Shown in Figure , there are again two types of cations corresponding to JT distortion through bond-length alternation or torsion. Similar to the parent spiropentadiene, 1,4-dichlorospiropentadiene cation relaxes with torsion to a minimum at 113 • . A constrained optimization where the molecular planes are kept perpendicular yields a higher energy structure (Figure ). In 1,2-dichlorospiropentadiene the optimized structure relaxes through bond-length alternation between equivalent bonds. Again, constraining the bond lengths yields a slightly higher energy (meta-stable) structure that relaxes through torsion of the molecular planes.