NMR SPECTROSCOPY

1. NMR SPECTROSCOPY

2. NMR

3. NMR INTRODUCTION
 All atomic nuclei can be characterized by a nuclear spin
quantum number, I. I can be ≥ 0 and any multiple of ½.
 Nuclei with I = 0 do not possess nuclear spin and
consequently are termed ‘NMR silent’.
 All nuclei with I ≠ 0 possess spin, charge, and angular
momentum P, resulting in a nuclear magnetic moment
 µ = γP
 Where γ is the magnetogyric ratio of the nucleus.

4. NMR

5. NMR QUANTUM CHEMISRY
 I = the nuclear spin quantum number


6. NMR QUANTUM CHEMISRY
 For Nuclei of: I = Example Odd Mass Half
Integer 1H, 13C
 Even Mass/Even Charge Zero 12C, 16O
 Even Mass/Odd Charge Integer 2H, 14N

 If I = 0, NMR Inactive
 If I ≥1, Quadrupolar (non-spherical nuclear charge
distribution)

7. NMR QUANTUM CHEMISRY
 I is quantized producing (2I + 1) discrete values of angular
momentum, mI.
 mI = I, I -1, …-I


8. NMR CONCEPT –SPIN STATE


9. NMR CONCEPT –SPIN STATE

10. NMR Concepts – Relaxation
 Once excited to the higher energy state by an rf pulse, the spins will return to their initial
equilibrium condition by means of two relaxation mechanisms, T1 and T2.
 T1 relaxation (longitudinal): Spin-lattice relaxation occurs by transfer
 of energy to the surroundings (heat); dipolar coupling to other spins. Results in recovery of Mz
to
 63% of original value.
 T2 relaxation (transverse):Spin-spin relaxation occurs by redistribution
 of energy among various spins of the system. Results in recovery of Mz to 37% of original
value.
 T2 ≤ T1
 T1 and T2 are routinely equivalent for most NMR experiments.
 NMR Linewidths ~1/ T2 for spin ½ nuclei
 Inorganic/Organometallic Linewidths -

11. NMR Concepts – Relaxation

12. NMR INSTRUMENTATION

13. Chemical shift
 different local chemical environments surrounding any
particular nuclei causes them to resonate at slightly different
frequencies. This is a result of a nucleus being more or less
shielded than another. This is called the chemical shift (δ).
One factor that affects chemical shift is the changing of
electron density from around a nucleus, such as a bond to an
electronegative group. Hydrogen bonding also changes the
electron density in 1H NMR, causing a larger shift.

14. Chemical shift
 The shift in the position of PMR signals resulting through
shielding and DE shielding by circulation of electrons in
chemical bonds is called the chemical shifts. • Protons in most
of the organic compounds absorb over a range of 700 Hz (cps)
at a field strength of 14100 Gauss. At this field strength protons
absorb at a frequency of about 60 x 106Hz. • The shielding and
DE shielding of protons by electrons produce very small change
in the strength of the applied magnetic field. These small
changes in the magnetic field strength cannot be determined
accurately. Therefore absolute position of the PMR signal
cannot be obtained. Therefore, chemical shifts of protons are
expressed in Hz or (cps) with reference to a particular standard.

15. Chemical shift
 The frequency and the strength of the magnetic field are related
by the equation • The most commonly used reference for PMR
spectroscopy is tetramethylsilane (TMS) (CH3 )4Si. It is chosen
as a reference compound for the following reason:- 1. It has 12
equivalent protons, therefore a very small amount of the TMS
produces a large single sharp signal. 2. TMS protons absorb at
a field much higher than the protons in most of the organic
compounds. This is due to the reason that silicon is more
electropositive than carbon. 3. It is chemically inert and is also
highly volatile (b.p. 300 K). Therefore, after the spectrum has
been scanned, the precious sample can be recovered by the
removal of TMS by evaporation. 4. It is highly miscible with most
of the organic solvents.

16. Units of Chemical Shif
 The commonly used unit is parts per million (ppm). • It is
dimensionless and are independent of the field strength or
oscillator frequency of the instrument. • It is the function of
the chemical environment of the protons in the organic
molecule.

17. Scales of the Chemical Shift
 It express in two scales. (i) The d (delta) scale (ii) The
(tau) scale = 10 - d On the d scale the position of TMS
signal is taken as 0.0 ppm and most of the chemical shifts
have d values between 0-10. Shielded protons have low d
values whereas DE shielded protons have large d values.

18. CHEMICAL SHIFT
 Paramagnetic contribution arises from non-spherical electron distribution
(nuclei with non-s orbitals). It is the dominating factor of chemical shift for all
nuclei other than protons.
 Magnetic anisotropy of neighboring bonds and ring currents – π
electrons of triple bonds and aromatic rings are forced to rotate about the
bond axis creating a magnetic field which counteracts the static field.
 Electric field gradients are the result of strongly polar substituents. The
distortion of the electron density alters the chemical shift.
 Hydrogen bonding can lead to a decrease in electron density at the proton
site resulting in a chemical shift to higher frequency. Hydrogen bonded
protons exhibit shifts that are highly dependent on temperature, solvent, and
concentration.
 Solvent effects are often exploited to separate overlapping signals of
interest in a spectrum. Large changes in chemical shift can be observed for
solvents that can selectively interact with one portion of a molecule (acetone
for it’s carbonyl group, and benzene for its ring currents)

19. Chemical shifts of different types of protons


20. FACTOR AFFECTING
CHEMMICAL SHIFT
 Inductive effects
 The proton chemical shifts increases as the
electronegativity of the atom attached to the
carbon atom bearing hydrogen increases.
Thus is due to -I effect of the electronegative
atom.
 The electron density around the proton
decreases which causes deshielding. Thus,
higher the electronegativity higher the
deshielding and hence higher the d value of the
chemical shifts.

21. Inductive effects
Compound Elemen
t
Electronegativit
y
Chemical shift
CH3–C F 4.0 d- 4.26
t- 5.74
CH3–O O 3.5 d-
3.5
t-
3.40

22. Greater the number of halogen
atoms, greater in the DE shielding
CH3
Cl
CH2C
l2
CHC
l3
d 3.0 d
5.30
d
7.27

23. 2. HYDROGEN BONDING
 Protons which exhibits hydrogen bonding (e.g. — OH and —
NH2 protons) show variable absorption position over a wide
range (0.5 - 5.0 ppm) since H– bonding decreases the
electron density in the O—H bond, therefore, the proton
involved in H– bonding gets deshielding.
 Further stronger the hydrogen bond more deshielded is the
proton. The extent of H– bonding depends upon
concentration, temperature and nature of the solvent. Thus in
concentrated solutions, H– bonding predominates and H–
bonded protons appear in the range 4-5.

24. ANISOTROPIC EFFECT

25. ANISOTROPIC EFFECT

26. SPIN –SPIN COUPLING
 Every set of equivalent protons gives are PMR signal. But the
spectra of most of the organic compounds are much more
complicated. For example : 1, 1, 2- tribromoethane (CH2Br-
CHBr2).
 This compounds contains two kinds of protons and hence its
PMR spectrum to show only two peaks. But spectra of this
compound show five peaks.

27. SPIN SPIN COUPLING

28. SPIN SPIN COUPLING
 Spin-spin or scalar coupling is the result of Fermi contact
interaction between electrons in the s orbital of one nucleus
and the nuclear spin of a bonded nucleus.
 The magnitude of coupling depends on the degree of
electron orbital overlap. The s-character of the orbitals relies
heavily on the hybridization of the nuclei involved.

29. SPIN SPIN COUPLING

30. SPIN –SPIN COUPLING
 Nuclei in a molecule are affected by the spins systems of
neighboring nuclei. This effect is observed for non-
equivalent nuclei up to 3 bond lengths away and is termed
spin-spin coupling or J coupling

31. SPIN SPIN COUPLING

32. APPLICATION OF NMR
1.Determination of Empirical Formulae.
 The transition of a proton from one spin state to the other is fundamentally
the same irrespective of the chemical environment of the proton. For this
reason the transition probability and hence intensity of absorption is the
same for every proton in the molecule, provided certain aspects of
experimental technique are observed. The intensity of absorption of a given
proton is determined by measuring the area under the absorption band
which it gives rise to in the n. m. r. spectrum. Thus, if the total area enclosed
by the nuclear magnetic resonance spectrum of a solution, containing a
known concentration of a compound of unknown structure, is compared with
the area of a solution containing known concentration of a compound of
known structure, it is possible to derive the percentage of hydrogen in the
former compound. To do this, the two determinations. must be carried out
under precisely the same conditions and it must be established that the
result is independent of the amount of radio-frequency power used in both
determinations*.

33. Determination of the Classes of
Protons in a Molecule. The
Chemical Shift.
 Since various commercial spectrometers operate at different
field strengths, it is not convenient to express the chemical shift
in c./sec. Instead, these separations are converted to field
independent units in the following way. The separation between
the tetramethylsilane absorption line and that of a proton in the
reference sample is defined as a positive quantity LI c./sec. if
the latter occurs at lower fields. (In the rare cases in which a
sample absorption occurs at higher fields Ll is taken as
negative.) One convenient field independent expression of the
chemical shift is then given as Lt. 106 !5-- spectrometer
frequency' This is known as the !5 scale and on this scale
tetramethylsilane is at zero parts per million.

34. . The Number of Protons in a
Given Class. Intensities.
 Having established the various classes present in a
molecule, the next problem is to count the number of protons
in each class. This is done by determining the intensities of
the absorptions arising from various classes of protons by
integrating the absorption bands. As discussed above, the
area under the absorption bands will be proportional to the
number of protons which give rise to the absorption, and for a
single molecular species this means that the intensities of the
various absorption bands arise from different classes of
protons in each class

35. The Sequence of Groups in
Molecules. Electron Coupled Spin-
Spin Interaction.
 The absorption bands arising from single protons or from
groups of equivalent protons (i. e. protons with exactly the
same chemical shift) frequently exhibit well defined fine
structure when examined under high resolution. Fig. 2 shows
the spectrum of acetaldehyde in which it can be seen that the
aldehyde proton gives rise to a quartet and that the protons
of the methyl group give rise to a doublet. It should be further
noticed that the quartet is symmetrical, the spacing of the
quartet components is the same as that of the doublet

36. Applications to the Elucidation of
Relative Stereochemistry and
Conformation.
 Both the chemical shift and the magnitude of spin-spin
coupling are critically'dependent on geometrical factors. In
cases in which this dependence is understood it is possible
to use chemical shift and coupling constant data to derive the
relative orientations of protons in molecules and, hence
determine the relative stereochemistry of groups. It is
important to realize that many organic molecules are
dynamic equilibrium mixtures of vanotts conformations and
that in almost all of such systems the observed nuclear
magnetic resonance parameters have values which
correspond to time averages over these conformations. T