Kenneth Harris Flashcards
Four anisotropic interactions
- Chemical shift anisotropy (shielding)
- Direct dipole-dipole interaction
- Indirect dipole-dipole interaction
- Quadrupolar interaction
Origins of Chemical shift anisotropy
- nuclei are shielded by the electrons surrounding the nucleus within a molecule
- Shielding is anisotropic (as the distribution of electrons is is usually not symmetrical
- shielding depends on the orientation of the molecule
Origins of Direct dipole-dipole
- Analogous to the direct (through space) interaction between two bar magnets
- Nuclear magnetic moments, similar interaction exists, either homo or hetero
Origins of Indirect dipole-dipole (J-coupling)
- Coupling mediated through chemical bonds connecting the spins
- Usually much smaller than direct DD
Origins of Quadrupolar
- For nuclei with I>1, nucleus has an electric quadrupole moment due to a non-spherical distribution of nuclear charge
- Electric quadrupolar moment of the nucleus interacts with the electric field gradient (EFG) at the nucleus
For the four anisotropic interaction, state the average value of the interaction under condition of rapid isotropic
Shielding -> isotropic chemical shift
Indirect dipole-dipole -> isotropic J-coupling
Direct dipole-dipole -> zero
Quadrupolar -> zero
Background to solid state 2H NMR spec
- Powerful technique to study dynamics
- 2H is quadrupolar (I=1) and quadrupolar is the dominant anisotropic interaction
- The shape of the 2H NMR powder pattern is very sensitive to: Rate of motion & Mechanism of motion
Method of 2H NMR with computation
1) record the experimental 2H NMR spectra as a function of temp
2) Use computer techniques to simulate the 2H NMR spectra for the proposed mechanisms and rates of motion
3) Finally deduce the set of simulated spectra that give the best match to the experimental spectra
This allows us to determine the mechanism of motion AND the rate (k) at each temp (T) studied
Activation energy from plot of ln(k) vs 1/T
Limitations of 2H NMR
Sensitive only to motions involving reorientation of the EFG tensor at the 2H nucleus (e.g reorientation of the X-H bond)
2H NMR is not sensitive to translational motion
Equation that define the quadrupole coupling
Chi = e^2 x Q x qzz / h
e = electric charge Q = quadrupole moment of the nuclues qzz = principle value of the electric field gradient tensor at the position of the nucleus h = plancks constant
When is quadrupolar interaction eliminated
In cubic symmetry, due to high symmetry of the site
Spinning side band
Gap between the the anisotropic peaks in a high resolution solid state NMR spectrum (in Hz)
Magic angle
54.7 (gives one peak)
Chi values for H-bonding and non-H-bonding
H-bonding —> Chi = 170-250kHz
Non-H-bonding —> Chi = 290-320kHz
Electric field gradient at the site of the nucleus (qzz) is significantly affected by the presence of hydrogen bonding
Spin = 1/2 nuclei
1H , 13C, 15N, 19F, 29Si, 31P
Spin = 1 nuclei
2H, 14N
Spin = 5/2 nuclei
17O, 27Al
Spin = 0 nuclei
12C, 16O
Abundant Nuclei
1H, 12C, 14N, 16O, 19F, 31P, 27Al
Non abundant Nuclei
2H, 13C, 15N, 17O, 29Si
Powder sample spectrum
The NMR spectrum is typically very broad because it represents the summation of the spectra for each individual crystal orientation
High-Resolution Solid-State NMR
The aim is to average the anisotropic NMR interactions
(by experimental techniques such as high-power
heteronuclear decoupling and/or MAS) to generate “isotropic” NMR spectra
Broad-Line Solid-State NMR
The aim is to study the anisotropic NMR interactions
as a source of information on the structural and dynamic
properties of solids
Purpose of MAS
To remove 13C chemical shift anisotropy
Purpose of High-Power 1H Decoupling
to remove direct 13C…1H dipole-dipole interaction
and indirect 13C…1H dipole-dipole interaction
High-resolution solid-state 13C NMR can be a powerful
technique for the following:
● Characterizing different polymorphs
● Determining the number of independent molecules
in a crystal structure
● Determining the symmetry of the site occupied by a
molecule in a crystal
● Detecting disorder in a crystal structure
● Identifying specific intermolecular interactions
● Determining aspects of molecular conformation
Multiple Pulse technique
- Homonuclear direct dipole-dipole interaction (1H……1H) cannot be removed by decoupling
- direct dipole-dipole can be eliminated by MAS, but many cases interaction too strong
- Clever multiple pulse technique can be used to average the effects of homonuclear dipole-dipole interaction by manipulating the spins in “spin space”
- This leads to the average interaction becoming zero
Three dynamic regimes for 2H NMR
Slow - k < 10^3 S^-1
Stable shape, cannot determine value of k
Intermediate - 10^3 < k < 10^7 S^-1
shape critically depend on mechanism/exact value of k
Fast - k > 10^7 S^-1
characteristic of mechanism of motion but does not change as k changes
“single pulse” 13C NMR with 1H decoupling
- apply a 90 degree single pulse to 13C (micro seconds)
- acquire the 13C NMR , while applying high power proton decoupling (eliminate 13…..1H DD interaction) (milli seconds)
- Wait! recylce delay to allow the equilibrium magnetization develope again along the z-axis in the rotating frame (time is x5 T1 value) (minutes)
Problems with “single pulse”
- 13C has low abundance-intensity is intrinsically low
- T1 (13C) is very long for organic solids
- requires long recycle delays
- takes long time to record spectra
1H–>13C “Cross polarisation” can be used instead
Cross Polarisation (CP)
- Involves transferring magnetisation from protons (abundant) to carbon (dilute) in solid
- apply 90degree pulse to 1H
- apply ‘spin lock B1H field along the y axis for 1H
- Also apply B1C field along y axis for 13C
- Magnetisation transfer (CP contact) occurs efficiently when the Hartmann-Hahn conditions are established
- Measure 13C spec when max magnetisation has developed on the y-axis
- MAS applied throughout
Hartmann-Hahn matching condition
Allows 1H and 13C spins to communicate with each other in their respective rotating frames
Advantages of CP method over single pulse method
-CP has an intensity gain factor of gammaH/gammaC = 4
-The recycle delay depends on the T1 (1H) and not T1(13C)
therefore can be repeated much more frequently
