Cation cyclopentadienyl

The cyclopentadienyl radical and the cyclopentadienyl cation are two well-known Jahn-Teller problems The traditional Jahn-Teller heatment starts at the D k symmetry, and looks for the normal modes that reduce the symmetry by first-01 second-order vibronic coupling. A Longuet-Higgins treatment will search for anchors that may be used to form the proper loop. The coordinates relevant to this approach are reaction coordinates. 

In the case of the cyclopentadienyl cation, there are only two electrons in the e" tt molecular orbitals that are degenerate in Dji, symmetry. The MO heatments... 

Figure 25. A suggested explanation for the pseudorotatioii motion around the degeneracy in the cyclopentadienyl cation. (Adapted from Ref. [72])...

Figure 31 shows the proposed Longuet-Higgins loop for the cyclopentadienyl cation. It uses the type-VI Ai anchors, with the type-VII B structures as transition states between them. This situation is completely analogous to that of the radical (Fig. 23). Since the loop is phase inverting, a conical intersection should be located at its center—as required by the Jahn-Teller theorem.

Figure 32, The main symmetric conical intersection of cyclopentadienyl cation, and five secondary conical intersections around it.

N t/ l Five cyclopentadienyl molecular orbitals 4f Cyclopentadienyl cation (four 77 electrons) 4f Cyclopentadienyl radical (five 77 electrons) -H- Cyclopentadienyl anion (six 7r electrons)... 

Two other systems that have been studied as possible aromatic or antiaromatic four-electron systems are the cyclopropenyl anion (59) and the cyclopentadienyl cation (60). In these cases also the evidence supports, antiaromaticity, not aromaticity. With respect to 59, HMO theory predicts that an unconjugated 61 (i.e., a single canonical form) is more stable than a conjugated 59, so that 61 would... 

Cyclopentadienyl cation (CPDC), phase-change rule, 467-472... 

The acid-catalysed ring-closure of divinyl ketones to cyclopentenones (equation 6), the Nazarov reaction6-8, represents a conrotatory electrocyclization of 4jr-cyclopentadienyl cations. The conrotatory course of the reaction was confirmed for the case of the dicyclo-hexenyl ketone 7, which yielded solely the tricyclic ketone 8 on treatment with phosphoric acid (equation 7)3b. Cycloalkanocyclopentenones 10 with c/s-fused rings are obtained from the trimethylsilyl-substituted ketones 9 (n = 1, 2 or 3) and iron(III) chloride and... 

Instead the allyl cation 97 was obtained. The reported experimental 13C NMR data (<513C(exp) 8 — 250/243, 153, 60 ppm) are in agreement with the cyclopentenyl cation structure 97, but differ significantly from the calculated 13C NMR chemical shift for both Jahn-Teller distorted valence isomeric cyclopentadienyl cation structures 95 and 96. 

---