RIKEN Review No. 44 (February, 2002): Focused on Magnetic Field and Spin Effects in Chemistry and Related Phenomena
Oscillating magnetic ﬁeld effects on chemical
Jonathan R. Woodward,∗1 Christiane R. Timmel,∗2 Peter J. Hore,∗2 and Keith A. McLauchlan∗2
Department of Chemistry, University of Leicester, UK
Physical and Theoretical Chemistry Laboratory, Oxford University, UK
The eﬀect of static magnetic ﬁelds on the recombination yield and kinetics of radical pair (RP) processes has
long been established. Our recent experiments demonstrate the control of RP reactions using magnetic ﬁelds
oscillating at frequencies that correspond to the hyperﬁne couplings of the RP members. Experimental data
is presented for eﬀects on radical ion pairs in viscous solution in both the absence and presence of a static
The generation of RPs in solution occurs with conservation
of electron spin angular momentum such that pairs are cre-
ated in non-stationary spin correlated states with the same
multiplicity (singlet or triplet) as the molecular precursor.
These pairs then oscillate coherently between these singlet
and triplet electronic states, which can react selectively to
give distinct chemical products. Thus, the singlet-triplet mix-
ing process controls the overall yield of the reaction. Static
magnetic ﬁelds have been employed to alter the rate and ex-
tent of this spin-mixing process either by energetically sep-
arating the T+1 and T−1 sublevels of the pair (via the Zee-
man interaction),1, 2) with magnetic ﬁelds of typically 1 mT
or more, or by enhancing the mixing process through the ap-
plication of smaller magnetic ﬁelds, the so-called Low Field
Hyperﬁne interactions are responsible for S-T mixing, and
thus we proposed that resonant eﬀects might be observed by
applying radiofrequency radiation at frequencies related to
the hyperﬁne couplings of the RP members.4, 5) A detailed
discussion of the mechanism by which resonant RF radiation
can inﬂuence spin mixing6) indicates that eﬀects similar in
magnitude to LFEs should be observable. Here we report
our recent experimental ﬁndings and theoretical calculations
for oscillating magnetic ﬁeld eﬀects (OMFEs) in radical ion
pair/exciplex systems. Fig. 1. Schematic diagram of the experimental apparatus used to
measure the eﬀect of oscillating magnetic ﬁelds on the yield of
radical ion pair recombination reactions.
A very sensitive detection system is required for the observa- collected using a liquid light guide orthogonal to the UV ir-
tion of OMFEs, as their magnitude tends to be small, even radiation beam and ultimately impinges on a photomultiplier
relative to the eﬀects of static ﬁelds of a few millitesla or more. tube. A phase sensitive detector is used to measure only that
Figure 1 shows the experimental apparatus used for the mea- part of the ﬂuorescence that oscillates at the audiofrequency
surements. In brief, radiofrequency and audiofrequency sig- at which the radiofrequency is modulated. This reduction
nals are mixed to produce a signal that consists of the desired of bandwidth provides the necessary sensitivity to measure
radiofrequency modulated at audiofrequency. This signal is the very small changes in ﬂuorescence intensity produced. A
ampliﬁed and supplied to a helmholtz pair containing the more detailed description of the experimental arrangement
ﬂowing sample. A xenon arc lamp provides UV radiation, can be found in Ref. 7).
which generates radical ion pairs inside the oscillating ﬁeld.
Singlet radical pairs give rise to exciplexes that ﬂuoresce, pro-
viding a measure of the singlet yield. This ﬂuorescence is
OMFEs in zero static ﬁeld Where Ij is the spin quantum number of nucleus j in radical
We have been successful in observing OMFEs in systems con-
sisting of pyrene and the isomers of dicyanobenzene, both The OMFE spectra reveal the individual contributions of the
in protonated and perdeuterated forms.8) More recently, we radical pair members, giving a much clearer picture of the RP
have performed a detailed study on the isotopomers of pyrene physics than is possible from static ﬁeld measurements, which
and dimethylaniline. Figure 2 shows frequency sweep spec- tend to be determined by the average hyperﬁne interaction
tra from 0–80 MHz for such systems along with simulations of the two radicals.10) Also, this technique may prove useful
calculated by a novel, rapid and insightful method. Time de- in the study of the ﬁeld sensitivity of enzymatic reactions
pendent perturbation theory is used to estimate the spectrum and electron-hole recombination in light emitting polymers
as a series of delta functions whose frequencies are the energy at concentrations below the detection limit of time resolved
level separations of the spin systems, and whose heights are EPR. It could also ﬁnd use as a diagnostic for the RP model
proportional to the ﬁeld induced change in singlet yield. The of photoreceptor based magnetoreception in birds.11)
stick spectra are then convolved with a Lorentzian lineshape
to mimic the broadening caused by oﬀ-resonance excitation.
Further details can be found in Ref. 9), along with details of OMFEs in an orthogonal static ﬁeld
the parameters employed.
In a traditional RYDMR experiment, a strong magnetic ﬁeld
There is excellent agreement between the experimental and (typically that of an X-band EPR spectrometer, 340 mT)
theoretical spectra. Each spectrum essentially shows reso- energetically isolates the T+1 and T−1 states from the S and
nances at the eﬀective hyperﬁne coupling of each of the RP T0 ones by the Zeeman interaction. Application of reso-
members, calculated by, nant microwave radiation excites transitions between these
two sets of states, altering the yield of the reaction in the
same sense as the OMFE experiments above.
aK = aKj 2 Ij (Ij + 1). (1)
We have performed experiments in which a static magnetic
ﬁeld of increasing magnitude is applied orthogonal to the ra-
diofrequency ﬁeld in an attempt to measure the intermediate
behaviour between our OMFE experiment and a traditional
RYDMR experiment. The apparatus described above was
modiﬁed by removing the mu metal box and encasing the en-
tire block, containing sample cell and RF coil, inside a large
pair of Helmholtz coils (radius 150 mm), such that static and
oscillating ﬁelds could be applied orthogonal to one another.
Figure 3 (a) shows radiofrequency sweep spectra obtained for
the photochemical reaction of pyrene and 1,3-dicyanobenzene
with increasing applied static ﬁeld.
The major changes associated with the application of the
static ﬁeld are as follows. (1) The resonance at approxi-
mately 35 MHz in the spectrum at B0 = 0, indicated by
squares, moves to higher frequency as B0 increases. (2) When
B0 = 0, there is a new feature, indicated by circles, whose
position and magnitude depends strongly on B0 . It is ob-
served between 10 and 20 MHz for B0 = 0.5 mT, between 20
and 30 MHz for B0 = 1.0 mT, and between 70 and 80 MHz
for B0 = 2.7 mT. These positions match the respective Lar-
mor frequencies: 14, 28 and 76 MHz. (3) The amplitude of
the spectrum increases by a factor of about eight as B0 is
increased from 0 to 2.7 mT. (4) The signal at most frequen-
cies is, as expected, negative. However, positive amplitudes
are observed above –55 MHz when B0 = 0, and at higher fre-
quencies in stronger ﬁelds. The 0.5 and 1.0 mT spectra have,
in addition, positive amplitudes at low radiofrequencies.
Figure 3 (b) shows calculations performed using the technique
described in Ref. 6). Once again, no account is made for elec-
tron exchange or dipolar interactions, spin relaxation or rad-
ical diﬀusion and the exponential model 1, 2, 5) is used for rad-
ical disappearence, with singlet pairs recombining and triplet
Fig. 2. Experimental (lower) and theoretical (upper) radiofrequency
sweep spectra for the isotopomers of pyrene (Py) and dimethy- pairs diﬀusively separating with equal rate constants (k =
laniline (DMA) in a 9 : 1 mixture of cyclohexanol: acetonitrile: 5.6 × 107 s−1 ). Despite these simpliﬁcations, the form of the
(i) deuterated Py, deuterated DMA, (ii) deuterated Py, proto- simulations agrees quite well with the experimental spectra.
nated DMA, (iii) protonated Py and deuterated DMA, and (iv) Further details of this system are included in Ref. 12).
protonated Py and protonated DMA.
Fig. 3. Experimental (a) and theoretical (b) radiofrequency sweep spectra for Py and 1,3-dicyanobenzene (DCB) in a 9 : 1 mixture of
cyclohexanol: acetonitrile under (i) zero static magnetic ﬁeld, (ii) 0.5 mT static ﬁeld, (iii) 1.0 mT static ﬁeld, and (iv) 2.7 mT static ﬁeld.
It is possible that this technique may be capable of dis- systems.
criminating radical pairs based on their diﬀusive trajecto-
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