy surfaces of substituted allenes and spiropentadienes through constrained optimizations that hint towards an understanding of the distortion through a simple MO picture. We relate the structural change to the helical and rectilinear representations of the frontier MOs, and further qualitatively assess helical electron delocalization in allene and spiropen-tadiene using the localized orbital locator (LOL). While helical πconjugation is negligible in neutral allene and spiropentadiene, we show that it is a decisive effect in their radical cations and we discuss the chemical intuition this provides.