1. Nuclear Magnetic Resonance
ANIMATED ILLUSTRATIONS
MS Powerpoint Presentation Files
Uses Animation Schemes as available in MS XP or MS 2003 versions
A class room educational material
File-8 FT NMR-I
2/24/ :38:15 PM
Dr.Aravamudhan

2. Frequency Domain Spectrum
This FID acquisition and the variations are considered in detail in the following slides
PULSED NMR
Acquire F.I.D.
Free Induction Decay
NMR detection soon after a strong pulse: precessing nuclear magnetization induces a signal in coil when it is free of the perturbing EM radiation
Acquisition is automatically in the digitized form
Computer memory
Address
Contents 15
1111 14
1110 8
1000 4
0100 7
0110 1
0001
0111
0000
Next Slide
Time domain 15 11
DIGITIZE
Analogue to Digital Converter A.D.C.
F.I.D.
Frequency Domain Spectrum
Computer output
FFT from FID
Computer input
This one-dimensional FT NMR spectrum is the same information as the C.W. NMR spectrum
2/24/ :38:16 PM
Dr.Aravamudhan

3. The FID2 is acquired after a delay one DW more compared to the FID1
Aquisition
Time domain 15 12 14 13 11 10 8
Pulse
This delay (after the first pulse/first scan) is called the receiver dead time. This is requirement due to response characteristics of the electronic system.
This first FID acquisition can be repeated without any necessities of alterations in delay times. This is typically the single pulse experiment repeated for summing the FIDs to improve signal to noise ratio.
FID 1
Receiver dead time/Acquisition delay
The FID2 signal from probe is the same.
But, the signal received is acquired after a larger delay than for FID1
Time domain 12 14 13 11 10 8
Aquisition
FID 2
Pulse
The delays of the type in FID2 are set by the experimenter to realize sequences of pulses for the case of multiple-pulse –sequence at the end of which the FID is ACQUIRED. These class of experiments dominate the advanced NMR techniques.
The time interval between the dotted (digitized) points is the DWELL (DW) time of the digitizer.
The FID2 is acquired after a delay one DW more compared to the FID1
2/24/ :38:16 PM
Dr.Aravamudhan

4. Apply Pulse
Acquisition of FID is started by sending a trigger pulse to the A to D converter/digitizer
It is possible to increment the delay, and acquire the FID at different phases.
Time domain 15 9 12 14 13 11 10 8
different phases
Trigger to ADC
cos
Start acquisition after a delay D1
Time domain 9 12 11 10 8
Acquisition
delay D1
Trigger to ADC
-sin
Trigger to ADC
-cos
Note D2 > D1
How will be the Fourier transformed frequency domain spectrum of these three time domain signals with different phases?
Start acquisition after a delay D2
Acquisition delay D2
2/24/ :38:16 PM
Dr.Aravamudhan

5. Time domain FID data: 32 points
Real Imaginary 16 data data points points Frequency domain spectrum
2/24/ :38:16 PM
Dr.Aravamudhan

6. t=0 +1
Value between +1 & 0
Provision is made in the data processing system, for routinely applying phase corrections
COS
Real
Imaginary
F.T
Real
Imaginary
F.T
SIN
Real
Imaginary
F.T
Arbitrary Phase
fc cos(2πνt) + fs sin (2πνt)
with fc2 +fs2 =1
2/24/ :38:16 PM
Dr.Aravamudhan

7. Time domain 15 12 14 13 11 10 8
Time domain 12 14 13 11 10 8
Time domain 9 12 13 11 10 8
Time domain 9 12 11 10 8
Time domain 9 11 10 8
Time domain 9 10 8
Time domain 9 8
Time domain 8
CONSIDER THE KIND OF DETAILS: for the variations in delays, phases and the frequency domain signals.
2/24/ :38:16 PM
Dr.Aravamudhan

8. PURE Cosine
Arbitrary Phase with sine and cosine
Mostly a negative of cosine
Negative cosine and negative sine

9. Cosine F.T. fC cos + fS sin Sine Cosine Sine Cosine Sine Cosine
Subtract out the sine component
F.T.
Sine
Cosine
Sine
Cosine
Sine
Cosine
Such requirements as subtracting a sine component from the magnetization can be effected by appropriate pulse widths and phases at the required time.

10. There has been no MULTIPLE PULSE SEQUENCE considered in this illustration. This only to illustrate the spectrometer settings required while setting up multiple pulse experiments. Two dimensional spectra contain rich information if the spin system is allowed to evolve under multiple pulse perturbations before acquisition of FID.

11. Preparation pulse: facilitates the following……..
Free evolution of spin system
Free evolution of spin system
FFT
Spectrum
Spin Echo Fourier Transform Spectroscopy x z y
Preparation pulse (π/2-pulse)
Echo x z y
Τ secs
Τ secs
Refocussing (Mixing?) pulse
Inverting pulse (π-pulse)
180° pulse
90° pulse
F.I.D
Refocus
Defocus t2
acquisition
The refocus period is somewhat similar to the Mixing Time in the pulse sequences for dimensional NMR Spectra. However the Mixing period is held fixed while varying the t1 period for 2D, as described in the next slide.
Rotating x,y axes :rotation about Lab z-axis
Preparation pulse: facilitates the following…….. Z X Y
Magnetization in XY plane appears stationary when viewed in Rotating Frame from within the rotating frame X Y X Y Z
Rotating system viewed from within that system: STATIONARY X Y
The description above of the spin echo spin system evolution was with the magnetization vectors in a rotating coordinate system
Transforming to Rotating coordinate system

12. tm t1 t2 Acquisition for duration t2 ω1 ω2
Evolution period t1
Mixing period tm (Fixed duration) t1 tm
Acquisition for duration t2 t2
Depends on the choice of the type of 2D experiment ω2 ω1
2D Spectrum Contour plot
2D NMR pulse sequence structure