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A flash-photolysis electron spinresonance study of radicals derivedfrom anhydrides of carboxylic acids;spin-polarized (CIDEP) spectraunder conditions of fast electronexchangeK.A. McLauchlan a & A.J.D. Ritchie aa Physical Chemistry Laboratory , South Parks Road, Oxford,OX1 3QZ, EnglandPublished online: 23 Aug 2006.
To cite this article: K.A. McLauchlan & A.J.D. Ritchie (1985) A flash-photolysis electronspin resonance study of radicals derived from anhydrides of carboxylic acids; spin-polarized (CIDEP) spectra under conditions of fast electron exchange, Molecular Physics: AnInternational Journal at the Interface Between Chemistry and Physics, 56:6, 1357-1367
To link to this article: http://dx.doi.org/10.1080/00268978500103091
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MOLECULAR PHYSICS, 1985, VOL. 56, No. 6, 1357-1367
A flash-photolysis electron spin resonance study of radicals derived from anhydrides of carboxylic acids;
spin-polarized (CIDEP) spectra under conditions of fast electron exchange
by K. A. M c L A U C H L A N and A. J. D. R I T C H I E
Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, England
(Received 13June 1985 ; accepted 18 August 1985)
The spectra of the radicals obtained by flash photolysis of solutions of two related organic anhydrides have been observed using the time- integration spectroscopy method. It is shown that the chemistry is complex, with several radicals observable under different conditions. The anions of the parent anhydrides exhibited spin-polarized spectra which were affected by electron exchange in the intermediate and fast ranges. They have been used to test the theory of the effect of electron exchange on observed polarized spectra.
1. INTRODUCTION
Electron exchange reactions may have profound effects upon the appearance, and development in time, of electron spin resonance spectra obtained from rad- icals created in an electron spin-polarized state, as a result of the action of a chemically-induced dynamic electron polarization (CIDEP) process [1]. Such effects can be observed at low exchange rates which cause no appreciable line- broadening in the spectra of equilibrated radicals which undergo similar exchange reactions. The behaviour in this slow-exchange limit has been demonstrated with the anion derived from 2,3,5,6-tetramethyl benzoquinone [2] and analysed in detail for anions obtained by photolysis of a series of carboxylic acids [3]. For a radical created in an emissively-polarized state through the action of the triplet mechanism (TM) of C I D E P , the characteristic behaviour in this case is that the lines invert in phase with time, as a result of relaxation, in the order of their degeneracies. In this situation the Bloch equations can be amended (see below) and solved analytically so that the spectrum at any time after the creation of the radicals in a photolysis flash can be calculated by considering lines of different degeneracies separately and by adding the (non-overlapping) lines to obtain the whole. A more stringent test of the theory of the effect of electron exchange on the spectra of spin-polarized radicals comes in the intermediate and fast exchange regimes in which the lines due to hyperfine structure in a non-exchanging radical first broaden then collapse to a single line under exchange.
Experimentally this behaviour has been observed in radical anions obtained by photolysis of the anhydrides of some of the carboxylic acids which formed the subject of a previous study of slow exchange effects [3], in particular benzene-
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1358 K . A . McLauchlan and A. J. D. Ritchie
1,2,4,5-tetracarboxylic acid dianhydride (pyromellitic dianhydride, P M D A ) and benzene- l ,2-dicarboxyl ic acid anhydride (phthalic anhydride, PAN). As in that study the application of t ime-resolved electron spin resonance spectroscopy has provided further understanding of the chemistry involved in the systems experi- mented upon.
2. THEORY
A degenerate electron exchange reaction is considered of the type
kET .41 +`4~- - " `4i- + A2, (1)
kET
where `41 and .42 are identical molecules and keT is the rate constant for electron exchange. It is assumed that the initial radical is created in an electron spin polarized state according to the triplet mechanism [1], so that the polarization ratio of each hyperfine component is the same. It is fur ther assumed that on the timescale of the observations of the polarized species, that is within its electron spin-lattice relaxation time, radical termination reactions may be neglected. This means that the radical concentration can be taken to be constant.
The Bloch equations, amended for electron exchange [4], can be written in matrix form [5] in a frame of reference rotating at the measurement frequency co as
1 1 - - M k r ( t ) = LkrMkr(t) + -~Z ~js" DMis(t) -- -z Mkr(t) + Feq, kr(t). (2)
Here Mkr(t ) is the magnetization associated with the rth component contr ibuting to the hyperfine line, k, of degeneracy DR, and similarly for Mjs(t); each has three orthogonal components . The matrix L kr is that for the Bloch equations in the absence of exchange
L kr
-- T f I Acok~ 0 )
--ACO k~ - - T ~ I col �9
0 -col - T [ 1
(3)
where it is assumed, as is usual with carbon-centred radicals, that the relaxation times are hyperf ine- independent ; col is the microwave field strength and Aco k" is the off-set of the resonance position of the line f rom the applied frequency.
D( = E i D j) is the total degeneracy and z is the mean lifetime of the species .4 - between electron jumps. I t is given by
"C = ( k E T [ . 4 ] ) - 1 (4)
The two terms in equation (2) which include z are those which account for the transfer of magnetization between states in the electron exchange process; their form has been discussed by Hore and McLauchlan [4].
T h e final term in the equation is to account for the contribution to the magne- tization f rom equilibrated radicals
0 '
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Spin-polarized (CIDEP) spectra 1359
kr Meq(t ) is a cons tan t if, as a s sumed , the radical concen t r a t i on does not va ry in t ime.
I t r ema ins to solve the fami ly of equa t ions (2) sub jec t to the cond i t i on tha t the radicals are c rea ted in an e l ec t ron s p i n - p o l a r i z e d s tate wi th in the m a g n e t i c field of the spec t rome te r , b y m e a n s of a pho to ly s i s flash wh ich can be c ons ide r e d ins tan- t aneous on the t imesca le of the evo lu t ion of the magne t i za t ion . T h i s creates a c o m p o n e n t M~,(O) of the magne t i z a t i on at zero t ime, in the d i rec t ion of the app l i ed field, which is g rea te r n u m e r i c a l l y than its value at t he rma l e q u i l i b r i u m ; the o r thogona l c o m p o n e n t s at th is t ime are zero.
In the s low exchange s i tua t ion var ious s imp l i fy ing a s s u m p t i o n s can be m a d e which al low the equa t ions to be so lved ana ly t ica l ly [4] , in pa r t i cu l a r tha t no t r ansve r se magne t i z a t i on is t r ans fe r r ed be tween radica ls wi th d i f fer ing nuc lea r spin conf igura t ions . T h i s is no longer the case in the fast exchange l imi t and the equa t ions can only be so lved numer i ca l ly . Such an a p p r o a c h is of course equa l ly va l id u n d e r all exchange cond i t ions b u t the n u m e r i c a l m e t h o d is c o m p u t a t i o n a l l y t i m e - d e m a n d i n g . T w o di f ferent n u m e r i c a l t e chn iques were inves t iga ted , a R u n g e - K u t t a - N I e r s e n one for d i rec t ly so lv ing l inked d i f ferent ia l equa t ions , and a d i rec t m a t r i x - s o l v i n g one. Both were i m p l e m e n t e d a r o u n d s t a n d a r d rou t ines p ro - v ided by the N u m e r i c a l A l g o r i t h m G r o u p ( N A G ) and run on a N o r s k Da ta N D - 5 2 0 c o m p u t e r . T h e two m e t h o d s gave ident ica l resul ts b u t whi l s t the ca lcu- la t ions were inva r i ab ly t i m e - c o n s u m i n g the d i rec t ma t r i x m e t h o d us ing N A G rou t ines was cons i s t en t ly the faster .
T h e t ime in tegra t ion s p e c t r o s c o p y ( T I S ) t e chn ique used in the e x p e r i m e n t s involves n u m e r i c a l s u m m a t i o n of a m e a s u r e d value of Mk(t) over a chosen t ime in terval . T h e theore t ica l s p e c t r u m is ob t a ined in a s imi la r fashion f rom the solu- t ion of the equa t ions ; for a s p e c t r u m of m lines, ca lcu la ted over the n s amp le in terva ls w i th in the s u m m a t i o n per iod , (n x m) values of Mk(t ) are ca lcula ted . T h e s e are a d d e d toge the r at each field pos i t ion to cons t ruc t the whole s p e c t r u m .
A n e x a m p l e is shown in f igure 1 of the appe a ra nc e of the s p e c t r u m of a rad ica l wh ich in the absence of exchange w o u l d y ie ld a 1 : 2 : 1 m u l t i p l e t wi th a h y p e r -
Figure 1. Theoretical T I S spectra calculated for an emissively-polarized hypothetical radical, with initial polarization of magnitude fifteen times that of the equilibrium polarization, as the radical undergoes electron exchange at various rates correspond- ing to exchange lifetimes of (a) infinity, (b) 10ps, (c) 1 Us, (d) 0"1 Us, (e) 0"02#s, ( f ) 0'01 Us, (g) 0'001 Us and (h) 0.0001/~s. Typical relaxation times of T 1 = 1,us and T 2 = 0'8ps, and a microwave field strength of 0-01 rad MHz, were used in the calculations which correspond to~ sampling between 5 and 7 Us after radical creation.
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1360 K . A . McLauchlan and A. J. D. Ritchie
fine coupling of 2"5gauss. The figure displays spectra over a wide range of exchange rates and they exhibit the normal characteristics associated with exchange in magnetic resonance. As the rate of exchange increases, the lines first broaden and then move together to form a single broad line which sharpens as the rate of exchange is increased further. The significance of this is that the exchange rate may be extracted from the spectra of radicals of very short life-time. Such radicals are almost invariably spin-polarized, and this fact facilitates their detec- tion whilst still allowing their analysis. Exchange processes can be studied conse- quently in short-lived species, and this extends the possible range of such studies greatly.
The spectra in figure 1 all correspond to one specific time after radical cre- ation, with the conditions chosen so that they appear normal and can be compared directly. As was shown before [-3] an important aspect of studying emissively- polarized radicals in the slow exchange condition is to observe the time- dependence of their spectra, in which selective phase inversions occur. This remains so as the exchange rate increases into the intermediate region so long as sufficient resolution remains between the broadened lines to distinguish the indi- vidual components. As with slow exchange, the microwave field strength can be adjusted to affect the effective spin-lattice relaxation time so as to bring this behaviour within the lifetime of the radical as a polarized species. In the fast exchange limit this ability is lost and no extra information is obtained by studying the t ime-dependence of the entire spectrum. It is however instructive to examine the underlying behaviour of the magnetization, sampled exactly on resonance, with time as the exchange rate is varied. A calculation made using the same parameters as in figure 1 is shown in figure 2.
Under these conditions the magnetization exhibits clear Torrey oscillations in the absence of exchange, but these become successively damped out as the
Figure 2. The variation in the magnetization of on-resonance signals from the spectra shown in figure 1 with time, the timescale being 30#s. The oscillations, which are damped out in the TIS spectra by integration, are observed most easily in the slow and fast exchange situations where the lines are sharp. In the intermediate range the effective T 1 becomes long, with polarization persisting for longer times, and the effective T 2 short.
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Spin-polarized ( C IDEP) spectra 1361
exchange rate increases and the lines in figure 1 broaden. In the intermediate exchange situation they may disappear, and then the polarization persists longer in the spectrum. As the exchange rate is increased further the oscillations re- appear. Although the curves are now similar to those in the slow exchange limit they, like the full spectrum, can only be analysed by a full calculation. Now however the calculation is much shorter than for a spectral simulation since only one decay curve has to be computed rather than the previous (n x m). In principle therefore the observation and analysis of a single decay curve is the most efficient method for extracting a rate constant. In practice exchange effects become appar- ent in any regime most immediately through their qualitative effects on complete spectra, often unexpectedly. Th is makes the type of analysis provided in this paper for the appearance of lines in the fast-exchange region essential. Some radical anions can be observed, for example, only as secondary species in the presence of precursors which cause them only to be observable under fast exchange conditions. An understanding of their spectra is then needed just to identify them.
The physical origin of the behaviour shown in figure 2 has been discussed previously in the slow-exchange situation [-2-4]. Here only one transition is in resonance at one t ime and the upper state, over populated by an emissive spin- polarization process, loses population at an effective relaxation rate determined by the natural spin-lattice relaxation t ime and the rate at which transitions are driven by the applied microwave field. In the exchange process, the electron hops f rom a radical in a specific nuclear hyperfine state to another molecule where it enters a hyperfine state at random, and radicals in these other states constitute a large reservoir f rom which the specific state may be re-filled. In consequence, as the effective relaxation process occurs the upper state gains population at a rate which depends upon the exchange process. As discussed previously, the net rate depends upon the degeneracy of the specific level and also the overall degeneracy of the spin system. I t is the inherent competi t ion between this exchange process and the microwave-pumping which makes the overall behaviour dependent upon the microwave field-strength. At a given field-strength, an increase in the exchange rate makes level depopulat ion less efficient and the effective relaxation time rises, as shown in figures 2 (d) and (e). I t is not possible to carry this simple description to the fast exchange limit since in this all transitions involving all hyperfine states are simultaneously in resonance. However instinct would suggest that at fast exchange the effect of exchange would tend to be nullified, as all the levels become connected efficiently, and that the overall spin system would relax at an effective rate mainly depending upon the natural relaxation process and the microwave pumping. This appears to happen, as shown by the comparison of figures 2 (g) and (h) with figures 2 (b) and (a).
3. EXPERIMENTAL
Spectra were obtained using the t ime-integrat ion spectroscopy (T IS ) method described previously [6, 7], using a broad-banded Bruker ER200D spectrometer and a Biomation 8100 transient recorder together with a dedicated micro- processor. Radicals were created by flash photolysis using the 308nm radiation f rom a XeC1 excimer in a Lambda-Phys ik E M G 100 laser, with a pulse width of about 20 ns and a repetition rate of 20 Hz. T h e spectra were obtained without use
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1362 K . A . McLauchlan and A. J. D. Ritchie
of modulat ion techniques and are not in derivative mode: any change in phase of a signal is genuine and results f rom electron spin polarization and relaxation p r o c e s s e s .
Solutions were made up f rom the purest substances available commercial ly and were de-oxygenated by passage of nitrogen for long periods before investiga- tion. They flowed through the irradiation region to avoid depletion of the photo- active molecules in the sample region.
4. RESULTS
Photolysis of low concentrations (<0"1 M) of the anhydrides P M D A and PAN gave spectra of the radical anions f rom the parent acids, pyromelli t ic acid (PMA) and phthalic acid (PA) respectively. These were the species P M A "'a- and PA ' 2 - whose spectra have been reported previously [3, 8]. At higher concentra- tions however the anions of the anhydrides themselves, P M D A " - and P A N " - were observed. T h e observation of the radical anions of the hydrolysed products at low concentration might have resulted in two possible ways. Firstly, the forma- tion of a pr imary polarized anhydride anion might involve, or be followed by, fast destruction of the anhydride ring. Secondly the anhydrides used may have been contaminated with acid, and the anhydride anions may be secondary species formed by reaction of the pr imary acid anions, for example by the reaction
P M A "'3- + PMDA,-~-PMA 2- + P M D A " - . (6)
All commercial sources of P M D A contain some of the parent acid. T h e nitrogen heterocyclic analogues of the anhydrides, phthal imide [9] and
pyromelli t ic di-imide have been shown in experiments performed in this labor- atory to yield pr imary anions without r ing-breaking, and it is difficult to see why the anhydrides should not behave similarly: nevertheless it is believed that the radicals observed in this study were secondary species. Th is follows f rom obser- vations made by adding an anhydride to a solution of its parent acid, whose photolysis on its own yielded the acid radical anion. As the concentration of anhydride was increased the spect rum changed to that of the anhydride anion; this is shown for the PA/PAN system in figure 3. Th is seems to demonstrate the occurrence of a secondary reaction, analogous to (6), so that even if pr imary ring scission should occur on the photolysis of the anhydride, the secondary reaction
Figure 3. (a) The TIS spectrum of the anion from phthalic acid, PA"-, observed in emission after its production by photolysis of a 0'1| M solution of PA in a methyl cyanide/triethylamine 3 : 1 mixture. (b) The spectra of the anhydride anion spec- trum, PAN"-, obtained by adding 0-28 M PAN to the acid solution, showing it to be produced as a secondary radical.
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Spin-polarized ( CIDEP) spectra 1363
Figure 4. (a) The spectrum of the pyromellitic dianhydride anion, PMDA"- obtained by photolysis of a fast-flowing 0-55 M solution of the parent molecule in a 3 : 1 methyl cyanide/triethylamine mixture, with sampling between 1-8 and 6"3/~s after the flash. (b) The spectrum obtained under similar conditions but with the PMDA concentra- tion reduced to 0 ' I M is that of the partially-hydrolysed anhydride anion, PMMA "'2-. (c) The appearance of the latter spectrum, on a reduced field scale, using a high microwave field strength, with sampling between 10 and 11 Us after the flash. It shows that the most intense central region has relaxed into absorption whilst the outer regions are still in emission, a characteristic of electron exchange behaviour outside of the fast exchange limit.
ensures that the anhydr ide ion is the species observed. Th is conclusion was con- firmed by investigating the variat ion in the nature of the radical observed with the rate of flow of sample through the irradiat ion region, as descr ibed below. All the anions yielded spectra in emission, with no hyperfine distort ion.
Photolysis of solutions of pyromel l i t ic d ianhydr ide of concentrat ion greater than 0 -2M in 1 : 3 solvent/amine mixtures, and using fast rates of flow ( > 2 ml min -1 ) through the flat E.S.R. cell used for photolysis, yielded the spec- t rum of the P M A " - ion, with a hyperfine coupling of 0"69 G [-10] ; a typical one is shown in figure 4 (a). The irradiat ion region of the cell had a volume of approx- imately 0" 1 ml.
0 0 II II / c ~ c \
~ ~-~ c/o II It 0 0
PMOA .r"
On lowering the concentrat ion whilst mainta ining the fast flow condit ions a different spect rum was obtained, shown in figure 4(b), which consisted of a t r iplet of doubles (g = 2"004, hyperfine couplings 0"6 G and 0-21 G). In this solu- tion decreasing the flow rate produced a th i rd spectrum, recognized as that of the radical t r ianion of the parent acid, P M A "'3-; its concentrat ion increased as the flow rate was decreased further, at the expense of the first radical. This behaviour is shown in the series of spectra, figures 5 (a)-(c). F r o m its hyperfine structure, and from a considerat ion of the chemist ry involved, the first spect rum is assigned to the par t ia l ly-hydrolysed mono-anhydr ide di-anion,
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1364 K . A . McLauchlan and A. J. D. Ritchie
0 O / 1 ~ 0
o - I ( ~1 0 \ l ~ -~ 0 0
,2- PMMA ~
An unidentified radical with the same coupling was reported by Sioda and Koski [11], whilst the four-negatively charged anion from pyromellitic acid is the most closely related known radical, and it also shows similar hyperfine couplings [8]. The relaxation time, T 1, of P M M A "'2- was measured in our experiments to be 12"4#s in the solution used, a lower value to that of the trianion PMA "'3- (18-6 #s) which is consistent with its lower charge and hence lower solvation.
This difference in relaxation times was exploited to confirm the analysis of the spectra from solutions which contained both radicals. Using the method of Basu and McLauchlan [12] the spectra were sampled about the time after radical creation where the intensity of one of the species, PMA "'3- relaxed through zero, as shown in figure 6. In figure 6 (a) its spectrum is still in emission, whilst that of P M M A ' 2 - has already relaxed into absorption; in figure 6 (b) its spectrum has disappeared, to leave that of P M M A "'2- alone, whilst in figure 6 (c) both radicals are observed in absorption. It is clear that only two species are present and also which lines originated in each. One caveat with this type of experiment is that the time at which a line relaxes through zero depends upon the size of its initial polarized signal at radical creation, besides upon its effective relaxation time; spectra may invert in the opposite order to the relaxation times of the radicals if the polarizations differ. This may not be uncommon where primary and second- ary radicals are observed simultaneously.
Under the fast-flow conditions of figure 4 it was possible to experiment on the P M M A ' 2 - species alone. Inset in this figure is a trace obtained (on a reduced field scale) using a high microwave field strength, and with the spectrum sampled after some relaxation has occurred. It shows that the central, most intense, feature inverts in phase before the outer components. This is characteristic of an electron exchange process, the reaction probably being with the preponderant ground-
tl
1G 4
C
Figure 5. Spectra recorded during the photolysis of a 0'08M solution of PMDA in a methyl cyanide/triethylamine 3 : 1 mixture as the flow rate through the flat irradia- tion cell is varied between (a) 2-22 ml min- t, (b) 1 "2 ml min- 1 and (c) 0-4 ml min- t The signals were integrated between 1-4 and 5-9 ps after radical formation. As the flow rate is changed, the most prevalent radical changes from PMMA '2- in (a) to PMA "'3- in (c).
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Spin-polarized ( CIDEP) spectra 1365
1G i
Figure 6. Spectra recorded on photolysis of a 0"008 M solution of PMDA in a methyl cyanide/triethylamine 3 : 1 mixture with an integration period of 1.2#s centred at (a) 31-0, (b) 32'4 and (c) 33"1 #s after the flash. The behaviour is described in the text. The microwave field strength was 0"5 rad MHz.
state P M D A molecules present . T h e effect could no t be invest igated fu r the r due to technical difficult ies involved in the use of variable t empera tu re e q u i p m e n t at high sample flow rates.
Similar behav iour is shown by the P M D A " - species itself, as shown in the
spectra ob ta ined at high microwave field s t rength in figure 7. T h e radical is observed in an in te rmedia te exchange s i tuat ion in which the l ines are b roadened , bu t the spec t rum is not collapsed into a single line. T h e d iagram shows how the
emissive spec t rum varies in t ime after radical creat ion, and displays how the
EXPERIMENTAL
Time I~s
I Emission
1"5/
CALCULATED
Figure 7. Observed and calculated spectra of the PMDA"- anion as it varies in time after the flash, with some contamination from the second radical species present. A sampling period of 1 #s was centred successively at 1"5, 4.5, 7"5 and 8"5 #s after the flash. Initially all the lines are in emission but, in this intermediate exchange situ- ation, the central component relaxes to absorption first. The microwave field strength was 0-5 rad MHz.
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1366 K . A . McLauchlan and A. J. D. Ritchie
strongest line changes phase first. Also shown in the figure are the theoretical spectra calculated using the full theory given above, using a T 1 value measured to be 9"7/~s and an exchange lifetime of 0"165 #s. The agreement with experiment is satisfactory although the presence of a small amount of a second radical in the sample makes it less than perfect. Nevertheless this system provides a more testing trial of the theory, with several components in the spectrum, than does a radical observed in the fast exchange condition. This latter situation occurs in the P M D A system if the parent molecule concentration is raised further.
It is however more clearly observed, with less interference from underlying spectra from other radicals, in the phthalic anhydride case. The spectrum of the radical anion P A N " - has been shown in figure 3, with the coupling constant measured as 2"7G; the broad lines in this intermediate exchange spectrum obviate observation of the small couplings [11].
0
0
On increasing the concentration of the anhydride the spectrum collapses to a single line, as shown in figure 8. As the concentration is increased from an initial value of 0-07 M, at which the line is a broad singlet, the line sharpens up as the electron exchange rate increases, as was predicted in figure 1. Attenuation of the
a b
d e
_J _J
C
f
Figure 8. TIS spectra in the fast exchange situation observed from the PAN"- anion in the usual solvent mixture, with integration between 2 and 5~us after the flash. The PAN concentrations were (a) 0"07 M, (b) 0"18 M, (c) 0"26 M, (at) 0-35 M, (e) 0'53 M and ( f ) 0.73 M. The microwave field strength was 0.17radMHz. The calculated spectra are presented below the observed ones, with the exchange lifetime the only variable altered between successive traces.
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Spin-polarized (CIDEP) spectra 1367
intensity of the light creating the radicals had no effect on the appearance of the lines, and confirmed the origin of the phenomenon as the chemical exchange process, rather than a Heisenberg one. Consideration of the concentrations of molecules present suggests that the reaction concerned is the degenerate process
PAN + P A N " - ~ P A N " - + PAN. (7)
Using an experimentally-determined value of T 1 (11"3/Is), each of the curves in figure 8 was simulated using the electron exchange lifetime, z, as the only variable between the various concentrations. Its value was found to vary inversely with concentration, as expected from equation (4), and a value of kET = (4 + 0"4) X 108 M - ~ s- ~ was derived for the electron transfer rate constant.
5. CONCLUSION
The application of time-resolved techniques to the study of chemical systems has once more disclosed a more complex chemistry than might have been expected from studies of systems at equilibrium, and has revealed the presence of several transient species. This chemistry had to be understood before it became possible to investigate the effects of electron exchange on the polarized spectra of the radicals. As in our previous study of radicals derived from carboxylic acids, many of them display these effects. In this paper we have shown that the theory presented can account satisfactorily for the appearance of spectra throughout the intermediate and fast exchange regimes. The slow exchange situation has been shown previously to be analysable in a way which yields equations which are much less demanding computationally than those used here. Throughout the entire range of exchange rates the observed spectra can be analysed to yield values for the rate constants. In the slow and intermediate exchange cases the behaviour most sensitive to the effects is the variation in the appearance of the spectrum with time. In the fast exchange condition it is the line shapes and how they vary with concentration which are the corresponding quantities.
We are grateful to the SERC for the support of the project and a maintenance award for A.J.D.R. It is a pleasure to thank Dr. P. J. Hore for his frequent advice.
REFERENCES [1] See for example, BUCKLEY, C. D., and McLAUCHLAN, K. A., 1985, Molec. Phys., 54,
1. [2] BASU, S., McLAUCHLAN, K. A., and RITCHIE, A. J. D., 1984, Chem. Phys., Lett., 105,
447. [3] McLAUCHLAN, K. A., and RITCHm, A. J. D., 1985, Molec. Phys., 56, 141. [4] HORE, P. J., and McLAUCHLAN, K. A., 1981, Molec. Phys., 42, 533. [5] PEDERSEN, J. B., 1973,ff. chem. Phys., 59, 2526. [6] BASU, S., McLAUCHLAN, K. A., and SEALY, G. R., 1984, ff. Phys. E, 16, 767. [7] BASU, S., McLAUCHLAN, K. A., and SEALY, G. R., 1984, Molec. Phys., 52, 1. [8] NETA, P., and FESSENDEN, R. W., 1973,if. phys. Chem., 77, 620. [9] BUCKLEY, C. D., GRANT, A. I., McLAUCHLAN, K. A., and RITCHIE, A. J. D., 1984,
Faraday Discuss. chem. Soc., 78, 257. [10] NELSEN, S. F., 1967, ff..4m, chem. Soc., 89, 5256. [11.] SIODA, R. E., and KOSKI, W. S., 1967, ff..4m, chem. Soc., 89, 475. [12"] BASU, S., and McLAUCHLAN, K. A., 1983,ff. magn. Reson., 51,335.
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