1. FT-NMR

2. Fundamentals Nuclear spin Spin quantum number – ½
Nuclei with spin state ½ are like little bar magnets and align with a B field.
Can align with (++) or against (+-) B
Small energy gap between + and – spin alignment (NMR insensitive/Boltzman dist)
Can probe difference with RW

3. (NMR insensitive/Boltzman dist)
Small population difference between +1/2 and -1/2 state
It is the small excess of nuclei in the -1/2 that produce NMR signal

4. Common NMR nuclei Protons, 1H 13C 15N 19F 31P
Sensitivity depends on natural isotopic abundance and g
DE = gћB0 , bigger magnet, greater sensitivity

5. Precession of nuclear dipolesz
+1/2
M0; net
magnetic moment
From small excess of
Nuclei in +1/2 state y M0
B0 from magnet x
-1/2

6. FT pulse Radiofrequency generator
A short, intense pulse generates a magnetic field in the x-y plane (excites all nuclei)
M0 of the nuclei interacts with the magnetic field produced by the pulse.
Tips M0 off axis
Θ = gB1tp
tp – length of pulse, 90 pulse

7. Vector Illustration of the pulse
+1/2 M0
RF pulse
B0 from magnet
RF coil
-1/2

8. Relaxation
T1 spin-lattice (relaxing back to precessing about the z axis)
T2 spin-spin (fanning out)

9. Induced current in coil
After pulse, nuclei begin to precess in phase in the x-y plane
Packet of nuclei induce current in RF coil
Relaxation is measured by monitoring the induced coil
→ FID (→ FT) NMR spectrum

10. FID

11. Noise reduction and increasing resolution
Apodization: Multiply the free-induction decay (FID) by a decreasing exponential function which mathematically suppresses the noise at long times. Other forms of apodization functions can be used to improve resolution or lineshape.
Zero filling

12. Chemical Shift Shielding
Electrons have spin, produce local B environments
Protons in different electronic environments experience different B, different precessing frequencies, DE = hu
Chemical shift proportional to size of magnet
ppm {(s-s0)/s0}*106

13. Spin-Spin Coupling
Adjacent nuclei have a 50/50 chance of being spin up (+1/2) or spin down (-1/2)
Each produce a small magnetic field that is either with or against B0
1 adjacent proton CHOCH3
CH3 is a doublet at frequencies
u0-ua, u0+ua (equal intensity), 1:1
CH is a quadruplet
1:3:3:1

14. Splitting Patterns J values
Quadruplet                        
Triplet        
Multiplets
1 2 1
¼ ½ ¼ ¾ 1 ½ ¾ ¾ 1 ½ ¾ ¼ ½ ¼

15. 13C NMR 13C frequency Different tuning folk Broadband Decoupling of 1H
No spin-spin coupling
NOE effect
Assignments based on chemical shift
Wider frequency range

16. Obtaining a 13C NMR Spectrum
1H Broadband decoupling
Gives singlet 13C peaks, provided no F, P, or D present in the molecule)
Continuous sequence of pulses at the 1H frequency causes a rapid reversal of spin orientation relative to the B0, causing coupling to 13C to disappear

17. Broadband Decoupling
1H channel
13C channel

18. H3C4-C3H=C2H-C1OOH
solvent
C-4
C-2
C-3
C-1 10
180

19. 13C Chemical Shifts Reference is TMS, sets 0 ppm A range of 200 ppm
Chemical shifts can be predicted
Empirical correlations
Ex. Alkanes
di = na + 9.4nb – 2.5ng + 0.3nd + 0.1ne + Sij
2-methylbutane
di = * *2 – 2.5* = 22.0 (22.3)

20. Signal averaging
13C experiment generally take longer than 1H experiments because many more FIDs need to be acquired and averaged to obtain adequate sensitivity.
NOE effect (enhancement/reduction in signal as a result of decoupling) N4 N4
13C 1H W2 N2 N2 N3 N3 W1 1H
13C N1 N1

21. NOE effect W2 (Enhancement) dominates in small molecules
Relevant for all decoupling experiments

22. Other more complex 1D Experiments
1H NOE experiment
Inversion Recovery Experiment; Determination of T1
J modulated Spin Echo
INEPT Experiment
DEPT Experiment

23. Targeted 1H Spin Decoupling
Continuous irradiation at a frequency (n2) that corresponds to a specific proton in the molecule during the 1H NMR experiment
All coupling associated with the protons corresponding to n2 disappears from the spectrum

24. 1H targeted decoupling (NOE)
n2 channel
1H channel

25. 1 3 2
TMS n2

26. NOE- nuclear Overhauser effect
Saturation of one spin system changes the equilibrium populations of another spin system
NOE effect can be positive or negative. In small molecules it is usually positive

27. Selective Heteronuclear Decoupling
Saturate at a specific frequency
Multiplets collapse reveal connectivity

28. More Complex NMR Pulse Sequences
J-Modulated Spin Echo experiment
Cq and CH2 down and CH3 and CH up
DEPT experiment
Q= 45, 90, 135
CH3 [DEPT(90)], CH2 [DEPT(45)-DEPT(135)], CH [DEPT(45)+DEPT(135)-0.707DEPT(90)]
2D-NMR
Het. 2D J resolved/Homo 2D J resolved
1H-1H COSY
1H/13C HETCOR

29. J-Modulated Spin Echo 13C channel 90x – t – 180x – t(echo)
1H channel _____________BBBBBBB
t = 1/J(C-H)
CH + CH3 (up)
C(q) + CH2 down

30. Neuraminic acid

31. CH and CH3
Cq and CH2

32. DEPT 13C ch 90x–t –180x–t–FID 1H ch 90x–t–180x–t – fy –t-BBBBBB
t = 1/2J(C-H)
fy = 90, 45, 135
CH: DEPT(90)
CH2: DEPT(45)- DEPT(135)
CH3: DEPT(45) + DEPT(135) DEPT(90)

33. DEPT DEPT(90) CH3 DEPT(45) – DEPT(135) CH2 DEPT(45)+DEPT(135)-
13C decoupled
spectra

34. HET 2D J Resolved 13C ch 90x – t – 180y – t - FID
1H ch BBBBBB__________BBBBBBB
t = 1/J(C-H)
Gives J(H-C) values as a function of 13C chemical shift

36. Homo 2D J Resolved 1H ch 90x – t – 180x – t - FID t = 1/J(H-H)
Gives J(H-H) values as a function of 1H chemical shift

38. H-H COSY 1H ch 90x – t1 – f- FID (t2) t = 1/J(H-H)
Coupled protons give cross correlation peaks off the diagonal

39. HOOC(1)-C(2)H(NH2)-C(3)H2-C(4)H2-C(5)OOH

40. HETCOR
Plots 13C chemical shifts as a function of 1H chemical shifts of the connected carbon/protons pairs.

41. F2 (13C NMR decoupled Spectrum)
H(3)
H(4)
H(2)
C(2)
C(4)
C(3)

42. Practical Aspects to “Running a sample”
Deuterated solvent
Air drop, sample height
Lock-in the deuterated peak (B drift)
Shimming the magnet; parallel magnetic field lines for limiting broadening of line width.
Setting the parameter; nuclei, spectral range, FID time, number of scans, and apodization, ect.