10530
J . Phys. Chem. 1993, 97, 10530-10538
An Electron Spin Polarization (CIDEP) Investigation of the Interaction of Reactive Free Radicals with Polynitroxyl Stable Free Radicals Nicholas J. Turro' and Igor V. Khudyakov Department of Chemistry, Columbia University, New York, New York 10027 David W. Dwyer Department of Chemistry, SUNY Brockport, Brockport, New York 14420 Received: May 4, 1993; In Final Form: July 26, 1993"
Time-resolved electron spin resonance (TR ESR) was employed to investigate the polarized ESR (CIDEP) spectra produced by interaction of mono- and polynitroxyls with reactive free radicals (r#) possessing net (single phase) electron polarization. Emissively polarized reactive radicals were produced by photolysis of benzil dimethyl monoketal and absorptively polarized reactive radicals were produced by photolysis of diphenyl(2,4,6-trimethylbenzoyl)phosphineoxide. The stable radicals were mono-, di-, tri-, and tetranitroxyl free radicals moiety as a radical unit. The C I D E P spectra obtained processing the 2,2,6,6-tetramethylpiperidine-N-oxyl under interaction with r# and polynitroxyls have the same net emissive (or absorptive) phase as r#. The observed CIDEP spectra of dinitroxyls that were obtained in strong exchange (spectroscopic triplets) are interpreted as the superposition of the C I D E P spectrum of the triplet dinitroxyl created by nonreactive polarization transfer from r# and the CIDEP spectrum of the adduct of r# and one of the nitroxyl moieties. These results provide a novel demonstration of net electron spin polarization transfer (ESPT) in a fast addition reaction of a polarized doublet free radical with diradicals. The observed C I D E P spectra involving r# and tri- and tetranitroxyl can also be interpreted in terms of a competition between reactive and nonreactive ESPT but require the postulate that nonreactive ESPT between r# and polyradicals of high multiplicity is relatively slow compared to ESPT between r # and doublet radicals.
Introduction The investigation of electron spin interactions between paramagnetic particles can provide information concerning the elementary processes of collisions between molecules in fluid solution.l.2 The strength of such interactions has been commonly measured from the dependence of the line broadening of continuous wave electron spin resonance (CW ESR) spectra as a function of the concentration of the paramagnetic species. Electron spin-spin interactions during collisions provides information relevant to other important chemical processes between molecules such as electron exchange, energy transfer, electron transfer and frontier orbital interactions that initiate bond formation and bond breaking.lq2 Time-resolved electron spin resonance (TR ESR) has recently provided a technique which probes nonreactive electron spin interactions between paramagnetic species by monitoring the generation of electron polarization and its transfer,2J electron spin polarization transfer (ESPT). For simplicity, in this report the various reactive and nonreactive phenomena producing electron spin polarization are grouped together under the term chemically induced dynamic electron polarization (CIDEP). To date there have been several mechanisms which are invoked to explain observed CIDEP spectra. The three most common mechanism^^^^^^^ are the radical pair mechanism (RPM), the triplet mechanism (TM), and radical-triplet pair mechanism (RTPM). The RPM generates CIDEP spectra which appear partially in emission and partially in absorption (E/A or A/E, termed multiplet CIDEP spectra), whereas T M generates CIDEP spectra which appear as pure emission of pure absorption (E or A, termed net CIDEP spectra). RTPM generates E or E + E/A spectra. Experimentally, it is not unusual for a system to display more than one mechanism for the generation of electron polarization. However, in many cases only one of the mechanisms dominates the CIDEP spectrum. e
Abstract published in Aduance ACS Abstracts, September 15, 1993.
The interaction of a polarized, reactive free radical with a paramagnetic species provides an opportunity to employ T R ESR to observe CIDEP and to investigate both reactive and nonreactive electron spin interactions betweenparamagnetic species. Electron polarization that has been generated by an mechanism can be transferred, in principle, to doublet radical^.^^^^^ In a number of cases polarization has been preserved in the reaction of a polarized reactive radical with a diamagnetic substrate to produce a polarized doublet product.' Most investigations reported in the literature involve the study of interactions between (doublet) free radicals. Only in few cases has the competition between spin exchange and chemical reaction between reactive free radicals been reported.IaX8 In a quantitative studyla involving small free radicals, it was demonstrated that spin exchange generally occurs faster than recombination of small radicals and that the effective radius of spin exchange, Re,, is several times larger than reaction radius, Rrxn.IaIt is generally expected that spin exchange between radicals will be accompanied by ESPT.2q3 Theoretical considerationslbVcsupport the conclusion that the spin exchange will generally be faster than the chemical reactions of free radicals. Experiments on ESPT between reactive and stable free radicals have provided indirect evidence for the conclusion that Re, is larger than Rrxn.2 We have reported2s6 the use of T R ESR to investigate the nonreactivecollisions of triplet states of ketones with mononitroxyl and polynitroxyls with the goal of understanding the factors that determine interactions between paramagnetic species of varying multiplicities. Polynitroxyls which exist in a state of strong electron exchange (defined as polynitroxyls which display C W ESR spectra which show spin-spin splittings due to exchange interactions) are particularly convenient objects for study of ESPT since they are stable species of high (triplet, quartet, quintet, etc.) spin multiplicity. The CIDEP spectra6 obtained by the interaction of triplet ketones in the presence of polynitorxyls possess the same structural form (number of lines and relative
QQ22-3654/93/2097lQ530$04.QQ/O 0 1993 American Chemical Society
Interaction of Reactive and Stable Free Radicals intensities) as is observed in the C W ESR of the polynitroxyls. In these examples the interaction responsible for generation of nitroxyl polarization was concluded to be the radical-triplet pair mechanism (RTPM). We have also employed photochemical excitation to selectively generate electron polarized reactive free radicals in prescence of stable polarization radicals (nitroxyls) that serve as polarization acceptors.2 This method can be extended to the investigation of the interactions of the polarized reactive doublet free radicals with paramagnetic species of higher multiplicity (polynitroxyls in strong electron exchange). This report is concerned with the investigation of three aspects of the interactions of reactive free radicals and high multiplicity nitroxyls. First, there is the issue of reactive versus nonreactive interactions. Is it possible that the attack of a reactive polarized free radical on a polynitroxyl can lead to both a bond formation and transfer ofpolarization to the remainingspins? Second, there is the issue of how the addition of a reactive free radical to a polynitroxyl competes with nonreactive ESPT to the polynitroxyl. Will the rate of reaction of a reactive free radical with a polynitroxyl simply differ from the rate of a mononitroxyl mainly by a statistical factor which takes the number of spins into account? Finally, it is possible that the multiplicity of the polynitroxyl will influence the rate of ESPT? If so, the competition between reaction and ESPT may be a function of acceptor multiplicity even though the rate of reaction is not a significant function of acceptor multiplicity. In order to address these issues experimentally, we have employed the technique of T R ESR to obtain CIDEP spectra of systems involving polarized reactive (doublet) free radicals and to investigate the reactive and nonreactive collisions of reactive radicals with stable paramagnetic species, mononitroxyl and polynitroxyls. The investigations employ the photolysis (a-cleavage) of carbonyl precursors to produce a reactive, polarized pair of doublet free radicals. The polarized radicals thus generated from the separated pair then interact with nitroxyls of varying multiplicity (doublet, triplet, quartet or quintet). In the case of mononitroxyls, two of the possible paths resulting from these interactions are directly accessible to experimental examination: (1) nonreactive interactions which result in electron spin polarization transfer, ESPT, from the initially polarized radicals, can be investigated by TR ESR; (2) reactive interactions of the reactive radical and the nitroxyl to form diamagnetic products can be investigated by time-resolved laser spectroscopy. In the case of the polynitroxyls of variable multiplicity a novel possibility for reactive ESPTexists in addition to the options of nonreactive ESPT. Reactive ESPT between a polarized reactive radical and a polynitroxyl produces a product which is still paramagnetic and can therefore be polarized. Such a process is directly observable, in principle, by T R ESR. In this report we provide support for (1) the occurrence of reactive ESPT, (2) a multiplicity dependence on thecompetition between the rate of reaction with polynitroxyls and the rate of ESPT to polynitroxyls, and (3) a multiplicity dependence of ESPT which changes markedly the proportion of reactive and nonreactive ESPT involving nitroxyls of higher multiplicity.
Experimental Section 1. Devices and Procedures. The details of the T R ESR measurements have been described elsewhere6~~f.9.The third harmonic of a Nd:YAG laser (A 355 nm) was employed for excitation of the sample. The laser energy employed was typically ca. 30 mJ/pulse at 20 Hz. In the CIDEP spectra presented here, control experiments demonstrate that there is no significant Boltzmann contribution to the reported spectra. The time delays for the reported spectra specify the interval, At = B - A , s, where A is the initial time of sampling after laser pulse, B is the time of the end of the sampling, and the B - A value is the sampling gate. Sampling gates of At = 50-300 ns were used in the T R
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
10531
ESR measurements of ESPT kinetics. Determination of polarization decay kinetics was performed in two ways. First, the intensity of CIDEPsignals taken at different times A were plotted against A . Second, a boxcar integrator was employed to monitor the decay of polarization in real time after the laser flash, and kinetic data were obtained. Both methods gave similar estimates of characteristic time for the polarization decay. Special control experiments on CIDEP spectra of dinitroxyls in strong exchange in the presence of IRG and DPO (see for abbreviations the next section) were performed with small laser frequency (1 Hz), short sweep time (200 s vs 500 s in standard experiments), and high flow rate (1 1 mL/min) in a cylindrical cell in order to prevent any accumulation of reaction product which could participate in secondary processes to generate ESPT. These experiments resulted in CIDEP spectra that were essentially identical to those observed under typical conditions with a flat cell, with which all the reported results were performed. C W ESR spectra were measured with a Bruker ESR 300 spectrometer which has been described previously.6 The laser flash photolysis system employing optical detection has also beendescribedelsewhere.lOAsin the TR ESRexperiments, the solutions in the laser flash photolysis experiments were photoexcited with the third harmonic of a Nd:YAG laser, with an energy of ca. 10-12 mJ/pulse. All experiments were performed at ambient temperature and all solutions were deoxygenated by prolonged bubbling with argon. 2. Reagents. All reagents were used as received: benzil dimethyl monoketal (Irgacure 65 1, IRG), from Ciba-Geigy; diphenyl(2,4,6-trimethylbenzoyl)phosphineoxide (DPO), from BASF; solvents, from Fisher. In the experiments, typical concentrations of DPO and IRG were 0.02-0.5 M, and for stable (po1y)radicals 10-3-10-2 M. In the laser flash photolysis experiments the concentration of IRG was 8 X 10-3 M. The abbreviations of the mono-, and polynitroxyl free radicals employed are prsented in Figure 1. The polynitroxyls were synthesized according to ref 11 and the synthesis of 2RXNH is described in ref 6.
-
Results
1. Generation of Spin Polarized Reactive Radicals. Terminology. By the appropriate selection of precursor, it is possible to selectively generate electron spin polarized reactive radicals possessing net emission (E) or net absorption (A).2as4 In each case, the initial spectrum will, of course, also possess a certain degree of contamination derived from other polarization mechanisms, and as time progresses different mechanisms which create and destroy polarization will operate. Throughout this report, the sign # represents electron spin polarization of a transient, non-Boltzmann population of electronic spins levels in magnetic to describe the field. We will use designations of *r# and polarization pattern of absorptively and emissively polarized radicals, r#, generated by photolysis two carbonyl compounds, IRG (eq 1) and DPO (eq 2). A clean source of net emissively polarized reactive radicals, Er#, is produced by the photolysis of IRG (eq l ) , which produces a triplet radical pair, consisting of a benzoyl radical and a dimethoxybenzyl radical,2,9J2in high yield. The triplet mechanism is believed to be responsible for the generation of this net emissive polarization. The CIDEP spectrum of the dimethoxybenzyl radical consists of a large number of sharp lines spread over 25 G ( g = 2.003) and the CIDEP spectrum of the benzoyl radical is a relatively broad singlet (a triplet at high resolution) with a width of ca. 7 G (g = 2.000).299J2 Since hte polarized benzoyl radical has a relatively rapid relaxation (TI < 1 ps2~9J2) compared to the benzyl radical, it is expected that the longer lived polarized benzyl radical will generally be more effective in ESPT processes provided that the interactions leading to ESPT are comparable.
Turro et al.
10532 The Journal of Physical Chemistry, Vol, 97, No. 41, 1993
I
TEMPO
2
~
~
VI1
2
~
IV,when n = 4
'(RXRJ
Figure 1. Structures, spin states, and abbreviations of the mono- and (po1y)nitroxylsemployed in this report. The spin states of diradicals I1 and IV were dependent upon the solvent. See text for further explanation.
\"
3r
IRG A clean source of net absorptively polarized reactive radicals, *r#, is produced by the photolysis of DPO (eq 2), which produces a triplet radical pair, consisting of a substituted benzoyl and a diphenylphosphinyl radical in high ~ i e l d . ~ ~As J 3in the case of IRG, the triplet mechanism is believed to be responsible for the generation of the observed electron polarization. The CIDEP spectrum of the radicals produced from photolysis of DPO is extremely simple: two sharp lines on either side of g = 2.004 separated by ca. 400 G due to the diphenylphosphinyl radical and a singlet near g = 2.001 due to the substituted benzoyl radical."J3 The difference in phase of the polarized spectra produced from IRG and DPO results from the difference in the selectivity of sublevel population in the SI-TI intersystem cross step for the two precursors.2~7f~9~'2.'3 2. Electron Spin States of Polynitroxyl Radicals. Terminology. An analysisof the C W ESR spectra of polynitroxyl radicals allows an assignment of their electronic spin states based on the form of the spectra (Le., the number of lines in the spectra)6J4 and provides spectroscopic experimental criteria to classify the dinitroxyls as being in the state of 'strong"or "weak" spin electron exchange. The basis for these assignments is discussed in detail in refs 6 and 14. We shall briefly summarize below the most important observations and features of the CW ESR of polynitroxyls. The simplest polynitroxyl is a dinitroxyl. In the limit of weak spin exchange a dinitroxyl will display a CW ESR spectrum that is characteristic of a mononitroxyl (but twice as intense at the same concentration), and (in nonviscous solvents) will typically consist of three lines of comparable intensity.6 The three lines
DPO
result from hyperfine coupling of the unpaired electron to the I4Nnucleus which possesses a nuclear spin of 1: high field to low field (left to right in conventional spectra) assignment M I = 1, 0, -1. For a dinitroxyl in weak exchange, the system may be treated as two independent doublets which happen to be apart of the same molecular structure. To represent a dinitroxyl in weak exchange, we use the notation 2RX2R, where the symbol X represents the 'template" on which the stable spins are grafted to form a single molecule, the symbol R represents the TEMPO moiety, and superscripts 2 indicate that the spin system is in weak exchange and consists of two weakly interacting doublets. However, in the limit of strong spin exchange the interaction of two nitroxyl electron spins causes a coupling of the electron spin system which manifests itself spectroscopically as a five-line CW ESR spectrum.6 The lines may be assigned according to the total nuclear quantum numbers resulting from hyperfinecoupling with the I4N nucleus from high field to low field: M I = 2, 1, 0, -1, -2. The intensity pattern provides indirect evidence that the
Interaction of Reactive and Stable Free Radicals TEMPO
+
IRG
-
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10533
-
hv
10 G
*RXNH + IRG 1
0
I1 3(RXR)
+ DPO
hv
2 1 0 -1 -2
3(RXR)#
-1
v TEMPO
-
11v Y +
IRG
‘R’XRr
hv
hv
Figure 2. TS ESR spectrum obtained under photolysis of solutions of polarization donors (a) IRG and (b) DPO. Conditions: acetonitrile solvent, TEMPO (2.5 X lC3M), At = 300-800 ns.
spin system is in an electronic triplet state of two strongly coupled electron spins. Such a superscript in similar formulas below indicates the spin multiplicity of a (po1y)radical. Experimentally, the components with M I = 2,0, 2 for a dinitroxyl, 3(RXR), are observed at essentially the same magnetic field as those with M I = 1, 0, -1 for TEMPO or for a dinitroxyl, 2RX2R. It is important to note that a single structure may display magnetic behavior that depends on the experimental conditions (solvent and temperature) and that a single structure may simultaneously display spectroscopic properties of two different multiplicities; Le., a RXR molecule may exist in structures which behave as a doublet state with two noninteracting spins [2RX2R] or as a triplet state [3(RXR)] or as a simultaneous mixture of the two. Furthermore, it must be kept in mind6J4 that in the limit of strong exchange for a single molecular structure, a polyradical may exist in more than one spin state, e.g., a dinitroxyl in strong exchange is actually a mixture of a paramagnetic, potentially ESR active triplet (-75%), 3(RXR), and a diamagnetic, ESR inactive singlet (-25%), O(RXR). In the case of strong exchange for a triradical (VI) or a tetraradical (MI), we employ the notation 4(RXR2)and 5(RXR3), where the superscript denotes a quartet and quintet multiplicity for three and four interacting TEMPO units, respectively.6 In these cases, the CW ESR spectrum of a 4(RXR2) trinitroxyl will display seven lines and a S(RXR3) will display nine lines. Employing the above spectroscopiccriteria, the multiplicity of the polyradicals employed in this investigationwas readily assigned from the CW ESR spectra and is indicated in Figure 1. 3. Interactions between Nitroxyls and Reactive Doublet Radicals Possessing Net Emissive Polarization. Figure 2 displays typical CIDEP spectra obtained by TR ESR under photolysis of IRG and DPO in the presence of TEMPO. The CIDEP spectrum obtained during the photolysis of IRG in the presence of sufficiently high (>(2-5) X l e 3 M) concentrations of either of two monoradicals (TEMPO and 2RXNH, Figure 1) or either of two diradicals, 2RX2R (I and V, Figure l), results in the appearance of a polarized spectrum of the nitroxyl that consists of three emissive lines of equal intensity (a representative example of three polarized lines of equal intensity is given in Figure 2 for TEMPO as polarization acceptor). Figure 3a shows the CIDEP spectrum of mononitroxyl 2RXNH which compare favorably with that obtained with TEMPO (Figure 2a). Thus, interaction of
VI1 4(RXR2)
+
IRG
hv
Figure 3. TR ESR spectrum obtained under photolysis of a solution of IRG in the presence of: (a) 2RXNH (2.5 X lC3M), At = 300-800 ns in acetonitrile; (b) I1 3(RXR), (2.5 X le3M), At = 300-800 ns in acetonitrile, stick spectra related to this spectrum are on the right of it, see text; (c) VI1 4(RXR2), (2.1 X M) in benzene, At = 1.0-1.5 ps.
the reactive radical, with a simple doublet nitroxyl monoradical or a diradical in weak exchangeconverts the net emissively polarized spectrum of Er# into an emissively polarized spectrum characteristic of a polarized doublet nitroxyl free r a d i ~ a l . ~ . ~ These results have the superficial appearance of predominantly electron spin polarization transfer (ESPT) from Er# to a single doublet nitroxyl center. The reactive radical can be imagined to “disappear” from the spectrum because electron polarization is being transferred (ESPT) to a nitroxyl and because only polarization is being monitored in the TR ESR measurements. However, the interactions of the reactive radicals and the nitroxyls, in addition to ESPT, are also expected to include reaction of the reactive radical with one of the doublet centers of the dinitroxyl moiety to form a CO bond. In the case of reaction with the monoradical, this will produce a diamagnetic product which is not detected by ESR. In this case the radicals truly “disappear” from the system. The removal of the radicals from the system by chemical reactions which produce diamagnetic products may be directly probed by TR optical pulsed laser spectroscopy (vide infra). The observations of ESPT from Er# in the photolysis of IRG to a nitroxyl center of a mononitroxyl or of a polynitroxyl in weak spin exchange, are unexceptional and correspond to literature p r e ~ e d e n t .However, ~~~ the CIDEP spectra (Figure 3c) produced by the photolysis of IRG in the presence of strongly spin coupled polynitroxyls are exceptional in that the form of the observed spectrum is more or less independent of the spectroscopic multiplicity of the polynitroxyl polarization acceptor. For example, the TR ESR spectrum obtained during the photolysis of IRG in the presence of sufficiently high (>(2-5) X l e 3 M) concentrations of a triplet diradical, 3(RXR) (11), results in the appearance of the polarized spectrum shown in Figure 3b. That the spectrum is in net emission is not remarkable, because this result is expected if ESPT is operating. What is striking, however,
Turro et al.
10534 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
is that the spectrum does not consist of five lines of comparable intensity expected from simple ESPT to produce a polarized triplet nitroxyl. Instead, the spectrum appears to consist of a mixture of the polarized spectrum of a polarized doublet nitroxyl and the spectrum of a polarized triplet dinitroxyl, with the former spectrum being the major component. From the assignment of the hyperfine lines given above, the 1, 0, and -1 lines of the polarized monoradical doublet would overlap with the 2,0, and -2 lines of the polarized triplet diradical. Similar spectra were observed with other 3(RXR); cf. Figure 1. Even more striking is the observation that a qualitatively similar spectrum (Figure 3c) is produced in the photolysis of IRG in the presence of the quartet triradical 4(RXR2) VII. The appearance and description of the spectrum, therefore, are similar to that for Figure 3b: a mixture of the polarized monoradical-like spectrum and a strong-exchange diradical nitroxyl, with the monoradical portion being the major component. Any contribution of the seven-line spectrum expected to result from direct ESPT to VI1 appears to be very weak, at best. A more complex spectrum is produced in the photolysis of IRG in the presence of the quintet tetraradical S(RXR3)VI11 as a polarization acceptor. The presence of lines assignable to the CIDEP spectra of complicates the CIDEP pattern to a much greater extent that in the case of the other nitroxyls investigated. However, an emissive CIDEP was observed, and the spectrum can be approximated as a mixture of a polarized monoradicallike nitroxyl spectrum and a strong-exchange triplet diradical nitroxyl, with the monoradical-like portion being the major component. However, the limited amounts of VI11 available prevented detailed investigation of this species. An apparently enhanced emission of the component with MI = -1 in TR ESR spectra of triplet dinitroxyls relative to the emission of the M I = 1 component was often observed a t short times or at low dinitroxyl concentration; cf. Figure 3b,c. This enhanced emission is assigned to the contribution of the benzoyl radical ( g = 2.000) which occurs close to the same field as the M I = -1 line of triplet nitroxyls.9J2 Attempts were made to observe the TR ESR spectra for different polynitroxyls under comparable conditions. However, it was found that for 4(RXR2) VI1 and to the greater extent for the S(RXR3)VI11 the observed spectra depended on experimental variables such as the laser intensity and flow rate of the sample through the cavity. For example, in the photolysis of IRG in the presence of S(RXR3) VI11 with relatively slow flow rates and high concentration of the tetranitroxyl, a CW ESR analysis showed substantial consumption of the tetranitroxyl and the observed spectrum displays five lines and is similar in form to that observed in the TR ESR experiment. It was found that, depending on conditions of laser intensity and flow rate, from 20 to 50% consumption of the polyradical occurs during photolysis of IRG or DPO (see section 4 below) in its presence. As a result, conditions were selected for which the reported spectra do not depend significantly on the laser intensity or flow rate of the sample through the cavity. The CW ESR spectrum of I1 (vide supra) can be transformed from the strong coupling limit to the weak coupling limit simply by variation of the composition of acetonitrile/formamide binary solvent. For a fixed concentration of I1 of 2.0 X le3M, the CW ESR spectrum of I1 gradually changes from a [strong exchange, 3(RXR)] five-line spectrum in pure acetonitrile to a [weak exchange, 2RX2R]three-line spectrum in 80%(v/v) formamide/ acetonitrile. The TR ESR spectra obtained during the photolysis of IRG in the presence of I1 in acetonitrile are similar to that of Figure 3b (decrease in the intensity of the MI = f l components of a five-component spectrum) and in 80% formamide is similar to that of Figure 2 and Figure 3a (three components of equal intensity). The diradical IV (Figure 1) demonstrated quite similar behavior as diradical 11.
A
2
1
1 0 -1 -2
'(RXR)'
I11 3(RXR)
+
hv
DPO
1
0
-1
'R'XRr
- G- I 10
observed
hv
VI1 '(RXR,) + DPO Figure 4. TR ESR spectrum obtained under photolysis of a solution of DPO in benzene in the presence of 111 3(RXR), ( 5 X M), At = 300-600 ns: (a) stick spectra related to this spectrum are on the right of it, see text; (b) VI14(RXR2),(5.0 X M) in benzene, At = 300-600 ns.
In summary, the polyradicals in strong spin exchange, [3(RXR), 4(RXR2), S(RXR,)], which display five, seven, and nine lines, respectively, in their CW ESR spectra, display a reduced number of emissively polarized lines in their TR ESR CIDEP spectra as the result of interactions with Er# produced from photolysis of IRG. It thus appears that the interaction or chemical reaction between Er# and the polynitroxyls is selective in producing polarization of species of lower spin than the original polynitroxyl. Thus, polarization is preserved after reaction of the reactive radical with one or more TEMPO units of the polyradical. 4. Interactions between Nitroxyls and Reactive Doublet Radicals Possessing Net Absorptive Polarization. The TR ESR spectrum of photoexcited DPO shows strong net absorptive polarization (videsupra, section 1). The results with *r# produced by photolysis of DPO and mono- and polynitroxyl are qualitatively similar to those employing produced from photolysis of IRG, except for the phase of polarization, which is enhanced absorption in all cases. For example, in the presence of sufficiently high (>(2-5) X M) concentrations of TEMPO, 2RXNH, or dinitroxyls in weak exchange (Figure 2b), the expected polarized three-line doublet nitroxyl spectrum is observed but in enhanced absorption. As in the case of photolysis of IRG as the generator of net emissive polarization, the CIDEP produced by the photolysis of DPO in the presence of strongly coupled di- and trinitroxyl polarization acceptor (Figure 4a,b). For example, the TR ESR spectrum obtained during the photolysis of DPO in the presence of sufficiently high (>(2-5) X le3 M) concentrations of a triplet diradical, 3(RXR) 11, results in the appearance of a polarized spectrum shown in Figure 4a. Although the nitroxyl spectrum is in net absorption, the spectrum appears to consist of a mixture of the monoradical-like polarization spectrum and a strongexchange diradical polarized spectrum, with the monoradicallike portion being the major component, Le., the same interpretation, except for the phase of polarization, as for the systems with IRG. Qualitatively similar results were observed for the quartet trinitroxyl VI1 as polarization acceptor (Figure 4b). The possible interpretation of the CIDEP spectra of VI1 (Figures 3c and 4b)
Interaction of Reactive and Stable Free Radicals is the following. This trinitroxyl 4(RXR2) contains a significant fraction (ca. 33W) of monoradical 2(RXRz),which can effectively accept polarization with formation of 2(RXR2)# (three lines). The reactive ESPT (eq 7) can occur also with the formation of diradical in strong exchange, i.e., "RXRr)# (five lines). As a result the observed spectra (Figures 3c and 4b) may be considered as a weighted sum of mono- and dinitroxyl in a strong exchange with larger contribution of the former spectrum. In the case of 5(RXR3) VI11 as a polarization acceptor, it was not possible to find conditions which produced the quality of signal to noise obtained with the other nitroxyls. This may be the result of the small number of experiments which could be performed with the limited material (see Results section 3 above) or to an inherently less efficient transfer of polarization. In summary, as in the case of the photolysis of IRG, the observations are made that qualitatively similar spectra are produced in the photolysis of DPO in the presence of the triplet diradicalS(RXR) 111and the quartet triradical 4(RXR2)VI1 as polarization acceptors (Figure 4a,b). The observed spectra show a major component from monoradical-like doublet polarization with a smaller contribution from triplet diradical-like polarization. 5. Kinetics of Polarization Transfer and Chemical Reactions in the Photolysis of IRG in the Presence of Nitroxyls. The growth and decay of the polarization of nitroxyl signal for the photolysis of IRG in acetonitrile in the presence of TEMPO and diradical 2RX2RI were studied. The component with M I= 1 in the T R ESR spectra of nitroxyls was monitored; this component does not overlap with the spectrum of Er#. In the time domain of ca. 100 to ca. 4000 ns, the shape and the number of lines in the CIDEP spectra of the polynitroxyls did not change. This result shows that interchange of polarization among possible multiplicities does not occur under our reaction conditions. In order to determine whether the observed dominance of polarized doublet nitroxyls in the observed spectra was related to differences in relaxation of polarization as a function of multiplicity, measurements of the TI relaxation (see the Experimental Section, 1) of the polarization of the mononitroxyl 2RXNH and the dinitroxylj(RXR) I11 were made. The results showed that these two species have comparable relaxation times T1 of ca. 2 ps,although TIof TEMPO was found to be considerably shorter (ca. 0.5 p s ) . These results indicate that a faster polarization relaxation of species of higher multiplicity is not responsible for the dominating appearance of the mononitroxyl in the observed spectra (Figures 3 and 4). In order to probe further the possible sources of the dominating mononitroxyl polarization, competitions between pairs of nitroxyls, one of low and one of high spin multiplicity, as polarization acceptors were examined. The competitions of ESPT from Er# to the pair TEMPO and to 3(RXR) IV, as well as to the pair 2RX2R (V) and 3(RXR) IV in acetonitrile, were investigated. The observed T R ESR spectra were similar to those of Figure 3b, i.e., mainly a polarized doublet with a smaller component of a polarized triplet. The ratio of intensities of components with M I= 2 to M I = 1 in the five-component spectrum was monitored as a function of [TEMPO]/[IV] and that of [V]/[IV]. No significant difference in the efficiency of polarization transfer to the doublet mono- or dinitroxyl and the triplet dinitroxyl '(RXR) was observed once an adjustment is made for the number of spins. Thus, we conclude that the rateof spin polarization transfer to polynitroxyl is proportional to the total number of spins in solution. 6. Time Resolved Laser Flash Photolysis. Short-lived free radicals have been established to react with nitroxyls with rate constants close to those for diffusion.I5 The rate constants of reaction of the dimethoxybenzyl radical with TEMPO and with IV '(RXR) were measured in acetonitrile by laser flash photolysis. The dimethoxybenzyl radical absorbs strongly in the range 380400 nm. Standard Stern-Volmer analysis with TEMPO and IV
The Journal of Physical Chemistry, Vol. 97,NO.41, 1993 10535 as quenchers of the absorption of the dimethoxybenzyl radical led to rate constants (20%error limits) of 5 X lo9 (TEMPO) and 6 X lo9 M-l s-I (IV). We conclude that the multiplicity of the nitroxyl does not significantly influence the rate of reaction of a reactive radical with a nitroxyl center.
Discussion
1. Polarization Transferfrom Reactive Doublet States Bearing Net Polarization. The interaction of Ar# and Er# with a mononitroxyl or a dinitroxyl in weak exchange leads to single phase absorptive or emissive CIDEP signals, respectively, of the corresponding nitroxyl. The observation of net ESPT is expected from the conservation of quantum numbers S and mSof a system under ESPT.2a*3v9In the following discussion, we assume that the major mechanism for production of polarized nitroxyls resulting from photolysis of DPO and IRG observed by T R ESR results from ESPT from and Er#, respectively. We now postulate that two ESPT processes (eqs 3 and 4) can occur in experiments with dinitroxyls. Polarization transfer which occurs with a concurrent chemical addition such as (3) is termed reactive ESPT, and polarization transfer which occurs without a chemical reaction (to produce a polarized polynitroxyl of the same multiplicity, eq 4) is termed nonreactive ESPT. The results indicate that the competition between these two processes is not sensitive to the phase of the polarization to be transferred (Figures 3b and 4a). In eqs 3 and 4, r# stands for either or Ar#.
k3/
r" + 3(RXR)
2R"XXRr
' T. 3(RXR)"
(3)
+
r
(4)
According to eqs 3 and 4, interaction of r# with 3(RXR) could lead to two polarizes species that could be simultaneously detectable by T R ESR: a polarized triplet dinitroxyl resulting from simple ESPT and a polarized doublet mononitroxyl resulting from concurrent addition and ESPT. It should be noted that even if both polarized doublet and a polarized triplet were produced simultaneously as indicated in eqs 3 and 4, the spectra would not be experimentally separable, because the lines of the mononitroxyls coincide with those of the M I= 2,0, and -2 lines of dinitroxyls in strong exchange. However, the presenceof the polarized triplet can be deduced from the enhanced absorption (emission) of lines of the M I= f l lines; cf. Figures 3b and 4a. In principle, the polarized double and triplet spectra might be time resolved. However, attempts to resolve the two spectra [2R#XRr and 3(RXR)#] in time were unsuccessful because of the similar relaxation times of the two polarized species. In spite of these complications, it is possible to estimate the relativecontributions ofZR#XRr and af'(RXR) # to the observed spectra, since the distribution of intensities of components in T R ESR spectra of dinitroxyls, which bear single phase polarization, are known.6J4 This distribution is shown as bars in the upper stick spectra in Figures 3b and 4a. The intensity of component I ( M ) in the observed spectra can be expressed as a weighted sum4 of two spectra bearing net polarization, i.e., of'(RXR)# and that of *R#XRr as given in eq 5. I(M) = uP"[~(RXR)#]
+ b P"et(2R#XRr); a
+ b = 1.0 ( 5 )
From the experimental spectra (Figure 3b and 4a demonstrate typical results) the ratio of intensity of spin-adduct spectrum to polarized dinitroxyl spectrum is computed to be b / a = 2-4. Our results are consistent with the conclusion that net polarization is preserved in fast chemical reactions such as eq 3. This conclusion has precedent in literature reports of the
Turro et al.
10536 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
conservation of multiplet78" and net2v7fpolarization in reactions involving the addition of polarized free radicals to a second diamagnetic reagent, Le., a molecule with C=C double bonds. The use of very reactive polynitroxyls allows their use in rather low concentrations, namely lO-3-lc2 M, compared to typical concentrations (0.1-1 M ) of diamagnetic The possible formation of a polarized reaction product (eq 3) is a distinguishing interaction of a polarized reactive radical with a dinitroxyl 3(RXR) from interaction of a polarized reactive radical with a mononitroxyl such as TEMPO (eq 6 ) . In the latter case the product of reaction (eq 6 ) is diamagnetic and invisible to ESR.
+
-
therefore spin polarization, between doublet radicals should occur faster than or at least competitive with bond formation.' The measurement of rates of spin exchange (kc,) and recombination (krK)in solutions of polyatomic radicals showed that k,, >, k,.* Let us consider now the competition between nonreactive and reactive ESPT for dinitroxyls (eqs 3 and 4). The ratio of rate constants of ESPT in these two processes can be estimated from experimental data through eq 5 since kJk4 = b / a = 2-4 for the 3(RXR) investigated.'6 In the estimation, it is assumed that the intensity of polarized component is directly proportional to rate of formation of the polarized species. The relative intensities of lines in the observed T R ESR spectra are expected to be determined only by ratio of rate constants for parallel bimolecular reactions if each is pseudo first order. The difference in intensities of lines of these two species cannot be due to differences in their electronic paramagnetic relaxation, because the experimental T I values are quite comparable (cf. Results section 5 ) . Since the competition in polarization transfer to two different dinitroxyls was observed to be simply related to the concentration of spins, (vide supra, section 5), ESPT is a bimolecular process and can be controlled by variations of the concentration of the polarization acceptor. At this point we conclude that some multiplicity dependent factor has been ignored in our analysis. We search for this factor in the efficiency with which a polarized donor spin system of lower multiplicity can transfer its polarization to an acceptor spin system of higher multiplicity. The following theoretical e q u a t i o n ~ hold ~ ~ Jtrue ~ for reactions 3 and 4, if both are diffusion controlled
r# TEMPO TEMPO-r (6) Our results provide no experimental evidence for significant formation of 4(RXR2)# and 5(RXR3)# in the interaction of r# with corresponding polyradicals. However, in thecase of 5(RXR) as a potential polarization acceptor, the experimental spectra are complicated due to significant contributions of in one case and from contributions of Ar# and poor signal to noise in the other case. However, it seems clear that a polarized product of the reaction with 4(RXR2) is the triplet diradical 3(RXR)#Rr. The dinitroxyl 3(RXR)#Rr probably can exist in two configurations (ina strong, 3(RXR)Rr andin a weakexchange, (ZRX2R)Rr, which could result in lower intensities of components with M I = f l compared to those with M I = f 2 in the five-component spectrum. In spite of these complications, the observation that ESPTin the products of reaction of r# with tri- and tetranitroxyls occurs with conservation of the phase of polarization of r# is direct and unambiguous. k3 = u34?rR,,,,D (loa) Competitive reactive and nonreactive ESPT involving polarized doublets with doublet, triplet, quartet, and quintet nitroxyls are k4 = u44?rR,,D ( 1Ob) major processes of interest in this investigation. When ESPT From our experimental results, eq 1Oc allows evaluation of k3/ occurs without a change in spin state, its mechanism has been considered to be related to that of electron spin e ~ c h a n g eTo . ~ ~ ~ ~k4. ~ the extent this analogy is correct, ESPT bears a mechanistic ( 1OC) k31k4 = U ~ R ~ X ~ / '=J ~2-4 R~X resemblance to processes such as electronic energy transfer which In the above equations D is a coefficient of mutual diffusion, u3.4 occurs by the electron exchange mechanisme2 are spin-statistical factors,17J8and u3 = I/3.l8 The experimental Consider the interactions of a polarized doublet radical, r# value of k3 (see Results section 6 ) seems to be equal within the and a polynitroxyl "(RXR,) which form a collision complex, accuracy of measurement and accuracy of estimation with the [(RXR,),r],possessing thecorrect totalspin (n- Z)foranallowed value predicted by eq 10a for a diffusion-controlled reaction.19 chemical reaction. Brackets here and below denote collision Evidently, encounters between a doublet radical, *R (S = I/2), complexes. In this case, we envision a competition to occur and a triplet diradical (S= 1) leads to different efficiency of spin between reactive ESPT (eq 7) and nonreactive ESPT (eq 8). exchange between such partners than that for spin exchange From our measurements of the rate constant for reactions such between two d0ub1ets.I~ The larger spin of a strong exchange as eqs 3 and 6 and from the l i t e r a t ~ r e , reaction '~ 7 is expected diradical, 3(RXR), is expected to be more efficient in reorientation to be close to diffusion limited. We therefore conclude that of spin of free radical, 2R, than is the spin of the radical in reaction 7, for reactive radical pairs (when it is allowed by spin reorientation of spin of a triplet diradi~a1.l~Since we have statistics), must compete favorably with reaction 8. monitored the T R ESR spectrum of 3(RXR), u4 corresponds to the statistics of spin exchange of 3(RXR). One of possible '-*(RXR,r)' (7) k 7 f theoretical estimatesof the maximalvalueof u4 is 0.22,I7JOwhereas / the maximal value of u for spin exchange between two doublet rN + "(RXR,) "-'[(RXRm), r l radicals is 0.5.17 The estimation of k4 for contact diffusion"(RXR,)# + r (6) controlled contact processlg according to eq 10b with u4 = 0.22 gives kd 4 X 109 M-%-I (nonviscous solvents, i.e., acetonitrile) However, if we consider the situations for which radical pairs and is close or several times larger than the experimental are formed between reactive radicals and nitroxyls in spin states estimationofk4=(1.5-3.0) X 1 0 9 M - W (cf-ref 19andeq 1Oc). for which reaction is forbidden (eq 9), the contribution of Therefore, we conclude that contrary to the results' on relative nonreactive ESPT from these pairs might be high due to absence rates of spin exchange and chemical reaction between doublet of competing channel of reactive ESPT. free radicals (see this section above), nonreactive ESPT between a polarized doublet radical and a triplet diradical occurs at a r # "(RXR,) "'[[(RXR,),r] "(RXR,)# + r (9) significantly slower rate than chemical reaction between thesame species and that Re, R,,,. Competition between nonreactive ESPT (eq 8) and chemical reaction (eq 6 ) has been established for mononitroxyls and small We propose that the main reason for smaller efficiency of free radicals. 1.2 The effective competition observed in these cases polarized polyradical formation via ESPT is the lower efficiency implies that the effective reaction radius for polarization transfer of reorientation of large spin (S= 1,3/2, and 2 for the polyradicals via spin exchange Re, is larger than the reaction radius Rrxn.lAs investigated) by a smaller spin (S = I/2) compared to theefficiency discussed above it is intuitively resonable that spin exchange, and of reorientation of two spins of the same m a g n i t ~ d e . 1This ~ factor
-
+
-
T
-
-
-
Interaction of Reactive and Stable Free Radicals
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10537
most likely accounts for the absence of significant CIDEP from 4(RXR2)# and 5(RXR3)# which could result from interactions of r# with corresponding polyradicals (see Results sections 3 and 4). A second possibility for the lack of significant CIDEP from 4(RXR2)# and YRXR3)# is that nonreactive ESPT (eq 9) actually occurs efficiently and produces these polarized species, but for some reason quartet-doublet (quintet-triplet, sextet-quartet) intersystem crossing (ISC) of the corresponding pairs occurs and produces a reactive pair for which chemical reaction of the pair competes efficiently with the escape of ESPT products from a cage. However, this possibility seems less probable because of the absence of any precedent of which we are aware that could lead to such effective ISC in the pair. The competition between chemical reaction with 2RXZRand ESPT to the same 2RXZRevidently takes place, and ESPT can lead to polarization of one radical terminus, see Results section 3. It is improbable that the polarization will be transferred to the second terminus under reactive ESPT in view of small exchange interaction between termini. The measurement of decay rates of T R ESR signals of reactive free radicals only allows determination of an effective paramagnetic relaxation time Tlpfo,which includes both the chemical decay rate of the polarized radical species in addition to the electron spin relaxation.12 However, the decay of T R ESR signals of stable free (po1y)nitroxyls is only due to paramagnetic relaxation. Thus the observation of decay of T R ESR signals of stable (po1y)nitroxyls allows the direct estimation of measurement of TI. The TI observed for TEMPO (0.5 ps, cf. Results section 5 ) is in agreement with the literature value of TI for a nitroxyl of similar structure, namely 2,2,6,6-tetramethyl-4-hydroxypyperidine-N-oxyl, for which TI is ca. 0.8 ps at room temperature in nonviscous solvent.21 The experimental estimations of the decay of polarized TEMPO, ZRXNH, different '(RXR)#, and ZR#XRr showed that TEMPO has a significantly shorter relaxation time than the other nitroxyls, all of which possess similar T I of ca. 2 ps. This result follows from the general observation that T I r3, where r is the van der Waals radius of mono- or diradical.22 TEMPO possesses a considerably smaller molecular radius (size), than the other nitroxyls investigated and is therefore expected to undergo faster paramagnetic relaxation.
-
Conclusions The present work has investigated polarization transfer from the reaction with free radicals bearing net absorptive (or emissive) polarization, r#, to mono-, di-, tri-, and tetranitroxyls. The most definitive results were obtained for r# interacting with dinitroxyls in weakand in strong exchange,2RXZRand 3(RXR),respectively. The conclusions from these results are summarized in Scheme I in which circles represent a polarized reactive radical and squares represent a nitroxyl moiety. For the dinitroxyls a dotted line connecting represents weak electron exchange between the nitroxyls, a straight line represents a conformation that separates the nitroxyls and keeps them in the state of weak exchange, and a curved line represents a conformation that brings the nitroxyl into a state of strong exchange. From previous results, the competition between electron exchange and chemical interactions of r# with a mononitroxyl leads to a polarized radical (path a in Scheme I), detectable by T R ESR, and to a diamagnetic product (path b in Scheme I) that is invisible to ESR spectroscopy, but which can be monitored by laser flash spectroscopy with optical detection. The results are consistent with an analogous competition when the interaction of r# is with a dinitroxyl in weak exchange 2RX2R (paths c and d in Scheme I ) . In contrast to the results with mononitroxyl and dinitroxyls in weak exchange, the interaction of r# with polynitroxyls in strong
SCHEME I
reaction
smng exchange
W reactive
ESPT
exchange allows the observation by T R ESR of the both processes in the same experiment: ESPT without chemical reaction to produce a polarized triplet (path e in Scheme I) and ESPT with chemical reaction to produce a polarizeddoublet (path f in Scheme 1) The relative rates of paths e and f favor the reactive path by a factor of 2-4. These results contrast with those involving the relative rates of spin exchange and chemical reaction between doublet free radicals (S = 1/2) for which spin exchange (path a in Scheme I) is generally of a comparable or faster rate than that of chemical reaction (path b in Scheme I). Thus, the results reported here appear to require that ESPT between a radical (S = ]/2) and a diradical in strong exchange (triplet state, S = 1) occurs at a significantly slower rate than chemical reaction between the same reagents. The conclusions concerning the interactions of r# with4(RXR2) and 5(RXR3), although based on less definitive experimental evidence, are also consistent with a slower rate of ESPT relative to chemical reaction. For these systems the relative formation of 4(RXR2)# and 5(RXR3)# is small compared to chemical reaction and significant polarization is observed only after one or two nitroxyls have reacted to produce species of lower multiplicity which then become polarized in secondary ESPT processes. The present results alsodemonstrate that the use of dinitroxyls as polarization acceptors has an important advantage over the use of mononitroxyls as polarization acceptors. The observation of polarized signals of mononitroxyl in the presence of reactive triplet state does not allow one to distinguish between polarization transfer from triplet state or from the reactive radical formed in the reaction of that triplet. The use of dinitroxyls allows one to discriminate between these two potential donors of polarization: the observation of strong three lines of monoradical in the T R ESR spectrum demonstrates that reactive free radical was a source of polarization, whereas observation of five components with the strongest central component demonstrates that the triplet state was the source of polarization.
Acknowledgment. The authors thank NSF, DOE, and AFOSR for their generous support of this research. We are grateful to Drs. K. Waterman and T. Noh for synthesis of a number of
10538 The Journal of Physical Chemistry, Vol. 97, No. 41 1993
Turro et al.
~
polyradicals and to Professor D. Doetschman for careful reading of the manuscript and fruitful discussions.
References and Notes (1) (a) Bartels, D. M.;Trifunac,A. D.; Lawler, R. G. Chem. Phys. Left. 1988, 152, 109. (b) Syage, J. A. J . Chem. Phys. 1987,87, 1022, 1033. (c) Adrian, F. J. J . Chem. Phys. 1988,88, 3216. (d) Closs, G. L.; Calcaterra, L. T.; Green, N.; Penfield, K. W.; Miller, J. R. J . Phys. Chem. 1988, 240, 440. (e) Closs, G. L.; Johnson, M. D.; Miller, J. R.; Piotrowiak, P. J. Am. Chem. SOC.1989, I l l , 3751. (2) (a) Jenks, W. S.; Turro, N. J. Res. Chem. Inrermed. 1990,13, 237. (b) Jenks, W. S.; Turro, N. J. J . Am. Chem. SOC.1990, 112, 9009. (3) (a) Imamura, T.; Onitsuka, 0.;Obi, K. J. Phys. Chem. 1986, 90, 6741. (b) Obi, K.; Imamura, T. Rev. Chem. Inrermed. 1986, 7, 225. (4) (a) McLauchlan, K. A. In Advanced EPR, Applicarion in Biology and Biochemistry; Hoff, J. A,, Ed.; Elsevier: Amsterdam, 1989; p 345. (b) McLauchlan, K. A. In Modern Pulsed and Cmrinuous- Wave Elecrron Spin Resonance; Kevan, L., Bowman, M. K., Eds.; Wiley: New York, 1990; p 285. ( 5 ) (a) Blatter, C.; Jent, F.; Paul, H. Chem. Phys. Letf. 1990,166,375. (b) Blatter, C.; Paul, H. Res. Chem. Inrermed. 1991,16,201. (c) Kawai, A,; Okutsu, T.; Obi, K. J . Phys. Chem. 1991, 95, 9130. (d) Kawai, A,; Obi, K. J . Phys. Chem. 1992, 96, 5701. (6) Turro, N. J.; Khudyakov, I. V.; Bossmann, S.H.; Dwyer, D. W. J . Phys. Chem. 1993, 97, 1138. (7) (a) McLauchlan, K. A.; Simpson, N. J. K. Chem. Phys. Lerr. 1989, 154,550. (b) McLauchlan, K. A.; Simpson, N. J. K. J . Chem. SOC.,Perkin Trans. 2 1990, 1371. (c) McLauchlan, K. A.; Stevens, D. G. Chem. Phys. Leu. 1985, 115, 108. (d) Akiyama, K.; Depew, M. C.; Wan, J. K. S. Res. Chem. Inrermed. 1989, 11, 25. (e) Ohara, K.; Murai, H.; Kuwata, K. Bull. Chem. SOC.Jpn 1992,65, 1672. (f) Turro, N. J.; Khudyakov, I. V. Chem. Phys. Lerr. 1992, 193, 546. (8) Nikitaev, A. T.; Nikitaeva, G. A.; Khudyakov, I. V.; Prokofev, A. 1.; Levin, P. P.; Burshtein, A. I. Dokl. Akad. Nuuk SSSR 1979, 247, 391. (9) Jenks, W. Ph.D. Thesis, Columbia University, New York, NY, 1991. (10) Zimmt, M. B.; Doubleday, C.; Turro, N. J. J . Am. Chem. SOC.1986, 108. 3618. (1 1) (a)Rozantsev,E.G.;Scholle,V.D.Synrhesisl971,190. (b)Glarum, S. H.; Marshall, J. H. J . Phys. Chem. 1967, 47, 1374.
(12) Jaegermann, P.; Lendzian, F.; Rist, G.; Mabius, K. Chem. Phys. 615. (13) (a) Majima, T.; Konishi, Y .; BBttcher, A.; Kuwata, K.; Kamachi, M.; Schnable, W. J. Photochem. Phorobiol. A: Chem. 1991,58.239. (b) Baxter, J. E.;Davidson, R. S.;Hageman, H. J.; McLauchlan, K. A,; Stevens, D. G. J. Chem. Soc., Chem. Commun. 1987, 73. (14) (a) Luckhurst, G. R. In Spin Labeling. Theory and Applicarions; Berliner, L. J., Ed.; Academic: New York, 1976; Vol. 6, p 133. (b) Parmon, V. N.; Kokorin, A. I.; Zhidomirov, G. M. Stable Biradicals; Nauka Publishers: Moscow, 1980; 240 pp. (in Russian). (c) Parmon, V. N.; Kokorin, A. I.; Zhidomirov, G. M . Zh. Srrucr. Khim. 1977, 18, 132. (15) (a) Beckwith, A. L. J.; Bowry, V. W.; Ingold, K. U. J . Am. Chem. SOC.1992, 114,4483. (b) Bowry, V. W.; Ingold, K. U.J. Am. Chem. Soc. 1992. 114. 4492. (16) The correct ratio of k3/k4 should be less to -25% in the view of ESPT in chemical reaction with the singlet state of dinitroxyl (see Results section 2). (17) Molin, Yu. N.; Salikhov, K. M.;Zamaraev, K. I. Spin Exchange. Principles and Applicarions in Chemistry and Biology; Springer: Berlin, 1980; p 242. (18) (a) Burshtein, A. I.; Khudyakov, I. V.; Yakobson, B. I. Prog. React. Kiner. 1984,13,221. (b) Saltiel, J.; Atwater, B. W. Ado. Phoiochem. 1988, 14, 1. (19) An estimation of k3 according eq 10a as well as the rate constant of any other contact diffusion-controlled reaction can be based on the Debye formula, which usually gives the lower estimateoftherateconstantfordiffusioncontrolled reaction.l*' We used eq 10a and the Debye formula and obtained k3 .- 6 X lo9 M-k-l. (20) The following simple estimation of maximal a4 can be performed. The probability of encounter of reagents in the quartet state Q (eq 9) is 2/3; thenonreactive ESPTcan occur in theQ*lpstatesand not inQ+/2 states.&Thus the maximal spin statistical factor for ESFT in an nonreactive encounter is u4 = ( ! / ~ ) ( ~ / 3 ) = '/a. If one accepts that nonreactive ESPT occurs with probability of unity in the reactive encounter in a doublet (D) state (eq 8) than for ESPT in Q and D states maximal u4 = l / 3 + = */,. (21) Schwarts, R. N.; Jones, L. L.; Bowman, M. K.J. Phys. Chem. 1979, 83, 3429. (22) Carrington, A.; McLachlan, A. D. Inrroducfion to Magnetic Resonance; Harper: New York, 1979; p 266. Left. 1987, 140,
