A time-resolved electron paramagnetic resonance investigation of the spin exchange and chemical interactions of reactive free radicals with isotopically symmetric (14N-X-14N) and isotopically asymmetric ( 14N-X-15</su

Article abstract of DOI:10.1021/ja0687670

Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl radicals (N-X-N) were studied by time-resolved electron paramagnetic resonance (TR-EPR). Reactive spin-polarized free radicals (r^#), with non-

Full text of DOI:10.1021/ja0687670

Published on Web 06/02/2007
A Time-Resolved Electron Paramagnetic Resonance
Investigation of the Spin Exchange and Chemical Interactions
of Reactive Free Radicals with Isotopically Symmetric
(14N-X-14N) and Isotopically Asymmetric (14N-X-15N)
Nitroxyl Biradicals
Elena Sartori,† Igor V. Khudyakov,‡ Xuegong Lei,† and Nicholas J. Turro*,†
Contribution from the Department of Chemistry, Columbia UniVersity, New York, New York
10027, and Bomar Specialties, Winsted, Connecticut 06098
Received December 7, 2006; E-mail: njt3@columbia.edu
Abstract: Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl
radicals (N-X-N) were studied by time-resolved electron paramagnetic resonance (TR-EPR). Reactive
spin-polarized free radicals (r#), with non-Boltzmann population of spin states were produced by laser flash
photolysis of benzil dimethyl monoketal or of (2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (the
superscript # symbol indicates electron spin polarization). Both isotopically symmetric nitroxyl biradicals
(14N-X-14N) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing 15N and the other
nitroxyl bearing 14N (14N-X-15N), were employed as probes of the spin exchange and chemical interactions
between r and the nitroxyl biradicals. The interaction of r# with the asymmetric ortho-nitroxyl biradical
(14N-O-15N), which exists in a condition of strong spin exchange, proved to be particularly informative. In
this case, spin polarized (14N-O-15N)# (product of spin exchange with r#) and two polarized monoradicals
(r14N-O-15N)# and (14N-O-15Nr)# (products of chemical reaction with r#) were observed. The latter three
species possess three distinct TR-EPR spectra with different line splittings. The relative cross sections for
spin exchange (Rex) and chemical reaction (Rrxn) were achieved through computer simulation of the TR-
EPR spectra. The cross section for spin exchange, Rex, between r# and (N-X-N) biradical is estimated to
be 4-6 times larger than the cross section of chemical reaction, Rrxn, between r# and (N-X-N). The para-
nitroxyl biradical (14N-P-15N) exists in weak spin exchange, and behaves as an equimolar mixture of 14
N
and 15N mononitroxyls.
between chemical reaction and spin exchange.4-9 We report a
study of the interaction of reactive free radicals r with nitroxyl
biradicals (N-X-N) by time-resolved continuous wave EPR
(TR-EPR) that allows a direct measurement of the ratio of the
cross sections Rrxn and Rex.
Introduction
Bimolecular collisions between two molecules represent one
of the most fundamental and common means of inducing
chemical reaction and energy exchange between two chemical
species. In general, nonreactive collisions between two mol-
ecules are “invisible” in the sense that there is no direct chemical
manifestation of the event, whereas a reactive collision is
“visible” through formation or cleavage of chemical bonds.
Electron paramagnetic resonance (EPR) has the potential to
detect both the cross sections, Rrxn, of chemical reactions
(reactiVe encounters) and the cross section, Rex, of spin exchange
(nonreactiVe encounters) between two paramagnetic reagents.
Nonreactive encounters can manifest themselves in EPR spectra1-3
owing to different mechanisms involving Heisenberg spin
exchange. An encounter of a reactive free radical with another
free radical or with a stable biradical leads to a competition
Nitroxyl radicals N and (N-X-N) are known to be excellent
scavengers of reactive carbon-centered radicals r.10-13a These
(4) Bartels, D. M.; Trifunac, A. D.; Lawler, R. G. Chem. Phys. Lett. 1988,
152, 109.
(5) Savitsky, A. N.; Paul, H.; Shushin, A. I. J. Phys. Chem. A 2000, 104, 9091.
(6) Jenks, W. S.; Turro, N. J. J. Am. Chem. Soc. 1990, 112, 9009.
(7) Turro, N. J.; Khudyakov, I. V.; Dwyer, D. W. J. Phys. Chem. 1993, 97,
10530.
(8) For review articles, see (a) Turro, N. J.; Khudyakov, I. V. Res. Chem.
Intermed. 1999, 6, 505. (b) Nagakura, S.; Hayashi, H.; Azumi, T. Dynamic
Spin Chemistry; Kodansha-Wiley: Tokyo, 1998. (c) Woodward, J. R. Prog.
React. Kinet. 2002, 27, 165.
(9) Turro, N. J.; Khudyakov, I. V.; van Willigen, H. J. Am. Chem. Soc. 1995,
117, 12273.
(10) Controlled Radical Polymerization. Matyjazewski, K., Ed.; ACS Sympo-
sium 685; American Chemical Society: Washington, DC, 1998.
(11) Sobek, J.; Martschke, R.; Fischer, H. J. Am. Chem. Soc. 2001, 123, 2849.
(12) Denisov, E. T.; Afanas’ev, I. Oxidation and Antioxidants in Organic
Chemistry and Biology; CRC: Boca Raton, FL, 2005.
(13) (a) Parmon, V. N.; Kokorin, A. I.; Zhidomirov, G. M. Stable Biradicals;
Nauka Publishers: Moscow, 1980 (in Russian). (b) Parmon, V. N.; Kokorin,
A. I.; Zhidomirov, G. M.; Zamaraev, K. I. Mol. Phys. 1973, 26, 1565. (c)
Luckhurst, G. R. Mol. Phys. 1966, 10, 543. (d) Hudson, A.; Luckhurst, G.
R. Chem. ReV. 1969, 69, 191.
† Columbia University.
‡ Bomar Specialties.
(1) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic Resonance;
Wiley: New York, 1994.
(2) Gerson, F.; Huber, W. Electron Spin Resonance Spectroscopy of Organic
Radicals; Wiley-VCH: Weinheim, Germany, 2003.
(3) Molin, Yu. N.; Salikhov, K. M.; Zamaraev, K. I. Spin Exchange; Springer-
Verlag: Berlin, 1980 (cf., in particular, p. 150).
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J. AM. CHEM. SOC. 2007, 129, 7785-7792
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A R T I C L E S
Sartori et al.
Chart 1. Structures and abbreviations of N, (N-X-N) and of
aromatic ketone (IRG) and phosphine oxide (DPO), which were
used to generate reactive free radicals r# (cf. Scheme 1).
Scheme 1. Photogeneration of Spin-Polarized Radicals r# from an
Aromatic Ketone (IRG, Reaction a) and from Phosphine Oxide
(DPO, Reaction b)
Scheme 2. Competition between Chemical Reaction (Cross
Section Rrxn) and Spin Exchange (Cross Section Rex) of Reactive
Polarized Radical r# and a Nitroxyl Radical N
reactions are important from both a fundamental point of view
and for practical applications such as inhibition of spontaneous
polymerization, controlled free-radical polymerization, and for
free-radical chemistry, in general.10-13a
In this report we generated spin-polarized reactive free
radicals r# from suitable precursors according to the reactions
shown in Scheme 1 (see Chart 1 for structures of precursors).
The superscript # symbol is used throughout this report to
indicate spin polarization, a term applied to situations for which
a paramagnetic species possesses a population of spins that is
different from the Boltzmann distribution at the temperature of
the experiment.
The rates of the competing processes of spin exchange (Rex)
and radical combination (Rrxn) have been studied in literature
by TR-EPR (Scheme 2).6,7,8a,9 In addition, the relative rates of
self-termination of r# and of their spin exchange were also
studied by TR-EPR and FT TR-EPR.4,5
EPR spectrometer. Several transient EPR signals evoked by laser pulses
were collected after the microwave bridge preamplifier (20 Hz-6.5
MHz) in the absence of magnetic field modulation. The transient EPR
signals were digitized and averaged by a digital oscilloscope triggered
on the laser pulse (LeCroy 9450A, 300 MHz) at each of 350 magnetic
field values in the sweep of interest. The oscilloscope and the magnetic
field advance were computer controlled by a home written LabView
program. The experimental technique and basics of TR-EPR are
described elsewhere in more detail.8,15 The signals were acquired at
low and high magnetic flux densities and the data matrix was produced
by subtracting from each transient a weighted average of the off-
resonance signals. The steady-state CW-EPR spectra were recorded
with a Bruker EMX EPR spectrometer.
2. Reagents. The following reagents were employed (see Chart 1
for structures): benzil dimethyl monoketal (Irgacure 651, IRG) was
obtained from Ciba Specialty Chemicals and diphenyl-2,4,6-trimeth-
ylbenzoyl phosphine oxide (DPO) was obtained from BASF. The
following reagents were from Aldrich: benzophenone (BP); phthalic
acid; phthalic anhydride; 4-formylbenzoic acid; 2,6-dimethyl-2,5-
heptadien-4-one (phorone); 2,2,6,6-tetramethyl-4-oxopiperidinyl-1-oxyl
(14N-TEMPON) and 2,2,6,6-tetramethyl-4-hydroxy piperidine-N-oxyl
(14N-TEMPOL); (15NH4)2SO4 (98+ atom %), Na2HPO4‚7H2O, NaCl,
NaOH, MgSO4, K2CO3, oxone (potassium peroxymonosulfate), and all
of the solvents. All commercial compounds were used as received.
3. Synthesis of N and (N-X-N). 4-Oxo-2,2,6,6-tetramethyl-1-
piperidine-15N (amine) was synthesized by a double Michael addition.16a
Phorone (9.70 g), (15NH4)2SO4 (3.0 g), and benzene (23 mL) were added
to a 100 mL glass pressure vessel (Chemglass). The mixture was frozen
with acetone/dry ice and then Na2HPO4‚7H2O (5.5 g) and NaOH (2.3
g) in 7.5 g of distilled water were added. The vessel was immediately
sealed and heated to 90 °C in an oil bath with continuous stirring. The
reaction was terminated after 10 days and allowed to cool to room
We describe a new method for the estimation of Rex/Rrxn
,
namely, the measurement of relative integrated intensities of
TR-EPR spectra corresponding to species which undergo both
spin exchange and reaction.7,8a,9 The ratio Rex/Rrxn is a funda-
mental quantity of considerable theoretical interest14 since it
directly compares cross sections (rates) of two fundamental types
of interactions of free radicals with one another: a chemical
interaction (bond formation) and a physical interaction (spin
exchange). In the present work we studied TR-EPR of aromatic
ketone IRG and of phosphine oxide DPO in the presence of
isotopically labeled nitroxyl biradicals, (14N-O-14N), (14N-
P-15N) and (14N-O-15N) (Chart 1).
Experimental Section
1. EPR Experiments. All experiments were performed at ambient
temperature. The TR-EPR measurements were obtained with a Bruker
ER 100D X-band spectrometer. Samples were excited with light pulses
(λ ) 308 nm, 10 ns, 10-15 mJ/pulse) from a Physik Lextra 50 XeCl
excimer laser. Irradiation took place in a flow flat quartz cell (internal
thickness of ∼0.3 mm) in a rectangular cavity (ER 4101 STE) of the
(15) (a) Koptyug, I. V.; Ghatlia, N. D.; Sluggett, G. W.; Turro, N. J.; Ganapathy,
S.; Bentrude, W. G. J. Am. Chem. Soc. 1995, 117, 9486. (b) Turro, N. J.;
Khudyakov, I. V. Chem. Phys. Lett. 1992, 193, 546. (c) Weber, M.;
Khudyakov, I. V.; Turro, N. J. J. Phys. Chem. A 2002, 106, 1938.
(16) (a) Hall, P. L.; Gilchrist, J. H.; Collum, D. B. J. Am. Chem. Soc. 1991,
113, 9571. (b) Liu, Z. Z. Ph.D. Thesis, Columbia University, New York,
2005.
(14) (a) Adrian, F. J. J. Phys. Chem. A 2003, 107, 9045. (b) Adrian, F. J. J.
Chem. Phys. 1988, 88, 3216. (c) Adrian, F. J. Res. Chem. Intermed. 1986,
7, 173.
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Interactions of Free Radicals and Nitroxyl Biradicals
A R T I C L E S
temperature. The mixture was washed twice with diethyl ether. The
ether extract was collected and dried over anhydrous MgSO4. After
the filtration, ether was removed under vacuum with the formation of
a viscous liquid of amber color. The product was purified by silica gel
flash column chromatography, eluting first with a methylene chloride/
THF mixture (from 40:1 to 20:1 v/v), and then with a methylene
chloride/methanol mixture (from 40:1 to 20:1 v/v). The fraction
containing the product was collected and dried under vacuum. 4-Oxo-
2,2,6,6-tetramethyl-1-piperidine-15N was obtained in the form of yellow
needlelike crystals in a yield of 66%. The structure of 4-oxo-
2,2,6,6-tetramethyl-1-piperidine was confirmed by GC, GC/MS (EI),
13C NMR (400 MHz, CDCl3), 1H NMR (300 MHz, CDCl3). 15N-
TEMPON was prepared by the known way of oxidation of 4-oxo-
2,2,6,6-tetramethyl-1-piperidine-15N.16b The product mixture was pu-
rified with silica gel flash column chromatography, eluting with a
hexanes/ethyl acetate mixture (from 4:1 to 3:1 v/v). The product
was collected as an orange-red solid in 39% yield. GC/MS (EI) m/z
(rel. intensities): 171 (M+, 100), 172 (39), 152 (24), 141 (35), 115
(48), 83 (61).
Figure 1. CW-EPR spectrum of (14N-P-15N) (0.5 mM) in hexane (solid
line) and its simulation (dotted line).
4-Hydroxy-2,2,6,6-tetramethyl-1-piperidine-15N (amine) was pre-
pared by reducing 4-oxo-2,2,6,6-tetramethyl-1-piperidine-15N with
NaBH4. 4-Oxo-2,2,6,6-tetramethyl-1-piperidine-15N (2.0 g) were dis-
solved in 35 mL of an ethanol/water mixture (1:1 v/v). The solution
was dripped into 10 mL of aqueous solution of NaBH4 (0.25 g) during
6 min while keeping the temperature of the reactive solution at 0 °C
(ice/water bath). The reactive vessel was removed from the bath, and
the solution was allowed to return to room temperature. NaCl was added
to the reaction mixture, and the product was extracted with 50 mL of
diethyl ether. The ether fraction was separated with a separation funnel
and was dried over anhydrous K2CO3. After removal of the solvent at
reduced pressure, a white solid of 4-hydroxy-2,2,6,6-tetramethyl-1-
piperidine-15N was obtained in 94% yield. The structure of 4-hydroxy-
2,2,6,6-tetramethyl-1-piperidine-15N was confirmed by the same meth-
ods as those of 4-oxo-2,2,6,6-tetramethyl-1-piperidine-15N described
above. 15N-TEMPOL was synthesized by a similar method as 15N-
TEMPON by oxidation of the corresponding amine. 15N-TEMPOL
was shown to be >98% pure by GC analysis. 15N-TEMPOL was
identified by GC/MS (EI) in a similar way as 15N-TEMPON described
above. The syntheses of 15N-TEMPON and 15N-TEMPOL are
described in detail elsewhere.16b
The nitroxyl biradicals (N-X-N) were prepared as follows. Phthalic
anhydride was reacted with 14N-TEMPOL with the resulting formation
of monoester of phthalic acid as described elsewhere.17 The monoester
reacted with 15N-TEMPOL with the resulting formation of diester
(14N-O-15N). The reaction of phthalic anhydride with 14N-TEMPOL
(1:2 on molar basis) yields to diester (14N-O-14N). Esterification of
4-formylbenzoic acid with 14N-TEMPOL results in 4-hydroxy-2,2,6,6-
tetramethylpiperidine-1-oxyl 4-formylbenzoate. The latter was oxidized
with oxone to 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl tereph-
thalic acid monoester, which reacted with 15N-TEMPOL to form (14N-
P-15N). All (N-X-N) were purified by chromatography on a silica
gel column with an ether/hexanes mixture (1:3 v/v).
Figure 2. CW-EPR spectrum of (14N-O-15N) (0.5 mM) in hexane (solid
line) and its simulation (dotted line).
The field position of the lines depends on the eigenvalues. The ratio
of the line intensities derives from the probability of transition, which
was obtained from the eigenstates calculated. The spectra were
generated by summing of the individual lines which were placed at
the proper field, with the proper intensity, width, and shape for each
transition.
Spectra of biradicals and monoradicals were summed to simulate
the observed TR-EPR spectra and their integrals were calculated. The
ratio of integrals of individual spectra contributing to the observed TR-
EPR spectra is considered to be equal to the ratio of the concentrations
of the corresponding paramagnetic species.
Results
CW-EPR Spectra of Nitroxyl Radicals and Biradicals. The
CW-EPR spectra of 14N-TEMPON [15N-TEMPON] were taken
in acetonitrile and in hexane. These spectra (not shown) consist,
as expected from the literature,2 of three [two] components with
a hyperfine coupling constant aN ≈ 1.5 mT [2.1 mT].
Figure 1 displays the CW-EPR spectrum of (14N-P-15N)
in hexane. The experimental spectrum of (14N-P-15N) (Figure
1) is the same within the accuracy of our measurements as the
EPR spectrum of an equimolar mixture of 14N-TEMPON and
15N-TEMPON (or 14N-TEMPOL and 15N-TEMPOL). TEMPO
fragments in (14N-P-15N) are spatially separated; as a result,
the exchange interaction, J, between unpaired electrons for this
biradical is much less than hyperfine coupling constants,
aN.1,2,13,18 The spectrum (Figure 1) was therefore simulated
considering J ) 0. Throughout this report, we use absolute
values of aN and of J.
4. Computer Simulation of EPR Spectra. We simulated experi-
mental EPR spectra of mono- and biradicals obtained in this work.
For the biradicals, the four uncoupled spin states |mS (14N),mS (15N)
(where mS has the usual meaning of projection of the electron spin S)
were taken as basis functions. Zeeman, hyperfine and exchange
interaction were calculated on the basis functions for each of the six
sets of the mI (14N), mI (15N) quantum numbers (where mI is the
projection of the nuclear spin I) and the six resulting matrices were
diagonalized.13c,d Eigenvalues and eigenstates were obtained corre-
sponding to specific values of the two hyperfine interactions a14
,
N
a15N and of the exchange interaction J, chosen to simulate the spectra.
(17) Hassner, A.; Alexanian, V. Tetrahedron Lett. 1978, 46, 4475.
(18) Komaguchi, K.; Iida, T.; Goh, Y.; Ohshita, J.; Kunai, A.; Shiotani, M.
Chem. Phys. Lett. 2004, 387, 327.
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A R T I C L E S
Sartori et al.
Figure 2 presents a CW-EPR spectrum of (14N-O-15N) in
hexane.19 The spectrum is clearly quite different from that of
(14N-P-15N) (Figure 1, solid line).
The difference in the spectra of (14N-P-15N) and (14N-
O-15N) is expected to be due to a stronger exchange interaction,
J, of the two closely positioned TEMPO fragments in the ortho-
nitroxyl biradical. The latter spectrum was simulated (Figure
2, dotted line) by diagonalization of the Zeeman, hyperfine and
exchange interaction (cf. Experimental Section, part 4 above),
and summing lines placed at the proper field, with the proper
intensity, width, and shape for each transition.
A simple combination of nuclear spins I ) 1/2 (15N) and I )
1 (14N) should lead to a six-component spectrum. However, this
result is only a first approximation, valid for J . aN. More
components arise for smaller J values, and indeed eight
components are observed under closer inspection (cf. Figure
2). A value of J ) 24 mT was obtained from the simulation.
The correct intensity of the experimental lines is obtained under
the assumption that the intramolecular exchange interaction is
modulated by a dynamic process (such as conformational
changes of nitroxyl fragments). In fact, to simulate the spectra
it is necessary to associate different widths to the EPR transitions
that belong to different sets of mI quantum numbers, which is
expected as an effect of the modulation of the exchange
interaction J.13,18 In the classical case of isotopically symmetric
nitroxyl biradicals containing only 14N (which possess, therefore,
just one hyperfine coupling constant) the effect is null for the
transitions occurring between states characterized by two
Figure 3. 2D-TR-EPR spectrum produced by photolysis of IRG (0.1 M)
in acetonitrile.
identical mI, the line width depending on a term a2 (mI(1)
-
14N
m(I2))2.13c,d The linewidths of the isotopically asymmetric ni-
troxyl biradicals are expected to depend on the mI (14N) and
Figure 4. 2D-TR-EPR spectrum produced by photolysis of DPO (0.025
M) in acetonitrile.
mI (15N) quantum numbers according to a term(a14 mI (14N) -
N
a15 mI (15N))2. Therefore in this case, all of the transitions are
Photolysis of IRG and DPO occurs via a triplet state as a
Norrish type I process and leads to well-documented TR-EPR
spectra of benzoyl and substituted benzyl radicals and of
2,4,6-trimethylbenzoyl and P-centered radicals, respectively
(Scheme 1).7,15b,c,20,21 The radicals r# produced by photolysis
of IRG show strong emissiVe net polarization (Figure 3),
whereas the radicals r# produced by photolysis of DPO show
strong absorptiVe net polarization (Figure 4).
The 2D-TR-EPR spectra produced by photolysis of IRG and
DPO (Figures 3 and 4) are consistent with the 1D spectra in
the literature. The triplet mechanism (TM) of CIDEP is
dominant in the observed polarization for photolysis of IRG
and DPO.7,8a,15b,c,20,21
TR-EPR Spectra Produced by Photoexcitation of Ben-
zophenone (BP) in the Presence of 14N-TEMPON and 15N-
TEMPON. To extract the cross sections for spin exchange and
chemical reaction, Rex and Rrxn, the concentrations of radicals
must be determined. Thus, an important issue to be addressed
is whether the experimental TR-EPR spectra yield the correct
relative concentrations of polarized radicals. With this aim, the
TR-EPR spectra of 14N-TEMPON# and 15N-TEMPON# were
obtained by photoexcitation of BP in the presence of an
equimolar mixture of 14N-TEMPON and 15N-TEMPON.
Triplet BP is known to produce emissive polarization of nitroxyl
N
affected by the modulation of J, though to a different extent,
1
with those associated with -1, /2 and 1, -1/2 being the most
affected. They correspond to the central components of the
spectrum which are indeed the most broadened.
There are five components expected for the EPR spectra of
the (14N-O-14N) biradicals in a strong exchange;1,2,7,13,18 the
expected five component spectra were experimentally observed
in this work (spectra not shown). The value of J obtained from
the simulation for (14N-O-15N) was confirmed with the
simulation of the spectrum of the isotopically symmetric (14N-
O-14N), whose lines in fact do not obey the 1:2:3:2:1 ratio
expected for J . aN. A modulation of J was also necessary in
this case to reproduce the experimental intensities.
As for the individual line shape, the Lorentzian line shape
leads to the best simulation in the case of (14N-O-14N) and
(14N-O-15N), while Gaussian line shape leads to the best
simulation in the case of (14N-P-15N). The para-nitroxyl
biradical (14N-P-15N) has a Gaussian line shape (see below)
as does the corresponding monoradical (spectrum not shown).
This difference in line shapes is consistent with the absence of
spin-spin interactions in the para-nitroxyl biradical, and the
presence of spin-spin interactions in the ortho-nitroxyl biradical
leads to homogeneous broadening in the latter.
TR-EPR Spectra of IRG and DPO. 2D-TR-EPR spectra
produced by photoexcitation of IRG and DPO were recorded.
(19) We observed minor differences in the CW-EPR spectra of (14N-P-15N)
and (14N-O-15N) in acetonitrile versus hexane. A discussion of these
differences is beyond the scope of this work.
(20) Jockusch, S.; Landis, M. S.; Freiermuth, B.; Turro, N. J. Macromol. 2001,
34, 1619.
(21) (a) Makarov, T. N.; Savitsky, A. N.; Mo¨bius, K.; Beckert, D.; Paul, H. J.
Phys. Chem. A 2005, 109, 2254. (b) Forbes, M. D. E.; Yashiro, H.
Macromol. 2007, 40, 1460.
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Interactions of Free Radicals and Nitroxyl Biradicals
A R T I C L E S
Figure 5. 2D-TR-EPR spectrum produced by photolysis of BP (0.1 M) in
the presence of 14N-TEMPON (0.5 mM) and 15N-TEMPON (0.5 mM) in
acetonitrile.
radicals by the radical triplet pair mechanism (RTPM).6,8,22 We
note that the CW-EPR spectrum of Figure 1 is very similar to
the TR-EPR signal of Figure 5, which is an equal molar mixture
of 14N-TEMPON and 15N-TEMPON. This is the expected
result, since isotopic substitution should have a negligible effect
on quenching of the triplet state of BP by TEMPON.
Isotopic substitution is expected to only influence the EPR
pattern of line intensities. However, the low-field components
of 14N-TEMPON and 15N-TEMPON are found to have
somewhat higher intensities than the corresponding high field
components (Figure 5) owing to a RTPM multiplet effect.8,22
We conclude that TR-EPR spectra manifest correct relative
concentrations of polarized radicals in the systems reported here.
Photolysis of IRG and DPO in the Presence of (14N-O-
14N) and of (14N-O-15N). Figure 6 displays the TR-EPR
spectra obtained under photolysis of IRG in the presence of
(14N-O-15N). The solid line in Figure 6b represents an
experimental spectrum extracted from a 2D-TR-EPR spectrum
like that in Figure 6a, corrected by subtraction of an experi-
mental spectrum of IRG (extracted from a 2D-TR-EPR
spectrum of IRG). The dotted line in Figure 6b is a simulation
based on the weighted sum in eq 1.7
Iobs ) a[I((14N-O-15N)#)] + 0.5b[I((r14N-O-15N)#)] +
0.5b[I((14N-O-15Nr)#)] (1)
In eq 1, Iobs is the observed intensity of the TR-EPR spectrum
(after subtraction of polarized IRG contribution, i.e., r#), I((14N-
O-15N)#) is the intensity ascribed to (14N-O-15N)#, I((r14N-
O-15N)#) and I((14N-O-15Nr)#) are the intensities ascribed
to polarized monoradicals. Integrals of I((14N-O-15N)#),
I((r14N-O-15N)#) and I((14N-O-15Nr)#) are normalized to the
same value and the values of a and b were adjusted to fit the
experimental spectrum. In the simulated spectrum the component
due to (14N-O-15N)# is obtained by using the same parameters
Figure 6. TR-EPR spectra produced by photolysis of IRG (0.1 M) in the
presence of (14N-O-15N) (1.0 mM) in acetonitrile: (a) 2D spectrum; (b)
1D spectrum at 1 µs after the laser flash (solid line) after subtraction of r#
(polarized IRG) contribution and its simulation (dotted line); (c) relative
contributions of each of three polarized species into the simulation spectrum
(sparse stripes filling, (r14N-O-15N)#; dense stripes filling, (14N-O-
15Nr)#, white filling, (14N-O-15N)#, gray filling, sum of the three
contributions; see the text for discussion). Spectra belonging to different
experiments have been chosen for Figure 6a and 6b according to their
suitability for 2D and 1D representation.
(a14 , a15 and J) employed to simulate the CW-EPR spectrum
N
of (14N-NO-15N) except for the line width, which in general is
not expected to be the same as that in TR-EPR spectra. The
monoradical components due to (r14N-O-15N)# and (14N-
O-15Nr)# are added to fit the spectra.
Table 1 summarizes the a/b values obtained by simulation
of the experimental data. The simulation of the experimental
spectrum is good but not perfect because of inaccuracies in the
subtraction of r# contribution into the observed spectrum (Figure
6b). All TR-EPR spectra of spin-adducts (rN-O-N)# are
satisfactorily approximated as symmetric spectra. We assume
(22) (a) Bla¨tter, C.; Jent, F.; Paul, H. Chem. Phys. Lett. 1990, 166, 375. (b)
Kawai, A.; Obi, K. Res. Chem. Intermed. 1993, 19, 865.
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A R T I C L E S
Sartori et al.
Table 1. Ratio of Products of Interaction of r# with (N-O-N) and
Ratio of the Exchange Cross Section to the Reaction Cross
Section
b
biradical
a/ba
Rex/Rrxn
(14N-O-15N)
(14N-O--4N)
1.6-1.1
2.4-1.6
∼4
∼6
a Range of ratios of integral of biradical (N-O-N#) signals to integral
of a sum of monoradical (rN-O-N)# signals (cf. eq 1, 2). Determination
error of individual value was 15%. Parameters were obtained by computer
simulation of TR-EPR spectra. b The estimation of Rex/Rrxn is based on a
number of assumptions. See the text for discussion.
Figure 8. 2D-TR-EPR spectrum produced by photolysis of IRG (0.1 M)
in the presence of (14N-O-14N) (2.5 mM) in acetonitrile.
Scheme 3. Competition between Chemical Reaction (Cross
Section Rrxn) and Spin Exchange (Cross Section Rex) of Reactive
Polarized Radical r# and a Nitroxyl Biradical (N-O-N)
Discussion
Biradicals (14N-X-15N). The biradicals (14N-O-15N) and
(14N-P-15N) differ only with respect to the occurrence of
different nitrogen isotopes at each nitroxyl center. Six to eight
components of the EPR spectrum of (14N-O-15N) are expected
according to the value of J for J > aN. As was described above,
computer simulation demonstrates excellent agreement with the
experimental spectra (Figure 2, dotted line).
Figure 7. 2D-TR-EPR spectrum produced by photolysis of DPO (0.025
M) in the presence of (14N-O-15N) (1.0 mM) in acetonitrile.
that the TR-EPR spectra of r# are the result of only action of
TM. In the presence of relatively high concentration of spin
acceptor, the polarization pattern is preserved in the spin-
adduct.15b,c The accuracy of such an assumption can be seen
by inspection of the goodness of the simulated fit of Figure 6b.
Polarization Transfer under Photolysis of Photoinitiators
in the Presence of (N-O-N). Scheme 3 is a modified Scheme
2 for a reaction of r# with biradical (N-O-N). Knowledge of
A representative TR-EPR spectrum obtained by photolysis
of a solution containing DPO and (14N-O-15N) is shown in
Figure 7. The TR-EPR spectrum obtained by photolysis of a so-
lution containing IRG and (14N-O-14N) is shown in Figure 8.
TR-EPR spectra resulting from the photolysis of DPO in the
presence of (14N-O-15N) show a residual contribution of r#
(Figure 7), which makes estimations of relative contributions
of paramagnetic species less accurate.
2
3
the relative concentrations of (rN-O-N)# and (N-O-N)#
provides a means of determining the ratios of the cross section
of spin exchange (Rex) to the cross section of chemical reaction
(Rrxn), that is, the ratio a/b in eqs 1 and 2.
The (N-O-N)# is formed by electron spin polarization
transfer (ESPT), through the spin exchange.6,7 The knowledge
of experimental a/b ratio allows evaluation of the value of Rex/
Rrxn under certain assumptions.
In the case of (14N-O-14N) we used eq 2 instead of eq 1:7
In previous experiments7 with (14N-O-14N) without com-
puter simulation, it was concluded that the magnitude of Rex is
close to that of Rrxn. The investigation of (14N-O-15N) and
computer simulation in experiments with (14N-O-14N) reported
here allows us to obtain a more reliable a/b ratio from
experimental data. The key feature of our isotopically asym-
metric system is that an interaction of a polarized reactive free
radical r# with (14N-O-15N) results in three distinct polarized
species with different sets of aN for each paramagnetic species
and not oVerlapping spectra (Scheme 4): (r14N-O-15N)#,
(14N-O-15Nr)# and (14N-O-15N)#. This situation is essentially
different from the previously studied case of (14N-O-14N),
where the components of the EPR spectrum of (14N-O-14N)#
coincide with components of the adduct (14N-O-14Nr)#
(Scheme 3).7,13,18
Iobs ) a[I((14N-O-14N)#)] + b[I((r14N-O-14N)#)]
(2)
From this point on we will use the designation (N-O-N),
to stand for both (14N-O-15N) and (14N-O-14N). The
experimental spectra of (N-O-N) were simulated as described
above.
In TR-EPR spectra simulation, Lorentzian line shape was
always chosen for the biradical (N-O-N)# contribution into
the spectrum, whereas for the spin-adduct (N-O-Nr)# con-
tributions both a Lorentzian and a Gaussian line shape lead to
satisfactory simulation. A certain range of values of a/b (Table
1), rather than a unique value, is obtained depending on whether
Gaussian or Lorentzian line shape is ascribed to (N-O-Nr)#
and on some experimental conditions (concentrations and time
of observation).
The value of a/b was found to slightly depend on the
assumption of the line shape of (rN-O-N)#. The line shape
depends upon the mechanism of the line broadening, and it is
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7790 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

Interactions of Free Radicals and Nitroxyl Biradicals
A R T I C L E S
of r# to (N-O-N) is expected to be diffusion-controlled or
very close to diffusion-controlled.7,11,12 The rate constant of a
diffusion-controlled reaction occurring on every encounter
between highly reactive species, in the absence of spin selection
and steric limitations, is described by the von Smoluchowski
equation:3,4,24,26
Scheme 4. The Three Distinct Spin Polarized Species Produced
by the Interaction of Polarized Free Radical r# with (14N-O-15N)
kdiff ) 4πRrxn
D
(5)
difficult to predict a priori.1,2,13 This well-known problem is
beyond the scope of the present work; however, we can estimate
the relative concentrations of paramagnetic species by signal
integrals. The values of a/b and Rex/Rrxn can be estimated within
a certain error (Table 1). We did not perform a detailed
investigation of the dependence of a/b values on solvent,
temperature, isotope substitution, source of polarized radicals,
etc. However, we note that there are no significant changes in
a/b with changes from hexane to acetonitrile or the type of (N-
O-N) employed; in addition, the measured a/b values depend
only slightly on the time of acquisition after the laser flash,
and the a/b values weakly depend on the relative concentration
of reagents.
where D is the mutual diffusion coefficient. In this common
approach, the reagents are considered as hard spheres of a certain
van der Waals radius, and a reaction occurs under their contact
at a distance equal to the cross section of the reaction or the
26
sum of van der Waals radii of reagents Rrxn
.
Thus, the total
rate constant of formation of (rN-O-N)# is given by25
2
1
krxn ) 4πRrxn D(1/4 + 3/4 /3)
(6)
1
where /4 stands for the relative concentration of the singlet
1(N-O-N), 3/4 stands for the relative concentration of the triplet
1
3(N-O-N) and /3 ) σrxn
.
Equation 7 gives the rate constant of the spin exchange, kex,
in the absence of steric limitations:3,26
Figures 6-8 clearly demonstrate the existence of a certain
amount of polarized (N-O-N)# species. This observation
requires that spin exchange (nonreactive ESPT) must occur with
a comparable rate to the rate of chemical reaction between the
same species (reactive ESPT).7,8a,23
kex ) σex3/44πRexD
(7)
We assume in all cases that spin exchange is “strong” or
“fast,” that is, J2τ2 . 1, where J is, in this case, the exchange
integral between 3(N-O-N) and r# and τ is the time of
According to Schemes 3 and 4, the ratio of concentration of
products of two pseudo-first-order reactions should be given
by a/b ) Rex/Rrxn. Since Rex/Rrxn is expected to be equal to kex/
krxn, the value of a/b can be treated as the ratio of two rate
constants, one for spin exchange and one for chemical reaction.
Thus, the measured integrated intensity of (rN-O-N)# and (N-
O-N)# in TR-EPR experiments are directly proportional to the
corresponding pseudo first-order rate constants, k, of their
formation. We assume that the relaxation times T1,2 of (rN-
O-N) are equal to the corresponding T1,2 of (N-O-N). CW-
EPR measurements demonstrated that consumption of (N-O-
N) during the course of the measurement in the flow system
was at most a few percent, so that the concentration of 1,3(N-
O-N) can be considered as constant during the measurements.
In the case of the formation of 2(rN-O-N)# we need to account
for a contribution of a reaction of r# with the EPR “invisible”
3
3
encounter of r# and (N-O-N).3,13 The coefficient /4 in eq 7
is the fraction of the total concentration of (N-O-N) partici-
pating in the spin exchange. The maximum value of σex is
expected to be /9.3 (Additional comments on the value of σex
2
can be found in the Supporting Information.)
The above analysis leads to the expression for the a/b ratio:
a/b ) [2/9 /44πRexD]/[4πRrxnD(1/4 + 3/4 /3)]
(8)
3
1
or
Rex/Rrxn ) 3a/b
(9)
Equation 9 allows the estimation of approximate values of
Rex/Rrxn (Table 1). If, in fact, reactions 3 and 4 occur with rates
lower than diffusion rates, then one needs to use eq 10 instead
of eq 6:26a,27
1
singlet biradicals, 1(N-O-N), which constitute ∼ /4 of the total
concentration of 1,3(N-O-N):1,7,13
r# + 1(N-O-N) f 2(rN-O-N)#
(3)
1
krxn ) 4πRrxnD(1/4 + 3/4 /3)fchem
(10)
Reaction 3 leads to a concentration of polarized monoradicals
that must be considered in our quantitative analysis of Rex and
Rrxn. Reaction 3 is spin-allowed.
where 0 < fchem < 1.0 is the probability of a chemical reaction
during the encounter.26a,27 Then eq 9 should be replaced by
Reaction 4 with EPR “visible” 3(N-O-N), which constitutes
Rex/Rrxn ) 3afchem/b
(11)
3
∼ /4 of the total concentration of (N-O-N),7,11,13 also leads
(24) Khudyakov, I. V.; Serebrennikov, Yu. A.; Turro, N. J. Chem. ReV. 1993,
93, 537.
(25) Buchachenko, A. L.; Ruban, L. V.; Step, E. N.; Turro, N. J. Chem. Phys.
Lett. 1995, 233, 315.
(26) Bimolecular chemical reactions and spin exchange with participation of
sterically hindered nitroxyl radicals are often characterized by a certain
steric factor feff, see for example: (a) Burshtein, A. I.; Khudyakov, I. V.;
Yakobson, B. I. Prog. React. Kinet. 1984, 13, 221. See also (b) references
3 and 12. We assume that feff values are close to each other for reaction
and for spin exchange and feff values cancel each other in eq 9.
(27) Heming, M.; Roduner, E.; Reid, I. D.; Schneider, J. W.; Keller, H.;
Odermatt, W.; Patterson, B. D.; Simmler, H.; Pu¨mpin, B.; Savic´, I. M.
Chem. Phys. 1989, 129, 335.
to polarized monoradicals:
r# + 3(N-O-N) f 2(rN-O-N)#
(4)
The spin-statistical factor for reaction 4 of the doublet r# with
3
1
the triplet (N-O-N) is σrxn ) /3.7,24,25 The rate of addition
(23) RTPM is not responsible for polarization of nitroxyls under photolysis of
DPO and IRG because these two compounds have extremely short-lived
triplet states (cf. ref 7 and references therein).
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7791

A R T I C L E S
Sartori et al.
Values of Rex/Rrxn of approximately 4-6 were determined
for the interaction between reactive radicals r# and stable
nitroxyl biradicals (N-O-N) (eq 9, Table 1). The values of
Thus, we conclude that spin exchange between a biradical (N-
O-N) and a polarized organic radical r# occurs at a relatively
large distance. If krxn is lower than kdiff, then an estimation of
Rex versus Rrxn are fundamental parameters for an understanding
4-6 is found as an upper limit of Rex/Rrxn.
of reactive and nonreactive contacts between two molecules,
an issue of great significance in chemistry. It was concluded in
pioneering work4 on this problem that Rex is 2-3 times larger
than Rrxn for a reaction between two radicals.5 The conclusion
that Rex will generally be greater than Rrxn is understandable:
spin exchange can take place when orbitals of reagents overlap
weakly, or exchange can occur even through diamagnetic solvent
molecules.3 On the contrary, formation of a covalent bond
requires substantial overlap of orbitals of the reacting pair and
cannot occur if an intervening molecule separates r and (N-
O-N). The value of Rex/Rrxn may be relatively high. It is
possible that reactions are not completely diffusion-controlled,
and the use of eq 11 will lead to smaller Rex/Rrxn values.
The study of TR-EPR with computer simulation of spectra
of the new biradicals (14N-O-15N) as well as spectra of the
known (14N-O-14N) biradicals during photolysis of aromatic
ketones has provided a deeper understanding of chemical and
physical processes with the participation of biradicals. In
addition to the fundamental importance of these measurements
toward an understanding of reactive and unreactive collisions,
the results and conclusions presented in this report provide an
understanding of the mechanisms and chemistry of nitroxyl
biradicals. The latter are used as antioxidants and UV-stabilizers
of polymers and interact with reactive thermo- or photogenerated
free radicals.12,13a Finally, we propose that isotopically asym-
metric biradicals such as (14N-O-15N) will find new applica-
tions as spin probes.
Conclusions
Acknowledgment. The authors thank the National Science
Foundation, Grant 04-15516, for generous support of this
research.
The relative values of Rex versus Rrxn were calculated
employing TR-EPR and computer simulation to estimate the
concentrations of products of spin exchange and chemical
reaction between the same reagents.7,9 It was assumed that the
exchange is “strong” and chemical reactions are diffusion-
controlled. A unique feature (Scheme 4) of the isotopically
asymmetric biradical (14N-O-15N) allows the extraction of
values of the ratio Rex/Rrxn to be ∼4 from the TR-EPR results.
Supporting Information Available: Additional information
on σex. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0687670
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7792 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007