High-Resolution Magic Angle Spinning and Cross-Polarization Magic

28 High-Resolution Magic Angle Spinning and Cross-Polarization Magic Angle Spinning SolidState NMR Spectroscopy Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch028

Analytical Chemical Applications C. A. FYFE, L. BEMI, H. C. CLARK, J. A. DAVIES, G. C. GOBBI, J. S. HARTMAN, P. J. HAYES, and R. E. WASYLISHEN University of Guelph, The Guelph-Waterloo Centre for Graduate Work in Chemistry, Guelph Campus, Department of Chemistry, Guelph, Ontario N1G 2W1 Canada

The techniques of cross-polarization and 'magic-angle' spinning used to obtain high-resolution solid-state NMR spectra are described and their application to inorganic systems illustrated. In general the experiments are complementary to diffraction techniques, being applicable to amorphous systems (such as surface immobilized species and glasses) where diffraction data cannot be obtained and to crystalline systems where diffraction measurements yield only partial structural data (as in the case of dynamic solid-state structures where the molecular motions are not detected, and zeolites where Si and Al atoms cannot be distinguished). In addition NMR spectroscopy provides a valuable 'bridge' between solid -state diffraction-determined molecular structures and those which exist in solution. There has been s u b s t a n t i a l i n t e r e s t i n recent years i n the chemical a p p l i c a t i o n s of h i g h - r e s o l u t i o n NMR spectroscopy o f solids. The experiments a r e the r e s u l t o f a s e r i e s of advances over a number of years and have now reached the routine stage with commercial systems being a v a i l a b l e . U s u a l l y they i n v o l v e some combination of high-power proton decoupling, c r o s s - p o l a r i z a t i o n and magic-angle spinning techniques. Below we review the e x p e r i ment and discuss i t s a p p l i c a t i o n i n p a r t i c u l a r to inorganic systems i n d i c a t i n g the r o l e o f each of the components of the t o t a l experiment. The

Experiment

NMR of s o l i d s u s u a l l y give broad f e a t u r e l e s s absorptions due to the d i p o l a r i n t e r a c t i o n s which, at l e a s t i n the case of protons, are orders of magnitude l a r g e r than the c h a r a c t e r i s t i c chemi c a l s h i f t s and s p i n - s p i n couplings used f o r s t r u c t u r e e l u c i d a t i o n 0097-6156/83/0211-0405$07.25/0 © 1983 American Chemical Society Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

406

INORGANIC C H E M I S T R Y : TOWARD T H E

21ST

CENTURY

etc. Scheme I i l l u s t r a t e s the way i n which the h i g h - r e s o l u t i o n c h a r a c t e r i s t i c s of the spectrum may be recovered i n highr e s o l u t i o n NMR of s o l i d s . Scheme I *H:

'Abundant H

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch028

13

Tot

C:

=

H

=

Tot

nucleus

Zeeman

'Dilute H

1

H

1

+

^-H

dipolar

+

H

i l ]

0ther (negligible)

nucleus

Zeeman

H

H-C

+ H Other ( s h i f t

dipolar

4" H

C-C

dipolar [2]

anisotropy)

Hydrogen i s an example of an 'abundant' nucleus. That i s , there i s a high concentration of n u c l e i with a nuclear isotope of high n a t u r a l abundance (*H, I = 99.8%) i n the sample. In t h i s case the d i p o l a r i n t e r a c t i o n s between the n u c l e i dominate the spectra g i v i n g broad f e a t u r e l e s s absorptions. In the case of C, a ' d i l u t e ' nucleus ( i . e . , there are few NMR a c t i v e n u c l e i i n the system), the s i t u a t i o n i s considerably s i m p l i f i e d . The H - C d i p o l a r i n t e r a c t i o n s are between d i f f e r e n t n u c l e i and may be removed by powerful oroton decoupling f i e l d s and most importantly, the homonuclear C - C d i p o l a r i n t e r a c t i o n s do not e x i s t because of the low concentration of C i n the system due to the low n a t u r a l abundance of C (I = h 1.1%). The l a r g e s t remaining i n t e r a c t i o n i s the 'chemical s h i f t anisotropy' which i s the three-dimensional magnetic s h i e l d i n g of the n u c l e i . In the s o l i d s t a t e , a s i n g l e nucleus w i l l give r i s e to a s i g n a l which i s dependent on the o r i e n t a t i o n of the c r y s t a l to the f i e l d and since a l l p o s s i b l e o r i e n t a t i o n s e x i s t f o r a p o l y c r y s t a l l i n e sample, a broad absorption (or s h i f t anisotropy pattern) w i l l r e s u l t . This i s averaged to the i s o t r o p i c average value by the random motion of the molecules i n s o l u t i o n . The same averaging may be achieved f o r a p o l y c r y s t a l l i n e sample by spinning i t r a p i d l y about an axis i n c l i n e d at an angle of 54°44' to the magnetic f i e l d v e c t o r (Figure 1), the s o - c a l l e d 'Magic-Angle' (_5, 6^7). Various designs are a v a i l a b l e for rapid sample spinning, most of which are based on the o r i g i n a l designs of Andrew (5) and Lowe (8). In a d d i t i o n , the technique of 'cross p o l a r i z a t i o n ' introduced and developed by Pines, Gibby and Waugh (9) i s used to increase the s i g n a l - t o - n o i s e r a t i o of the spectrum. The proton magnetizat i o n i s 'spin-locked' along the y' a x i s with a s p i n - l o c k i n g f i e l d Hfl and the carbons subjected to an RF pulse chosen such that the two f i e l d s f u l f i l l the 'Hartmann-Hahn' c o n d i t i o n (10), equation [3] (Figure 2). 13

1

13

13

3

1 3

1 3

9

V H

=

Y

H

C C

Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

[3]


28.

FYFE ET A L .

Magic Angle

407

Spinning

Figure 1. Representation of the geometric arrangement for a sample spinning at the magic angle to the magnetic field vector H . (Reproduced with permission from Ref. 40. Copyright 1982, Royal Society of London.) 0

H decoupling

H channel

l-contact H timeTc

acquisition time

1

13

C channel

Figure 2. Cross-polarization timing diagram (see text for discussion). (Reproduced with permission from Ref. 40. Copyright 1982, Royal Society of London.)


408

INORGANIC

CHEMISTRY:

TOWARD T H E 21ST C E N T U R Y

The net e f f e c t i s that the carbon n u c l e i are p o l a r i z e d by the proton magnetization and the S/N13 i s increased both d i r e c t l y by the r a t i o YH/"YC case of and i n d i r e c t l y because the experiment now depends only on the recovery of the proton magnetizat i o n which w i l l u s u a l l y be much f a s t e r than the C r e l a x a t i o n . The combined CP/MAS experiment was f i r s t a p p l i e d by Schaefer and S t e j s k a l (11, 12) and has s i n c e found wide a p p l i c a t i o n i n a number of areas. Figure 3 shows the e f f e c t of s p i n n i n g and the kind of r e s o l u t i o n which may be obtained from very c r y s t a l l i n e m a t e r i a l s (13). Spectra with t h i s degree of r e s o l u t i o n obviously contain s u b s t a n t i a l chemical information. Several p o i n t s may u s e f u l l y be made at t h i s stage concerning the experiment : F i r s t l y , although C was used i n the above example and i n the p r a c t i c a l a p p l i c a t i o n s , the experiment w i l l work f o r any nucleus which s a t i s f i e s the requirements of being ' d i l u t e . In some cases, eg. C (I = 1.1%) or S i (I = h> 4.7%) t h i s occurs a u t o m a t i c a l l y because of the low abundance but w i l l work even f o r P as long as there are not too many of them, too c l o s e to each other. In f a c t , apart from *H and p o s s i b l y F i n 'concentrated' samples any s p i n h nucleus w i l l f u l f i l l the NMR r e q u i r e ments of being ' d i l u t e ' . Results reported to date on other n u c l e i suggest that many are amenable to study i n the s o l i d s t a t e . Secondly, i n many important i n o r g a n i c systems, a very cons i d e r a b l e s i m p l i f i c a t i o n e x i s t s i n that o f t e n there are no protons present which are d i r e c t l y incorporated i n t o the l a t t i c e (eg. z e o l i t e s , glasses and calcogenides) and the only mechanism of line-broadening i s the chemical s h i f t a n i s o t r o p y , which can be r e moved by MAS. Further, c r o s s - p o l a r i z a t i o n i s not p o s s i b l e and the removal of the need f o r high-power decoupling means that i t i s p o s s i b l e to use a conventional h i g h - r e s o l u t i o n spectrometer and the experiment i s thus e a s i l y w i t h i n the reach of most NMR spectroscopists. In many cases, there are s u b s t a n t i a l advantages of working at high f i e l d s i n the s u p e r i o r homogeneity and s t a b i l i t y of superconducting s o l e n o i d magnets, and a MAS probe f o r narrow bore superconducting magnet systems has been described (14). Examples of s p e c t r a of d i f f e r e n t n u c l e i obtained i n t h i s way are shown i n Figure 4. T h i r d l y , i n the case of quadrupolar n u c l e i with n o n - i n t e g r a l spins (eg. A 1 , B , 0 ) there are very d e f i n i t e advantages i n working at the highest p o s s i b l e f i e l d . The (h «-* -h) t r a n s i t i o n i s not subject to quadrupolar i n t e r a c t i o n s to f i r s t order (Figure 5), but i s a f f e c t e d by second order quadrupolar i n t e r a c t i o n s , r e s u l t i n g i n line-broadening and s h i f t s . This i n t e r a c t i o n i s i n v e r s e l y p r o p o r t i o n a l to the magnetic f i e l d s t r e n g t h (equation [4]) where i s the l i n e w i d t h a t h a l f - h e i g h t of the peak, VQ i s the quadrupolar frequency and V i s the Larmor frequency, and i s thus minimized at high f i e l d s when the chemical s h i f t i s maximized. MAS reduces, but does not completely remove, the i n t e r a c t i o n and s u b s t a n t i a l improvement i s found at high f i e l d s (Figure 6). I t i

n

C)

t r i e


1 3

1 3

1

13

2 9

3 1

1 9

2 7

: 1

1 7

L



Figure 3.

13

C-CP/MAS solid-state spectrum of the cation shown illustrating the resolution obtainable from crystalline samples.



«

-550

15

'

1

ι

4

2

1

-600 -650 ppm from Pb(CI0 )

s

— ι

-ι

27

200

0

1———ι

,

15

-200 ppm from NO3

1

s

9

,

15

s

11

Figure 4. Solid-state magic angle spinning spectra. Key: a, Al spectra of zeolite-Y at 23.5 MHz and at 104.2 MHz; b, Β spectrum of Corning 7070 glass at 128.4 MHz; c, *"Pb spectra of Pb(NO ) at 83.4 MHz (the powder pattern is shown above the spinning spectrum); and d, N spectrum of NHi NO at 40.5 MHz. The NO portion of the spectrum, magnified 22 times, is also shown. (Reproduced with permission from Ref. 14. Copyright 1982, J . Magn. Reson.)

«


,

-400 0 § Η g ^

H

*2

a

28.

FYFE ET AL.

Magic

Angle

411

Spinning

5

/2

5/2

3/

3/

2

2


Vl /2 -Vl -/2 -3/2

" 3/2

-54 -5/2 Figure 5. Energy level diagram for a spin 5/2 nucleus showing the effect of the first-order quadrupolar interaction on the Zeeman energy levels. The (m = V2 ±? m = -V2) transition (shown in bold) is independent of the quadrupolar interaction to first order.

104.22 MHz

(a)

200

-100

100

23.45 MHz

100

0

-100

6 [ppm] Figure 6. High-resolution micrograph together with corresponding scalar structural drawing, computed image, and appropriate diffraction pattern showing structural resolution of Bi W O with some Bi W O intergrowths (24). 2

2

g

2

3

ïg


INORGANIC C H E M I S T R Y : TOWARD THE

412

CENTURY

21ST

2


ωι

[4]

(quadrupolar) =

should be noted that the majority of NMR a c t i v e n u c l e i are quadru p o l a r with n o n - i n t e g r a l spins and h i g h - f i e l d MAS experiments w i l l be a p p l i c a b l e to a whole range of s o l i d systems c o n t a i n i n g such n u c l e i , e s p e c i a l l y since t h e i r r e l a x a t i o n times are t y p i c a l l y short. Thus, by the c o r r e c t choice of experiment, i t i s p o s s i b l e to obtain h i g h - r e s o l u t i o n spectra from a wide v a r i e t y of n u c l e i i n the s o l i d s t a t e and spectra of t h i s type have many a p p l i c a t i o n s to inorganic chemistry. A p p l i c a t i o n s i n Inorganic

Chemistry

In general, h i g h - r e s o l u t i o n NMR of s o l i d s i s found to be very much complementary to d i f f r a c t i o n techniques i n the i n v e s t i g a t i o n of s o l i d - s t a t e s t r u c t u r e s . The a p p l i c a t i o n s may be c l a s s i f i e d i n t o three very general groupings: F i r s t l y , f o r amorphous systems (some of which are of con s i d e r a b l e commercial importance) the NMR data provides e s s e n t i a l information as d i f f r a c t i o n techniques are not a p p l i c a b l e at a l l due to the lack of order i n the l a t t i c e . Secondly, f o r some c r y s t a l l i n e systems, the s t r u c t u r e ob tained by d i f f r a c t i o n techniques may be incomplete. For example, i n some cases the d i f f r a c t i o n data may not r e v e a l dynamic aspects of the s o l i d - s t a t e s t r u c t u r e (as i n the case of f l u x i o n a l organom e t a l l i c s ) and i n others i t may not be p o s s i b l e to d i s t i n g u i s h c l e a r l y between d i f f e r e n t atoms (as for example A 1 and S i in z e o l i t e s ) and a combination of the NMR and x-ray data w i l l y i e l d a more complete and meaningful d e s c r i p t i o n of the s t r u c t u r e . T h i r d l y , i n cases where the NMR spectra can be obtained both i n s o l u t i o n and i n the s o l i d s t a t e , the NMR studies w i l l act as a valuable 'bridge between the s t r u c t u r e c h a r a c t e r i z e d by d i f f r a c t i o n techniques and that which e x i s t s i n s o l u t i o n . Where gross s t r u c t u r a l changes do occur, as f o r example, where exchange equi l i b r i a are present i n s o l u t i o n , the s o l i d - s t a t e s p e c t r a w i l l serve as 'benchmark' values f o r an i n t e r p r e t a t i o n of the s o l u t i o n studies. In the f o l l o w i n g sections we w i l l i l l u s t r a t e a p p l i c a t i o n s of the technique w i t h i n the general groupings described above, using examples from our own work and r e l a t i n g the choice of the experimental conditions to the c h a r a c t e r i s t i c s of the n u c l e i being studied as discussed i n Part 2 above. 27

9

1

Amorphous Systems (Polymer and 'Inorganic' Glasses)

Surface Immobilized C a t a l y s t s

and

Immobilized Polymers. There has been considerable i n t e r e s t i n recent years i n the synthesis and use of reagents c o v a l e n t l y



28.

FYFE

ET

Magic Angle

AL.

413

Spinning

immobilized on i n s o l u b l e support m a t e r i a l s such as glass and polymers because of t h e i r p o t e n t i a l advantages i n terms of ease of separation, conservation of valuable m a t e r i a l s , c o n t r o l of noxious reagents e t c . (15,16,17). The immobilization of t r a n s i t i o n metal c a t a l y s t s has been e x t e n s i v e l y studied as a method of combining the most d e s i r a b l e c h a r a c t e r i s t i c s of homogeneous and heterogen eous c a t a l y s t s . Systematic research e f f o r t s i n t h i s f i e l d have been severely r e s t r i c t e d because of the lack of s u i t a b l e a n a l y t i c a l techniques f o r the a n a l y s i s and most importantly, s t r u c t u r a l c h a r a c t e r i z a t i o n of surface immobilized species ( d i f f r a c t i o n mea surements c l e a r l y not being a p p l i c a b l e ) . Many of the complexes contain phosphine donor l i g a n d s and P CP/MAS s p e c t r a may be used to probe t h e i r s t r u c t u r e and geometry. Immobilization on glass surfaces can be c a r r i e d out by e i t h e r routes [5] or [6] i n Scheme 2 both y i e l d i n g the immobilized com p l e x (1), r e a c t i o n [5] being the procedure most commonly used. 3 1

Scheme 2 (s)-OH + (EtO) SiCH2CH PPh ^ 0-O-Si-CH CH PPh2 3

2

2

2

2

(S/-0-S i-C H C H-P-ML _ I Ph 2

2

(n

χ )

(1) Ph [MLjJ + ( E t O ) S i C H C H P P h 3

2

2

I

2

+ (EtO) Si-

High-Resolution Magic Angle Spinning and Cross-Polarization Magic

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