Basic principle and instrumentation along with spectral interpretation

1. Presented by
Uswa Arshad
Hamza Shoukat
M.Phil Chemistry (II)
Department of Chemistry
1

2. Contents
2
 Introduction
 1H NMR (PROTON NMR)
 Principle
 Instrumentation
 Terms used in NMR
1. Reference solvent
2. Relaxation process
3. Chemical shift
4. Shielding & Deshielding
5. Splitting of signals
6. Peak shapes with respect to pascal triangle
7. Coupling Constant
• NMR spectra and interpretaion
3
4
5-7
8-12
13
14
15
16
17-20
21-22
23-25
26-35
Slide no.

3. Introduction to NMR
 It is the study of absorption of radiofrequency radiation by nuclei in a
magnetic field is called Nuclear Magnetic Resonance.
 Nuclear magnetic resonance spectroscopy is basically another form of
absorption spectrometry. It involve change of the spin state of a nucleus,
when the nucleus absorb electromagnetic radiation in a strong magnetic
field.
 The source of energy in NMR is radio waves which have long
wavelengths, and thus low energy and frequency.
Keeler, J. (2011). Understanding NMR spectroscopy: John Wiley & Sons.

4. Proton NMR
4
 It is a technique which is based on the absorption of electromagnetic
radiation in the radio frequency region 4 to 900 MHz by nuclei of the
atoms.
 It is used to study a wide variety of nuclei: 1H ,15N, 19F, 13C, 31P.
 The most common form of NMR is based on the hydrogen-1 (1H), nucleus
or proton.
 Shows how many kinds of nonequivalent hydrogen’s are in a compound.
 Equivalent H’s have the same signal while nonequivalent are “different”
and as such may cause additional splitting.
Keeler, J. (2011). Understanding NMR spectroscopy: John Wiley & Sons.

5. Principle
 Protons in different environments absorb at slightly different frequencies, so they
are distinguishable by NMR.
 The frequency at which a particular proton absorbs is determined by its electronic
environment.
 The size of the magnetic field generated by the electrons around a proton
determines where it absorbs.
 Modern NMR spectrometers use a constant magnetic field strength B0, and then a
narrow range of frequencies is applied to achieve the resonance of allprotons.
 Only nuclei that contain odd mass numbers (such as 1H, 13C, 19F and 31P) or odd
atomic numbers (such as 2H and 14N) give rise to NMR signals.
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction to
spectroscopy: Cengage Learning.

6. absorb
-spin-spin flipping
Release ∆E
Signals detected by NMR
6
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction to
spectroscopy: Cengage Learning.

7. Principle of NMR
7

8. Instrumentation
RF oscillator, SweepRF field it consists of Sample holder,
generator and RF receiver.
 A detector, to process the NMR signals.
 A recorder, to display the spectrum.
Basically NMR instrumentation involves the following units.
 A magnet to separate the nuclear spin energystate.
 Two RF channels, one for the field/frequency stabilization and one to
supply RF irradiating energy.
 A sample probe, containing coils for coupling the sample with the
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction to
spectroscopy: Cengage Learning. 8

9. Fig.01 Schematic diagram of NMR spectrophotometer
9
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction to
spectroscopy: Cengage Learning.

10. Instrumentation Animation
10

11. Types of instruments
Two instruments are used for NMR spectroscopy:
1. The Continuous-Wave (CW) Instrument
2. The Pulsed Fourier Transform (FT) Instrument.
Fig.02 Schematic diagrame of FT NMR instrument
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction
to spectroscopy: Cengage Learning. 11

12. Principle of FT NMR Spectroscopy
• This converts time domain signal into
frequency domain signal.
• Time domain signal is un-interpretable
by eye so it is mandatory to convert
into frequency domain signal of
normal spectrum.
• FT comes in, its mathematical
operation that convert Time signal into
Frequency signal.
Fig.03 Principle of FT
Spectroscopy
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction
to spectroscopy: Cengage Learning. 12

13. Terms used in NMR
1. Reference solvent
2. Spin-spin relaxation
3. Chemical shift (𝛿)
4. Up-field and Down-field
5. Shielding & De-shielding
a) Local diamagnetic effect
b) Magnetic Anisotropy
6. Spin-spin splitting
a. Coupling Constant (J)
13

14. Reference Solvent
• TMS is used as reference solvent in
NMR.
• This is chemical inert.
• All protons are magnetically
equivalent.
• Miscible with most organic liquids
so used as external reference
standard.
NMR spectrum of
Tetramethylsilane
-1012
PPM
Younas, M. (2015). Organic Spectroscopy and Chromatography. Ilmi Kitab Khana, Lahore,
Pakistan.
14

15. Relaxation Process
Two kinds of relaxation processes are:
 Spin-spin relaxation: This takes place by transferring energy to neighboring
nucleus. A nucleus in the upper energy state can transfer its energy to a
neighboring nucleus by mutual exchange of spin.
 Spin- Lattice Relaxation: It involves the transfer of energy from the nucleus
in its higher energy state to the molecular lattice, the energy is transferred to
the components of lattice as the additional translational, vibrational &
rotational energy, and this process keeps the excess of nuclei in the lower
energy state which is necessary for NMR Phenomenon. 15

16. Chemical Shift
 “Chemical shift is the difference between the absorption position of the
sample proton and the absorption position of reference standard.”
 The position in NMR spectrum where signal occurs is called chemical shift.
 It indicates how far the signal is from the TMS reference peak.
Chemical Shift, ppm ( )= Shift from TMS in Hz
Spectrometer frequency(Mhz)
𝛿
16

17. Shielding & Deshielding
Shielding:
Fig.04 Shielding effect in compound
17

18. Factors effecting shielding
shielding
Magnetic
anisotropy
Local diamagnetic
effect
hybridization
electronegat
ivity
sp3
sp2
sp
H-Bonding
18

19. Examples
Electronegativity:
Element : CH3Cl
E.N of X: 3.1
Chemical Shift: 3.05
Substitution effect: CHCl3 𝛿 7.27
Hybridization:
sp3= R-CH3 (0.7-1.3)
R-CH2-R (1.2-1.4)
R3CH (1.4-1.7)
sp2 = R-C=C-R (1.6-2.6)
sp = (1.7-2.7)
Hydrogen Bonding:
H-bonding shielding signal will be more towards downfield.
RCCH
C2H5OH in
variouse forms
Polymeric
5.34 ppm
Diameric
2-4 ppm
Vapour
0.5 ppm 19

20. Magnetic anisotropy
20

21. Spin-Spin Splitting
 Each signal in an NMR spectrum represents one kind or one set of protons
in a molecule.
 It is found that in certain molecules, a single peak (singlet) is not observed,
but instead, a multiplet (groups of peaks) is observed.
 Splitting followed by (n+1) rule.
 E.g. A molecule of CH3CH2Br, ethyl bromide.
21

22. Pascal’s Triangle
n(no of neighboring H) Name Signal Row
0 Singlet 1
1 Doublet 1 1
2 Triplet 1 2 1
3 Quartet 1 3 3 1
4 Quintet 1 4 6 4 1
5 Sextet 1 5 10 10 5 1
6 Septet 1 6 15 20 15 6 1
7 Octet 1 7 21 35 35 21 7 1
22

23. Shapes of peaks
23

24. 24

25. 25

26. Coupling Constant
• Distance between peaks in a multiplet is called coupling constant.
• Expressed by ‘J’
• This spacing between multiplet peaks is measured in scale Hz.
• Variouse types of ‘J’ , associated with Two-bond( 2j) three-bond( 3j)
four-bond ( 4j).
J= 1.5div ×
5 𝐻𝑧
1 𝑑𝑖𝑣
= 7.5 𝐻𝑧
H-NMR spectrum of ethyl iodide (60 MHz) 26

27. Calculation of J Value
H1NMR Spectrum of Ethyl Bromide
Frequency: 400MHz.
0123
PPM
27

28. Two-bond Coupling
• Usually Found in Geminal
protons.
• Protons present at same carbon
atom will coupled and then
split with reference to each
other.
• Represented as 2J2
J
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction
to spectroscopy: Cengage Learning.
28

29. Three-bond coupling
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction to
spectroscopy: Cengage Learning. 29

30. 1H NMR Spectra
1H NMR spectra consist of signals originated from hydrogens or protons
present in molecule.
 Only those hydrogens will give signals which are having
different environment.
 Hydrogens having same chemical environment will only appear as a single
signal.
30

31. Steps in 1H spectra interpretation
many different kinds ofIndicate how
protons present.
1.Number of signals
2.Position of signals
3.Relative intensity of signals
4.Splitting of signals (spin spin coupling)
Indicate something about (chemical shift),
magnetic (electronic) environment of
protons.
Proportional to number of protons present
Indicate the number of near by nuclei
usually protons.
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction to
spectroscopy: Cengage Learning. 31

32. Chemical Shift table for different groups
32

33. 33

34. Example of NMR spectra
 CH3CH2COCH3, butan-2-one
Singlet = δ 2.09 (CH3)
Triplet = δ 1.06, 2H (CH 3 )
Quartet= δ 2.49, 3H (CH 2)
a b
c
c
a
b
34

35. Toulene:
(a)Singlet at 2.35
(b)Multiplet at 7-7.5
35

36. a b a
 Octane ,CH3(CH2)6CH3
(a)Triplet at 0.88
(b)Singlet at 1.26
a
b
36

37. Acetamide
a b a
b
37

38.  Ester, H3C O
O
H2
C
CH3
38

39. a b c
 Propanoic acid, CH3CH2COOH
a
b
c
(a)Triplet at 1.2
(b)Quartet at 2.3
(c)Singlet at 11.2
c
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Vyvyan, J. A. (2008). Introduction to
spectroscopy: Cengage Learning. 39

40. c
a
b
40

41. 41