Transcript Chapter 11: Solid State NMR

Solid-State NMR
Impact of Structural Order on NMR Spectrum
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Factors that average to zero in solution due to random motion are now factors
in solid state NMR
T1 is long  lack of motion and modulation of dipole-dipole interaction
T2 is short  mutual spin flips occurring between pairs of spins
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Each nucleus is “fixed” in the crystal lattice
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Each nucleus produces a rotating magnetic field as it precesses in the
applied magnetic field  lifetime of spin state is reduced
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Each spin has a static field component that influences Larmor frequency of
neighbors
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Spin directions vary randomly
Range of frequencies that add to line-width
Chemical shift anisotropy
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Chemical shift varies with orientation relative to B0
Contributes to line broadening
Solid-state
(ordered structure)
Bo
Solution-state
(random-orientation)
Solid-State NMR
Broad Structureless Resonances
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Requires Different Approaches Compared to Solution State NMR
Contains Unique Information Relative to Solution State NMR
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Peak width is caused by dipole-dipole interaction which is distance related
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Peak width can monitor motion within the crystal lattice
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13C
Solid state NMR spectrum can be used to obtain internuclear distances
Slowly increase temperature
Line-width transactions indicates introduction of motion
NMR of glycine
O
H2N
OH
glycine
solution-state
solid-state
Angew. Chem. Int. Ed. 2002, 41, 3096-3129
Solid-State NMR
Powder vs. Crystal
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Crystal – regular uniform and repeat lattice structure
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Powder – consists of very many crystals all with different orientations
Solid-State NMR
Powder Pattern
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Dipolar coupling
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Interaction of nuclear magnetic moments of two different
nuclear spins (I & S)
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The local magnetic field at spin S will be affected by spin I
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Changes resonance frequency of spin S
The degree by which spin I affects the magnetic field at
spin S is determined by the dipolar coupling constant (d):


H IS  d 3 cos 2 q  1 I z S z
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where q is the angle between Bo and the internuclear
distance (r)
The dipolar constant is dependant on the distance
between the nuclear spins and their gyromagnetic ratios
 o   I  S
d   3
 4  rIS
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z
B0
Through space interaction  structural information
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In solution, random motion averages dipolar coupling to
zero
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In solids, orientations are static  defined by crystal
lattice
q
r
1H
13C
y
x
Angew. Chem. Int. Ed. 2002, 41, 3096-3129
Solid-State NMR
Powder Pattern
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Dipolar coupling
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Contains structural information ( r, q)
Dipolar coupling provides distance information
 o   I  S
d   3
 4  rIS
Orientation relative to B0


H IS  d 3 cos 2 q  1 I z S z
Angew. Chem. Int. Ed. 2002, 41, 3096-3129
Solid-State NMR
Powder Pattern
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Chemical Shift Anisotropy
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Chemical shift is dependent on orientation of nuclei in the solid
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Distribution of chemical shifts
Averaged to zero for isotropic tumbling
Leads to extensive line-width broadening in solid-state NMR
Progress in Nuclear Magnetic Resonance Spectroscopy 6 46 (2005) 1–21
Solid-State NMR
Temperature Dependence
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Crystal Lattice Mobility Changes with Temperature
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Changes in bond rotations
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Large changes in line-shape depending on mobility in lattice
Rotation about C-N bond
Rotation of NMe3
Whole molecule rotates
and diffuse within crystal
Solid-State NMR
Magic Angle Spinning (MAS)
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Nucleus with magnetic moment () will create a field at a second nucleus at a
distance r away
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Magnetic field will have a z component (Bz) in direction of Bo direction
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Influences the frequency of the second nucleus
Couples the two spins
Magnitude of Bz will depend on the angle of the magnetic moment relative
to B0
K
BZ  3 3 cos 2 q  1
r


z
B0
q
r
1H
13C
y
x
Solid-State NMR
Magic Angle Spinning (MAS)
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Zero z component (Bz) if the angle (q) relative to B0 is 54.44o
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All dipolar interactions disappear at this angle
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All chemical shift anisotropy disappear at this angle
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Quadrupole broadening is also reduced
Bz = 0
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Simulate a uniform distribution of magnetic moments in a powder by
spinning the sample very fast at 54.44o
Solid-State NMR
Magic Angle Spinning (MAS)
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Spin Samples at 54.44o to reduce line-width
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Spinning speed must be greater than static line-width to be studied
(powder pattern width)
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Normal speed limit is 35 kHz
rotor at MAS
Sample holder
Sample holder at MAS
MAS probe
rotor
Solid-State NMR
Magic Angle Spinning (MAS)
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13C
Impact of Spinning Speeds at MAS
NMR of glycine powder
O
Similar to Solution Spectrum
H2N
glycine
Increasing Spinning Speed
OH
Number of lines are reduced
with increase in spinning speed
as it approaches static line-width
Lines are separated by
spinning speed
Powder Pattern
Angew. Chem. Int. Ed. 2002, 41, 3096-3129
Solid-State NMR
Spin ½ Nuclei with Low Magnetogyric ratios (13C, 15N, 29Si, 31P, 113Cd)
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Combine MAS with high power 1H decoupling
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Double resonance technique
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High power is required because of very large 1H line-widths
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Long T1 requires slow pulse rates to avoid saturation of signal
Low sensitivity of nuclei requires long acquisition times
MAS reduces linewidth
from 5000 Hz to 200 Hz
Increase in sensitivity
(NOE, spin-splitting)
High power decoupling
reduces linewidth from
5000 Hz to 450 Hz
MAS & high power decoupling
reduces linewidth from 5000
Hz to 2 Hz
Similar to liquid state sample
Solid-State NMR
Cross-polarization combined with MAS (CP-MAS)
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Exchange polarization from 1H to 13C
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Similar in concept to INEPT experiment
2 ms
50 ms
• 1H 90o pulse generates xy magnetization (B1H)
• Spin-lock pulse keeps magnetization in xy plane
precessing at:
HB1H/2 Hz
13
• C pulse generates xy magnetization that precesses
at:
CB1C/2 Hz
• Polarization transfer occurs if:
HB1H/2 Hz = CB1C/2 Hz
Hartmann Hahn matching condition
Polarization transfer
1Hb
HB1H/2
1Ha
13Cb
CB1C/2

DE =  h Bo / 2
13Ca
Solid-State NMR
Cross-polarization combined with MAS (CP-MAS)
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Simultaneously pulse 1H to 13C
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Use RF energy to equilibrate energy states
HB1H/2 Hz = CB1C/2 Hz
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The increase in the 13C signal depends on the strength of the dipolar
interaction and the duration of the mixing or contact time
Maximum enhancement is H/C
Solid-State NMR
Cross-polarization combined with MAS (CP-MAS)
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Example of CP-MAS 13C spectrum
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Cross-polarization increases the 13C population difference by the factor H/C
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Increases signal sensitivity
Solid-State NMR
Spin ½ Nuclei with High Magnetogyric ratios (1H, 19F)
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Homonuclear interactions are very strong
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Difficult to remove by MAS
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Highest field strength and spinning rates can reduce a 10 kHz line-width to
1500 Hz
Static line-widths are very large and chemical shifts are small
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Obtaining resolution is challenging
Simulate MAS spinning by a series of RF pulses (MREV-8)
Shift magnetization quickly between the three orhogonal axes
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Hop around magic angle and reduce dipole-dipole interaction
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Does not affect CSA or heteronuclear interactions
MAS can be used to remove CSA
CRAMPS – combines MAS with MREV-8
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Solid-State NMR
Spin ½ Nuclei with High Magnetogyric ratios (1H, 19F)
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1H
Example of CRAMPS
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Resolution on the order of 180 Hz is possible
NMR of aspartic acid powder
O
CRAMPS
OH
HO
NH2
O
aspartic acid
MAS with increasing
spinning rates
Static Spectrum with
Broad Line-widths
Solid-State NMR
Two-Dimensional NMR Spectrum
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Can run similar solution state 2D NMR experiments
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Have to account for larger band-width, higher energy longer T1 and shorter T2
Example of 2D 1H EXSY experiment using CP-MAS 13C spectrum
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[(Me3Sn)4Ru(CN)6]
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Six unique methyl resonances, two distinct SnMe3 groups, exchange
identifies which methyls belong to which group
Exchange
between Methyls
Solid-State NMR
Two-Dimensional NMR Spectrum
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2D HETCOR
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Correlates closely spaced 1H
and 13C resonances
Similar to HSQC and HMQC
experiments
Solid-State NMR
Two-Dimensional NMR Spectrum
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2D REDOR
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Dipolar coupling contains distance information
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MAS yields sharps lines, but eliminates dipolar coupling
Reintroduces dipolar coupling information while maintaining sharp lines
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Can not turn spinning on and off
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Can synchronize spinning with RF to reintroduce dipolar coupling
Magnitude of dipolar coupling
The integral of the dipolar
coupling averages to zero for
each rotation
Apply 180o pulses at regular
intervals that disrupts the
trajectory of the dipolar
coupling so the integral is no
longer zero during a complete
rotation.
Solid-State NMR
Two-Dimensional NMR Spectrum
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2D REDOR
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A reference spectra is collected without the  pulses (S0)
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A series of spectra are collected with increasing mixing time (tm)
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Measure magnetization decay (S) as a function of tm
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Dipolar coupling is measured by fitting the S/So vs. tm plot
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A distance can be measured from:
    
d   o  I3 S
 4  rIS
d = 195 Hz,
13C-15N
= 2.47 Ǻ
O
H2N
OH
glycine
Solid-State NMR
Two-Dimensional NMR Spectrum
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2D REDOR
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Can also be used to generate chemical shift correlations
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Similar to HSQC, HMQC experiments
HETCOR: MAS effectively removes 13C-15N couplings
13C-15N
15N
13C
correlations for a peptide