JOURNAL OF MAGNETIC RESONANCE 125, 245–258 (1997)
ARTICLE NO. MN971111
Time-Reverse ODESSA. A 1D Exchange Experiment for Rotating
Solids with Several Groups of Equivalent Nuclei
D. REICHERT,* H. ZIMMERMANN,† P. TEKELY,‡ R. POUPKO
§ AND Z. LUZ §
*
Martin-Luther Universit a¨ t Halle-Wittenberg, Fachbereich Physik, 06108 Halle, Germany; †Max-Planck-Institut f u¨ r Medizinische Forschung,
AG Molek u¨ lkristalle, 69028 Heidelberg, Germany; ‡Laboratoire de M e´ thodologie RMN, URA CNRS, 406 Universit e´ H. Poincar e´ ,
Nancy 1, France; §Weizmann Institute of Science, 76100 Rehovot, Israel
Received September 3, 1996
A one-dimensional exchange experiment is proposed for rotor period, and the resulting cross peaks provide quantita-
magic-angle-spinning samples with several groups of equivalent
nuclei undergoing internal exchange, such as pure reorientation,
as opposed to mutual exchange. The method, which we term
time-reverse ODESSA, is an extension of the recently proposed
tive information on dynamic processes even if only equiva-
lent nuclei are involved (6–8).
Such two-dimensional experiments are often very time
consuming, and it is therefore desirable to use, whenever
possible, equivalent 1D methods. In these 1D experiments,
the spin system is prepared in a nonequilibrium state that is
modified during the following mixing time by some dynamic
process. The first such 1D exchange experiment for equiva-
1
D ODESSA experiment for a single group of exchanging nuclei.
When several different groups of spins are present, as is usually
the case for carbon-13 in polymers and molecular crystals, the
normal ODESSA spectrum yields phase-twisted spectra which
are difficult to analyze quantitatively. This problem is solved in
the time-reverse ODESSA experiment which yields pure absorp- lent nuclei in rotating solids was designed by Yang et al.
tion spectra for all families of side bands, as long as only internal (9). It consists of a preparation period during which the
exchange need be considered. The experiment consists of the spinning side bands are suppressed by a TOSS sequence, a
usual three pulse sequence of 2D exchange, P1—t
P3—t (acquisition), except that the evolution time is fixed at
half a rotation period, t /2, the mixing time is set to an
Å T
odd number of half rotation periods, /2, and
Å (2G 0 1)T
the acquisition starts at t /2 after the detection pulse,
1 m
—P2— t —
variable mixing period
m
t , during which spin exchange will
2
mismatch the TOSS conditions, followed by a detection pe-
riod in which the restoration of the spinning side bands is
monitored. The experiment is, however, very sensitive to
the exact setting of the TOSS conditions.
1
R
t
m
R
2
Å T
R
P3. The method is demonstrated using the carbon-13 spectra of
An alternative MAS 1D exchange experiment, suitable
for monitoring spin exchange between equivalent nuclei, was
recently proposed by G e´ rardy-Montouillout et al. (10). The
experiment was termed ODESSA for one-dimensional ex-
change spectroscopy by sideband-alternation. It consists of
the usual three-pulse sequence of 2D exchange spectroscopy,
P1—t — P2— tm —P3—t (acquisition), but with the
dimethyl sulfone and an enriched sample of tropolone, and is
applied to the study of the
p flip of the inner benzene ring of
1
,4-diphenoxybenzene. The scope and limitations of the method
are discussed. ᭧ 1997 Academic Press
INTRODUCTION
1
2
preparation time fixed at half a rotation period, t
Chemical or spin exchange between different types of and the mixing time set to an integral number of rotation
carbon-13 (or other) nuclei in the solid state, in the slow- periods, tm GT , as shown in Fig. 1a. Under these experi-
exchange regime, where 1/T 1/T , can be studied mental conditions, the initial magnetic polarization at tm
1
Å
T
R
/2,
Å
R
2
ú
k
Ռ
1
Å
under magic-angle spinning (MAS) by various polarization- 0 associated with odd spinning side bands is reversed with
transfer experiments (1–3). For example, the approach to respect to the even ones. Dynamic processes during the mix-
equilibrium can be monitored after selectively inverting the ing time redistribute the polarization between the various
magnetization of one component in a spin multiplet. The spinning side bands, resulting in a modified MAS spectrum.
extension to pure reorientation, or internal spin exchange Analysis of such spectra as a function of the mixing time
within the same group of equivalent nuclei, is more compli- provides information about the mechanism and kinetic pa-
cated since it involves polarization transfer between different rameters of the dynamic processes. The ODESSA experi-
spinning side bands belonging to the same chemical species. ment works well if the sample consists of a single set of
The most direct way to do this is by the rotor-synchronized chemically equivalent spins having the same isotropic chem-
2
D exchange method, first proposed by Veeman and co- ical shift. However, for a system consisting of several in-
workers (4) and later modified by Hagemeyer et al. (5). In equivalent nuclei with different isotropic chemical shifts,
this experiment, the mixing time is synchronized with the the above procedure results in phase-twisted signals. This
2
45
1
090-7807/97 $25.00
1997 by Academic Press
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