Magnetic isotope effect in the photolysis of organotin compounds

Article abstract of DOI:10.1021/jp060592t

Photolysis of organotin molecules RSnMe₃ is shown to be a spin selective radical reaction accompanied by fractionation of magnetic, ^117,119Sn, and nonmagnetic, ^118,120Sn, isotopes between starting reagents and products. A primary photolysis process is a homolytic cleavage of the C-Sn bond and generation of a triplet radical pair as a spin-selective nanoreactor. Nuclear spin dependent triplet-singlet conversion of the pair results in the tin isotope fractionation. Experimentally detected isotope distribution unambiguously demonstrates that the classical, mass-dependent isotope effect is negligible in comparison with magnetic, spin-dependent isotope effect.

Full text of DOI:10.1021/jp060592t

3857
2006, 110, 3857-3859
Published on Web 03/02/2006
Magnetic Isotope Effect in the Photolysis of Organotin Compounds
Anatoly L. Buchachenko,* Vladimir L. Ivanov, Vitaly A. Roznyatovsky, and Yuri A. Ustynyuk
Chemical Department of M. V. LomonosoV Moscow State UniVersity, Moscow 119992, Russia
ReceiVed: January 27, 2006; In Final Form: February 21, 2006
Photolysis of organotin molecules RSnMe3 is shown to be a spin selective radical reaction accompanied by
fractionation of magnetic, 117,119Sn, and nonmagnetic, 118,120Sn, isotopes between starting reagents and products.
A primary photolysis process is a homolytic cleavage of the C-Sn bond and generation of a triplet radical
pair as a spin-selective nanoreactor. Nuclear spin dependent triplet-singlet conversion of the pair results in
the tin isotope fractionation. Experimentally detected isotope distribution unambiguously demonstrates that
the classical, mass-dependent isotope effect is negligible in comparison with magnetic, spin-dependent isotope
effect.
Introduction
Fractionation of isotopes in chemical and biochemical reac-
tions is induced by two isotope effects; their origin and physical
nature are shown in Figure 1. A birth of molecule, as a central
event in chemistry, is controlled by two physical factorssenergy
and angular momentum. The two interactions, different in nature
and scalesCoulomb and magnetic, stand behind these factors.
The former creates classical, mass-dependent isotope effect; the
latter is a source of magnetic, nuclear spin-dependent isotope
effect. After the first isotope effect stands an old, widely
recognized and deserved mass-selective isotopy; the magnetic
isotope effect as a fundamental phenomenon introduces in
chemistry and related sciences a new, nuclear spin-dependent
isotopy.
The physics of these two isotopies are completely different:
the former is based on the difference in energy of isotopic
molecules; the latter appeals to the difference in nuclear spins
and nuclear magnetic moments of reagents. The classical isotope
effect (CIE) sorts isotopic nuclei on their masses; the magnetic
isotope effect (MIE) demonstrates nuclear spin selectivity of
reactions and separates isotopic nuclei according to their
magnetic moments.1
Since the discovery in 1976,2 MIE was detected in many
chemical reactions, for many isotopic pairs and triads (H-D,
12C-13C, 16O-17O-18O, 28Si-29Si-30Si, 32S-33S-34S, 72Ge-
73Ge-74Ge, 235U-238U, 198,200Hg-199,201Hg, 24Mg-25Mg-26-
Mg). In all these cases chemical reactivity of reagents with
magnetic nuclei is strongly different from that of reagents with
nonmagnetic, spinless nuclei.3 An attempt to detect MIE-induced
fractionation of tin isotopes was made in 1979 and seemed to
be successful;4 however, in 1985 the same authors in more
careful experiments did not confirm the results of previous
publication.5 The purpose of this paper is to declare reliably
detected MIE on the tin nuclei in the photolysis of tin organic
compounds.
Figure 1. Scheme illustrating the origin and physical nature of classical
and magnetic isotopies.
Experimental Section
(1-Naphthylmethyl)trimethyltin (1) and (9-fluorenyl)trimeth-
yltin (2) were synthesized and purified according to a known
procedure.6,7 Photolysis of both starting compounds 1 and 2 was
carried out in tetrahydrofurane-d8 solutions, 0.1 M, in Pyrex
NMR tubes 5 mm in diameter, sealed under vacuum. NMR tube
with solution was placed in a quartz vessel filled with water at
25 °C and was rotated with a rate of 2 rpm. Irradiation was
performed with a light from a high-pressure mercury lamp using
a filter to select spectral line at 365 nm.
Both chemical and isotope composition of the reaction
1
* Corresponding author. E-mail: abuchach@chph.ras.ru.
mixture were analyzed by H and 13C NMR spectroscopy on
10.1021/jp060592t CCC: $33.50 © 2006 American Chemical Society

3858 J. Phys. Chem. A, Vol. 110, No. 11, 2006
Letters
the Bruker AVANCE-600 spectrometer using procedure de-
scribed previously.8,9
Results and Discussion
1
Photolysis Products and NMR Spectra. H and 13C NMR
spectra of the starting compounds 1 and 2 were monitored during
the photolysis as well as reaction products 3 and 4 from 1 and
3, 5, and 6 from 2. In the latter case the yields of 5 and 6 were
nearly equal. No other products whose concentration would
exceed more than 5% of the starting compounds were detected
at the conversion 60-70%.
As an example, the 13C NMR spectrum with signal assign-
ment in the high-field region is shown in Figure 2. The signal
Figure 2. Fragments of the 13C-{1H} NMR spectra of compounds 1,
3, and 4 (a) and 2, 3, 5, and 6 (b). Arrows indicate the 119Sn and 117Sn
satellites, a star denotes signals from admixtures.
from 1 and 2 is easily identified as the two pairs of 13C-117,119
-
Sn satellites (nuclear spins of both tin isotopes are identical,
1/2; magnetic moments are only slightly different, -1.0 and
-1.046 µB for 117Sn and 119Sn respectively, so that spin-spin
coupling constants J(13C-117Sn) and J(13C-119Sn) differ by 5%
only). Some pairs belong to the 13CH2Sn(CH3)3 fragment of 1
(at 18.5-16.0 ppm) and 13CHSn(CH3)3 fragment of 2 (at 38.5-
40.5 ppm); other pairs refer to 13CH3Sn(CH3)2 fragments in 1
and 2, and they fall in the region from -9 to -12 ppm (see
Figure 2a,b).
the reaction and (A0*) is that in the starting compounds 1 and
2 initially. Figure 3a exhibits the isotope fractionation between
the starting reagent 1 and product 3; it shows the dependence
of S on the chemical conversion F in terms of eq 2 (see later).
As the starting reagent 1 decomposes, its remaining fraction is
enriched with 117,119Sn magnetic isotopes (S > 1), whereas the
reaction product 3 is depleted with these isotopes (S < 1) and,
respectively, enriched with nonmagnetic 118,119Sn nuclei.
A similar fractionation of magnetic and nonmagnetic isotopes
was detected in the photolysis of 2 (Figure 3b). The only
difference is that for 2 the measurement of isotopic contents of
117Sn at the conversions more than 30% was not successful
because of NMR lines overlapping.
The 13C NMR signal of 3 consists of four pairs of satellites:
the two inner pairs belong to fragment 13CH3-Sn-117,119Sn-;
the outer pairs refer to the 13CH3-117,119Sn-Sn- fragment
(Figure 2). Central NMR lines of satellites belong to the same
fragments with nonmagnetic, spinless tin nuclei, mainly to
116,118,120Sn. Some lines in the spectra of 1-3 and 2 and 3 are
superimposed or overlapped.
Chemical conversion F was determined from the ratio of
signal intensities of 1 and 3 (for 1) and of 2 and 3 (for 2) under
the assumption that the Sn(CH3)3 group is transferred from the
starting reagents into the products as a whole. Isotope composi-
tion was determined by iteration analysis of the total line shape
in the 13C NMR spectral region containing signals of the Me3-
Sn groups, taking into account the residual inhomogenity of
the polarizing magnetic field and phase distortions using the
INTSPECT-2 program, as described elsewhere.8,9
Isotope fractionation unambiguously demonstrates that the
photolysis of organotin compounds is a spin-selective reaction
in which magnetic isotope effect functions. The reaction scheme
for both compounds 1 and 2 can be presented in generalized
form as follows:
hν
R-SnMe3 {
} [R˙ Sn˘ Me3]
8 RR, RH, Me3SnSnMe3
117,119Sn
118,120Sn
where R denotes the organic radical in 1 and 2, RR corresponds
to 4 and 6, and RH is 5.
The spin-selective monoreactor is a radical pair in the triplet
spin state,10 generated by homolytic cleavage of the C-Sn bond.
Recombination of radicals in this nanoreactor is spin forbidden;
however, the pairs with magnetic nuclei 117,119Sn undergo very
fast triplet-singlet spin conversion and recombine in the singlet
spin state, regenerating starting reagents enriched with magnetic
Isotope Fractionation. Isotope enrichment (or depletion) S
is determined as the ratio
S ) (A*)/(A0*)
(1)
where (A*) is the concentration of certain isotopomer during

Letters
J. Phys. Chem. A, Vol. 110, No. 11, 2006 3859
Equation 2 is known to be rigorously fulfilled for magnetic
isotope effects of all elements, wiht the exception of tin (Figure
3), for which this dependence strongly deviates from the linear
one. The reason of exception is a reaction of exchange by SnMe3
groups:
RSnMe3 + S˙ nMe3 f RSnMe3 + S4 nMe3
which produces also isotope exchange and tends to equalize
isotope composition of starting reagents and products. A
competition between these two processes, isotope fractionation
in the primary reaction and isotope scrambling in the secondary
reactions, decreases the of efficiency of isotope separation
already at rather low chemical conversion, about 20-30%.
Another problem, as seen from Figure 3, is that at first sight
there is no isotope balance between starting reagents and
products. The reason for this apparent imbalance is a limitation
of isotope analysis by NMR because of differences in nuclear
relaxation times and partial NMR lines overlapping. The same
limitation results in an apparent difference in fractionation of
117Sn and 119Sn despite the fact that both these nuclei have
almost identical magnetic moments (the difference is about 5%),
so that the difference in fractionation is expected to be not more
than 5%.
Conclusion
For the first time we observed the magnetic isotope effect
on the tin nuclei in the photolytic reactions of (1-naphthylm-
ethyl)trimethyltin and (9-fluorenyl)trimethyltin. The reactions
are nuclear spin selective and fractionate magnetic and non-
magnetic tin isotopes (117,119Sn and 118,120Sn, respectively): the
former are accumulated in the starting molecules, the latter are
concentrated in the reaction product hexamethyldistannane. One-
step enrichment coefficients, estimated at low chemical conver-
sions, are 1.16 and 1.15 for the two organotin compounds;
however, at higher conversions the exchange reaction reduces
the efficiency of isotope separation and tends to equalize the
isotope composition of starting reagents and products.
Figure 3. Isotopic composition as a function of chemical conversion
F at the photolysis of 1 (a) and 2 (b). Open circles refer to 119Sn, filled
circles to 117Sn in 1 and 2; triangles refer to 3: open ones to 119Sn,
filled ones to 117Sn.
Acknowledgment. This work was financially supported by
the Russian Foundation for Basic Research (Grant No. 03-03-
32652) and by the Ministry of Science and Education of Russia
(Grant No. 1221.2003.3).
isotopes. Triplet-singlet spin conversion is induced by strong
magnetic electron-nuclear interaction (hyperfine coupling)
between an unpaired electron and the 117,119Sn nuclei in the
SnMe3 radical; its magnitude is 4.3 × 109 and 4.5 × 109 Hz
for 117Sn and 119Sn nuclei, respectively.11 These frequencies are
adequate to the rate of triplet-singlet spin conversion of the
pairs with magnetic tin nuclei.
References and Notes
(1) Buchachenko, A. L. J. Phys. Chem. A 2001, 105, 9995.
(2) Buchachenko, A. L.; Galimov, E. M.; Ershov, V. V.; Nikiforov,
G. A.; Pershin, A. D. Dokl. Akad. Nauk USSR 1976, 228, 379 (in Russian).
(3) Buchachenko, A. L. Chem. ReV. 1995, 95, 2507.
(4) Podoplelov, A. V.; Leshina, T. V.; Sagdeev, R. Z.; Molin, Yu, N.;
Gol’danskii, V. I. JETP Lett. 1979, 29, 380.
(5) Podoplelov, A. V.; Sen Chel Su; Sagdeev, R. Z.; Shtein, M. S.;
Moralev, V. M.; Gol’danskii, V. I.; Molin, Yu., N. Russ. Chem. Bull. 1985,
34, 2041.
(6) Builpitt, M.; Kitching, W.; Adcock, W.; Doddrell, D. J. Organomet.
Chem. 1976, 116, 187.
The rate of spin conversion of the pairs with nonmagnetic
tin nuclei is by 2 orders of magnitude slower than that of the
pairs with magnetic nuclei, so that these pairs mostly dissociate
and result in reaction products of escaped radicals and one of
them, SnMe3, carries an excess of nonmagnetic tin nuclei. As
a result, magnetic tin nuclei are accumulated in the starting
molecules 1 and 2.
The theory of magnetic isotope effect1 predicts a linear
relation between S and F in coordinates log S and log(1 - F):
(7) Knjazhanski, S. Ya.; Moreno, G.; Cadenas, G.; Belsky, V. K.;
Bulychev, B. M. Tetrahedron 1999, 55, 1639.
(8) Roznyatovsky, V. A.; Ustynyuk, Yu., A.; Vorob’ev, A. Kh.;
Zemlyanskii, N. N.; Borisova, I. V. Russ. J. Phys. Chem. 2004, 78, 1277.
(9) Roznyatovsky, V. A.; Roznyatovsky, V. V.; Ustynyuk, Yu., A. Russ.
Chem. Bull. 2004, 10, 2196.
(10) The direct evidence of the triplet spin state of the radical pair follows
from the sign of magnetic isotope effect; it is a new and the most reliable
test on the spin multiplicity.1 In particular, if the spin multiplicity of the
radical pair is a singlet, we would observe the tin isotope distribution
between starting molecules and products opposite to that observed
experimentally.
1 - R
R
log S )
log(1 - F)
(2)
(
)
where R is a one-step enrichment coefficient:
R ) (1 - P)/(1 - P*)
(3)
Here P and P* are recombination probabilities of the pairs with
nonmagnetic and magnetic nuclei; because P* > P, R > 1.
(11) Lehnig, M. Bull. Soc. Chim. Belg. 1980, 89, 907.