Enhancement of Spin Diffusion of Quadrupolar Spins in Solids under Magic - Angle - Spinning Po-Chi Huang, Zhen Wu, Shangwu Ding Department of Chemistry, National Sun Yat-sen University ISMAR Conference 2007 Kenting, Taiwan, October 14-19, 2007
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Enhancement of Spin Diffusion of
Quadrupolar Spins in Solids under
Magic-Angle-Spinning
Po-Chi Huang, Zhen Wu, Shangwu Ding
Department of Chemistry,
National Sun Yat-sen University
ISMAR Conference 2007 Kenting, Taiwan, October 14-19, 2007
Introduction
Recoupling Schemes:
Recoupling Pulses in Quadrupole Channel
Recoupling Pulses in Proton Channel
Simultaneous Recoupling Pulses
Optimized MQMAS/STMAS experimental parameters
Optimizing Initial State
An MQMAS spectrum enables us to measure the principal components of the
EFG tensor of a quadrupolar nucleus, but does not offer information about the
orientation of this tensor. With the cross peaks of a spin diffusion spectrum,
this information can be elucidated. However, the sensitivity of a spin diffusion
spectrum of quadrupolar system can be rather low. New techniques are needed
to improve it.
In this work, we present two approaches each of which can help increase the
sensitivity of an MQMAS/STMAS spin diffusion spectrum:
The EFG TensorThree Principal Values (Two Independent Quadrupolar Coupling Constant, Asymmetry Parameter)
V33V22
V11
X
Y
ZPlus Orientation V11+V22+V33=0
The Relative Orientation Between Two
Quadrupolar Tensors
What MQMAS/STMAS May Tell
V33,B
YB
V33,A
V11,A
XA
YA
V11,B
V22,A
V22,B
XB
ZAZB
Two Spin-3/2 MQMAS-Spin Diffusion Spectrum
MQMAS PeaksCross Peaks
(3/2,-3/2)(1/2,-1/2)
)45,0,90(),,(
.1.0,4.1
,5.0,5.2
0
22,
11,
oo
q
q
MHzC
MHzC
Exchange Enhancement by Recoupling
and Optimizing Initial State
A(m,-m)B(1/2,-1/2), m>1/2
P1 t1 P2 tm P3 t2S
I
…
Optimizing initial state
reduces recycle delay
τR
Recoupling pulses
increase mixing
Na2B4O7．10H2O
MQMASMQMAS-SD
No recoupling
MQMAS-SD
0.1 tRMQMAS-SD
0.6 tR
200 MHz, 7 kHz
Prec = 4 μs, 55.3 dB
Na2B4O7．10H2OMQMAS-SD
0.3 tR
200 MHz, 7 kHz
Prec = 4 μs, 55.3 dB
0.7 tR0.8 tR 0.9 tR
0.5 tR0.6 tR0.4 tR
0.2 tR0.1 tR
pw3q1= 6 us
pw3q2= 2.2 us
power= 54.8 dB
mixing time= 70 ms
pws2=18 us
power=29.8 dB
Na2B4O7．10H2OMQMAS-SD
200 MHz, 7 kHz
Prec = 8 μs, 55.3 dB
0.3 tR
0.7 tR 0.9 tR
0.5 tR0.1 tR
pw3q1= 6 us
pw3q2= 2.2 us
power= 54.8 dB
mixing time= 70 ms
pws2=18 us
power=29.8 dB
Na2B4O7．10H2O MQMAS-SD
200 MHz, 7 kHz
Prec = 12 μs, 55.3 dB
0.3 tR
0.7 tR 0.9 tR
0.5 tR0.1 tR
pw3q1= 6 us
pw3q2= 2.2 us
power= 54.8 dB
mixing time= 70 ms
pws2=18 us
power=29.8 dB
Na2B4O7．10H2OMQMAS-SD
200 MHz, 7 kHz
Prec = 30 μs,17.3 dB
0.3 tR
0.7 tR
0.5 tR0.1 tR
pw3q1= 6 us
pw3q2= 2.2 us
power= 54.8 dB
mixing time= 70 ms
pws2=18 us
power=29.8 dB
Na2B4O7 ．10H2O200 MHz, 7 kHz
Prec = 65 μs, 17.3 dB
0.5 tR
0.1 tR
pw3q1= 6 us
pw3q2= 2.2 us
power= 54.8 dB
mixing time= 70 ms
pws2=18 us
power=29.8 dB
MQMAS-SD
0.3 tR
Na2B4O7 ．10H2OMQMAS-SD
200 MHz, 7 kHz
Prec = 65 μs, 37.3 dB
0.6 tR
0.2 tR
pw3q1= 6 us
pw3q2= 2.2 us
power= 54.8 dB
mixing time= 70 ms
pws2=18 us
power=29.8 dB
Three-site Case: Na2B4O7．10H2O
MQMAS-SD
500 MHz, 10 kHz
Prec = 4 μs, 55.3 dB
0.3 tR
0.5 tR0.1 tR
mixing time= 70 ms
pws2=2.5 us
power=48.8 dB
pw3q1= 4.8 us
pw3q2= 1.6 us
power= 54.8 dB
MQMAS
Three-site Case: Li2B4O7．10H2O
MQMAS MQMAS-SD
70 ms
MQMAS-SD
300 ms
MQMAS-SD
Prec=12 μs
tm=70 ms
MQMAS-SD
Prec=12 μs
tm=150 ms
MQMAS-SD
Prec=12 μs
tm=200 ms
Topic 2: Enhancement by Optimizing
Initial State
A(m,-m)B(1/2,-1/2), m>1/2
P1 t1 P2 tm P3 t2S
I
…
Optimizing initial state
reduces recycle delay
τR
Recoupling pulses
increase mixing
Optimizing Initial State
pw0= 0 36 72 143 287 574 1148 2296 4592 μs
Power = 10
Power = 20
Power = 30
Na2B4O7．10H2O
Optimizing Initial State
Power = 10
Power = 20
Power = 30
Na2B4O7．10H2O
d1=0.2 0.4 0.8 2.0 3.0 5.0 sd1=0.2 0.4 0.8 2.0 3.0 5.0 s
d1=0.2 0.4 0.8 2.0 3.0 5.0 s
pw0=0 us pw0=287us (= 2 tR)pw0=72 us (= 0.5 tR)
Optimizing Initial State
Power = 10
Power = 20
Power = 30
Li2BO3．5H2O
d1=0.2 0.4 0.8 2.0 3.0 5.0 8.0 sd1=0.2 0.4 0.8 2.0 3.0 5.0 8.0 s
d1=0.2 0.4 0.8 2.0 3.0 5.0 8.0 s
pw0=0 us pw0=574 us (= 4 tR)pw0=72 us (= 0.5 tR)
Optimizing Initial State: MQMAS
pw0=287 us (= 2 tR)
d1=2s
Normal MQMAS
d1=2s
Li2BO3．5H2O
Optimizing Initial State: MQMAS-SD
pw0=587 us (= 4 tR)
d1=3s
Normal MQMAS-SD
d1=3s
Na2BO7．10H2O
Experimental Procedure for
Optimizing Initial State
▲Set up a typical MQMAS/STMAS exp
▲Get the first slice of a series of MQMAS/STMAS spectra
by arraying recycle delay, the length and amplitude of the
preparatory pulse.
▲Choose the optimal values of recycle delay, the length and
amplitude of the preparatory pulse by viewing the spectra.
▲Use the above values for your MQMAS/STMAS exp
Experimental Procedure for Optimizing
Initial State and Recoupling Pulse
▲Set up a typical MQMAS/STMAS exp
▲Get the first slice of a series of MQMAS/STMAS spectra
by arraying recycle delay, the length and amplitude of the
preparatory pulse.
▲Choose the optimal values of recycle delay, the length and
amplitude of the preparatory pulse by viewing the spectra.
▲Use the above values for your MQMAS/STMAS-SD exp
▲ Get the first slice of a series of MQMAS/STMAS-SD spectra
by arraying the position, length and amplitude of the
recoupling pulse. Find the optimal values for the recoupling pulse.
Conclusion The wide applications of MQMAS/STMAS spin
diffusion require higher sensitivity of the cross peaks. The enhancement of cross peaks can be achieved with recoupling approaches and/or optimal initial states.
For recoupling during mixing, a pulse applied at the beginning or end of each rotor period performs better than other cases, but enhancement can always be achieved with the recoupling pulse applied at almost anywhere if recoupling power and length are adjusted properly (depending on sample).
For initial state optimization, it is found that the pulse length of a few rotor period is desired and the pulse power should be appropriately chosen.
Acknowledgment: $ National Science Council of ROC, NSYSU
Other Considerations
Amplitude/phase/frequency modulation
of the initial pulse
Speed effect
Spin-5/2
3D
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