Enrichment of germanium-73 with the magnetic isotope effect on the hydrogen abstraction reaction of triplet benzophenone with triethylgermane in an SDS micellar solution

Article abstract of DOI:10.1021/ja9739493

Using the hydrogen abstraction reaction of triplet benzophenone with triethylgermane in an SDS micellar solution, we succeeded in enriching ⁷³Ge with the magnetic isotope effect. At zero magnetic field, 0.5% enrichment of ⁷³Ge was ob

Full text of DOI:10.1021/ja9739493

J. Am. Chem. Soc. 1998, 120, 3227-3230
3227
Enrichment of Germanium-73 with the Magnetic Isotope Effect on
the Hydrogen Abstraction Reaction of Triplet Benzophenone with
Triethylgermane in an SDS Micellar Solution
Masanobu Wakasa,† Hisaharu Hayashi,*,† Katuyoshi Ohara,‡ and Takeo Takada‡
Contribution from the Molecular Photochemistry Laboratory, The Institute of Physical and Chemical
Research (RIKEN), Wako, Saitama 351-01, Japan, and Department of Chemistry, Faculty of Science,
Rikkyo UniVersity, Nishiikebukuro, Toshima, Tokyo 171, Japan
ReceiVed NoVember 17, 1997
Abstract: Using the hydrogen abstraction reaction of triplet benzophenone with triethylgermane in an SDS
micellar solution, we succeeded in enriching 73Ge with the magnetic isotope effect. At zero magnetic field,
0.5% enrichment of 73Ge was observed for the cage product (diphenyl(triethylgermyl)methanol). With increasing
magnetic field strength (B), the enrichment of 73Ge increased initially, attaining the maximum value of 5.3%
at 500 G. At higher fields (500 G < B e 10 kG), it decreased gradually, becoming 1.0% at 10 kG. The
magnetic field dependence of the enrichment of 73Ge can be explained by the relaxation mechanism.
Introduction
enriched by the MIEs on the reactions involving heavy-atom-
centered radicals, although a small degree of the enrichment of
29Si was reported with a Si-substituted carbon-centered radical.10
There was a report of an MIE of an Sn-centered radical,11 but
this was subsequently withdrawn.12
It is important, therefore, to find general reaction systems
which can be extended to the enrichment of other heavy atoms.
In the present study, we have found that 73Ge can be much more
efficiently enriched by the MIE on the hydrogen abstraction
reaction of triplet benzophenone with triethylgermane in an SDS
micellar solution than by the MIE on our previous reaction.7
Since the hydrogen abstraction by excited carbonyl compounds
is one of the best-known photochemical reactions, one can
expect that many other heavy isotopes will be enriched by the
MIE on the reactions of this type.
Magnetic field effects (MFEs) on chemical reactions of radical
pairs have been studied extensively and constitute a rapidly
developing field encompassing chemistry, physics, and biol-
ogy.1,2 This is a new branch which may be called “spin
chemistry”.3 The magnetic isotope effect (MIE) is one of the
most interesting applications of spin chemistry.4-6 Here,
reaction rates of radical pairs can be changed, not by the mass
of isotopes, but by the hyperfine interaction between electron
and nuclear spins. It is, therefore, possible to enrich magnetic
isotopes in the reactions which occur through radical pairs.
Large MFEs and MIEs have been observed during reactions
of light-atom-centered radicals such as carbon-, nitrogen-, and
oxygen-centered ones.1,2,4,5 MFEs and MIEs, however, decrease
drastically with increasing atomic number. This is because the
spin-orbit interaction of heavy atoms, which enhances the spin
conversion of radical pairs, is insensitive to external magnetic
fields. In 1993, we found that the photolysis of methyltriphen-
ylgermane (Ph3MeGe) in a Brij35 micellar solution gave 4%
enrichment of 73Ge.7 At that stage, 73Ge was the heaviest mag-
netic isotope enriched via the MIE from samples of natural abun-
dance. After our report, Khudyakov and Buchachenko reported
on the enrichment of 235U by the MIE from samples of natural
abundance.5 To the best of our knowledge, only three magnetic
isotopes (33S,8 73Ge,7 and 235U5,9) have ever been found to be
Experimental Section
Materials. Triethylgermane (Et3GeH) was synthesized as described
in the literature.13 Diphenyl(triethylgermyl)methanol was synthesized
from (triethylgermyl)lithium with benzophenone in hexane.14 Hexa-
ethyldigermoxane was synthesized by the treatment of triethylchloro-
germane with H2O.15 Benzopinacole and benzophenone (BP) were
(8) Step, E. N.; Buchachenko, A. L.; Turro, N. J. Chem. Phys. 1992,
162, 189.
(9) Rykov, S. V.; Khudyakov, I. V.; Skakovsky, E. D.; Tychinskaya, L.
Yu.; Ogorodnikova, M. M. J. Photochem. Photobiol., A 1992, 66, 127.
(10) Step, E. N.; Tarasov, V. F.; Buchachenko, A. L. Chem. Phys. Lett.
1988, 144, 523.
(11) Podoplelov, A. V.; Leshina, T. V.; Sagdeev, R. Z.; Molin, Yu. N.;
Gol′danskii, V. I. JETP Lett. 1979, 29, 380.
(12) Podoplelov, A. V.; Medvedev, V. I.; Sagdeev, R. Z.; Salikhov, K.
M.; Molin, Yu. N.; Moralev, V. M.; Gol’danskii, V. I. Proc. Int. Conf.
Chemically Induced Spin Polarization and Magnetic Effects in Chemical
Reactions; Sagdeev, R. Z., Ed.; Institute of Chemical Kinetics and
Combustion: Novosibirsk, USSR, 1981; p 81.
(13) Anderson, H. H. J. Am. Chem. Soc. 1957, 79, 326.
(14) (Triethylgermyl)lithium was synthesized by the treatment of trieth-
ylhydrogermane with t-BuLi in THF at -10 °C. Reaction of (triethylger-
myl)lithium with benzophenone was carried out similarly as described in:
Gladyshev, E. N.; Vyazankin, N. S.; Fedorova, E. A.; Yuntila, L. O.;
Razuvaev G. A. J. Organomet. Chem. 1974, 64, 307.
† The Institute of Physical and Chemical Research (RIKEN).
‡ Rikkyo University.
(1) (a) Steiner, U. E.; Ulrich, T. Chem. ReV. 1989, 89, 51. (b) Steiner,
U. E.; Wolff, H.-J. Photochemistry and Photophysics; CRC Press: Boca
Raton, FL, 1991; Vol. 3, Chapter 1.
(2) Hayashi, H. Photochemistry and Photophysics; CRC Press: Boca
Raton, FL, 1990; Vol. 1, Chapter 2.
(3) S. Nagakura Festschrift; El-Sayed, M. A., Ed. J. Phys. Chem. 1997,
101 (4).
(4) Buchachenko, A. L. Prog. React. Kinet. 1984, 13, 163.
(5) Buchachenko, A. L. Chem. ReV. 1995, 95, 2507.
(6) Salikhov, K. M. Magnetic Isotope Effect in Radical Reactions;
Springer: Wien, New York, 1996.
(7) Wakasa, M.; Hayashi, H.; Kobayashi, T.; Takada, T. J. Phys. Chem.
1993, 97, 13444.
(15) Anderson, H. H. J. Am. Chem. Soc. 1950, 72, 2089.
S0002-7863(97)03949-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/17/1998

3228 J. Am. Chem. Soc., Vol. 120, No. 13, 1998
Wakasa et al.
Table 1. Relative Isotope Enrichment, δ(72Ge) and δ(73Ge),
Observed upon Photolysis of Benzophenone with Triethylgermane
in SDS Micellar Solution at Room Temperature in the Absence of
an External Magnetic Field
Scheme 1
entrya
δ(72Ge)/%b
δ(73Ge)/%b
1
2
3
0.0 ( 0.5
0.1 ( 0.5
0.0 ( 0.5
0.5 ( 0.3
0.4 ( 0.6
0.5 ( 0.5
a In each entry, the δ values were measured 10 times, the error being
the relative standard deviation (RSD) of such measurements. b δ(ZGe)
) ((ZGe/74Ge)/(ZGe/74Ge)0) - 1.
Thus, we investigated the magnetic isotope effects (MIEs)
on product 1. After photolysis of the SDS micellar solution
containing Et3GeH and BP under 95% conversion in the absence
of an external magnetic field, the ratios of 72Ge/74Ge and
73Ge/74Ge in product 1 were measured by the ICP-MS. The
relative enrichment of ZGe (δ(ZGe)) can be represented as
follows:
obtained commercially and recrystallized from ethanol. Sodium
dodecyl sulfate (SDS) was recrystallized from a 1:1 (v/v) mixture of
methanol and ethanol. Water was distilled and deionized.
δ(ZGe) ) ((ZGe/74Ge)/(ZGe/74Ge)0) - 1
(1)
Photochemical Reaction. The concentrations of Et3GeH, BP, and
SDS in the employed micellar solution were 1.0 × 10-2, 3.0 × 10-3
,
and 8.0 × 10-2 mol dm-3, respectively. The irradiation of the sample
was carried out with a 500 W Xe lamp at room temperature under
nitrogen atmosphere.
Here, (ZGe/74Ge)0 and (ZGe/74Ge) represent the isotope ratios
Z
of Ge/74Ge observed before (on starting compound Et3GeH)
and after (on product 1) reactions, respectively. Typical results
observed for the δ(ZGe) values at zero field are listed in Table
1. This table shows that the δ(73Ge) values are slightly larger
than the experimental errors, but the δ(72Ge) ones are almost
zero. This means that enrichment of magnetic 73Ge was
observed slightly in the absence of an external magnetic field,
but that no appreciable enrichment of nonmagnetic 72Ge was
observed at zero field.
Isotope Analysis. For analysis of the germanium isotope after
photolysis, the reaction products and starting compounds (Et3GeH and
BP) had to be isolated from the micellar solution, but this isolation
was very difficult. We isolated them by using a JAI-LC908 recycle
HPLC with a Hitachi Kasei GL-W252 gel permeation chromatography
(GPC) column. The isotope ratios were measured using a VG-
ELEMENT PQ-Ω inductively coupled plasma mass spectrometer (ICP-
MS).7
In the absence of an external magnetic field, the enrichment
of magnetic 73Ge is somewhat small. This may be due to the
slow recombination reaction of the singlet radical pair. Ac-
cording to the irreversible reaction system shown by Scheme
1, the isotope enrichment, S, in the cage product can be
represented as follows:4
Results and Discussion
First, product analysis for the photochemical reaction of
benzophenone (BP) with triethylgermane (Et3GeH) in the SDS
micellar solution was carried out. Results are described in
Scheme 1. Irradiation of the micellar solution for 20 min gave
diphenyl(triethylgermyl) methanol (1, 25%16), benzopinacol (2,
36%16), and hexaethyldigermoxane (3) under 95% conversion
of BP. These products were separated by preparative GLC and
identified in comparison with authentic samples by MS and
NMR. As a minor product, a coupling product of benzophenone
ketyl radical and alkyl radical from SDS was also obtained.
Here, it is considered that product 1 is a cage product and
products 2 and 3 are escaped ones. As shown in Scheme 1,
upon photolysis of the SDS micellar solution containing
Et3GeH and BP, the benzophenone triplet state (3BP*) is pro-
duced, followed by the hydrogen abstraction reaction to produce
a triplet radical pair of the triethylgermyl radical (Et3Ge•) and
benzophenone ketyl radical (BPH•) in the micellar supercage.
The triplet radical pair in the cage converts to the singlet radical
pair, and the radicals in the radical pairs partially escape from
the cage, forming escaped radicals. Since triethylgermyl radical
is unstable in H2O, the escaped radical immediately reacts with
H2O, resulting in product 3. On the other hand, two escaped
ketyl radicals are coupled with each other, forming product 2.
From the singlet radical pair, cage recombination resulting in
product 1 can occur. Here, the triplet-singlet (T-S) spin
conversion of the radical pair involving magnetic 73Ge (I ) 9/2)
is much faster than that involving nonmagnetic 70Ge, 72Ge,
74Ge, and 76Ge. Consequently, the 73Ge isotope can be enriched
in product 1.
S ) δ(ZGe) + 1 ) γ(k1 + k2)/(k1 + k2*)
γ ) k2*/k2
(2)
(3)
where k1 represents the rate constant of escape the process and
k2, and k2* represent those of the recombination of nonmagnetic
and magnetic radical pairs, respectively. The γ value character-
izes the efficiency of isotope selection. In such reactions as
the present one where reactions occur from T-precursors, the
recombination process (k2 and k2*) includes the T-S spin
conversion process (kT-S) and the reaction of the singlet radical
pair (krec) as shown in Scheme 1. In the case of a fast reaction
(krec > kT-S), the isotope enrichment (S) only depends on kT-S
.
Since the T-S spin conversion of the radical pair involving
magnetic 73Ge is much faster than that involving nonmagnetic
72Ge, a large enrichment of magnetic 73Ge is expected to occur.
However, when the reaction of the singlet radical pair is
comparable to or slower than the spin conversion process (krec
e kT-S), the enrichment may be depressed by this slow process.
This is due to the fact that the rate-determining process is not
the spin conversion but the reaction of the singlet radical pair
which is independent of nuclear spins. The small enrichment
of the present reaction at a zero field can be explained by the
latter case.
Next, the magnetic field dependence of δ(72Ge) and δ(73Ge)
was also measured in the magnetic field range of 0-10 kG.
(16) The product yield was determined by GLC on the basis of consumed
benzophenone.

Enrichment of 73Ge with the Magnetic Isotope Effect
J. Am. Chem. Soc., Vol. 120, No. 13, 1998 3229
involves a 73Ge-centered radical. Thus, the T(1-S conversion
of RPm is not blocked at B ) B1 as shown by case B of Figure
2. The δ(73Ge) value at B ) B1, therefore, becomes larger
than that at B ) 0 G. With increasing B from B1, a gradual
blocking of the T(1-S conversion starts to occur for RPm. Thus,
δ(73Ge) value starts to decrease with increasing B from B1.
Finally, the decrease in the δ value is saturated at B ) B2 ≈
2B1/2* as shown by case C of Figure 2. The magnetic field
dependence of the δ(73Ge) value due to the HFCM, therefore,
can be summarized as follows: The δ value increases with
increasing B (0 G e B e B1), attaining the maximum value at
B ) B1 (≈2B1/2). With increasing B from B1, the δ value
decreases, becoming saturated at B ) B2 (≈2B1/2*) as shown
in Figure 2a.
Figure 1. Magnetic field dependence of the relative isotope enrichment
δ(ZGe): open circles, δ(72Ge); filled circles, δ(73Ge). After photolysis
of SDS micellar solutions containing benzophenone (BP) and trieth-
ylgermane (Et3GeH) under 95% conversion in the magnetic field range
of 0-10 kG.
In the present reaction, the B1/2 value of RPn is calculated to
be 18.3 G with the reported hyperfine coupling constants (aCH
3
) 0.56 G and aCH ) 4.75 G for Et3Ge• 19 and ao ) 3.21 G, am
2
) 1.23 G, ap ) 3.64 G, and aOH ) 2.91 G for BPH• 20). On
the other hand, the hyperfine coupling constant of 73Ge of
Et3Ge• has not yet been reported, but it seems to be similar to
that of the trimethylgermyl radical which has been reported to
be 84.7 G.21 Using this value, we estimate the B1/2* value of
RPm to be 828 G. Therefore, if the blocking of the T(1-S con-
version occurs through the HFCM, the δ(73Ge) value is ex-
pected to attain the maximum value around 40 G (≈B1), but
the decrease of the δ(73Ge) value is expected to be saturated
around 1.7 kG (≈B2). In the present study, however, the
maximum δ(73Ge) value was obtained at 500 G and the decrease
of the δ value was not saturated even at 10 kG as shown in
Figure 1. Thus, the magnetic field dependence observed for
the δ value in the present reaction cannot be explained by the
HFCM.
On the other hand, according to the RM, the T(1-S con-
version through the RM occurs by the anisotropic HFCs. In
this mechanism, the T(1-S conversion rates of RPn and RPm
decrease gradually with increasing B from 0 G, but their
conversion should not be blocked at B1 and B2, respectively.
However, with increasing B from B1, the T(1-S conversion of
RPn should be almost blocked at B ) B1′ > B1 as shown by
case D of Figure 2, because the T(1-S conversion rate of RPn
becomes negligible at B ) B1′. At this stage, the T(1-S
conversion of RPm is not blocked as shown by case E of Fig-
ure 2. Thus, the δ value increases with increasing B from 0 G
to B1′ as shown in Figure 2b. At higher fields (B > B1′), a
gradual blocking of the T(1-S conversion also starts to occur
for RPm. Thus, the δ value starts to decrease with increasing
B from B1′ as shown in Figure 2b. Finally, the T(1-S
conversion of RPm is almost blocked at B ) B2′ > B2 as shown
by case F of Figure 2.
The observed δ(73,72Ge) values are plotted against the magnetic
field strength (B) in Figure 1. Here, the δ(72Ge) value shows
no appreciable change beyond the experimental error. With
increasing B from 0 to 500 G, however, the δ(73Ge) value
increases initially, attaining the maximum value of (5.3 ( 0.3)%
at 500 G. With increasing B from 500 G to 10 kG, the
δ(73Ge) value decreases gradually, becoming (1.0 ( 0.3)% at
10 kG. The magnetic field dependence of the isotope enrich-
ment of 73Ge shown in this figure is strong evidence for the
enrichment by the MIE.
The observed magnetic field dependence of the isotope en-
richment of 73Ge can be explained by the blocking of the T(1-S
conversion by external magnetic fields. Such MFEs on the
T(1-S conversion occur through the hyperfine coupling mech-
anism (HFCM)1,2,4,5 and the relaxation mechanism (RM).17
Figure 2 schematically shows the energy diagram of singlet and
triplet radical pairs and the magnetic field dependence of the
isotope enrichment (δ) expected for the T(1-S conversion
through the HFCM and RM. Here, RPn represents a radical
pair involving a nonmagnetic 70Ge, 72Ge, 74Ge, or 76Ge and RPm
that involving a magnetic 73Ge.
According to the HFCM, the half-saturation field (B1/2) of
the MFE is given as follows:18
2
B1/2 ) 2(A12 + A2 )/(A1 + A2)
(4)
The individual Ai value characterizing the radical (i ) 1, 2) is
given by
2 1/2
A ) ( I (I + 1)a )
(5)
∑
i
ij ij
ij
j
In the present study, the magnetic fields of B1′ and B2′ cannot
be calculated exactly, because the anisotropic HFCs of Et3Ge•
and BPH• have never been observed. However, the magnetic
field dependence of the isotope enrichment observed for the
present reaction can qualitatively be explained by the RM for
the following reasons: (1) Since the radical pair generated in
the present reaction involves the triethylgermyl radical, the
anisotropic HFC of 73Ge seems to be fairly large. (2) The
maximum δ(73Ge) value was obtained at 500 G which is much
larger than that (40 G) expected from the HFCM. (3) The
decrease of the δ value was not saturated even at 10 kG. Thus,
Here, aij is the isotropic hyperfine coupling constant of the jth
nuclei in radical i. In Figure 2a, the half-saturation fields of
the MFEs of RPn and RPm are represented by B1/2 and B1/2*,
respectively. At zero field, since the triplet and singlet radical
pairs are degenerate, the T-S conversions of both RPn and RPm
occur. Because the T-S conversion of RPm is much faster than
that of RPn as mentioned above, the enrichment of 73Ge is
observed for the cage product. An increase of B from 0 G to
B1 (≈2B1/2) primarily affects RPn, in which its T(1-S con-
version is induced by small aij values of protons. Therefore,
the T(1-S conversion of RPn is almost blocked at B ) B1 ≈
2B1/2 as shown by case A of Figure 2. In RPm, however, the
B1/2* value is much larger than the B1/2 value, because RPm
(19) Sakurai, H.; Mochida, K.; Kira, M. J. Am. Chem. Soc. 1975, 97,
929.
(20) Bowers, P. R.; McLauchlan, K. H.; Sealy, R. C. J. Chem. Soc.,
Perkin Trans. 2 1976, 72, 915.
(17) Hayashi, H.; Nagakura, S Bull. Chem. Soc. Jpn. 1984, 57, 322.
(18) Weller, A.; Nolting, F.; Staerk, H. Chem. Phys. Lett. 1983, 96, 24.
(21) Lloyd, R. V.; Rogers, M. T. J. Am. Chem. Soc. 1973, 95, 2459.

3230 J. Am. Chem. Soc., Vol. 120, No. 13, 1998
Wakasa et al.
Figure 2. Schematic energy diagram of singlet and triplet radical pairs and the magnetic field dependence of isotope enrichment (δ) by the HFCM
and RM: (a) in the case of the HFCM, (b) in the case of the RM. Here, RPn represents a radical pair involving a nonmagnetic 70Ge, 72Ge, 74Ge,
or 76Ge and RPm that involving a magnetic 73Ge.
the present magnetic field dependence of the δ value was
concluded to be due to the RM. Such magnetic isotope en-
richment by the RM was theoretically proposed by Salikhov,6
but no experimental result has ever been reported until now.22
Therefore, the present study is the first experimental report on
the isotope enrichment due to the RM.
In a partially reversible reaction system such as the previous
one,7 the isotope enrichment depends on the conversion yield.
Thus, the larger the isotope enrichment, the higher the conver-
sion yield (and therefore the lower the product yield). Actually,
in the previous reaction,7 we had to continue the reaction until
93% conversion in order to enrich 73Ge by 4%. This means
that the yield of the product involving enriched 73Ge is as small
as 7%. On the other hand, in an irreversible reaction system,
the isotope enrichment is independent of the product yield but
only dependent on the γ value () k2*/k2). We, therefore, can
expect to obtain a large isotope enrichment with a high product
yield if we choose a suitable irreversible reaction. In the present
study, the isotope enrichment (5.3%) and the yield of the cage
product (25%) were certainly larger than those observed in the
previous study7 for a partially reversible reaction system. In
the present study, we have found that the hydrogen abstract
reactions of triplet carbonyls are suitable for the magnetic
isotope enrichment of heavy atoms.
Conclusion
In the present study, we have succeeded in enriching 73Ge
using a general photochemical reaction system of hydrogen
abstraction of triplet benzophenone. The observed magnetic
field dependence of the isotope enrichment has the following
characteristic features: (1) The magnetic field of the maximum
δ (73Ge) value is fairly large. (2) The decrease of the δ(73Ge)
value is not saturated even at 10 kG. This magnetic field
dependence can be explained by the RM. The irreversible
reaction system such as the present one is suitable for the large
isotope enrichment with high product yield. The enrichments
of many other heavy isotopes are expected to be observed with
similar hydrogen abstraction reactions of triplet carbonyls.
(22) Similar large experimental B1/2 values were reported by Turro et
al. (Turro, N. J.; Chow, M.-F.; Chung, C.-J.; Weed, G. C.; Kraeutler, B. J.
Am. Chem. Soc. 1980, 102, 4843. Turro, N. J.; Mattay, J. J. Am. Chem.
Soc. 1981, 103, 4200), but they explained their magnetic field dependence
only with the HFCM. Although Steiner suggested1b that the experimental
B1/2 values reported by Turro et al. were significantly larger than those
calculated with the HFCM, he did not refer to the RM.
JA9739493