4002 J. Phys. Chem. A, Vol. 107, No. 19, 2003
Volkova et al.
the electron spin exchange between the stable radical and the
short-lived one.
studied the photodecomposition of 7-silanorbornadiene in the
presence of the stable radical in the magnetic fields of different
strengths, by means of laser pulse photolysis.
Some recent publications also discuss the results of the
investigations of magnetic field effects in multispin systems.11-14
Unlike the papers on spin catalysis, the majority of these
investigations involve systems where the stable radicals are
bound to one of the reagents and influence the spin evolution
via the electron-dipole interaction. The systems with stable
radicals bound to one of the reagents are of special interest from
the chemical viewpoint. From our perspective, these systems
might be of interest from the viewpoint of the potentiality of
the stable radicals and biradicalsscommonly used as paramag-
netic scavengerssto form complexes with the reagents of the
radical reactions. It is necessary to emphasize that the formation
of weak (the dissociation energy of the chemical bond being
about several kilocalories) donor-acceptor π-π, π-σ, and n-π
complexes between the paramagnetic particles, such as stable
radicals and oxygen, and a number of diamagnetic compounds
is a well-known fact.15 There are also certain evidences of the
influence of similar complexation on the parameters of chemical
reactions.11,14 Thus, one cannot exclude that, in the systems
where RP partners are not chemically bounded to the stable
radicals/biradicals, the influence of the third spin could manifest
itself via complexation. Although it is still not clear whether
the interactions of the paramagnetic species in these complexes
are a manifestation of the spin catalysis, it is necessary to stress
that the effects of the third spin were observed only for a limited
number of the experimental systems. Therefore, new experi-
mental evidences of the influence of paramagnetic species on
the radical reactions should provide the answer to the above-
mentioned question. The exploration of the chemical manifesta-
tions of selected types of the interactions inherent in some
paramagnetic particles, in particular spin catalysis, and in real
chemical processes is believed to be a very prospective field of
research.
Experimental Section
The laser pulse photolysis technique was used to study the
photodecomposition of the solutions of 7,7-dimethyl-1,4,5,6-
tetraphenyl-2,3-benzo-7-silanorbornadiene (1) ((0.9-1.0) × 10-3
M in hexane) in the presence and in the absence of stable
4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyle radical (4-OH-
TEMPO), in aerated and deaerated solutions. 4-OH-TEMPO
was a kind gift from Prof. I. A. Grigoriev (Novosibirsk Institute
of Organic Chemistry) and was recrystallized prior to all
experiments. Hexane (JSC Mosreaktiv) was purified by distil-
lation. The samples were photolyzed with an excimer (XeCl)
laser: λ ) 308 nm, pulse duration 15 ns, average pulse energy
ca. 10 mJ, delay time 50 ns. The detailed description of the
setup could be found elsewhere.17 The concentration of 4-OH-
TEMPO was varied in the range (0.2-1.5) × 10-2 M.
According to reference data,18 in aerated solution in hexane,
the equilibrium oxygen concentration amounts to 2 × 10-3 M.
To prepare the deaerated solution, the samples were purged by
the argon flow for 15 min to displace the dissolved oxygen.
All experiments were performed at ambient temperature and at
atmospheric pressure.
The external magnetic field was created by the permanent
Sm-Co magnet with one sliding pole to ensure the variation
of the magnetic field strength. The magnetic field effect was
monitored by the level of optical absorption of the end photolysis
productstetraphenylnaphthalene (TPN). As has been shown
earlier,19 the change in the optical density of the solution of 1
after the laser pulse (∆D at λ ) 334 nm) is determined by the
optical absorption of the resulting TPN. The MFE was calculated
according to the formula
MFE ) [(∆D - ∆DH)/∆D] × 100%
Some reference data also point to the potential complexation
between two paramagnetic speciessoxygen and short-lived
biradicals.13,16 It was suggested16 that the intermediate biradicals
generated through the photolysis of aromatic ketones via the
Norrish Type II mechanism could reversibly form weak
complexes with molecular oxygen. This conclusion was based
on the effect of oxygen on the biradical lifetime as well as on
the yield of the reaction products. Another example is the
influence of O2 on the magnitude of the magnetic field effect
(MFE) on the yield of products in the photolysis of 7-silanor-
bornadiene in solution.13 The reaction mechanism proposed on
the basis of laser pulse photolysis experiments and the analyses
of CIDNP effects and the MFE suggests that the observed MFE
is formed in short-lived silicon-centered 1,6-biradicals. In the
presence of equilibrium concentration of the dissolved oxygen,
the MFE demonstrates a more than 2-fold increase as compared
to the case of the deaerated solutions, and the maximum of the
MFE dependence on the external magnetic field strength shifts
toward the higher field region. According to the radical pair
theory, such a shift of the extreme suggests that in aerated and
deaerated solutions the MFE is generated in the paramagnetic
intermediates of different structure. In the aerated solutions this
shift has been attributed to the complex of the 1,6-biradical with
O2.13 To check the hypothesis about the complexation of two
paramagnetic particles and to explore whether the influence of
oxygen could be regarded as an example of spin catalysis, it
would be reasonable to investigate the influence of other
paramagnetic species on the photolytic decomposition of
7-silanorbornadiene. To this end, in the present paper we have
where ∆D (∆DH) is the absorption change detected under the
geomagnetic (external) field, respectively. Data points of the
MFE dependencies show the results of standard statistical
processing of seven to nine kinetic traces taken in laser pulse
photolysis experiments during the photodecomposition of 1. The
error bars represent the standard deviation.
Results and Discussion
The scheme of the photodecomposition of 7-silanorbornadiene
(1) in solution has been formulated earlier13,14 on the basis of
the laser pulse photolysis data and from the analysis of CIDNP
and the MFE observed in this reaction (see Scheme 2).
According to this scheme, the magnetosensitive stage respon-
sible for the formation of the MFE is the singlet-triplet con-
version in 1,6-biradicals (3) formed through the reaction of the
initial 1 with the dimethylsilylene Me2Si: (DMS) in triplet
excited and singlet ground states. Since the sign of the observed
MFE is dependent on the multiplicity of the starting 1,6-biradical
(3T or 3S, respectively; see Scheme 2), it will be stipulated by
the ratio of the reaction rates of initial 1 with the T- and S-states
of DMS and the rate of its triplet to singlet conversion, kT-S. It
has been shown that in the absence of quenchers and in the
presence of oxygen the contribution of the reaction of triplet
excited DMS becomes prevalent, while in the presence of the
diamagnetic scavengers of DMSstriphenylphosphine (Ph3P) and
carbon tetrachloride (CCl4)swe have observed the changes in
magnitude and the alteration of the sign of the MFE.13,20 This