1. Nuclear Magnetic Resonance Spectroscopy
Chapter 14
Nuclear Magnetic Resonance Spectroscopy
“study of interaction of light with molecules”

2. 14.1 Proton Magnetic Resonance Spectroscopy
Two or more E states are generated when external magnetic field is applied
radio wave absorption between these states
NMR spectroscopy
Hydrogen nucleus (1H) and the isotope of carbon (13C)
 1H NMR and 13C NMR
Three types of information for H NMR spectrum
 Chemical shift: the carbon to which proton is attached
 Multiplicity( # of the peaks in each group): neighboring hydrogens
 Integral: # of hydrogens

4. 14.2 Theory of 1H-NMR
Two spin states are generated when external magnetic field is applied ( 546 Figure 14.3)
E (E difference between the two states) = hBo/2
h = plank constant,
 = magnetogyric ratio (characteristic value for each nucleus)
Bo = external magnetic field

5. Magnetic field , resolution of NMR 
Typical values of E of proton (reference TMS)
External magnetic field 14,000 gauss (or 1.4-tesla)
then E =10-6 kcal/mol 60 x 106 s-1 or 60 MHz
46,700 gauss (4.67-tesla) magnet  200 MHz (in this book)
70,000 gauss  300 MHz
9.33-tesla magnetic  400 MHz (in this book)
Magnetic field , resolution of NMR 
 to get high magnetic field superconducting maget is needed (superconducting magnet is operated using liquid helium)
Magnetic field , $ 

6. 13C, 19F, 2H and 31P have nuclear spins
 NMR technique can be applied
12C and 16O do not have nuclear spins
 no NMR absorption
Why? Ask god!

7. 14.3 The Chemical Shift
 the field required for the hydrogen to resonance
 varies slightly with the chemical environment of the hydrogen
The required field difference between the hydrogen of the sample and that of TMS (tetramethylsilane)
Chemical shift () =
ex) 200MHz ( 46,700 gauss) 1H-NMR : aceton peak at 436 Hz
(436 x 106/200x106 =2.16  or 2.16 ppm
in 300 MHz NMR, this peak will be detected at 654 Hz
 548 Figure 14.4

9. Factors on Chemical Shift ( )
downfield: left on an NMR spectrum  deshielded 
increased external field  higher 
upfield: right on an NMR spectrum  shielded 
decreased external field  lower 
Inductive effect

10. Exchangeable protons: O-H/N-H (H-bonding)
Pi Electron Effect
deshielding effect  higher 
Exchangeable protons: O-H/N-H (H-bonding)
O-H: 2~5 (ppm), N-H: 1~3, CO2H: ~12 (10~15)
 peak position varies

11. Chemical equivalence  Three NMR absorptions
Protons with chemically identical environment
Three different sets of chemically equivalent protons
 Three NMR absorptions

12.  Three NMR absorptions
Enantiomer: identical chemical shift
Diastereomer: different chemical shift
Two different sets of chemically equivalent protons
 two NMR absorptions
Three different sets of chemically equivalent protons
 Three NMR absorptions

13. one NMR absorption peak
two NMR absorption peaks
five NMR absorption peak

14. A useful summary of the chemical shifts

15. Estimation of the chemical shift
approximate chemical shift of hydrogens

16. Empirical Correlation for predicting Chemical Shifts
 Shoolery’s Rule (old text book)
Ex) To the value from Table 14.1, add
0.3 ppm if CH2
0.7 ppm if CH
2-5 ppm according to concentration (actual value= 2.3 ppm)
O-CH3 = 3.3 ppm, (due to CH2) = 3.6 ppm
(actual value= 3.5 ppm)
C-CH3 = 0.9 ppm, ( CH2) (CH3) = 1.5 ppm
(actual value= 1.5 ppm)
0.9 ppm (C-CH3 )

17. Practice

18. 14.4 Spin Coupling (H,H Coupling)
Geminal coupling = Two-Bond Coupling
If they are diastereotopic, then have different chemical shifts
Vicinal coupling = Three-Bond Coupling
Very thoroughly studied, both experimentally and theoretically
Long-range Coupling: very small (not easily detected)

19. spin coupling: change in the local magnetic field by nearby H atoms having different d
multiplicity: Bo  BH;  559 & Figure 14.6~7
splitting pattern: Jax in 0~20 Hz (typical 6); s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad)
Differing spin relationships between HA and HB

20. The origin of spin-spin spliting in proton A’s NMR spectrum
NMR spectrum of this compound

22. Spin-spin splitting in ethyl iodide (CH3CH2I)
Splitting pattern of methyl hydrogens
Splitting pattern of ethyl hydrogens

24.  Peak number and area (height)
Spin-spin splitting rule: n+1 rule
Two neighboring
give triplet (three peaks)
n+1=3
One neighboring
give douplet two peaks
n+1=2
Pascal’s triangle
 Peak number and area (height)

25. The Coupling Constant (J)
In 1H-NMR, peak area indicates the number of hydrogen for the corresponding absorption
See  559 Fig. 14.6~7

26. Proton is coupled to nonequivalent protons

27. 14.5 Complex Coupling
Leaning
 The downfield peak of the triplet is slightly larger than the upfield peak.
 The quartet leans toward the triplet
As the difference in chemical shift becomes smaller, the leaning effect becomes larger.  562 Figure 14.8 vs  569 Figure 14.10

28. 14.6 Chemical Exchange NMR spectrum at RT  one peak
NMR spectrum at low temp
 two peaks
No coupling in NMR spectrum of methanol (no splitting) unless the methanol is extremely pure
In general, H in oxygen or nitrogen are subject to the rapid exchange and do not couple to nearby H’s
 SLOW CAMERA EFFECT

29. 14.7 Deuterium Solvent for NMR
 no signal not to interfere the spectrum of the sample
CCl4 can be used  poor solubility
Deuterated solvents (CDCl3, (CD3)2SO, C6D6) are used in NMR
Deuterium is invisible in NMR
1H-NMR measures E + small chemical shift
E (E difference between the two states) = hBo/2
(magnetogyric ratio) of the nucleus of deuterium is different from that of hydrogen  it does not appear in 1H-NMR
carbocations by NMR:  565 Focus On

30. 14.8 Interpretation of 1H-NMR Spectra
Step 1. Examine the general position of the peak.
Step 2. Examine the integral for the ratios of the different kinds of hydrogen.
Step 3. Examine the coupling pattern.
Step 4. Construct a tentative structure
Step 5. Determine whether all the information is consistent with this structure

33. 14.9 13C-NMR Spectrum Then in 13C-NMR, all peaks are singlet
Chemical shift range: 0~240 ppm
Broader than that of 1N-NMR
resolution is much higher
There is no, if any, 13C -13C coupling natural abundance: 13C 1.1% & 12C 98.9%
There is 13C –1H coupling, while this is removed by broad band decoupling technique
Then in 13C-NMR, all peaks are singlet

36. Other 13C-NMR Techniques
Off-resonance decoupling which allows hydrogens and carbons that are directly bonded to couple
CH3:quartet, CH2:triplet, CH:doublet, C:singlet
 Old technique
DEPT-NMR: Three spectra are obtained: New technique
Broad band decoupled spectrum
DEPT 90o spectrum: only CH’s appear
DEPT 135o: CH’s and CH3’s appear as positive absorptions and CH2’s appears as negative absorptions
see next page

38. Chemical equivalence in 13C-NMR