8/12/2019 Conte Et Al. GIDRM 2006
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RAPID MEASUREMENT OF 1 H 90 PULSE IN THE SOLID STATE NMR VIA
CROSS POLARIZATION
P. Conte, R. Spaccini, A. Piccolo
Dip. di Scienze del Suolo della Pianta e dellAmbiente, via
Universit 100, 80055 Portici (Na) ItalyE-Mail:
[email protected]
Cross polarization magic angle spinning (CPMAS) 13C NMR
spectroscopy is based onthe excitation of carbons via proton
nuclei. Namely, a 90 pulse applied on the protonsis followed by a
spin lock sequence on both 1H and 13C in order to excite the
carbons. Aproton decoupling sequence is then applied while the 13C
signals are acquired. TheCPMAS 13C sequence is the basis of more
complicated and powerful experiments suchas those based on the
Lee-Goldburg decoupling. Key factors of the CPMAS 13C
NMRspectroscopy are: 1. the matching of the Hartmann-Hahn
condition, and, 2. the precision
of the proton 90 pulse. If one of the two factors are not
matched, NMR artefacts mayarise. The present work deals with the
precise calibration of the proton 90 pulse viacross polarization on
standard molecules and natural organic matter. The normal
methodused to calibrate the proton 90 pulse via cross polarization
is the use of an optimizationmethod on a standard system such as
the simple glycine. Namely, the proton pulselength is varied until
13C NMR signals are undetected. The undetectability of the
carbonsignals is due to the application of a proton pulse
corresponding to a 180 1Hmagnetization inversion. The application
of a 180 1H pulse prevents the 1H- 13C cross-polarization. The main
disadvantages of the standard 90 pulse measurements arerelated to:
1. the NMR pulse trapezoidal shapes that shorten the real pulse
length to beapplied in CPMAS 13C-NMR experiments, 2. the use of a
recycle delay that can be
inadequate as the proton pulse length is increased during the 90
proton pulsemeasurements, 3. the use of a simple organic standard
that can have a NMR behaviourdifferent from that of the natural
organic matter. In order to account for all theseproblems, a new
method for proton 90 pulse calibration was developed. The
classicalCPMAS 13C NMR pulse sequence was modified in order to
apply a pulse train made byfour 90 pulses on protons. The use of
such a modified sequence minimizes the errorsdue to the trapezoidal
pulse shapes, and allows the application of a very short
recycledelay, thereby shortening the 90 pulse measuring time. In
fact, the 4x90 pulse traincorresponds to a rotation of 360 of the
1H magnetization. Being close to theequilibrium state, an unusually
short recycle delay can be applied. Moreover, theabsence of 13C NMR
signals is a confirmation that the correct 90 1H pulse wasmeasured.
The modified 4x90 pulse sequence can be also used to rapidly
measure thecorrect 90 1H pulse on natural organic matter (NOM),
thereby preventing the use ofstandard organic systems that differ
from NOM and can provide imprecise NOM 90 1Hpulses.
