K.-P. Zeller, P. Schuler, P. Haiss
FULL PAPER
known, although its dialkyl esters are well characterized
and are widely used as reagents in synthetic chemistry.[19]
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Experimental Section
General Remarks: The 13C NMR spectroscopic data were obtained
with a Bruker AMX 600 spectrometer by single-pulse excitation
without proton decoupling (flip angle 30°, delay time 10 s, acqui-
sition time 5 s). During the intervals between the measurements of
the crossover experiment the tightly closed NMR tube was stored
at 20 Ϯ 0.5 °C. Sodium [13C]carbonate (99% 13C) was purchased
from Aldrich. [18O]Water recycled from 18F production in the PET
center, University Hospital, Tübingen, with an 18O enrichment of
80% was used in the synthesis of 18O-labeled sodium carbonate.
[3]
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Sodium [18O]Carbonate: The 18O-labeled carbonate was prepared
according to the method of Hutchinson and Mabuni[20] for the syn-
thesis of sodium [18O]acetate. A rigorously dried flask equipped
with a septum was charged with tetraethyl orthocarbonate (5 mL,
23.8 mmol) and dehydrated 4-toluenesulfonic acid (1Ϫ2 mg).
[18O]Water (200 µL, 9.1 mmol, 80% 18O) was then injected under
magnetic stirring. After a few minutes of exothermic reaction, the
mixture became homogeneous. A 0.5 m solution of sodium methox-
ide in dry methanol (30 mL, 15 mmol) and [18O]water (350 µL,
15.9 mmol) was added to the resulting 18O-labeled ethyl carbonate
and the mixture heated to 70Ϫ75 °C for 30 h. A white crystalline
mass precipitated. The reaction mixture was dried first in a rotary
evaporator (70 °C/30 Torr) and then with an oil pump (160 °C/
10Ϫ3 Torr); yield 1 g (98%). The degree of 18O-enrichment was de-
termined by 13C NMR spectroscopy of a 0.5 m solution in D2O
containing 1 equiv. of NaOD to suppress oxygen exchange with
the solvent D2O. Four equidistantly separated signals (∆δ ϭ
0.016 ppm) could be detected for the carbonate ion in an intensity
ratio of 46:41:12:1 caused by 18O3, 18O216O, 18O16O2 and 16O3
species.
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[19]
Measurements of the dependence of the isotopic effect on the 13C
chemical shift of aqueous sodium carbonate are given in the Sup-
porting Information. In addition to Figures 2 and 3 of the main
text the full set of time-dependent intensity distributions of the
2Ϫ
13C16O3Ϫn18On ions (n ϭ 0Ϫ3) are presented as schematic dia-
2Ϫ
grams. The rate constant for the addition of CO3 to CO2 is esti-
mated based on a linear relation between log k and pKa.
[20]
R. Hutchinson, C. T. Mabuni, J. Labelled Compd. Radiopharm.
1977, 13, 571Ϫ574.
Acknowledgments
Received May 27, 2004
Early View Article
Published Online November 10, 2004
We thank Prof. Dr. H.-J. Machulla, Abt. Radiopharmazie, Univer-
sitätsklinikum Tübingen, for a generous gift of [18O]water.
172
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Eur. J. Inorg. Chem. 2005, 168Ϫ172