Cyclopentadienyl cation, triplet states

The parent cyclopentadienyl cation 1+ has been calculated at various levels of theory [6, 7], and has been found to have a triplet ground state. Its existence as a triplet species in an SbFs matrix at 75 K had previously been proved by ESR spectroscopy [8]. Prior to that, the penta-chlorocyclopentadienyl cation 7+ (Figure 2) had been generated, also in an SbFs matrix at 77 K, and identified as a triplet on the basis of its observable ESR spectrum [9]. The penta-arylcyclopentadienyl cations 8+ [10], especially those with donor-substituted aryl substituents [11], are stable in solution up to 233 K, coexisting as singlet and triplet species due to the small energy difference between them. 

E.s.r. spectra of this cation indicated that it has a triplet ground state [261,263]. Electrochemical determination of the pKp+ value were hampered by a number of uncertainties but indicate that it is exceedingly low, and much lower than the value for the allyl cation [264]. Calculations of the ionic heats of formation of the cyclopentyl, cyclopentenyl and cyclopentadienyl cations from the measured ionisation potentials of the corresponding radicals and the heats of formation of these radicals show that whereas introduction of one double bond stabilises the cation, introduction of the second double bond destabilises the system [265]. E.s.r. spectra of the pentaphenyl [258,259] and pentachloro [259,266] derivatives show that the former has a ground state singlet structure but with a triplet state less than 1 kcal.mol" higher, while the latter has a triplet ground state [252,259]. 

In contrast, the less strained four-7r-electron cyclopentadienyl cation is very unstable. Its p r+ has been estimated as --40, using an electrochemical cycle. The heterolytic bond dissociation energy to form the cation from cyclopentadiene is 258 kcal/mol, which is substantially more than for formation of an allylic cation from cyclopentene but only slightly more than the 252 kcal/mol for formation of an unstabilized secondary carbocation. " Solvolysis of cyclopentadienyl halides assisted by silver ion is extremely slow, even though the halide is doubly allylic. When the bromide and antimony pentafluoride react at -78°C, the EPR spectrum observed indicates that the cyclopentadienyl cation is a triplet. Similar studies indicate that pentachlorocyclopentadienyl cation is also a triplet, but the ground state of the pentaphenyl derivative is a singlet. 

Gallup and Norbeck have made valence bond ab initio calculations on the electronic structure of cyclopentadienyl cation and anion with Dj symmetry. The lowest singlet- and triplet-state energies were calculated for each ion, from which it was concluded that, as expected, the cation has a 2 ground state and the anion ground state, i.e. antiaromatic and aromatic respectively. 

The cyclopentadienyl cation (5) was generated in a matrix and observed by ESR spectroscopy. Simple Hilckel theory predicts this species will have a triplet ground state with a symmetrical Dzt, geometry 5a, and this is confirmed by recent ab initio calculations the ESR spectrum of 5 in a matrix is in agreement... 

Fluorenyl cations are, however, quite readily formed and observed in flash photolysis experiments. The ease of formation of fluorenyl cations from triplet precursors and the calculated greater stability of triplet compared to singlet cyclopentadienyl cation argue for ground-state destabilization of the fluorenyl cation due to antiaromaticity. 9-Arylfluorenyl cations react with nucleophilic solvents with rate constants approximately 2 orders of magnitude greater than those for the corresponding monosubstituted triaryl cations. Such kinetic instability was one of the early criteria for antiaromaticity, and the parent fluorenyl cation 28 has only been directly observed, on a nanosecond time scale, in the very weakly nucleophilic hexafluoroisopropyl alcohol or in zeolites. ... 

Despite many controversial arguments regarding the definition and physical origin of aromaticity [1,15-17], the concept of aromaticity has crossed the boundary of benzenoid hydrocarbons [with (4n + 2)ji -electrons] to include heterosystems [50] like pyridine, thiophine, cations such as tropylium [12] and cyclopropenium [13], anions like cyclopentadienyl [51], organometallic systems, namely ferrocene [52], purely carbon-free systems [53,54], namely Pj, [(P5)2Ti]. The three-dimensional aromaticity of boron-based clusters [55] and of fullerenes [56], the homoaromaticity of cationic systems [57], aromaticity of triplet state annulenes [58], and pericyclic transition states [59] has enlarged the concept of aromaticity. Extension of aromaticity... 

In antiaromatic annulene ions with 4m 1 or 4 2 atoms the two degenerate MOs have atoms in common. Consequently, because these MOs are nondisjoint, at the geometry of highest symmetry the triplet is calculated to lie well below the singlet in energy. Thus, as predicted by Hund s rule, many antiaromatic annulene ions (e.g., cyclopentadienyl cation and hexachlorobenzene dication) have been found experimentally to have triplet ground states. 

Cyclopentadienyl cation has Civ symmetry but adopts a A symmetry in the triplet state. As expected for a An molecule, it has a small singlet-triplet gap, and here, the triplet state is calculated to have the lower energy,in agreement with experimental observations. In contrast, the aromatic cyclopentadienyl anion with D h symmetry has its symmetry reduced to Cg in the triplet state. Here, the singlet—triplet gap is 68 kcal/mol. This is similar to that for benzene. 

Cycloheptatrienyl cation has Djj, symmetry, which is reduced to Cz in the triplet state. The calculated singlet-triplet gap is 73 kcal/mol, similar to that for benzene and cyclopentadienyl anion. The An anion has Cz symmetry, and this is increased to Djt, in the triplet state.The singlet-triplet gap is -2 kcal/ mol, with the triplet having the lower energy. 

Benzene satisfies the Am + 2 rule. Ilie cyclopentadienyl radical "CsHs is one electron short of satisfying (16.52) the cyclopentadienyl anion C5H5 satisfies the 4m + 2 rule the cation C5H5 is predicted to have a triplet ground state and be highly reactive. These predictions are borne out C5H 5 is found to be considerably more stable than either QH or CsHs. Similarly, C7H7 should be more stable than 07117 or C7H7, as is verified experimentally for example, the salt C H Br" is readily prepared. 

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