NMR spectroscopy

UG and PG

1. 1H Nuclear Magnetic
Resonance

By
G. P. Sadawarte
Asst. Professor & Head, Dept. of Chemistry
B. P. Arts, S.M.A. Sci. and K.K.C. Com. College
chalisgaon.

Email: [email protected]
B.P. Arts, S.M.A. Science And K.K.C.
Commerce College, Chalisgaon,

2. Introduction
•electromagnetic radiation in the radio-
frequency region (~4-900 MHz)
•1937 Rabi predicts and observes NMR
•NMR was first experimentally observed by
Bloch and Purcell in 1946 -received Nobel
Prize in 1952
•preferred technique rapidly elucidate the
chemical structure of most organic
compounds
•Wavelength 1000- 10000 m (low energy)

3. NMR Sensitivity
~$8,00,000 ~$20,00,000
~$45,00,000

4. Principal of NMR
Nucleus of an atom possesses magnetic moment
Nucleus – P+ N ie nucleus is positively charged
This charged particles also spin about their own
axis
Generate a magnetic moment along the axis of the
spin so that nuclei act as tiny bar magnet
Depending upon nucleons certain nuclei posses spin
When placed in external magnetic field spin
flipping occurs and signal recorded by instrument

5. Magnetic and Non magnetic nuclei
•Nuclei with spin (Magnetic Moment, Quantized
Spin Angular Momentum, Magnetic Dipole) have a
certain number of “Spin States.”

•The number of “Spin States” a nuclei can have is
determined by its “Spin Quantum Number I,” a
physical constant, which is an intrinsic (inherent)
property of a spinning charged particle.

•The Spin Quantum Number (I) is a non-negative
integer or half-integer (0, 1/2, 1, 3/2, 2, etc.).

•The Spin Quantum Number value for a given nuclei
is associated with the Mass Number and Atomic
Number of the nuclei.

6. Magnetic and Non magnetic nuclei
•Nucleus I=0 (Zero Spin) Nonmagnetic nuclei Even
Mass No / Even Atomic No Ex 12C6, 16O8, 32S16

•Nuclei with I >0 posses spin angular momentum
and generated magnetic field are magnetic nuclei
a) Odd Mass / Odd Atomic No - 1/2, 3/2, 5/2 Spin
b) Odd Mass / Even Atomic No - 1/2, 3/2, 5/2 Spin
1H1, 13C6, 19F9, 31P17 --------(I=1/2)

•The nuclei with Even Mass no and Odd Atomic No
have Integral (1, 2, 3) Spin quantum no (I=1) are
also magnetic nuclei 2H1, 14N 7

7. NMR
absence of external
magnetic field
B
o
strong external magnetic
field

8. NMR
absence of external
magnetic field
strong external magnetic
field

9. Nuclear Resonance
A
B
n = g B
o / 2p
RF
DE = g h B
o / 2p
B
o – external magnetic field
h – Planck’s constant
g – gyromagnetic ratio
B0
Orientation of spin state =(2I+1)
I=1/2 for proton(H)
No of Orientation for H = 2(1/2)+1
= 2
Two possible orientation for H
A Parallel B anti parallel arrangement

10. Processional Motion of nucleus
M
o
z
x
y y
x
z
B
o B
o
•Nuclei either align with or against external magnetic
field along the z-axis.
•Since more nuclei align with field, net magnetization
(M
o, M
Z) exists parallel to external magnetic field.
•Net Magnetization along +Z, since higher population
aligned with B
o.
•Magnetization in X,Y plane (M
X,M
Y) averages to zero

11. NMR Signal
DE = h n
a
b
•Strength of the signal depends on the population
difference between the a and b spin states

•The population (N) difference can be determined from
the Boltzmann distribution and the energy separation
between the a and b spin states:
https://upload.wikimedia.org/wikipedia/commons/3/38/NMR_EPR.gif

12. Instrumentation

13. Instrumentation
20 mg
Dissolve in CDCl3, DMSOD6

14. Really Old : Continuous wave (CW)
40 MHz NMR spectrometer (1960)
1964 spectrum

15. Chemical shift
The distance in (delta) values from TMS to each
signal (peak) in the spectrum called as chemical
shift for proton(s) giving that signal
The chemical shift in δ units expresses the
amount by which a proton resonance is shifted
from TMS, in parts per million (ppm)
•Chemical shift measured in Hz

•Sophisticated instrument 60,90,100------600MHz

•Chemical recorded in Hz but it vary with spectrometer

16. Chemical shift Scale
•It expressed in Tau(τ) or Delta(δ) Scale




10 9 8 7 6 5 4 3 2 1 0 δ scale (ppm)
0 1 2 3 4 5 6 7 8 9 10 τ scale (ppm
TMS
Chemical Shift (δ) = v sample - v TMS x 10 ppm
v instrument
6
•Inert
•Volatile (B.P. 27 )
•Unique line position
•12 H strong, sharp, peak
Proton shielded – up field
•Soluble in most of the
organic compound H
3C SiCH
3
CH
3
CH
3

17. Shielding and de-shielding
•Molecule placed in external magnetic field B0

•Electron around proton – forced to circulate

•This circulation produce a secondary magnetic
field around the nucleus

•When secondary magnetic field around the
nucleus is opposite direction of external/applied
magnetic ie it try to protect nuclei from applied
magnetic field is called as diamagnetic Shielding

18. Shielding and de-shielding
•When nucleus of proton is not protected by
induced magnetic field then de-shielding takes
place
•de-shielding depend upon electron density
around the proton

B0
Circulation of e generate
induced magnetic field
Nucleus

19. Magnetic Anisotropy
•Six C atom remain perpandicular
to external magnetic field
• Pi electron generate induced
magnetic field
•The field felt by this proton is less
•Down field (up to 7.5 δ)
Diamagnetic anisotropy in benzene
Diamagnetic anisotropy in Acetylene
•Acetylene Molecular axis parallel to
external magnetic field
•Direction of induced magnetic field
opposite to applied magnetic field
•Acetylene proton highly shielded up
field (2.5) δ

20. Environment of proton/eq. set of proton CH
4
CH
3-CH
3 H
3C
H
2
CCH
3 H
3C
H
2
CCl H
3C
H
2
COH H
3C C CH
3
O
01 01
02
02
01 03
All of the protons found in chemically identical environments
within a molecule are chemically equivalent, and they
often exhibit the same chemical shift. ????

21. Environment of proton/eq. set of proton H
3CCH
O H
3C
H
2
CO
H
2
CCH
3 H
C
HC
HC
C
H
CH
CH H
C
HC
HC
C
H
CH
C
CH
3
02
02
01
02

22. Integration
•NMR spectrum furnishes a valuable type of
information on the basis of the number of different
peaks observed
•that is, the number of peaks corresponds to the
number of chemically distinct types of protons in
the molecule.
•Often, protons that are chemically equivalent are
also magnetically equivalent.
•Note, however, that in some instances, protons
that are chemically equivalent are not
magnetically equivalent

23. Integration
•Note that the height of the integral line does not give the
absolute number of hydrogens. It gives the relative
number of each type of hydrogen.

• For a given integral to be of any use, there must be a
second integral to which it may be referred.

•Benzyl acetate provides a good example of this. The first
integral rises for 55.5 divisions on the chart paper; the
second, 22.0 divisions; and the third,32.5 divisions.

•These numbers are relative. One can find ratios of the
types of protons by dividing each of the larger numbers by
the smallest number

24. Integration
ratio of the
protons of all
the types is
2.52:1.00:1.48
ie
5:3:2

25. Integration
•NMR signal are not equal intensity

•Area of each signal/ peak height α No of mag.eq. H

•NMR spectrum of
toluene – the area
ratio 3:5due to CH3
and C6H5 protons

•The ratio of peak
height and ratio of
proton are identical

26. Spin spin Splitting/ Coupling
•Low resolution – a number of broad absorption peak

•High resolution signal shows multiplicity

•Splitting of signal into two, three, four or many peak

•Multiplicity is due to spin spin coupling

spin–spin splitting, can be explained empirically by the so-called n + 1
Rule. Each type of proton “senses” the number of equivalent protons (n)
on the carbon atom(s)next to the one to which it is bonded, and its
resonance peak is split into (n + 1) components.

27. Spin spin Splitting/ Coupling
•Coupling occur between to nuclei of neighboring
proton which magnetically non equivalant
C
H
H
Geminal coupling-
two bond
Vicinal coupling
– three bond
Long range coupling-
four bond C C
H H C C C
H C

28. Rules of Spin spin Splitting/ Coupling
•Coupling occur between to nuclei of neighboring
proton
•Coupling occur between to nuclei which are
magnetically non equivalent
•Magnetically equivalent nuclei doesn’t couple with
each other
•Total no of line in signal is given by (n+1) rule n=
number of proton on adjacent/ neighboring C
•Intensity ratio of mutilate are symmetric and given
by pascal triangle

29. Mechanism/TH Spin spin Splitting/Coupling

30. Acetaldehyde
https://scilearn.sydney.edu.au/OrganicSpectroscopy/?type=NMR&page=Coupling

31. Coupling Constant J
•Distance between two adjacent peak in multiplate.

•It is measured in Hz

•0 to 18 Hz

•If Δv/ J > 6 Simple pattern obtained

•If Δv/ J < 6 Complex pattern obtained

32. Coupling Constant J
JAB = 6.1Hz
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(Bruice)/1
4%3A_NMR_Spectroscopy/14.12%3A_Coupling_Constants_Identify_Coupled_Protons

33. Coupling Constant J

34. Application of NMR
•Structure Determination- different type of proton -no signal,
intensity ie area gives ratio of proton, chemical shift – shielded or
deshielded, multiplicity – no of neighbouring proton
•Geometrical Cis trans isomerisum
•NMR and MRI: Applications in Medicine
•Determination of Hydrogen bonding
•Drug screening and designing
•Metabolic analysis
•Coupling constant – study more complex

35. Application of NMR
https://www.youtube.com/watch?v=1W1PHEnlUgk

36. Problems

37. Problems

38. Problems

39. Problems

40. Problems

41. Problems

42. Problems

43. References
•Introduction to Spectroscopy, Donald L. Pavia Gary
M. Lampman George S. Kriz James R. Vyvyan..

•Spectroscopy by Y.R. Sharma.

• Spectrometric Identification of Organic Compounds,
R. M. Silverstein and F. X. Webster , John Wiley and
Sons.

•Nirali Publication & prashant publication textbook.

44. Any ?