1
Solid‐state NMR Training Course
Experimental Basics
1
Solid‐state NMR Training Course
Experimental Basics
2
The Building Blocks
High‐resolution solid‐state NMR
Magic‐angle spinning
Direct‐polarisation (DP)
[aka SPE and pulse‐acquire]
Cross‐polarisation (CP)
Direct‐polarisation
recycle delay
free induction decay
recycle delay
relaxation delay
pulse delay
recycle
free induction decay
FID
signal
4
When to use DP
Quantitative measurements –
spin‐½ nuclei (
31
P (100%),
29
Si(4.7%))
Soft solids
Heterogeneous materials
Quadrupoles (
27
Al,
11
B,
23
Na)
5
When to use DP
Quantitative measurements (
31
P,
29
Si)
Soft solids (high molecular mobility)
Heterogeneous materials
(containing “soft” and “hard” phases)
Quadrupoles (
27
Al,
11
B,
23
Na)
Deliberately selective conditions
6
When to use DP
Quantitative measurements (
31
P,
29
Si)
Soft solids
Heterogeneous materials
Quadrupoles (
27
Al,
11
B,
23
Na)
7
Optimising a DP experiment
t
time constant T
1
(spin‐lattice
relaxation time)
90
o
90
o
Recovery of
equilibrium
magnetisation
after a pulse
8
Optimising a DP experiment
repetitions
(constant
experiment time)
recycle / T
1
relative
S/N
8
5
1.00
12
3.33
1.19
32
1.2
1.41
80
0.5
1.25
400
0.1
0.68
tip‐angle = cos
‐1
(e
‐t/T
1
)
“Ernst” angle
tip‐angle < 90/(I+½)
quadrupoles
t
90
o
90
o
9
Cross‐polarisation
…. for sensitivity enhancement
requires:
an abundant nucleus – usually
1
H
coupled network of abundant nuclei (ideally)
coupling between the abundant spin and
n
X
10
Cross‐polarisation – the pulse sequence
B
0
z
y
x
90
x
1
H
90
x
1
H
spin-lock
y
n
X
spin-lock
y
contact
90
x
1
H
γ
H
B
1
H
= γ
X
B
1
X
Hartmann‐
Hahn match
condition
11
90
x
1
H
flip-back
-x
decouple
y
FID
contact time
acquisition time
n
X
spin-lock
y
contact
Cross‐polarisation – the pulse sequence
12
The advantages and disadvantages of CP
9 Recycle ∝ T
1
H
(not T
1
X
) and T
1
H
<< T
1
X
9 Signal enhancement (potentially γ
H
/γ
X
)
9 Fewer problems with background signals
9 Can probe relationship between nuclei
8 Loss of quantitivity
8 Potentially selective
Advantages
Disadvantages
9 Potentially selective
13
90
1
H
decouple
n
X
spin-lock
y
-y
-y
x
x
y
-y
y
-y
y
y
x
x
y
-y
x
-x
Receiver
Cross‐polarisation – the phase cycling
• A full phase‐cycle removes the signal generated directly from the contact pulse
14
The advantages of cross‐polarisation ‐ illustrated
recycle 3 s, 448 repetitions
experiment time 22 minutes
DP
CP
DP
recycle 120 s, 448 repetitions
experiment time 15 hours
(T
1
(C) >> T
1
(H))
15
Why CP is not (necessarily) quantitative
n
X
spin-lock
y
contact
Energy transfer
depends on
relationship of X
and H
COOH
H
3
C
H
2
N
CH
3
H
H
16
How to make CP quantitative
17
How to make CP quantitative
COOH
H
3
C
H
2
N
CH
3
H
H
COOH
Rise:
0.2 and
1.1 ms
Decay:
2.2
ms
Absolute intensity: 23.4
CH
Rise: 32 μs
and 0.33 ms
Decay:
2.1
ms
Absolute intensity: 30.5
• Dilute H or high spin‐rate introduces extra complexity
T
1ρ
(H)
18
Decoupling
phase
continuous wave (cw)
phase
p
180
two‐pulse phase‐modulated (tppm)
p
180
19
Matching
low spin‐rate
2 kHz
high spin‐rate
22 kHz
‐1 0 +1 +2
centreband
sideband
20
Matching (fast MAS)
21
1
H
n
X
spin-lock
contact
dephasing
delay
180
1/spin-rate
1/spin-rate
Spectral editing (dipolar dephasing)
22
Spectral editing (dipolar dephasing)
Difference (CH)
Quaternaries (and anything mobile)
All carbons
(two molecules in the crystallographic asymmetric unit)
23
H
X
1
H ‐
n
X WIdeline SEparation (WISE)
rigid
mobile
24
1
H ‐
13
C (wideline) correlation
25
Chemical shifts: solid vs solution
26
Instrument (probe) issues
7.5 mm
4 mm
S/N on HMB
350:1
80:1
Time
1 hour
19 hours
Decoupling
60 kHz
125 kHz
Spin‐rate
7 kHz
18 kHz
Sample volume
450 μl
52 μl
13
C (1.11%)
13
C minor component
15
N (0.37%)
29
Si (4.70%)
27
Al (100%)
119
Sn (8.58%)
27
Li
Be
B
C
N
O
F
Na
Mg
Al
Si
P
S
Cl
Xe
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Accessible elements
Easy!
Feasible with varying degrees of difficulty
Special