enhancement of spin diffusion of quadrupolar spins in...

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


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


200 MHz, 7 kHz


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


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


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


pw0= 0 36 72 143 287 574 1148 2296 4592 μs

Power = 10

Power = 20

Power = 30

Na2B4O7．10H2O


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)


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


▲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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