Spiropentadiene has D 2d symmetry, and like allene we assess it in quasi-D 2 symmetry. When the symmetry is reduced, the πsystems mix and the occupied π-MOs become helical (Figure ) with a continuous π-nodal plane spanning the entire length of the molecule. As we have discussed in detail in recent work, the (near-)degenerate sets π → π * transitions are separated by symmetry, and the two HOMO→LUMO transitions shown in Figure do not mix. The HOMOs differ in helicity, while the LUMO is a non-helical spiroconjugation MO. Their products show weak linear and circular charge displacement and consequently both ⃗ µ and ⃗ m are relatively small in magnitude, and it is hard to visually identify any helical components in the transition density. ⃗ µ and ⃗ m both point between the two molecular planes. The two transitions are energetically degenerate and the opposite signs of R thus cancel out in an ECD spectrum; consequently the unsubstituted molecule is optically inactive. The same is true for the HOMO→LUMO+1 transitions (ESI Figure ).