3466
J . Phys. Chem. 1991, 95, 3466-3411
FEATURE ARTICLE Study of Radical Ions in the Condensed Phase by Fiuorescence-Detected Magnetic Resonance D. W. Werst and A. D. Trifunac* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: November 21, 1990)
Magnetic resonance studies of transient condensed-phase radical ions have been realized for the first time without the use of a stabilizing matrix. The occurrence, structure, and reactivity of radical cations and radical anions in the condensed phase using time-resolved fluorescence-detected magnetic resonance (FDMR) is reviewed. FDMR observes the time-resolved EPR spectra of spin-correlated radical-ion pairs on the 10-8-104 time scale. FDMR studies have elucidated the identity and fate of primary radical cations in saturated hydrocarbons, alcohols, and other solvent media exposed to ionizing or photoionizing radiation. In photoionization, FDMR allows the determination of the photophysical pathway, Le., the photon order and the multiplicity of the state which is ionized. The formation of secondary radical cations via electron transfer from solutes to solvent radical cations allows the FDMR/pulse radiolysis method to study a wide range of organic radical cations. Radical-cation reactions that have been studied include unimolecular dissociation, charge transfer, aggregate formation, and ion-molecule reactions. Ion-molecule reactions such as proton transfer are the principal channels besides neutralization for radical-cation decay in the condensed phase.
1. Introduction
Radical ions are important intermediates in many areas of chemistry-radiation chemistry, photochemistry, electrochemistry, and sundry processes involving electron transfer. Radical cations in particular are ubiquitous in chemistry induced by energetic radiation such as ionizing and photoionizing radiation. Their widespread Occurrence is becoming increasingly appreciated with the advent of faster and better tools for detecting transient species generated by pulsed excitation methods. In studies of highly reactive, short-lived species in the condensed phase the problem of identification often transcends that of detection. In many instances multiple transient species can occur simultaneously, making the identification of individual components difficult. Nonselective detection methods are bedeviled by the challenge of unraveling signals which are the sum of several subcomponents. Selectivity and structural information become indispensible aids for the characterization of reactive intermediates in complex condensed-phase systems. Few detection methods approach the degree of structural-information content for condensed-phase species provided by magnetic resonance techniques. EPR spectroscopy, for example, can provide well-resolved spectra containing hyperfine structure which, in many instances, allows a straightforward assignment of the identity of odd-electron species such as radical cations, radical anions, and neutral radical3 and gives considerable structural insights about the molecules. The shortcomings of EPR are limited sensitivity and time resolution. Radical-cation studies using EPR almost invariably employ matrix-isolation techniques to stabilize radical cations at low temperature.I4 This paper describes studies using fluorescence-detected magnetic resonance (FDMR), a time-resolved variant of EPR spectroscopy, to observe radical ions in liquid or solid solutions. FDMR uses optical detection which makes it orders of magnitude more sensitive than conventional EPR spectroscopy. In the next section we examine the processes of ion-pair creation and re(1) Shiotani, M . Mugn. Reson. Rev. 1987, 12, 333. (2) Symons, M.C. R. Chem. SOC.Rev. 1984, 13, 393.
(3) Shida, T.; Haselbach, E.;Bally, T. Acc. Chem. Res. 1984, 17, 180. (4) Knight, Jr., L. B. Acc. Chem. Res. 1986, 19, 313.
0022-3654/91/2095-3466$02.50/0
combination which are responsible for the FDMR m e t h d and' account for the unique selectivity of FDMR for radical ions. The ability of FDMR to selectively and exclusively detect spin-correlated radical ions in the presence of much greater concentrations of other neutral radical intermediates is a vital advantage. The starting point for the investigation of the structure and reactivity of radical cations in the condensed phase is the extensive body of data from EPR studies of radical cations in low-temperature matrices.I4 For many types of radical cations we have considerable information on structure rearrangements, and some aspects of condensed-phase reactivity, albeit in the environment of a given matrix. In the FDMR studies we have made the next step which is to begin to examine condensed-phase radical-cation reactions in more "realistic" chemical systems where normal fast reactions are allowed to occur. Only in this way can one determine which of the many reaction possibilities enumerated in low-temperature-matrix studies can play a significant chemical role. The answers we obtain can help us better understand the early chemical events that occur when ionizing or photoionizing radiation interacts with matter and ultimately provide insights/details of chemistry responsible for biological effects of such high-energy radiation. In the following sections we explore the occurrence of radical ions in various solvent media exposed to electron-beam radiolysis or UV-laser photolysis. The goal is to understand the complete life history of radical cations from ion-pair creation, through subsequent transformations and culminating in recombination. In the final section we illustrate studies of several unusual radical-cation species, some of which, e.g., cubane'+, were observed for the first time by FDMR. 2. FDMR Detection of Radical-Ion Pairs 2.1. Radical-Ion-Pair Spin Dynamics. Radical-ion pairs which are born from the same precursor molecule ("geminate" pairs) are spin-correlated since they possess a well-defined relative orientation and spin phasing (singlet or triplet) at the moment of radical cation/radical anion pair creation. The radical cation may be the initial radical cation, or it may be descended from the initial radical cation via electron-transfer hole scavenging or some other radical-cation transformation. The radical anion may 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3467
Feature Article
"geminate" pair
T
1-
$8
/
-1.1
(a)
(b)
-1
I=
F v 1. Energy-level diagram showing the mixing of radical-pair singlet and triplet states: (a) zero magnetic field and (b) high magnetic field. EPR denotes rmnance transitions in a microwave field, and hfi denotes transitions induced by the hyperfine interaction. be the ejected electron or a radical anion resulting from the attachment of the electron to an acceptor molecule. Singlet pairs develop triplet character, and vice versa, due to mixing of the singlet-pair spin state IS) and the three triplet-pair spin states ITo), IT+,1, and IT-,) mainly because hyperfine interactions cause the unpaired electrons to experience different magnetic environments on their respective radical ions (Figure 1). In a static magnetic field Hothat is large compared to the radical-ion hyperfine couplings, mixing of IS) with IT+,) and IT-I) is turned off. The mixing between these states is reestablished, and accelerated, by an alternating magnetic Weld Hi perpendicular to the static field. This is a resonant process, and the transitions between pair spin states of IS) f ITo) character and those of IT+I) or IT-, ) character correspond to EPR transitions of the radical ions constituting the pair (Figure 1). If, as is often the case, ion recombinatioin results in excited-state formation, then EPR spectra of radical-ion pairs can be detected as a resonant change in (singlet or triplet) excited-state yield. The FDMR experiment uses fluorescence detection to observe the resonant change in excited-singlet-state yield.s The delayed fluorescence from radical-ion recombination is proportional to the number of singlet-recombining pairs per unit time. EPR transitions are detected as a decrease or increase in the fluorescence intensity, depending on whether the geminate ion pairs were created predominantly in the singlet state or triplet state, respectively. The initial spin multiplicity (singlet or triplet) of a radical-ion pair depends on the multiplicity of the precursor molecule. In radiolysis, ionization of ground-state molecules results in the predominance of singlet pairs. In a photoionization process involving sequential absorption of two photons, the initial ion-pair multiplicity depends on the multiplicity of the intermedite excited state. It is possible to have photoionization occur to give a mixture of singlet and triplet pairs, the relative amounts of which will depend on such factors as the intersystem-crossing rate and the photon flux (Figure 2). Geminate Recombination. Once created, radical-ion pairs may geminately recombine or separate and undergo homogeneous recombination. Only geminate recombination preserves spin coherence which carries the memory of the precursor state; homogeneous recombination is a random process and yields a statistical ratio of singlet states to triplet states (1:3). FDMR only detects radical-ion pairs which recombined geminately, and the fraction of radical-ion pairs that do so depends strongly on the solvent polarity.6 In nonpolar solvents most radical ions recombine geminately, whereas in polar solvents the fraction can be considerably less than half. The average geminate recombination lifetime can be exceedingly short, especially in solvents with high electron mobility.6 In nonpolar solvents geminate recombination of cation/electron pairs is a picosecond process. This is insufficient time to allow pulsed-microwave perturbation of the radical-ion-pair spin states. In practice FDMR is capable of detecting only radical-ion pairs ( 5 ) Smith, J. P.;Trifunac,A. D. J . Phys. Chem. 1981,85, 1645. Trifunac, A. D.;Smith, J. P. Chem. Phys. Lett. 1980, 73, 94. (6) Warman, J. M. In The Study ofFasr Processes and Transient Species
by Electron Pulse Radiolysis; Baxendale, J. H., Busi, F., Eds.; Reidel: Boston, 1981; p 433.
so
I
so
I
Radiolysis Photoionization Figure 2. Energy-level diagram comparing geminate ion-pair creation via radiolysis and sequential, two-photon photoionization. I , is the threshold for ionization.
where geminate recombination is slowed down, a t least into tens or hundreds of nanoseconds when the electron is converted to a less mobile radical anion or the electron becomes solvated. FDMR will also exhibit greater sensitivity for the relatively longer lived recombining pairs which were created with longer separation distances. FDMR is foremost a spectroscopic tool excelling in the detection and structural characterization of transient radical ions. Dynamical information can also be obtained, especially with the time-resolved variant of FDMR discussed here. Kinetic information can be obtained from the time evolution of the spectrum or from spectral line width effects-effects common to conventional EPR spectra (e.g., spin exchange). Furthermore, radical-cation lifetimes can be estimated when the radical-cation decay is faster than the geminate ion recombination. When radicalcation decay competes with geminate recombination the FDMR intensity is reduced and is directly related to the relative radical-cation longevity. In the limit that the radical-cation lifetime is shorter than the minimum time necessary to perturb the ion-pair spin states (20 f 10 ns), no FDMR signal due to that radical cation can be observed. A quantitative theory of FDMR that would predict the FDMR amplitude in terms of the percent modulation of fluorescence (at resonance) is presently beyond our scope. Such a theory must include a full treatment of the nonhomogeneous ion-pair recombination kinetics as well as the pair spin dynamics in the magnetic field and response to the microwave excitation. This is possible in principle but involves a multitude of system-specific parameters (temperature-dependent diffusion coefficients, scavenging rate constants, pair-separation distribution function, excited singlet lifetime, hyperfine coupling constants, g factors, spin-relaxation times, etc.) which magnifies the task immensely. And while many experimental observations (e.g., in hydrocarbon radiolysis) agree fairly well with model pair-separation distribution functions, the exact form of these is not known. (The problem is that the recombination dynamics are sampled during a delayed time window and only scavenged ion pairs are detected, and thus the dynamics are not sensitive to the initial pair distribution. The sampled recombination dynamics only reflect the limiting r i I 2 behavior characteristic of a geminate process.s) Preliminary attempts to model the FDMR intensity (for a non-time-resolved experiment) as a function of diffusion rate, pair distribution, and microwave power have been carried out? but at too rudimentary a level to extract, e.g., radical-cation lifetimes, yields, or pair distributions from experimental data. All things considered, strenuous efforts to quantitatively model FDMR signals are probably a departure from the best use of the method, Le., spectroscopy, since more direct methods exist that (7) Smirnov, S. N.;Rogov. V. A.; Shustov, A. S.;Sheberstov, S. V.; Panfilovitch, N. V.; Anisimov, 0. A.; Molin, Yu.N. Chem. Phys. 1985, 92,
381.
3468 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
I
I
t=O
1 PS
Pulse
.-
zEPR
I
Box-
Microwave
Ionization
Werst and Trifunac
car
I
1,
variable
t, W
t,+w
=
20
- lOOOns
-
100 200 n s (typical) Figure 3. Experimental timing diagram for time-resolved FDMR.
can measure such quantities as radical-cation lifetimes and yields with better accuracy and time resolution (e.g., time-resolved fluorescence and time-resolved absorption spectroscopy). Comparison of the FDMR intensity in closely related systems does allow the longevity of radical cations to be qualitatively assessed and provides important insights about the fate of radical cations, as we will illustrate in later sections. 2.2. Time-Resolved FDMR. The FDMR experiment is carried out in the cavity of an X-band EPR spectrometer. Early FDMR experiments were performed in Novosibirsk using continuous ionization and C W microwave e x c i t a t i ~ n . ~ At . ~ Argonne we time-resolve the experiment by using a pulsed ionization source (pulsed excimer laserlo or pulsed 3-MeV electron Van de Graaff accelerators) and pulsed microwave excitation. The time-resolved variant of FDMR is versatile and provides both spectral and dynamical information. The FDMR apparatus has been described in detail e l s e ~ h e r e . ~ Fluorescence emitted from the sample in the EPR cavity is measured with a photomultiplier. Provision can be made for wavelength discrimination (monochromator or colored filter). In liquid-phase experiments the sample is continuously flowed and recirculated, a measure necessitated by the heating effects and sample degradation caused by repetitive pulsing by the ionization source. In solid-phase experiments these same effects are partially obviated by vertical translation of the sample." Temperature control is achieved by means of either a variable-temperature bath acting as a heat exchanger (liquid-phase experiments) or a liquid-helium-transfer cryostat (solid-phase experiments). Figure 3 shows the timing diagram for the time-resolved FDMR experiment. Time zero is defined as the arrival time of the ionization pulse which is 5-15 ns in duration. A microwave pulse of width w is applied immediately after the arrival of the ionization pulse or after some variable delay, tl. The fluorescence intensity is then integrated during the desired time interval after the end of the microwave pulse by a boxcar detector. An FDMR spectrum is obtained by fixing the microwave-pulse and boxcar-gate timing and sweeping the magnetic field at constant microwave frequency. In the following section we illustrate FDMR detection of radical cations in electron-irradiated alkanes. The electron-beam pulse (8) Anisimov, 0. A,; Grigoryants, V. M.; Molchanov, V. K.; Molin, Yu. N. Chem. Phys. Lett. 1979.66, 265. (9) Molin, Yu.N.; Anisimov, 0. A.; Grigoryants, V. M.; Molchanov, V.
K.; Salikhov, K. M. J . Phys. Chem. 1980, 84, 1853. (10) Percy, L.T.; Bakker, M. G.; Trifunac, A. D. J . Phys. Chem. 1989,
93, 4393. (11) Werst, D. 82, 588.
1
L
W.; Percy, L. T.; Trifunac, A. D. J . Magn. Reson. 1989,
H-
Figure 4. Stick EPR spectra of a hypothetical radical anion A*- and radical cation RH,'+,and the simulated FDMR spectrum resulting from the recombination of RH,'+ and A*-. creates solvent radical cation/electron pairs, eq 1. A small amount RH
+ e- beam
RH'+
+ e-
(1)
( 104-10-3 M) of an aromatic scintillator A is present to scavenge
the highly mobile electrons, eq 2. FDMR signals are observed e-
A
A'-
(2)
due to recombination reactions between A'- and various solvent-derived radical cations in which the fluorescent excited state of the scintillator is formed, e.g., eq 3. RH'+
+ A'-
-
RH
+ )A*
(3)
For illustration Figure 4 shows a simulation of the FDMR spectrum resulting from recombination of A'- and a hypothetical hydrocarbon radical cation with four equivalent protons (RH,") below the simulated stick EPR spectra of the respective radical ions. The small hyperfine splittings in the EPR spectra of aromatic radical anions are usually not resolved by FDMR, so the stick EPR spectrum of A'- is drawn as a single unresolved line. (The resolution of FDMR is typically 2-4 G because of the pulsed nature of the excitation and high microwave powers typically used. Some improvement of the frequency resolution can be obtained, but at the sacrifice of sensitivity and time resolution, by using low microwave fields (HI < 0.5 G) and very wide pulses (2500 nshS) The stick EPR spectrum of RH.,'+ is a quintet with the same g-factor as A*-. If RH4'+ had a different g factor, then its spectrum would be shifted with respect to the center of the A'spectrum. The FDMR spectrum is the superposition of the A' and RH4'+ EPR spectra and is registered as a decrease (singlet pairs, t = 0) in the fluorescence intensity. The center line is the sum of two resonances and is very intense. The spectrum has the appearance of a normal absorption spectrum rather than the familiar firstderivative form of field-modulated EPR spectra. By convention the fluorescence axis is usually defined so that the spectrum consists of peaks instead of dips. In summary, (1) FDMR only observes geminate radical ions; randomly-paired radical ions, neutral radicals and diamagnetic ions are not observed. (2) The phase of the FDMR signal depends on the spin multiplicity of the geminate-ion-pair precursor state: singlet precursor-fluorescence decrease; triplet precursorfluorescence increase. (3) The time evolution of processes causing radical-ion transformation is only observable if it is comparable to the geminate-ion pair recombination time which is the internal clock in the FDMR experiment.
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3469
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3. Occurrence of Radical Cations in Radiolysis An important difference between generating ions with ionizing radiation (fast electrons, y-rays, X-rays) and near-UV photons is the nature of the energy absorption process. Interaction of ionizing radiation results in the ejection of electrons from the atoms of the medium.I2 This process is not sensitive to molecular structure, and most of the energy is absorbed by the solvent. The energy of near-UV photons, on the other hand, is sufficient only to interact with valence electrons. This process is very dependent on the electronic/molecular structure of the absorbing species. A strongly absorbing solute may be ionized (via absorption of one or more photons) selectively, without any absorption of energy by the surrounding solvent. In radiolysis the initial radical cation is the radical cation of the solvent. In the presence of solute molecules more easily ionized than the solvent electron-transfer hole scavenging converts solvent radical cations into solute radical cations. In this way positive charge cascades down until it is in the lowest energy trap in the system or is neutralized by ion recombination. In photolysis the ionized species often is the lowest energy trap, and hole transfer is less common. Radiolysis is a versatile method for generating a wide range of radical cations. Absorption of energy by the solvent followed by energy transfer and charge transfer results in a plethora of species (solvent ions, solute ions, excited states, neutral radicals), but the selectivity of FDMR makes all species but radical ions invisible. Studies of neat solvents have been carried out to learn about the formation and subsequent chemistry of the solvent radical ~ations.'~-'' In other studies the solvent was utilized as a convenient host medium and the solvent radical cation was used as an effective oxidant for solute molecules of Inevitably, the solvent also affects to a varying degree the chemistry of the solute radical cations. 3.1. Observation of Radical Catiom in Alkanes. Low polarity and relatively high ionization potentials make alkane solvents well-suited for FDMR studies of radical cations. Alkane radical cations are among the most fundamental organic radical cations, and the radiation-induced chemistry in alkanes is the obvious model for learning about the effects of radiation on organic materials. The FDMR studies which have been carried out in neat alkanes can be divided into liquid-phase and solid-phase experiments. The principal goal was to characterize the initial solvent radical cations by direct, real-time EPR detection by using FDMR. Methods previously used to investigate radical cations in alkanes, such as conductivity and optical spectroscopy, do not provide a unique structural signature for radical cations as does EPR, and thus identification of radical cations by these methods is tenuous. Conventional EPR does not possess sufficient sensitivity or time resolution to detect radical cations in neat alkanes. FDMR studies of pulse radiolysis of liquid alkanes reveal olefin radical ctions which result from the elimination of H2 from the initial solvent radical cation.I3*l4The parent radical cations cannot be observed, even in spectra observed at the earliest time possible by applying the miicrowave pulse immediately after the arrival ~~
(12) Chatterjee, A. In Radiation Chemistry; Farhataziz, Rodgers, M. A. J., Eds.; VCH Publishers: New York, 1987; p 1. (13) Werst, D. W.; Desrosiers, M. F.; Trifunac, A. D. Chem. Phys. Lert. 1987, 133, 201. (14) Werst, D. W.; Trifunac, A. D. J . Phys. Chem. 1988, 92, 1093. (15) Werst, D. W.; Bakker, M. G.; Trifunac, A. D. J . Am. Chem. SOC. 1990, 112.40.
(16) Trifunac, A. D.; Werst, D. W.; Percy, L. T. Radiat. Phys. Chem. 1989, 34, 547.
(17) Percy, L. T.; Werst, D. W.; Trifunac. A. D. Radiat. Phys. Chem. 1988, 32, 209.
(18) Dcsrosiers, M. F.;Trifunac, A. D. Chem. Phys. Lett. 1985,118,441. (19) Desrosiers, M. F.; Trifunac, A. D. J . Phys. Chem. 1986, 90,1560. (20) Lefkowitz, S. M.; Trifunac, A. D. J . Phys. Chem. 1984, 88, 77. (21) Werst, D. W.; Trifunac. A. D. J . Phys. Chem. 1991, 95, 1268. (22) Qin, X.-Z.; Werst, D. W.; Trifunac, A. D. J. Am. Chem. Soc. 1990, I 12, 2026. (23) Qin, X.-Z.; Trifunac, A. D. To be published.
Figure 5. FDMR spectra observed at 35 K in (a) n-hexane, (b) n-heptane, (c) n-octane, and (d) n-nonane. The anthracene-& scintillator concentration was M. The hyperfine parameters used for the stick EPR spectra are given in ref 15. The structure indicates the two protons (black circles) with largest coupling in the fully extended conformer of the n-hexane radical cation.
of the electron-beam pulse. The presence of the olefin-radicalcation signals in the FDMR spectrum shows that they are formed by prompt dissociation of the parent radical cation, eq 4. The RH'+
-
+
Ole+ H2
(4)
dissociation of RH'+ to give Ole+is by no means quantitative, but the absence of an alkane-radical-cation FDMR signal suggests that the solvent radical cation no longer exists on the time scale accessible to FDMR, Le., times later than 10-20 ns after ion-pair creation. In contrast to the results in liquid alkanes, alkane radical cations are observed in the FDMR spectra obtained in solid-phase experiments at low temperature.lsJ6*?4JS Figure 5 shows the FDMR spectra observed in a series of n-alkanes at 35 K. Each spectrum consists of the usual intense central peak superimposed on a triplet EPR spectrum the hyperfine splitting of which decreases with chain length. This is just what is expected for the linear alkane radical cations with planar-extended structure.26 The singly occupied molecular orbital (SOMO) is a cT-delocalized orbital with the highest unpaired-electron density in the two in-plane C-H bonds at the chain ends. The hyperfine coupling to the two terminal protons decreases with increasing number of carbon atoms as was previously shown in matrix EPR studies.26 Thus, by lowering the temperature, the lifetimes of the solvent radical cations in alkanes increase. Solvent radical cations can also be observed by FDMR in many simple cyclic and branched-alkane solids at low temperature. However, considerable variations in the intensity of the FDMR response (more than 2 orders of magnitude) in different alkane solids indicates great diversity in the lifetimes of the solvent radical cations in different alkanes. 3.2. Observation of Solute Radical Cations. There are many reasons which make it desirable to generate radical cations by dissolving in a solvent (e.g., alkanes) the parent compound which can then be oxidized by reacting with solvent radical cations formed by radiolysis. The parent compound may be a solid, or for various reasons the neat liquid may fail to give a satisfactory FDMR signal. At high concentrations the parent compound may quench the scintillator excited state. A complicating factor, which (24) Melekhov, V. 1.; Anisimov, 0. A.; Veselov, A. V.; Molin. Yu.N. Chem. Phys. Lert. 1986, 127,97. ( 2 5 ) Tadjikov, B. M.; Melekhov, V. 1.; Anisimov, 0. A.; Molin, Yu.N. Radiat. Phys. Chem. 1989, 34, 353. (26) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Phys. Chem. 1986, 90, 6836.
3470 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
100G
Figure 6. FDMR spectra observed at 185 K in n-pentane containing lo4 M anthracene-& and IO-* M (a) cis-decalin, (b) 2,2,3,3-tetramethylbutane, and (c) bicyclopentyl. The hyperfine parameters used for the stick EPR spectra are given in ref 15.
we consider in depth in section 4, is that reactions can occur between a radical cation and its neutral parent molecules. An important fact is that a radical cation prepared by hole transfer will possess less initial excess energy than radical cations formed by direct excitation by the electron-beam pulse. Many radical cations studied by FDMR in solution were previously only accessible to EPR by using matrix isolation, especially in freons.14 A few radical cations have been observed for the first time by FDMR. The advantage of FDMR over the matrix-isolation methods is its ability to provide structural and dynamical information about radical cations over a large temperature range in a nonviscous, noninteracting solvent where fast reactions of the radical cations still take place. 3.2.1. Alkane Radical Cations. Alkane radical cations are observed in liquid solution by FDMR when diluted sufficiently in another alkane solvent with a higher ionization potentia1.'4J5*24 Figure 6 shows spectra of three alkane radical cations observed in n-pentane solvent at 190 K. The alkane solute concentrations were 1W2 M. Simple alkanes (n-hexane, n-pentane, cyclopentane) are excellent solvents because of their relatively high ionization potentials, and formation of their radical cations via hole transfer in alkane mixtures is problematic for the same reason. The cis-decalin radical-cation spectrum (Figure 6a) observed in n-pentane by FDMR is in excellent agreement with that observed in a freon matrix by EPR.27 FDMR results show that the EPR parameters of the cis-decalin radical cation are constant over the temperature range 10-290 K.I4 The tetramethylbutane and bicyclopentyl radicalcation spectra are very temperature dependent.Is Torsional motions (methyl rotation, ring puckering) which give rise to a dynamical averaging of the proton hyperfine couplings with increasing temperature become effectively frozen (on the EPR time scale) at low tem(27)
Werst, D.W.; Desrasien, M. F.; Trifunac, A. D. Unpublished results.
Werst and Trifunac perature (
