Radical cations electronic doublet states

The primary process taking place in a PE spectrometer is best viewed as a reaction in which a closed-shell diene or polyene M in its electronic (singlet) ground state 1 >Po reacts with a photon of energy hv to yield a radical cation M+ in one of its electronic doublet states 2excess energy kinj ... 

As shown in this figure, the quartet and doublet states consisting of the cation radical and the anion biradical ( A "-Ri ) are generated after the electron transfer. From both of the quartet and doublet states, the radical and the biradical escape from solvent cages, forming the escape radical and biradical. On the other hand, the cage recombination only occurs from the doublet state, forming D and A-Ri . 

The situation is different for radical ions in which the unpaired electron is situated in a degenerate (or nearly degenerate) MO (Figure 5.12, right). Here, the doublet ground state of the radical ion will be degenerate (or nearly degenerate) and the lowest excited state may be one with three unpaired electrons. Indeed, quartet states and quartet —> doublet phosphorescence of such systems have been observed, for example, for the decacyclene radical anion,440 441 or for the radical cations of acetylene and benzene.442,443... 

The PE spectroscopic evidence for silaethene as the retrodiene dissociation product51 rests on the coincidence between precalculated and observed ionization energies, on the band intensity increase upon addition of argon to the gas stream, which apparently reduces secondary reactions such as oligomerization, and on the vibrational fine structure of the first PES band, which resembles that of the iso(valence)electronic ethene and precludes with high probability the presence of carbene or silylene isomers. Possibly, the silaethene radical cation H2Si=CH 2e possesses a twisted ground-state equilibrium structure like H2C=CH 2 with the two lowest doublet states coupled via the torsional vibration v6 (cf. Reference 10). 

Oxidation The radiation-chemically induced ionization of chlorinated hydrocarbons, i.e., dichloroethane (DCE) leads to the initial generation of the corresponding solvent radical cation, [DCE] ". The electron affinity of the latter is sufficient to oxidize the fullerene moiety ([60]fullerene E1/2 = +1.26 versus Fc / Fc ). Pulse radiolytic experiments with [60]fullerene in nitrogen-saturated or aerated DCM solutions yielded a doublet with maxima at 960 and 980 nm (Figure 1) (12-18). This fingerprint is identical to that detected in photolytic oxidation experiments and that computed in CNDO/S calculations. Rate constants for the [60]fullerene oxidation are typically very fast with estimated values IC7 > 2 x 10 ° M s. The 7t-radical cation is short-lived and decays via a concentration-dependent bimolecular dimerization reaction with a ground state molecule (kg = 6 x lO M s ) (13). 

Fig. 4.9 The two-state reaction manifold as formulated by Shade and coworkers [68,69]. The ferryl radical cation of compound I (7) has two unpaired electrons in iron d orbitals and one in the Si2u porphyrin orbital. This electron configuration can give rise to either a quartet state ( Aju) if all spins are unpaired or a doublet if the spin of the electron in the aju orbital is inverted. A hydrogen atom is abstracted fi om the substrate in the first step of the reaction and an electron is transferred to either the iron, producing the ferrous state (as shown), or to the porphyrin, neutralizing the radical cation. A quartet or doublet state is...

DFT calculations suggest that the doublet-and quartet-spin states of the P450 compound I ferryl porphyrin radical cation are energetically close. According to these calculations, both the doublet and quartet species oxidize ethylene by addition of the ferryl oxygen to one carbon, leaving an unpaired electron on the second carbon of the double bond (Fig. 4.30) [185]. This intermediate radical also exists in doublet and quartet... 

The majority of neutral, chemically stable, molecules possess a closed shell. As a result, ionization leads to a radical cation, i.e. to an odd-electron ion (OE +). In spectroscopic terms, a neutral singlet state leads to an ionic doublet state. 

Consider for example, the nucleophilic cleavage of one-electron bonds in Scheme 5 which shows the principal FMVB configurations needed to discuss the problem. The reactant state is described by 23 which involves two electrons in the orbital of the nucleophile and one electron in the Oa. orbital of the cation radical. Following statements 1-3, the promoted state in the SCD is 24 which involves a triplet situation of the C-X electrons and a single electron on the nucleophile the three electrons are coupled to a total of doublet spin. Following the VB mixing rules, the mixing between the... 

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