- Email: [email protected]
Solid state FDMR studies of ion-molecule reactions in radiolysis of saturated hydrocarbons
Volume 153, number
I
CHEMICAL
PHYSICS
LETTERS
SOLID STATE FDMR STUDIES OF ION-MOLECULE IN RADIOLYSIS OF SATURATED HYDROCARBONS
2 December
1988
REACTIONS *
D.W. WERST, L.T. PERCY and A.D. TRIFUNAC Chemistry Divaion, Argonne National Laborator_s, Argonne, IL 60439, USA Received 29 August 1988: in final form 21 September
1988
Studies of radical cations produced in pulse radiolysis of frozen hydrocarbon solutions have heen carried OULusing time-resolved fluorescence-detected magnetic resonance (FDMR). These first time-resolved solid-state FDMR expcrimcnts wcrc performed with a spectrometer modified to allow sample translation and coohng 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 mformation about the dynamics of charge pair recombination in solids.
1. Introduction The description of the mechanism of alkane radiolysis has been a goal of many radiation chemistry studies. Early insights into the hydrocarbon chemistry induced by ionizing radiation were gained from product yield and scavenging studies, and optical spectroscopy of y-irradiated alkane glasses. Our understanding of the early events and the nature and identity of transient species has been greatly advanced by time-resolved techniques such as transient absorption and emission, conductivity and timeresolved fluorescence-detected magnetic resonance (FDMR), and by EPR studies of alkane radical cations in inert matrices. Magnetic resonance methods excel because they allow unequivocal identification of the species observed and provide structural information on paramagnetic transients. Of these, only optically detected EPR techniques have proven sensitive enough to detect short-lived alkane radical cations in pure hydrocarbon systems. When ionizing radiation interacts with matter, charge pairs consisting of an electron and a positive hole (i.e. solvent radical cation) are created. Energy * Work performed
under the auspices of the OiXce of Basic Energy Sciences, Division of Chemical Science, US DOE under contract number W-31-109-ENG-38.
0 009-2614/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
in excess of that needed for ionization is deposited in the charge pair as excess excitation (electronic and/or vibrational) of the radical cation and/or kinetic energy of the electron. In alkanes, owing to the low dielectric constant greater than 95% of the initial radical cations recombine with their “geminate” electron partner. The high electron mobility in many hydrocarbons drives this process to completion very quickly (picoseconds). Hence, much of the ensuing chemistry is determined by the consequences of such charge pair recombination as it yields excited states which may further react. It is known that alkyl radicals and olefins are formed by excited state fragmentation [ l-31. Excited states may also relax to the ground state with the emission of a photon. In time-resolved FDMR experiments, radical cations are observed whose geminate electron partners have been scavenged and converted into less mobile radical anions. When charge pair recombination is slowed down, it becomes possible to study reactions of the alkane radical cations themselves. In previous FDMR studies of pulse-irradiated liquid alkancs (RH, ), we have observed radical cations of solute molecules possessing a lower ionization potential than the solvent, which are formed via electron transfer to solvent radical cations [ 4-9 1. Olefin radical cations are observed in solutions of saturated hydrocarbons due to elimination of H1 from alkane B.V.
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[ 9, lo] _We have shown that this dissociation reaction depends on the energy content of the initial alkanc radical cation. Detection of alkane radical cations by FDMR in lzeat alkane liquids has been elusive. FDMR spectra of a small number of alkane radical cations have been detected under conditions of high dilution in an alkane solvent which has a higher ionization potential than that of the solute. The cis-decalin radical cation was detected in a lop2 M solution of cis-decalin in 3-methylpentane at 140 K by Melekhov et al. by static optically detected EPR [ 11 1. WC have published time-resolved FDMR observations of the cis-decalin and norbomane radical cations in various alkane host solvents at solute concentrations less than 0.1 M and over the temperature range 140 K to room temperature [ 9 1. At solute concentrations much greater than 0.1 M, the solute alkane cation signals disappeared. Recent efforts have shown that such observations can be extended to the radical cations of several other alkanes [ 121. However, there are conspicuous exceptions which are not observed. A notable one is the trans-decalin radical cation which is not observed at all in the liquid phase. Is there a significant decay mechanism of alkane radical cations which competes with geminate recombination? Evidence suggests (vidc infra) that it is necessary to isolate RH$ . from RH2, implying that an ion-molecule reaction between the alkane radical cation and neutral alkanc molecules may be responsible for making alkane radical cations very shortlived. The reaction that is usually considered is proton transfer: radical
cations
RH:‘tRH2+RH’tRH:.
(1)
In this paper we discuss the importance of alkane radical cation decay via proton transfer reactions and our first observations of alkane radical cations of decalins in frozen hydrocarbon solutions. We have recently developed the capability to do FDMR experiments in low-temperature solids.
2. Method and materials The time-resolved FDMR spectrum is a “snapshot” of the geminate radical ions present at a given instant in time. In the FDMR experiment, fluores46
2 December 1988
CHEMICAL PHYSICS LETTERS
cence is detected from irradiated hydrocarbon solutions of aromatic scintillators. The relevant ionic processes and scavenging events are depicted in scheme 1. Application of a microwave pulse at resonant magnetic field accelerates mixing between singlet and triplet pair states by inducing EPR transitions between the doublet levels of the separated radical ions. Thus, by measuring the fluorescence intensity as a function of magnetic field strength, EPR spectra of radical ions which were present during the microwave pulse and which recombined geminately to give scintillator excited singlet states are observed [ 41. The apparatus and technique for obtaining timeresolved FDMR spectra in low-temperature solids are similar to those previously described for liquid phase experiments [ 4,131. A pulsed 3 MeV electron van de Graaff serves as the ionizing source. The sample (contained in a 4 mm outer diameter cylindrical Suprasil cell) is located in the resonant cavity of a pulsed X-band EPR spectrometer which has been modified for optical detection. Fig. 1 shows the most relevant portion of the experimental set-up. The new modifications for solidstate studies are the addition of an Air Products LTR3 liquid transfer “Heli-Tran” refrigerator for temperature control and a stepper-motor-driven device for translating the solid sample through the electron beam. The latter measure was necessary to obviate heating effects and sample degradation due to irradiation by repetitive pulses. The temperature can be conveniently controlled between approximately 20 K and room temperature. A more detailed description of the low-temperature modifications will be published elsewhere [ 141. A 5 ns electron beam pulse of z lo- ‘I C or less was used at a repetition rate of 30 to 60 pulses/s. The arrival of the electron beam pulse defines the zero of Scheme I Ionic processes (A=aromatic scintillator) RH2 t electron beam + RH:. fe-
ionization
A+RH: -4A+‘+RH, A+e-+A-’
scavenging
A+‘+A-‘+‘A*+A RH,t’+A-‘+‘A*+RH2
geminate recombination -+fluorescence
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PHYSICS LETTERS
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1988
3. Results
Fig. 1.Experimental apparatus for time-resolved magnetic resonance studiesof solids: (A) quartz sample tube, (B) EPR cavity, (C) translation mount, (D) stepper motor drive, (E) helium refrigeratoridewar, (F) liquid helium inlet, (G) waveguide, (H) lens, (I ) EPR magnet.
time in the FDMR experiment, and the application of the microwave pulse is denoted by, e.g. MW (0, 100 ) ns, meaning a microwave pulse from t=O to 1~100 ns. Similarly, BC( 100, 200)ns denotes the boxcar detector viewing time to be from t= 100 to t =200 ns. An FDMR spectrum is obtained by fixing the timing of the microwave pulse and boxcar gate and measuring the fluorescence intensity as a function of the applied magnetic field. Cis- and trans-decalin (Aldrich) and squalane (Aldrich) were purified by passing through a 0.75 m column of silica gel which had previously been activated at ~250°C. 3-methyloctane (Wiley Organits), iso-octane (Burdick and Jackson) and cyclohexane (Burdick and Jackson) were used without further purification. Anthracene-d,, was used as received from Aldrich. All frozen samples were degassed by the freeze-pump-thaw method prior to radiation.
We have observed FDMR signals in a variety of frozen hydrocarbon solutions (containing millimolar concentrations of anthracene-d,,, scintillator), both in neat alkanes and in alkane mixtures, in glassy and polycrystalline solids. The intensity of the scintillator fluorescence and the FDMR signals observed is solvent dependent. At the present level of understanding, the alkanes which have been studied can be very roughly categorized as solvents which give strong (e.g. cis-decalin, squalane, 3-methyloctane) or weak FDMR signals (e.g. trans.-decalin, cyclohexane, isooctane) when frozen [ 12 1. The time dependence of the scintillator ion FDMR intensity measured in three different solvents is shown in fig. 2. The baseline points at negative times were collected many microseconds before the electron beam pulse. Time zero corresponds to MW(0, lOO)ns, BC(l00,200)ns, and the microwave pulse and boxcar delays were swept simultaneously in increments of 30 ns. The data points represent the fluorescence intensity measured with the microwave pulse on minus the fluorescence intensity measured with the microwave pulse off. The normalized decay curves shown for squalane, 3methyloctane and cis-decalin (T= 45 K, scintillator
I
I
0
I
I t
l.! PS
Fig. 2. Time dependence of the central FDMR peak intensity (T=45 K, scintillator concentration= IO-’ M) in squalane, cisdecalin and 3-methyloctane. Time zero corresponds to MW(0, lOO)ns, BC( 100,200)ns. The microwave pulse and boxcar delay were swept simultaneously in 30 ns steps.
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concentration= 1Ow3 M) are not significantly different. Data collected in a mixture of 30% (v/v) cisdecalin in squalane were indistinguishable from the curves for the pure solvents. The inset in fig. 2 shows a log/log plot of one of the decay curves (data for 3-methyloctane). The plot gives a reasonably straight line with a slope of approximately - 1. This is in contrast with liquid-phase FDMR where a t -3’2 dependence is observed (e.g. in cyclohexane) as expected if diffusion-controlled geminate ion-pair recombination is the dominant process for excited state production and radical ion decay [4]. Stolzenburg et al. have observed t-’ law behavior for the decay of delayed fluorescence in photoionization studies of polyvinylcarbazole [ 15 ] _ They attribute such behavior to inherent energetic disorder of hopping sites in amorphous systems where diffusive motion of radical ions is mediated by trapping. We may be witnessing a similar phenomenon in the hydrocarbon glasses. Here, we focus on EPR spectral data obtained in cis- and trans-decalin and in alkane mixtures containing decalins. The radical cations of cis- and transdecalin have simple, well-resolved EPR spectra which have been previously assigned (see references in ref. [ 9 ] ). Cis- and trans-decalin have both been studied in the liquid phase by time-resolved FDMR [ 9-l l ]_ The behaviors of the cis- and trans-decalin radical cations contrast sharply and provide important insights for the mechanism of alkane radical cation decay. The cis-decalin radical cation can be observed under a variety of conditions in both liquid and solid solutions. Table 1 summarizes the present results in Table 1 FDMR results in decalins
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PHYSICS LETTERS
frozen samples along with results from our previous studies in liquids. The observation of cis-decalin+. in the solid state is facile. Fig. 3a shows the time-reFDMR (MW(0, 100)ns; spectrum solved BC( 100,200)ns) obtained at 45 K in pulse-irradiated cis-decalin containing 1O-’ M anthracene-d, D, The previously assigned, five-line spectrum (51 G coupling constant) of cis-decalin+‘ is superimposed on an intense central line due to the unresolved EPR lines of the scintillator radical ions. Cis-decalin+’ could be observed in neat glassy cis-decalin at temperatures below approximately 150 K. The signal intensity at 45 K in a polycrystalline sample (obtained by annealing above 150 K) was comparable to that of glassy cis-decalin at the same temperature. The cis-decalin+ ’ spectrum was also observed in mixtures. Fig. 3c shows the FDMR spectrum (low field only) obtained at 45 K in a sample of 30% (v/v) cis-decalin in squalane. The spectrum obtained in neat squalane (fig. 3b) exhibits no resolved features attributable to the solvent radical cation, only a slightly broadened scintillator ion peak. Preliminary concentration dependence studies show that cis-decalin+’ can be observed at cis-decalin concentrations as low as 1% (in 3-methylpentane at 45 K). Previous
attempts
neat decalin b, B 0.2 M decalin/alkane solvent 1O-3-1O-’ M decalin/alkanesolvent neat decalin decalin/squalane 50: 50 neat decalin decalin/squalane 50: 50
Temperature range (K)
the trans-decalin
263-290 140-290 140-290 100-150 100-170 <75 < 100
Cis-decalin+‘“’
Trans-decalin+’
Y Y Y Y Y
Y Y
a) Y in this column indicates that the radical cation was observed under the stated conditions. b, That is, cis-decalin or trans-dccalin.
48
to observe
radical cation in the liquid phase and solid phase were unsuccessful (see table I, refs. [ 9- 111). Our liquid phase studies explored many solvent systems and covered a large range of trans-decalin concentrations, but trans-decalin+. could not be observed in the time window of our observation, i.e. tens to
(see also ref. [ 9 ] )
Sample
I988
‘)
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PHYSICS LETTERS
2 December
1988
magnitude less than that observed in neat cis-decalin. Dilution of trans-decalin with a second alkane solvent (3-methylpentane or squalane) significantly enhanced the trans-decalin+’ signal intensity. Enhancement of the cis-decalin+’ signal intensity was not noticeable upon dilution of cis-decalin with 3methylpentane or squalane. Clearly, cis-decalin” is easier to observe than trans-decalin+‘. Cis-decalin+’ can also be observed at higher temperatures than trans-decalin+’ in neat decalins and in mixtures. All of these results are summarized in table 1.
4. Discussion
c
50 G
d
50 G
I
l-iFig. 3. Time-resolved FDMR spectra obtained in frozen solutions: (a) cis-decalin, (b) squalane, (c) 30% (v/v) cis-decalin in squalane, (d) 30% (v/v) trans-decalin in squalane. The scintillator concentration (anthracene-dIo) was lo-’ M. T=45 K; MW(O,lOO)ns,BC( 100,200)ns. The spectra shown in (b)-(d) are low field only. See text for assignments.
hundreds of nanoseconds after the electron beam pulse. Fig. 3d illustrates our recent observations of transdecalin + in low-temperature solids. It shows the FDMR spectrum obtained at 45 K in a 30°16 (v/v) mixture of tram-decalin in squalane. This is the first observation of trans-decalin+’ outside a freon matrix, and the coupling constant (52 G) obtained here agrees with the value measured from the EPR spectrum observed in a CFC& matrix [ 91. The intensity of the FDMR spectrum observed in neat trans-decalin at 45 K (identical to fig. 3d) was an order of
The contrast in the behaviors of the two seemingly similar radical cations, cis-decalin+’ and trans-decalin + ., evident from earlier liquid phase studies [ 91, is further emphasized by the observations in frozen solutions. The results (summarized in table 1) are consistent with the conclusion that cis-decalin+’ and trans-decalin” decay by proton transfer, the rate of which increases with temperature and concentration, The differences between cis-decalin+’ and transdecalin+. are attributed to differing propensities for undergoing proton transfer to neutral decalin molecules or to alkane solvent molecules. Decay of alkane radical cations via ion-molecule reactions with their neutral parent molecules has previously been proposed to explain a variety of experimental observations. The stability which can in some cases be gained from isolating RH: ’from RH, was noticed in early experiments by Louwrier and Hamill, who observed the optical absorption spectra of alkane radical cations in y-irradiated mixed alabkane glasses at 77 K [ 16,171. Time-resolved sorption experiments in pulse-irradiated alkanes have shown that alkane radical cations decay faster than the solvated electrons [ 18-221, indicating that alkane radical cations disappear by some reaction other than recombination_ Thermal or photoconversion of alkane radical cations to neutral alkyl radicals is a very common (though matrix dependent) observation in EPR studies of alkane radical cations in halogenated matrices [ 23-261. While these experiments do not unequivocally distinguish between ion-molecule and unimolecular dissociation reactions, re49
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lated EPR studies using synthetic zeolites as the host matrix have shown that alkyl radical formation does not occur in the absence of a neutral alkane molecule to act as proton acceptor [ 271. This is consistent with the FDMR results on the concentration dependence of radical cation signals in mixtures (e.g. for cis-decalin+. and norbomane+’ (see table 1 and ref. [ 91). Ultrafast resonant charge transfer has occasionally been hypothesized in the past as a mechanism which may prevent the observation of resolved EPR spectra of alkane radical cations in hydrocarbons. Such a mechanism is inconsistent with our FDMR results. This picture envisions the alkane radical cation as a spin-delocalized state due to fast electron hopping from neutral alkane molecules to the radical cation. Kn the limit of fast electron hopping, the EPR spectrum would be narrowed to a single unresolved line. This picture also implies a solvent hole with an anomalously high mobility. Fast electron transfer cannot explain why the FDMR spectra of some alkane radical cations, including cis-decalin+-, are observed in alkane mixtures but not that of transdecalin+‘, since it is known from conductivity experiments that solvent mixtures will inhibit fast hole transport [ 281. Furthermore, we observe no evidence for the onset of narrowing of the cis-decalin +EPR spectrum at intermediate solute concentrations, only a diminution of the FDMR signal intensity. The existence of delocalized solvent radical cations even in neat alkanes has not been shown. Conductivity experiments which have observed fast positive charge mobility (e.g. in trans-decalin and cyclohexane) track such behavior for hundreds of nanoseconds ( < 0.5 ps) [2X-30]. The evidence discussed here suggests that alkane radical cation lifetimes in all neat alkanes and shorter than this by at least an order of magnitude. A more plausible mechanism to explain the conductivity experiments, suggested by us previously [ 3 11, involves proton transfer (i.e. RH: ). Substitution of “proton transfer” for “ion-molecule” reaction reflects a bias that has not been unambiguously proven by experiment. Our results cannot rule out the symmetric reaction, which is H abstraction by the radical cation, However, EPR studies indicate that alkyl radical formation (eq. ( 1) ) shows position selectivity and probably depends on 50
2 December
1988
the spin densities in the initial radical cations [ 27,321. Such observations favor a mechanism involving deprotonation of the radical cation. More experiments are needed to distinguish between these two mechanisms. In summary, we conclude the following: ( 1) Cis-decalin+’ and trans-decalin+‘, and probably all alkane radical cations, decay via proton transfer reactions which compete with geminate charge pair recombination. undergoes more facile proton (2) Trans-decalin+transfer than cis-decalin+‘. Cis-decalin+- does not undergo proton transfer to other alkane solvent molecules, while trans-decalin+undergoes proton transfer to other alkane solvent molecules as well as to trans-decalin. (3) The propensity of alkane radical cations in general to undergo proton transfer is likely to show great diversity. A growing number of examples studied by us bears out this expectation [ 121. (4) Proton transfer seems to require little or no excess energy. Our results show that even solute radical cations formed via electron transfer (as opposed to direct ionization by the electron beam pulse) undergo proton transfer. (5) Some alkane radical cations are thermally stable.
Acknowledgement We thank R.H. Lowers for his expert operation of the van de Graaff and other technical support. J. Gregar provided us with the quartz sample cells, and P. Walsh assisted with graphics.
References [ 1] P.Ausloos, R.E. Rebbert, F.P. Schwarz and S.G. Lias, Radiat. Phys. Chem. 21 (1983) 27. [2] F.P. Schwarz, D. Smith, S.G. Liasand P. Ausloos, J. Chem. Phys. 75 ( I981 ) 3800. [3] J. Fafisi-Movaghar and Y. Hatano, _I. Phys. Chem. 78 (1974) 1899. [4] J.P. Smith and A.D. Trifunac, J. Phys. Chem. 85 (1981) 1645. [ 51S.M. Lefkowitz and A.D. Trifunac, .I. Phys. Chem. 88 (1984) 77.
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[ 6 ] M.F. Desrosiers and A.D. Trifunac, Chem. Phys. Letters 118 (1985) 441. [ 7 ] M.F. l&rosters and A.D. Tnfunac, Chem. Phys. Letters 12 1 (1985) 382. [8 ] M.F. Desrosiers
and A.D. Trifunac,
(1986) 1560. [9] D.W. Wcrst and A.D. Trifunac,
I. Phys. Chem.
J. Phys. Chem. 92
90
( 1988)
1093. [IO] D.W. Werst, M.F. Desrosiers and A.D. Trifunac, Chem. Phys. Letters 133 (1987) 201. [ 11 ] V.I. Melekhov, O.A. Anisimov, A.V. Veselov and Yu.N. Molin, Chem. Phys. Letters 127 (1986) 97. Werst, M.G. Bakker and A.D. Trifunac, to be published. [ 131 A.D. Trifunac and J.P. Smith, in: The study of fast processes and transient species by electron pulse radiolysis, eds. J.H. Baxendale and F. Busi (Reidel, Dordrecht, 1981) p. 179. [ 14 ] D.W. Werst, L.T. Percy and A.D. Trifuanc, J. Magn. Reson., submitted for publication. [ 151 F. Stolzenburg, B. Ries and H. Bfssler, Ber. Bunsenges. Physik. Chem. 91 (1987) 853. [16]P.W.F. Louwrier and W.H. Hamill, J. Phys. Chem. 72
[ 12lD.W.
(1968)
3878.
[ 17 ] P.W.F. Louwrier
and W.H.
(1970) 1418. [ 181 N.V. Klassen andG.G. 326.
Hamill,
J. Phys. Chem.
74
Teather, J. Phys. Chem. 83 (1979)
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[ 191 G.G. Teather and N.V. Klassen, J. Phys. Chem. 85 (1981) 3044. [ 201 N.V. Klassen and G.G. Teather, J. Phys. Chem. 89 (1985) 204X. [ 2 1 ] R. Mehnert, 0. Brede and W. Naumann, Ber. Bunsenges. Physik. Chem. 88 (1984) 7 I, [ 22 ] R. Mehnert, 0. Brede and G. Cserep, Radiat. Phys. Chem. 26 (1985) 353. [ 23 ] K. Toriyama, K. Nunomc and M. Iwasaki, J. Chcm. Phys. 77 (1982) 5891. [24] M. Iwasaki, K. Toriyama and K. Nunome, Faraday Discussions Chem. Sot. 78 ( 1984) 1. [ 25 ] K. Toriyama, K. Nunomc and M. Iwasaki, J. Phys. Chcm. 90 (1986) 6836. [ 26 ] K. Nunome, K. Toriyama and M. Iwasaki, Tetrahedron 42 (1986) [27
6315.
] K. Toriyama,
K. Nunome and M. Iwasaki, J. Am. Chem. Sot. 100 (1987) 496. [ 281 J.M. Warman, in: The study of fast processes and transient species by electron pulse radiolysis, eds. J.H. Baxendale and F. Busi (Reidel, Dordrecht, 1981) p. 433. r29jM.C. Sauer Jr., A.D. Trifunac, D.B. McDonald and R. Cooper, J. Phys. Chem. 88 (1984) 4096. [ 301 MC. Sauer Jr. and K.H. Schmidt, Radiat. Phys. Chem. 32 (1988) 281. [ 3 I ] A.D. Trifunac, MC. Sauer Jr. and CD. Jonah, Chem. Phys. Letters 1 13 (1985) 316. [32]M. Iwasaki, K. Toriyama, M. Fukaya, l-l. Muto and K. Nunome, J. Phys. Chem. 89 ( 1985) 5278.
51
I
CHEMICAL
PHYSICS
LETTERS
SOLID STATE FDMR STUDIES OF ION-MOLECULE IN RADIOLYSIS OF SATURATED HYDROCARBONS
2 December
1988
REACTIONS *
D.W. WERST, L.T. PERCY and A.D. TRIFUNAC Chemistry Divaion, Argonne National Laborator_s, Argonne, IL 60439, USA Received 29 August 1988: in final form 21 September
1988
Studies of radical cations produced in pulse radiolysis of frozen hydrocarbon solutions have heen carried OULusing time-resolved fluorescence-detected magnetic resonance (FDMR). These first time-resolved solid-state FDMR expcrimcnts wcrc performed with a spectrometer modified to allow sample translation and coohng 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 mformation about the dynamics of charge pair recombination in solids.
1. Introduction The description of the mechanism of alkane radiolysis has been a goal of many radiation chemistry studies. Early insights into the hydrocarbon chemistry induced by ionizing radiation were gained from product yield and scavenging studies, and optical spectroscopy of y-irradiated alkane glasses. Our understanding of the early events and the nature and identity of transient species has been greatly advanced by time-resolved techniques such as transient absorption and emission, conductivity and timeresolved fluorescence-detected magnetic resonance (FDMR), and by EPR studies of alkane radical cations in inert matrices. Magnetic resonance methods excel because they allow unequivocal identification of the species observed and provide structural information on paramagnetic transients. Of these, only optically detected EPR techniques have proven sensitive enough to detect short-lived alkane radical cations in pure hydrocarbon systems. When ionizing radiation interacts with matter, charge pairs consisting of an electron and a positive hole (i.e. solvent radical cation) are created. Energy * Work performed
under the auspices of the OiXce of Basic Energy Sciences, Division of Chemical Science, US DOE under contract number W-31-109-ENG-38.
0 009-2614/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
in excess of that needed for ionization is deposited in the charge pair as excess excitation (electronic and/or vibrational) of the radical cation and/or kinetic energy of the electron. In alkanes, owing to the low dielectric constant greater than 95% of the initial radical cations recombine with their “geminate” electron partner. The high electron mobility in many hydrocarbons drives this process to completion very quickly (picoseconds). Hence, much of the ensuing chemistry is determined by the consequences of such charge pair recombination as it yields excited states which may further react. It is known that alkyl radicals and olefins are formed by excited state fragmentation [ l-31. Excited states may also relax to the ground state with the emission of a photon. In time-resolved FDMR experiments, radical cations are observed whose geminate electron partners have been scavenged and converted into less mobile radical anions. When charge pair recombination is slowed down, it becomes possible to study reactions of the alkane radical cations themselves. In previous FDMR studies of pulse-irradiated liquid alkancs (RH, ), we have observed radical cations of solute molecules possessing a lower ionization potential than the solvent, which are formed via electron transfer to solvent radical cations [ 4-9 1. Olefin radical cations are observed in solutions of saturated hydrocarbons due to elimination of H1 from alkane B.V.
45
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[ 9, lo] _We have shown that this dissociation reaction depends on the energy content of the initial alkanc radical cation. Detection of alkane radical cations by FDMR in lzeat alkane liquids has been elusive. FDMR spectra of a small number of alkane radical cations have been detected under conditions of high dilution in an alkane solvent which has a higher ionization potential than that of the solute. The cis-decalin radical cation was detected in a lop2 M solution of cis-decalin in 3-methylpentane at 140 K by Melekhov et al. by static optically detected EPR [ 11 1. WC have published time-resolved FDMR observations of the cis-decalin and norbomane radical cations in various alkane host solvents at solute concentrations less than 0.1 M and over the temperature range 140 K to room temperature [ 9 1. At solute concentrations much greater than 0.1 M, the solute alkane cation signals disappeared. Recent efforts have shown that such observations can be extended to the radical cations of several other alkanes [ 121. However, there are conspicuous exceptions which are not observed. A notable one is the trans-decalin radical cation which is not observed at all in the liquid phase. Is there a significant decay mechanism of alkane radical cations which competes with geminate recombination? Evidence suggests (vidc infra) that it is necessary to isolate RH$ . from RH2, implying that an ion-molecule reaction between the alkane radical cation and neutral alkanc molecules may be responsible for making alkane radical cations very shortlived. The reaction that is usually considered is proton transfer: radical
cations
RH:‘tRH2+RH’tRH:.
(1)
In this paper we discuss the importance of alkane radical cation decay via proton transfer reactions and our first observations of alkane radical cations of decalins in frozen hydrocarbon solutions. We have recently developed the capability to do FDMR experiments in low-temperature solids.
2. Method and materials The time-resolved FDMR spectrum is a “snapshot” of the geminate radical ions present at a given instant in time. In the FDMR experiment, fluores46
2 December 1988
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cence is detected from irradiated hydrocarbon solutions of aromatic scintillators. The relevant ionic processes and scavenging events are depicted in scheme 1. Application of a microwave pulse at resonant magnetic field accelerates mixing between singlet and triplet pair states by inducing EPR transitions between the doublet levels of the separated radical ions. Thus, by measuring the fluorescence intensity as a function of magnetic field strength, EPR spectra of radical ions which were present during the microwave pulse and which recombined geminately to give scintillator excited singlet states are observed [ 41. The apparatus and technique for obtaining timeresolved FDMR spectra in low-temperature solids are similar to those previously described for liquid phase experiments [ 4,131. A pulsed 3 MeV electron van de Graaff serves as the ionizing source. The sample (contained in a 4 mm outer diameter cylindrical Suprasil cell) is located in the resonant cavity of a pulsed X-band EPR spectrometer which has been modified for optical detection. Fig. 1 shows the most relevant portion of the experimental set-up. The new modifications for solidstate studies are the addition of an Air Products LTR3 liquid transfer “Heli-Tran” refrigerator for temperature control and a stepper-motor-driven device for translating the solid sample through the electron beam. The latter measure was necessary to obviate heating effects and sample degradation due to irradiation by repetitive pulses. The temperature can be conveniently controlled between approximately 20 K and room temperature. A more detailed description of the low-temperature modifications will be published elsewhere [ 141. A 5 ns electron beam pulse of z lo- ‘I C or less was used at a repetition rate of 30 to 60 pulses/s. The arrival of the electron beam pulse defines the zero of Scheme I Ionic processes (A=aromatic scintillator) RH2 t electron beam + RH:. fe-
ionization
A+RH: -4A+‘+RH, A+e-+A-’
scavenging
A+‘+A-‘+‘A*+A RH,t’+A-‘+‘A*+RH2
geminate recombination -+fluorescence
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3. Results
Fig. 1.Experimental apparatus for time-resolved magnetic resonance studiesof solids: (A) quartz sample tube, (B) EPR cavity, (C) translation mount, (D) stepper motor drive, (E) helium refrigeratoridewar, (F) liquid helium inlet, (G) waveguide, (H) lens, (I ) EPR magnet.
time in the FDMR experiment, and the application of the microwave pulse is denoted by, e.g. MW (0, 100 ) ns, meaning a microwave pulse from t=O to 1~100 ns. Similarly, BC( 100, 200)ns denotes the boxcar detector viewing time to be from t= 100 to t =200 ns. An FDMR spectrum is obtained by fixing the timing of the microwave pulse and boxcar gate and measuring the fluorescence intensity as a function of the applied magnetic field. Cis- and trans-decalin (Aldrich) and squalane (Aldrich) were purified by passing through a 0.75 m column of silica gel which had previously been activated at ~250°C. 3-methyloctane (Wiley Organits), iso-octane (Burdick and Jackson) and cyclohexane (Burdick and Jackson) were used without further purification. Anthracene-d,, was used as received from Aldrich. All frozen samples were degassed by the freeze-pump-thaw method prior to radiation.
We have observed FDMR signals in a variety of frozen hydrocarbon solutions (containing millimolar concentrations of anthracene-d,,, scintillator), both in neat alkanes and in alkane mixtures, in glassy and polycrystalline solids. The intensity of the scintillator fluorescence and the FDMR signals observed is solvent dependent. At the present level of understanding, the alkanes which have been studied can be very roughly categorized as solvents which give strong (e.g. cis-decalin, squalane, 3-methyloctane) or weak FDMR signals (e.g. trans.-decalin, cyclohexane, isooctane) when frozen [ 12 1. The time dependence of the scintillator ion FDMR intensity measured in three different solvents is shown in fig. 2. The baseline points at negative times were collected many microseconds before the electron beam pulse. Time zero corresponds to MW(0, lOO)ns, BC(l00,200)ns, and the microwave pulse and boxcar delays were swept simultaneously in increments of 30 ns. The data points represent the fluorescence intensity measured with the microwave pulse on minus the fluorescence intensity measured with the microwave pulse off. The normalized decay curves shown for squalane, 3methyloctane and cis-decalin (T= 45 K, scintillator
I
I
0
I
I t
l.! PS
Fig. 2. Time dependence of the central FDMR peak intensity (T=45 K, scintillator concentration= IO-’ M) in squalane, cisdecalin and 3-methyloctane. Time zero corresponds to MW(0, lOO)ns, BC( 100,200)ns. The microwave pulse and boxcar delay were swept simultaneously in 30 ns steps.
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concentration= 1Ow3 M) are not significantly different. Data collected in a mixture of 30% (v/v) cisdecalin in squalane were indistinguishable from the curves for the pure solvents. The inset in fig. 2 shows a log/log plot of one of the decay curves (data for 3-methyloctane). The plot gives a reasonably straight line with a slope of approximately - 1. This is in contrast with liquid-phase FDMR where a t -3’2 dependence is observed (e.g. in cyclohexane) as expected if diffusion-controlled geminate ion-pair recombination is the dominant process for excited state production and radical ion decay [4]. Stolzenburg et al. have observed t-’ law behavior for the decay of delayed fluorescence in photoionization studies of polyvinylcarbazole [ 15 ] _ They attribute such behavior to inherent energetic disorder of hopping sites in amorphous systems where diffusive motion of radical ions is mediated by trapping. We may be witnessing a similar phenomenon in the hydrocarbon glasses. Here, we focus on EPR spectral data obtained in cis- and trans-decalin and in alkane mixtures containing decalins. The radical cations of cis- and transdecalin have simple, well-resolved EPR spectra which have been previously assigned (see references in ref. [ 9 ] ). Cis- and trans-decalin have both been studied in the liquid phase by time-resolved FDMR [ 9-l l ]_ The behaviors of the cis- and trans-decalin radical cations contrast sharply and provide important insights for the mechanism of alkane radical cation decay. The cis-decalin radical cation can be observed under a variety of conditions in both liquid and solid solutions. Table 1 summarizes the present results in Table 1 FDMR results in decalins
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frozen samples along with results from our previous studies in liquids. The observation of cis-decalin+. in the solid state is facile. Fig. 3a shows the time-reFDMR (MW(0, 100)ns; spectrum solved BC( 100,200)ns) obtained at 45 K in pulse-irradiated cis-decalin containing 1O-’ M anthracene-d, D, The previously assigned, five-line spectrum (51 G coupling constant) of cis-decalin+‘ is superimposed on an intense central line due to the unresolved EPR lines of the scintillator radical ions. Cis-decalin+’ could be observed in neat glassy cis-decalin at temperatures below approximately 150 K. The signal intensity at 45 K in a polycrystalline sample (obtained by annealing above 150 K) was comparable to that of glassy cis-decalin at the same temperature. The cis-decalin+ ’ spectrum was also observed in mixtures. Fig. 3c shows the FDMR spectrum (low field only) obtained at 45 K in a sample of 30% (v/v) cis-decalin in squalane. The spectrum obtained in neat squalane (fig. 3b) exhibits no resolved features attributable to the solvent radical cation, only a slightly broadened scintillator ion peak. Preliminary concentration dependence studies show that cis-decalin+’ can be observed at cis-decalin concentrations as low as 1% (in 3-methylpentane at 45 K). Previous
attempts
neat decalin b, B 0.2 M decalin/alkane solvent 1O-3-1O-’ M decalin/alkanesolvent neat decalin decalin/squalane 50: 50 neat decalin decalin/squalane 50: 50
Temperature range (K)
the trans-decalin
263-290 140-290 140-290 100-150 100-170 <75 < 100
Cis-decalin+‘“’
Trans-decalin+’
Y Y Y Y Y
Y Y
a) Y in this column indicates that the radical cation was observed under the stated conditions. b, That is, cis-decalin or trans-dccalin.
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radical cation in the liquid phase and solid phase were unsuccessful (see table I, refs. [ 9- 111). Our liquid phase studies explored many solvent systems and covered a large range of trans-decalin concentrations, but trans-decalin+. could not be observed in the time window of our observation, i.e. tens to
(see also ref. [ 9 ] )
Sample
I988
‘)
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magnitude less than that observed in neat cis-decalin. Dilution of trans-decalin with a second alkane solvent (3-methylpentane or squalane) significantly enhanced the trans-decalin+’ signal intensity. Enhancement of the cis-decalin+’ signal intensity was not noticeable upon dilution of cis-decalin with 3methylpentane or squalane. Clearly, cis-decalin” is easier to observe than trans-decalin+‘. Cis-decalin+’ can also be observed at higher temperatures than trans-decalin+’ in neat decalins and in mixtures. All of these results are summarized in table 1.
4. Discussion
c
50 G
d
50 G
I
l-iFig. 3. Time-resolved FDMR spectra obtained in frozen solutions: (a) cis-decalin, (b) squalane, (c) 30% (v/v) cis-decalin in squalane, (d) 30% (v/v) trans-decalin in squalane. The scintillator concentration (anthracene-dIo) was lo-’ M. T=45 K; MW(O,lOO)ns,BC( 100,200)ns. The spectra shown in (b)-(d) are low field only. See text for assignments.
hundreds of nanoseconds after the electron beam pulse. Fig. 3d illustrates our recent observations of transdecalin + in low-temperature solids. It shows the FDMR spectrum obtained at 45 K in a 30°16 (v/v) mixture of tram-decalin in squalane. This is the first observation of trans-decalin+’ outside a freon matrix, and the coupling constant (52 G) obtained here agrees with the value measured from the EPR spectrum observed in a CFC& matrix [ 91. The intensity of the FDMR spectrum observed in neat trans-decalin at 45 K (identical to fig. 3d) was an order of
The contrast in the behaviors of the two seemingly similar radical cations, cis-decalin+’ and trans-decalin + ., evident from earlier liquid phase studies [ 91, is further emphasized by the observations in frozen solutions. The results (summarized in table 1) are consistent with the conclusion that cis-decalin+’ and trans-decalin” decay by proton transfer, the rate of which increases with temperature and concentration, The differences between cis-decalin+’ and transdecalin+. are attributed to differing propensities for undergoing proton transfer to neutral decalin molecules or to alkane solvent molecules. Decay of alkane radical cations via ion-molecule reactions with their neutral parent molecules has previously been proposed to explain a variety of experimental observations. The stability which can in some cases be gained from isolating RH: ’from RH, was noticed in early experiments by Louwrier and Hamill, who observed the optical absorption spectra of alkane radical cations in y-irradiated mixed alabkane glasses at 77 K [ 16,171. Time-resolved sorption experiments in pulse-irradiated alkanes have shown that alkane radical cations decay faster than the solvated electrons [ 18-221, indicating that alkane radical cations disappear by some reaction other than recombination_ Thermal or photoconversion of alkane radical cations to neutral alkyl radicals is a very common (though matrix dependent) observation in EPR studies of alkane radical cations in halogenated matrices [ 23-261. While these experiments do not unequivocally distinguish between ion-molecule and unimolecular dissociation reactions, re49
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lated EPR studies using synthetic zeolites as the host matrix have shown that alkyl radical formation does not occur in the absence of a neutral alkane molecule to act as proton acceptor [ 271. This is consistent with the FDMR results on the concentration dependence of radical cation signals in mixtures (e.g. for cis-decalin+. and norbomane+’ (see table 1 and ref. [ 91). Ultrafast resonant charge transfer has occasionally been hypothesized in the past as a mechanism which may prevent the observation of resolved EPR spectra of alkane radical cations in hydrocarbons. Such a mechanism is inconsistent with our FDMR results. This picture envisions the alkane radical cation as a spin-delocalized state due to fast electron hopping from neutral alkane molecules to the radical cation. Kn the limit of fast electron hopping, the EPR spectrum would be narrowed to a single unresolved line. This picture also implies a solvent hole with an anomalously high mobility. Fast electron transfer cannot explain why the FDMR spectra of some alkane radical cations, including cis-decalin+-, are observed in alkane mixtures but not that of transdecalin+‘, since it is known from conductivity experiments that solvent mixtures will inhibit fast hole transport [ 281. Furthermore, we observe no evidence for the onset of narrowing of the cis-decalin +EPR spectrum at intermediate solute concentrations, only a diminution of the FDMR signal intensity. The existence of delocalized solvent radical cations even in neat alkanes has not been shown. Conductivity experiments which have observed fast positive charge mobility (e.g. in trans-decalin and cyclohexane) track such behavior for hundreds of nanoseconds ( < 0.5 ps) [2X-30]. The evidence discussed here suggests that alkane radical cation lifetimes in all neat alkanes and shorter than this by at least an order of magnitude. A more plausible mechanism to explain the conductivity experiments, suggested by us previously [ 3 11, involves proton transfer (i.e. RH: ). Substitution of “proton transfer” for “ion-molecule” reaction reflects a bias that has not been unambiguously proven by experiment. Our results cannot rule out the symmetric reaction, which is H abstraction by the radical cation, However, EPR studies indicate that alkyl radical formation (eq. ( 1) ) shows position selectivity and probably depends on 50
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the spin densities in the initial radical cations [ 27,321. Such observations favor a mechanism involving deprotonation of the radical cation. More experiments are needed to distinguish between these two mechanisms. In summary, we conclude the following: ( 1) Cis-decalin+’ and trans-decalin+‘, and probably all alkane radical cations, decay via proton transfer reactions which compete with geminate charge pair recombination. undergoes more facile proton (2) Trans-decalin+transfer than cis-decalin+‘. Cis-decalin+- does not undergo proton transfer to other alkane solvent molecules, while trans-decalin+undergoes proton transfer to other alkane solvent molecules as well as to trans-decalin. (3) The propensity of alkane radical cations in general to undergo proton transfer is likely to show great diversity. A growing number of examples studied by us bears out this expectation [ 121. (4) Proton transfer seems to require little or no excess energy. Our results show that even solute radical cations formed via electron transfer (as opposed to direct ionization by the electron beam pulse) undergo proton transfer. (5) Some alkane radical cations are thermally stable.
Acknowledgement We thank R.H. Lowers for his expert operation of the van de Graaff and other technical support. J. Gregar provided us with the quartz sample cells, and P. Walsh assisted with graphics.
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[ 6 ] M.F. Desrosiers and A.D. Trifunac, Chem. Phys. Letters 118 (1985) 441. [ 7 ] M.F. l&rosters and A.D. Tnfunac, Chem. Phys. Letters 12 1 (1985) 382. [8 ] M.F. Desrosiers
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1093. [IO] D.W. Werst, M.F. Desrosiers and A.D. Trifunac, Chem. Phys. Letters 133 (1987) 201. [ 11 ] V.I. Melekhov, O.A. Anisimov, A.V. Veselov and Yu.N. Molin, Chem. Phys. Letters 127 (1986) 97. Werst, M.G. Bakker and A.D. Trifunac, to be published. [ 131 A.D. Trifunac and J.P. Smith, in: The study of fast processes and transient species by electron pulse radiolysis, eds. J.H. Baxendale and F. Busi (Reidel, Dordrecht, 1981) p. 179. [ 14 ] D.W. Werst, L.T. Percy and A.D. Trifuanc, J. Magn. Reson., submitted for publication. [ 151 F. Stolzenburg, B. Ries and H. Bfssler, Ber. Bunsenges. Physik. Chem. 91 (1987) 853. [16]P.W.F. Louwrier and W.H. Hamill, J. Phys. Chem. 72
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