Electronic absorption radical ions

This technique for the study of a fast reaction is gas phase or liquid phase was developed by Norrish and Poster. This is an example of Pulse method which initiates a reaction by creating new reactive species—excited electronic states, radicals, ions in the system under study. The method uses a light flash of high intensity for a very short duration (10- s) to produce atoms or free radicals or excited species in a system. These are at a fairly high concentration and undergo further reactions which are followed spectroscopically. A spectroscopic flash of light is followed by the initial flash by some fraction of a millisecond. The absorption spectra of all the species that are formed within the system can be recorded. One cannot only get indications of what species are formed but also how these species give rise to others. Thus a very direct picture of the kinetic behaviour of a fast reaction can be obtained. 

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

Figure 12. Electronic spectra and the results of open-shell PPP-like semiempirical calculations for radical ions. The vertical lines represent the allowed transitions, the wavy lines with arrows the forbidden ones. The right side scales denote the calculated spectral intensities, where f stands for the oscillator strength. Top left the absorption curve (146) redrawn to the log e vs. 0 (cm ) form calculations are taken from (59). Top right taken from (11). Bottom left taken from (143). Bottom right taken from (136), the absorption curve redrawn to the log e vs, 0 (cm" ) form.

Shida, T. (1988). Electronic Absorption Spectra of Radical Ions. Elsevier, New York... 

The broad emission and low-fluorescence quantum yield of PPS suggested a distribution of trapping sites in the Si skeleton, which were also considered responsible for the lower-than-expected conductivity. The far-IR spectrum of PPS suggested the existence of cyclohexasilane rings connected by linear chains.361,362 Subsequent investigations by Irie et al. on the electronic absorption spectra of radical ions of poly(alkylsilyne)s were taken to indicate the presence of various cyclic silicon species, in corroboration of this conclusion.363 The large Stokes shift and broadness of the fluorescence emission indicate a range of fluorophore structures, different from the chromophore structures. This is... 

Shida, T. (1988). Electronic Absorption Spectra of Radical Ions. Elsevier, New York Shiner, Jr, V. J. (1970). In Isotopes Effects in Chemical Reactions (ed. Collin, C. J. 

The nitration reagents (NO2 Y) for electrophilic aromatic nitration span a wide range and contain anions Y such as nitric acid (Y = OH-), acetyl nitrate (Y = OAc-), dinitrogen pentoxide (Y = NO3-), nitryl chloride (Y = Cl-), TV-nitropyridinium (Y = pyridine) and tetranitromethane [Y = C(N02)3-]. All reagents contain electron-deficient species which can serve as effective electron acceptors and form electron donor-acceptor (EDA) complexes with electron-rich donors including aromatic hydrocarbons107 (ArH, equation 86). Excitation of the EDA complexes by irradiation of the charge-transfer (CT) absorption band results in full electron transfer (equation 87) to form radical ion... 

The photocatalytic system is shown in Scheme 5, where BNAH is oxidized by the ZnP + moiety in the radical ion pair ZaP -Ceo (ki) produced upon photoirradiation of ZnP-Ceo, whereas HV " is reduced to HV by the Ceo" moiety of ZnP +-C6o ki). These individual electron-transfer processes compete, however, with the BET in the radical ion pair (/cbet)- This pathway was experimentally confirmed by photolysis of the ZnP-Ceo/BNAH/HV and ZnP-H2P-C6o/BNAH/HV + systems with visible light (433 nm) in deoxyge-nated PhCN [70], For instance. Fig. 4 depicts the steady-state photolysis in deoxy-genated PhCN, in which the HV absorption band (X ax = 402 and 615 nm) increases progressively with irradiation time. By contrast, no reaction occurs in the dark or in the absence of the photocatalyst (i.e., ZnP-Ceo or ZnP-H2P-C6o) under photoirradiation [70]. Once HV+ is generated in the photochemical reaction, it was found to be stable in deoxygenated PhCN. The stoichiometry of the reaction is established as given by Eq. (3), where BNAH acts as a two-electron donor to reduce two equivalents of HV [70] ... 

The first intermediate to be generated from a conjugated system by electron transfer is the radical-cation by oxidation or the radical-anion by reduction. Spectroscopic techniques have been extensively employed to demonstrate the existance of these often short-lived intermediates. The life-times of these intermediates are longer in aprotic solvents and in the absence of nucleophiles and electrophiles. Electron spin resonance spectroscopy is useful for characterization of the free electron distribution in the radical-ion [53]. The electrochemical cell is placed within the resonance cavity of an esr spectrometer. This cell must be thin in order to decrease the loss of power due to absorption by the solvent and electrolyte. A steady state concentration of the radical-ion species is generated by application of a suitable working electrode potential so that this unpaired electron species can be characterised. The properties of radical-ions derived from different classes of conjugated substrates are discussed in appropriate chapters. 

Analyses of the electronic and electron spin resonance (ESR) spectra of the radical cation and anion of polysilanes make it possible to elucidate the structure of HOMO and LUMO, because an unpaired electron in the radical anion or cation occupies HOMO or LUMO, respectively. As schematically depicted in Fig. 10, the radical ions of polysilanes show absorption bands in UV and near-IR regions [29 31]. The former band corresponds to intraband transitions between valence and conduction bands. The latter band corresponds to transitions within the valence or the conduction band [32,33]. Because the near-... 

The electronic structures of polysilane radical ions have also been studied by pulse radiolysis of the liquid solution [35-40]. However, due to short lifetime of the radical ions, the measurement is limited to electronic absorption spectroscopy. 

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

Dicyclopentadiene forms a radical cation (20 ) in which one of the bonds linking the monomer units is cleaved. The species contains two allyl moieties attached to a C4 spacer . Structure 20 + rests on an unmistakable CIDNP pattem " and is supported by an analysis of the electronic absorption spectmm. The large energy gap in the OS of this ion (AE = 1.67 eV) is incompatible with the photoelectron spectrum of the parent molecule (AE = 0.15 eV), but it fits the ring-opened structure 20 +. 

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