Magnetic Field inhomogeneity

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Magnetic Field inhomogeneity
Bz-DB so wL-Dw
B0 is not homogeneous over entire sample, so in
sample some spins precess faster than the Larmor
frequency, others slower
Bz, wL
BzDB so wLDw
sample
MAGNETIC FIELD INHOMOGENEITIES REFOCUSSED AT TIME
2t
2
Chemical Shift
Two species (isochromats - MA and MB) with
different chemical shift
CHEMICAL SHIFT REFOCUSSED AT TIME 2t
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(No Transcript)
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Homonuclear AX System
J coupling causes MAXa and MAXb to precess at
different rates. In the rotating frame

• MAXa precesses in an anti-clockwise direction
  (since precesses at - ½JAX (slow))
• MAXb precesses in an clockwise direction (since
  precesses at ½JAX (fast))

population in X system now also inverted
MAXa
MAXa
MAXa
MAXb
MA inverted about y-axis
y
MAXb
MAXb
MAXb
MAXa
x
iv) After time 2t
iii) 1800y(A)(X) at time t. Inverts population of
A and X systems.
i) Immediately after 900x(A)
ii) After time t
J COUPLING IS NOT REFOCUSSED IN HOMONUCLEAR CASE
- DEPHASING AT TIME 2t
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How can we produce J modulated spin echoes for a
Heteronuclear AX System ?
MAXa
MAXa
y
y
MAXb
MAXb
x
i) Immediately after 900x(A)
ii) After time t
iv) After time 2t
X population now also inverted by 1800y(X)
MAXa
MAXb
iii) Apply a 1800y(A) pulse at the same time as a
1800y(X) pulse
BY APPLYING TWO 180 PULSES (1800y(A) at the same
time as a 1800y(X)) J COUPLING IS NOT REFOCUSSED
- DEPHASING AT TIME 2t
WE CAN USE SUCH A SEQUENCE TO PRODUCE A 2D
J-RESOLVED NMR SPECTRUM