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Appl. Magn. Reson. 17, 609-614 (1999)
AppliedMagnetic Resonance
Springer-Verlag 1999Printed in Austria
MRI Study of Spatial Distribution ofPhotochemical Reaction
Products
A. A. Obynochny', A. G. Maryasov', K. A. I1'yasov 2 ,O. I.
Gnezdilov 3, and K. M. Salikhov 3
'Institute of Chemical Kinetics and Combustion, Russian Academy
of Sciences,Novosibirsk, Russian Federation
'Kazan State University, Kazan, Russian Federation' Kazan
Physical-Technical Institute, Russian Academy of Sciences, Kazan,
Russian Federation
Received October 22, 1999
Abstract. Spatial distribution of molecules with chemically
induced dynamic nuclear polarization hasbeen studied by nuclear
magnetic resonance imaging. It is shown that heating of a system
during thephotolysis can cause highly nonuniform distribution of
reaction products due to a convective effect.
1 Introduction
To perform exhaustive kinetic analysis of chemical reactions one
should knowthe spatial distribution of concentrations C(r, t) of
different molecules: initial re-agent molecules, intermediate
particles, and products. The formation of interme-diate particles
and products during photolysis or radiolysis is in general
casenonuniform throughout a sample. For example, the light
intensity decreases deepinto the sample by the Lambert-Beer law
J(x) = Joexp(ax), (1)
where x is the distance from the surface, a is determined by the
extinction co-efficient and concentration of absorbing molecules.
Thus, intermediates and prod-ucts of photochemical reactions are
expected to be distributed nonuniformly. Theremight be other
reasons for nonuniform distribution of chemical reaction
products.
The spatial distribution of products, C(r, t), can be found by
nuclear mag-netic resonance (NMR) tomography methods [1, 2]. In
these experiments, theNMR signal is detected from a slice of a
sample. The slice width 5 determinesspatial resolution. However,
when S decreases, sensitivity decreases as well. TheNMR sensitivity
increases if nuclear spins are polarized. Suppose that the
nuclear
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610 A. A. Obynochny et al.
spin polarization exceeds the equilibrium polarization by P
times. In this case, thespatial resolution of magnetic resonance
imaging (MRI) increases P times, i.e.,if spins are polarized then
the spatial resolution equals S q/P, where .5. charac-terizes the
spatial resolution of MRI for equilibrium polarization of nuclear
spins.
Photochemical reactions often proceed via a formation of radical
pairs and,as a result, the chemically induced dynamic nuclear
polarization (CIDNP) mani-fests for reagents and products. The
CIDNP effect can increase the NMR sensi-tivity by several orders of
magnitude (see, e.g., [3]). Thus, one can expect thatin a case of
photochemically induced radical reactions in solutions the
spatialdistribution of reagents and reaction products can be
observed with NMR to-mography. Note, there is another circumstance
which favors an increase in thespatial resolution of the
distribution of reagents and radical reaction products.In radical
reactions, nonuniformly distributed radicals will contribute
nonuniformlyto paramagnetic relaxation of nuclear spins,
interaction with free radicals andradical pairs will nonuniformly
shorten the relaxation times T l
and T2 . Thus, freeradicals can serve as contrasting agents and
can increase the spatial resolutionof NMR imaging experiments.
This paper aims to study the spatial pattern of
methyl-tert-butylketone pho-tolysis in tetrachloride carbon by
means of the NMR tomograph. We hope togain the sensitivity due to
the CIDNP effect.
2 Experimental
Our experimental setup consists of the excitation system,
reaction cuvette, andrecording system. Ultraviolet (UV) radiation
of a superhigh-pressure mercury lampDRSh-1000 was used as a source
of excitation. The UV radiation is passingthrough a system of
focusing lenses to the cuvette with the sample under study.A system
of spherical mirrors was employed to collect most of the UV
radia-tion. The greatest number of quanta in the cuvette in the
absorption band ofketones was 6. 10" quanta per s. In experiments
two types of cuvettes were used,cylindrical and spherical. The
diameter of the spherical cuvette was 50 mm, thatof the cylindrical
cuvettes was 45 and 15 mm. The length of the cylindrical cu-vette
was 50 and 110 mm, respectively. Radiation entered the cuvettes
throughflat windows. In experiments, a 3 by 10 mm' diaphragm was
used. Series ofexperiments were performed without diaphragm. The
experimental setup is pre-sented schematically in Fig. 1. BMT-1100S
NMR tomograph (Bruker) was usedfor recording. In experiments, a
transmit-receive radio-frequency coil for a hu-man shin was used.
We have employed the pulse sequence to obtain tomogramsand sections
in different directions and the sequences for obtaining
one-dimen-sional NMR spectra. The mechanism of CIDNP formation was
studied with HX-100 (Bruker) and FX-90Q (Jeol) spectrometers. CIDNP
was recorded by stan-dard techniques. In methyl-tert-butylketone
photolysis the concentration of ke-tone was 0.5 M/l. Before
irradiation, samples were bubbled with argon for 30min. All
substances and solvents were purified by standard techniques. In
ex-
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NMR Study of Spatial Distribution of Reaction Products 611
by
Fig. 1. The experimental setup showing a part of the cuvette
studied with an NMR tomograph.
periments performed on NMR spectrometers, the deuterated
reagents of the firm"Isotope" were used as inner standards. In this
case, the enrichment in deute-rium was 99% and the content of basic
substances was 98.5%. The deuteratedreagents were not purified.
3 Results and Discussion
The mechanism of the methyl-tert-butylketone photolysis in CC14
is schematicallyshown in Fig. 2. The characteristic feature of the
photolysis of aliphatic ketonesis that it proceeds through the
formation of a radical pair (R,CO R 2). Accordingto this scheme the
NMR spectra obtained on the photolysis of methyl-tert-butyl-ketone
solution in tetrachloride carbon manifest the CIDNP effect. The
totalpolarization for all substances involved in the reaction is
negative and exceedsthe total intensity of the signals of the
sample NMR spectra before irradiation.In an NMR tomograph the
modulus of the total intensity of all NMR signalswithin a certain
slice is detected.
Figure 3 shows the tomograms of the cuvette with
methyl-tert-butylketonephotolized in CC14 . In Fig. 3, the results
are presented when the photolysis wasinduced with the diaphragm
protecting the cuvette from heating. Figure 3bh pre-
byCH3C(0)C(CH 3)3 0 (CH3C(0)C(CH3)3)*
t 'I,CH3CO C(CH3)3 ^ CH 3CHO + (CH 3)2C=CH2r1
CH3CO + C(CH3)32CH3CO > CH 3COCOCH3 / J HC(CH3)3, (CH
3)2C=CH2
2C(CH3)3 > (CH3) 3CC(CH3) 3CC14
Fig. 2. Scheme of methyl-tert-butylketone photolysis in
CCl4.
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612 A. A. Obynochny et al.
Fig. 3. a Tomogram of the experimental cuvette fragment with
maximal UV absorption obtained onmethyl-tert-butylketone solution
in tetrachloride carbon. Number of scans is 16. 1 is the region
ofmaximal absorption and maximal CIDNP signal. 2 is the
intermediate region. 3 is the region of theequilibrium intensity of
the NMR lines. b, g Transverse cross sections of a in the region 2
close tothe region 1. c Transverse cross section of a in the region
1. d, e, f Longitudinal cross sections ofa. h Transverse cross
section of a in the region 3. AB indicates the location of the
cross section. 4shows the corresponding NMR spectrum. Note the
intense sharp line in the center of c and in the
left-hand side of e giving the NMR signal from the region 1.
sents the tomogram cross sections, as indicated; it shows the
contrast nonuni-form spatial distribution of the products of the
photochemical reaction studied.In the case exposed we observe a
dark rectangular region with a width of 0.3mm and a length of 10 mm
surrounded from three sides by a prominent lightregion with a width
of 2 mm after which there is a smooth transition to theregion in
which no reaction occurs. The cross sections of different parts of
thecuvette show that the dark rectangle region contains a strong
signal which ex-ceeds several times the equilibrium signal. In the
light region the signal sharplydrops to zero and further the signal
intensity varies from zero to the equilib-rium value (see Fig.
3).
The result presented in Fig. 3 is interpreted rather
straightforwardly. Indeed,the light is absorbed in some layer. In
this area the CIDNP is created. In a bulkof a sample where light
does not penetrate no CIDNP is formed, nuclear spinsare in thermal
equilibrium. In the system studied the total CIDNP effect is
nega-tive. Thus, there should be a region where the CIDNP effect
just compensatesthe equilibrium polarization of nuclear spins.
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NMR Study of Spatial Distribution of Reaction Products 613
Fig. 4. Tomogram obtained on photolysis of
methyl-tert-butylketone solution in tetrachloride carbonplaced in a
spherical cuvette in the absence of a screen. AA indicates the
cross section of tomo-
gram, 1 is the region of maximal absorption.
The MRI pattern drastically changes if the diaphragm protecting
the cuvettefrom heating is removed. In this case the dark region
(the region with a strongNMR signal) is shifted to the upper part
of the cuvette. For example, Fig. 4 pre-sents the tomogram obtained
for the spherical cuvette in the absence of the screen.It is
clearly seen that on the top of the cuvette the NMR signal is
maximal. TheMRI pattern depends on the shape of the cuvette. These
results indicate the pres-ence of a convective flow during the
photolysis experiment performed withoutdiaphragm. When the
diaphragm was removed the system is heated, this heatingis
nonuniform, only a thin surface layer of the liquid will be heated.
The heatedregion is subjected to the Archimedian force and the
heated liquid rises to thesurface which causes convective
instability of the medium. The convective liquidflow will carry
molecules with polarized nuclei. The nuclear spin
polarizationdecreases due to the spin-lattice relaxation in the
time scale around 10 s. For thistime the polarized molecules can be
carried by the convective flow to a distanceof several
centimeters.
4 Conclusions
Our observations demonstrate that NMR tomography allows one to
study the spa-tial pattern of a reaction zone in photochemical
reactions accompanied by thechemically induced nuclear spin
polarization. At the same time, it was found thatthe spatial
distribution of reaction products can be essentially affected by
the con-vective flow. In this case the spatial distribution of
reaction products is deter-mined not by the structure of a reaction
zone but by hydrodynamic phenomena.
Acknowledgements
This work was supported by the Russian Foundation for Basic
Research (grants96-03-32956, 96-03-32485, 96-03-40043, 96-15-97444)
and INTAS (grant 96-1269).
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614 A. A. Obynochny et al.: NMR Study of Spatial Distribution of
Reaction Products
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Authors' address: Key M. Salikhov, Kazan Physical-Technical
Institute, Russian Academy of Sci-ences, Sibirsky trakt 10/7,
420029 Kazan, Russian Federation