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Physical Chemistry - FID, One Pulse NMR Method, Fourier Transform, Nuclear Relaxation
Processes, Spin lattice relaxation and spin-spin relaxation

The FID
- The resulting pattern which develops overtime is caled Free Induction Decay (FID)
- FID contains all spectral information ('frequency domain') encoded in decay pattern ('time
domain')
- Each pulse - recover - measure cycle is the equivalent to one entire spectrum scan, but can be
accomplished much faster
...
The precise timing of the pulse varies and must be adjusted accordingly (full
π/2 angle gives largest response signal)
...

- Allow enough time for nuclei to return to normal (i
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aligned with main field) while collecting
the FID - this is very important
...
This is achieved by Fourier
transform calculation
...
However, data must be
collected and prepared carefully - in particular the FID must decay to zero and not be cut off (or
pulse peaks can appear in the spectrum)
...

- In order to encompass a wide frequency (or chemical shift) range initial pulse needs to be very
brief - tp is typically a few micro seconds
...

- Note that magnetic vector of nucleus is still precessing as its axis is tipped into the y plane by
the pulse - this composite motion is called mutation
...
The energy loss follows an exponential decay f(t) α e-R, t
 R is the rate constant of spin lattice relaxation
- Associated longitudinal relaxation constant, T, is given by:
T1 = 1/R1
T is about 10-2-10-4s for solids and 10-2-10s for liquids
...
This can
lead to overall magnetisation of zero
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- However, apparent spin-spin relaxation time constant can be affected by the operation of NMR
machine itself, the apparent values are labelled R2* and T2*
...
10-4s) and in liquids T2 approximately equals T1
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