Time-resolved Fluorescence-detected Magnetic Resonance Research Articles

Self-exchange reactions of radical anions in n-hexane

The formation and reactions of radical anions in n-hexane at 190 K were investigated by pulse radiolysis and time-resolved fluorescence-detected magnetic resonance (FDMR). Electron attachment was found to occur for compounds with gas-phase electron affinities (EA) more positive than −1.1 ± 0.1 eV. The FDMR concentration and time dependence are interpreted as evidence for self-exchange electron-transfer reactions, indicating that formation of dimer radical anions is not prevalent for the range of molecules studied. FDMR detection of radical anions is mainly restricted to electron acceptors with EA less than approximately 0.5 eV.

Electron Transfer Reactions and Mobile Holes in Radiolysis of Squalane. Time-Resolved FDMR Study

Fluorescence detected magnetic resonance (FDMR) is applied to study ion−molecule reactions in squalane (2,6,10,15,19,23-hexamethyltetracosane). Several time-resolved pulsed FDMR experiments are suggested to measure rate constants of scavenging by aromatic solutes. These constants vary from 5 × 108 to 1.6 × 109 mol-1 dm3 s-1. The ion−molecule reactions of radical cations are somewhat slower than reactions of radical anions. The scavenging of the alkane hole is significantly faster than ion−molecule reactions of molecular cations. The mobile hole decays on a time scale of several tens of nanoseconds. These FDMR results are in accord with our recent work on transient absorption spectroscopy in radiolysis of squalane.

(Iso)Pagodane Radical Cations in Liquid Hydrocarbons: “Time-Resolved Fluorescence-Detected Magnetic Resonance” Study of Valence Isomeric Radical Cations

The `tight` (T), cyclobutanoid, and the `extended` (E), {pi}-complex-like, four-centred-three-electron radical cations (4c/3e) derived from (iso)pagodanes and from the valence isomeric (iso)dienes can be viewed as well-separated stationary points on the reaction coordinate for the ethylene/ethylene radical cation [2+1]cycloaddition. So far, experimentally verified examples (ESR, CV, PE) are the extended [1.1.1.1](2{sup .+}) and iso-[2.2.1.1](4{sup .+})ions, persistent in fluid solution at room temperature, and the tight iso-[1.1.1.1]{sup .+} and [2.2.1.1]{sup .+} ions, observable only in Freon matrix (CFCl{sub 3}, {gamma}-rays, 77K). According to calculations, these are the thermodynamically more stable ones of the respective pairs of valence isomers. In this communication the direct observation of tight 1{sup .+} and the activation barrier for its isomerization into extended 2{sup .+} are reported. 16 refs., 3 figs., 1 tab.

FDMR in Low-Temperature Solids. 2. Observation of Weakly Bound Electrons

Time-resolved fluorescence-detected magnetic resonance (FDMR) was used to study radiolysis of organic solids at low temperatures (4-30 K). The mobility of charges generated in the primary events of solid-state radiolysis may be accounted for by two mechanisms: single-step tunneling and relatively slow (10[sup 6]-10[sup 7] s[sup [minus]1]) resonant charge transfer involving the solvent hole. Acting together, these mechanisms engender a variety of unusual and previously unknown spectral features. Although the tunneling satisfactorily explains FDMR behavior in general, quantitative agreement is poor, particularly for donor-acceptor systems. It appears that the origin of this discrepancy lies in an underestimation of the tunneling radii in reactions of negative charges. The latter is rationalized in terms of a selectivity of FDMR toward mobile weakly bound electrons and excited radical anions formed after electron scavenging. The possible nature of these weakly bound electronic states is discussed. 50 refs., 13 figs., 2 tabs.

FDMR in Low-Temperature Solids. 1. Theory

Various mechanisms of geminate recombination in spin-correlated radical ion pairs produced by radiolysis of organic solids under cryogenic conditions are considered and compared with experimental data. Single-step electron tunneling and slow resonant hole transfer are examined from the viewpoint of their manifestation in pulsed time-resolved fluorescence-detected magnetic resonance (FDMR). Two theoretical models treating the phenomenon are developed, and their predictions are compared with the experimental results. These models are found to be consistent with many observed features: t[sup [minus]1] decay kinetics, unusual evolution of the FDMR spectra with time, narrow radial distribution of the FDMR-active pairs, and efficient spin memory transfer from the primary pairs. However, with very few exceptions the tunneling theories fail to reproduce the exact scale of the effect and its dependence on solute concentration and thermalization distance of electrons. This discrepancy suggests that the negative charge is more mobile than predicted by the single-step tunneling model. 31 refs., 8 figs.

Solid-state FDMR from sundry poly(oxyethylene) alkylphenyl ethers: a verification of the single-step electron tunneling as a cause of the charge mobility in frozen solids at 4-30 K

Observations of the time-resolved fluorescence-detected magnetic resonance (FDMR) from bridged radical ion pairs generated by pulse radiolysis of micellar aqueous and homogeneous alcohol solutions of poly(oxyethylene) (chain length 5-40) alkylphenyl ethers at 430 K are reported. Theoretical analysis of the spectra favors single-step through-bond electron tunneling as a mechanism of the geminate recombination in the pairs at these low temperatures

Solvent effects in nonpolar solvents: Radical anion reactions

Dimer radical anions, formed by anion—molecule reactions between octafluoronaphthalene and 1,2,4,5-tetrafluorobenzene and their respective radical anions, are observed in n-hexane solvent. The radical anions are characterized by their EPR spectra obtained by the time-resolved fluorescence-detected magnetic resonance technique. In toluene solvent the dimer anions are not formed. Rapid electron-exchange reactions occur instead in at least one case. Possible reasons why the solvent affects the anion reaction mechanism are discussed.

Radical-cation complexes formed by .pi.-lone pair interactions

Time-resolved fluorescence-detected magnetic resonance (FDMR) is used to observe radical cations of thioethers in alkane and aromatic hydrocarbon solvents. Monomeric sulfide radical cations are stabilized at temperatures less than {approximately} 250 K in aromatic solvents such as toluene by electron donation from the solvent. The resulting arene-thioether complex radical cations are examples of mixed complex radical cations involving the interaction between lone-pair and {pi}-orbitals. The solvent interaction inhibits reactions of thioether radical cations (dimer formation and probably proton loss) and retards other radical cation reactions, such as dimer exchange. 39 refs., 7 figs., 4 tabs.

Medium effect on the Jahn-Teller distortions of the tetramethylallene radical cation

The Jahn-Teller distortion of the delocalized {pi}-radical cation tetramethylallene{sm bullet}{sup +} (TMA{sm bullet}{sup +}) was found to be medium dependent. TMA{sup {sm bullet}} was studied in liquid hydrocarbons (195-295 K) by time-resolved fluorescence-detected magnetic resonance (FDMR) spectroscopy and in freon matrices (80-160 K) and zeolite Na-Y (80-298 K) by EPR spectroscopy. The observed coupling constants for the twelve protons of the four methyl groups of the radical cation are 11.3, 8.1, and 14.4 G, respectively. AM1 and INDO calculations suggest that different coupling constants can be explained by different twist angles in TMA{sup {sm bullet}+}. It is suggested that interaction between TMA{sup {sm bullet}+} and the chlorine or fluorine atoms of freon matrices or between TMA{sup {sm bullet}+} and the sodium atoms of the zeolite can influence the charge and spin distribution in the radical cation so that the structures that differ from that found in hydrocarbon solution can be stabilized. This is the first example of a Jahn-Teller-active radical cation that has been studied in several media.

Solid state FDMR studies of ion-molecule reactions in radiolysis of saturated hydrocarbons

Studies of radical cations produced in pulse radiolysis of frozen hydrocarbon solutions have been carried out using time-resolved fluorescence-detected magnetic resonance (FDMR). These first time-resolved solid-state FDMR experiments were performed with a spectrometer modified to allow sample translation and cooling to near liquid-helium temperature. The stability of solvent radical cations in decalins increases at lower temperatures in solid phase. The contrasting behavior of the cis- and transdecalin radical cations is indicative of the importance of ion-molecule reactions of alkane radical cations. Experiments of this type provide information about the dynamics of charge pair recombination in solids.

Olefin radical cations in pulse radiolysis of hydrocarbons.time-resolved fluorescence-detected magnetic resonance

Olefin radical cations are characterized by time-resolved fluorescence-detected magnetic resonance (FDMR) in pulse radiolysis of saturated hydrocarbons. Studies of both neat and mixed saturated hydrocarbon fluids by FDMR at low temperature(to≈ 100°C) suggest that olefin cations originate in the fragmentation of (vibrationally) excited (parent) saturated hydrocarboncations.

Observation of transient cyclohexene and 1,4-cyclohexadiene radical cations. Time-resolved fluorescence-detected magnetic resonance

Transient radical cations of cyclohexene and 1,4-cyclohexadiene are characterized in pulse radiolysis in liquid hydrocarbons. Time-resolved fluorescence-detected magnetic resonance allows observation and EPR spectral assignment of these very short-lived radical cations. The olefin radical cations are created by charge transfer from saturated hydrocarbon solvent cations to the solute olefins.

ChemInform Abstract: TIME‐RESOLVED FLUORESCENCE‐DETECTED MAGNETIC RESONANCE AND FLUORESCENCE STUDIES OF TRIALKYLAMINES IRRADIATED BY PULSE RADIOLYSIS IN ALKANE SOLVENTS

Time-resolved fluorescence-detected magnetic resonance (FDMR) studies of irradiated alkane solutions of trialkylamines and scintillators reveal the EPR spectra of the trialkylaminium radicals, formed by scavenging solvent radical cations. A qualitative kinetic analysis indicates that the growth of the triethylaminium radical (TEA/sup +/-) FDMR signal occurs on similar time scales in both n-hexane and cyclohexane, suggesting that, in cyclohexane, TEA/sup +/- is formed by scavenging the lower mobility ''trapped'' cyclohexane radical cations. Fluorescence results indicate that TEA quenches both scintillator fluorescence and total FDMR intensities to a greater extent than is expected from amine scavenging of solvent holes. TEA also exhibits an intense, relatively long-lived fluorescence which is apparently not produced by radical ion recombination or energy transfer. 6 figures

Time-resolved fluorescence-detected magnetic resonance and fluorescence studies of trialkylamines irradiated by pulse radiolysis in alkane solvents

Time-resolved fluorescence-detected magnetic resonance (FDMR) studies of irradiated alkane solutions of trialkylamines and scintillators reveal the EPR spectra of the trialkylaminium radicals, formed by scavenging solvent radical cations. A qualitative kinetic analysis indicates that the growth of the triethylaminium radical (TEA/sup +/-) FDMR signal occurs on similar time scales in both n-hexane and cyclohexane, suggesting that, in cyclohexane, TEA/sup +/- is formed by scavenging the lower mobility ''trapped'' cyclohexane radical cations. Fluorescence results indicate that TEA quenches both scintillator fluorescence and total FDMR intensities to a greater extent than is expected from amine scavenging of solvent holes. TEA also exhibits an intense, relatively long-lived fluorescence which is apparently not produced by radical ion recombination or energy transfer. 6 figures

ChemInform Abstract: OBSERVATION OF SHORT‐LIVED RADICAL ION PAIRS BY PULSE RADIOLYSIS OF ALKANE SOLUTIONS BY TIME‐RESOLVED FLUORESCENCE‐DETECTED MAGNETIC RESONANCE. ESR SPECTRA OF ALKANE RADICAL CATIONS

Chemischer InformationsdienstVolume 14, Issue 4 Preparative Organic Chemistry ChemInform Abstract: OBSERVATION OF SHORT-LIVED RADICAL ION PAIRS BY PULSE RADIOLYSIS OF ALKANE SOLUTIONS BY TIME-RESOLVED FLUORESCENCE-DETECTED MAGNETIC RESONANCE. ESR SPECTRA OF ALKANE RADICAL CATIONS J. P. SMITH, J. P. SMITHSearch for more papers by this authorS. LEFKOWITZ, S. LEFKOWITZSearch for more papers by this authorA. D. TRIFUNAC, A. D. TRIFUNACSearch for more papers by this author J. P. SMITH, J. P. SMITHSearch for more papers by this authorS. LEFKOWITZ, S. LEFKOWITZSearch for more papers by this authorA. D. TRIFUNAC, A. D. TRIFUNACSearch for more papers by this author First published: January 25, 1983 https://doi.org/10.1002/chin.198304123AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume14, Issue4January 25, 1983 RelatedInformation

Observation of short-lived radical ion pairs by pulse radiolysis of alkane solutions by time-resolved fluorescence-detected magnetic resonance. ESR spectra of alkane radical cations

Time-resolved fluorescence-detected EPR studies of electron-irradiated solutions of 2,5-diphenyloxazole (PPO) in alkane solvents have shown that there are two distinct germinate radical ion pairs, distinguished by their lifetimes and EPR spectra. While the central EPR peak belongs to the aromatic radical ions, the broader components are assigned to the parent radical cation of the alkane solvent. Analysis of the resolved hyperfine structure indicates the presence of a trapped solvent hole in cyclohexane, which does not undergo unusually rapid charge exchange. The kinetic analyses of the EPR spectra show that scavenging events preserve the relative orientation of the unpaired electron spins in these geminate ion pairs.