Nuclear magnetic resonance NMR spectroscopy | Basics about NMR spectroscopy | Construction and working of NMR instrument

What is nuclear magnetic resonance (NMR) spectroscopy?

Nuclear magnetic spectroscopy i.e. NMR Spectroscopy is one of the most important and powerful techniques used in organic chemistry for structural elucidation of Organic (and inorganic) compounds and biomacromolecules. NMR spectroscopy provides information about desired molecules and the environment based on the interactions of nuclear magnetic moments with electromagnetic radiation.

Magnetic Moment

A proton is charged particle and it spins due to which a magnetic field is produced giving rise to a magnetic movement ‘μ’ the direction of μ will be along the axis of spin, depending on whether the spin is clockwise or anticlockwise (the spin quantum number is + ½ or – ½ respectively). If  I = 1/2, μ is along the axis in an upward direction and if     I = – ½, then μ is along the axis in the downward direction as shown.

μ  =Magnetic moment

I  = Spin quantum number

H = Strength of external magnetic field

E = Energy of the system

I = ± ½

I = + ½ clockwise spin of proton, μ  is directing downwards.

I = – ½ Anticlockwise spin of proton, μ  is directing upwards.

If such a spinning proton is placed in an external magnetic field then there are the following two possibilities

1)  If the spin is such that the magnetic field (aligned orientation) then the energy of the system is lowered and the system is stable. (E = -μH),       ‘-ve sign’ indicate lowers the energy system.

2)  If the spin is such that the magnetic field, produced by the proton is in the opposite direction as the external magnetic field (opposing orientation) then the energy of the system is increased and the system is unstable [E = + μH), ‘+ ve sign’ indicates higher energy system.

Most of the protons would prefer to remain in the aligned orientation which is more stable.  However, if a proton in the aligned orientation absorbs energy and goes from now more stable aligned orientation to a less stable opposed orientation, it is called the flipping of the proton. Which is the basic principle of NMR spectroscopy.

Gyromagnetic ratio/g-factor

‘g’ factor is the most important quantity derived from NMR spectra of odd electron systems from the relation,

v=g_e m_B B/h

Therefore, g_e=hv/m_B B

‘g’ factor is essentially a measure of the ratio between frequency and magnetic field. For free electron, g_e = 2.303. The organic radicals, such as methyl radicals, have a g-value very close to the free-electron g_e value. Here, only the spin of the electron contributes to g_e.

The deviation of ‘g’ from g_e= 2.303 depends on the applied magnetic flux density to induce local currents in the free radical and therefore, its value gives the same information about the electronic structure.

In the case of the transition metal ions and their complexes, there is considerable interaction between the spin and the orbital motion of the electron, which prevents the complete quenching of the orbital contribution. Therefore, for such systems, the g-value departs from g_e value.

In transition metal complexes containing d-shells less than half-filled, ‘g’ is less than g_e, while for shells more than half-filled, ‘g’ is greater than g_e. Also, the g-value is anisotropic, i.e. its magnitude depends upon the direction of measurement. The anisotropy in g-value is shown by systems in solid-state, where conditions of restricted motion exist. In systems containing axial symmetry, the g-value along two dimensions, i.e. the x and y directions, are equal, while the g-value along the z-direction is different. If, for instance, an octahedral complex, wherein two diametrically opposed ligands are pulled or pushed symmetrically, while the remaining four ligands remain undisturbed. This is a case of axial symmetry. If the magnetic field is applied along the z-direction, the g-value is designated as g11. If the field is applied perpendicular to the z-axis, the g-value is designated as g1, in solutions, because of free rotating motion, the g-factor is isotropic, i.e. it is the average of the three g-values in the x,y, and z directions.

Larmor’s precessional motion

When a spinning proton is placed in an external magnetic field then the axis of spin inscribes a circular path about the axis of the external magnetic field and such a motion is called Larmor’sprecessional motion.

The number of precession per second is called asLarmor’sprecessional frequency. Since ‘2π radians’ are θ covered in one precession, processional frequency equals ‘W/2π’. The angle between the actual axis and the imaginary angle is called as precessional angle θ.

Relaxation process

• More number of proton remains in the aligned orientation as compared to the opposite orientation this is the condition necessary for the resonance.

• However, the proton undergoes resonance and gets excited from aligned to opposed orientation state at a certain stage would be equal this is called as ‘Saturation stage’ beyond which NMR does not occur but this does not happen due to ‘relaxation process’.

• A relaxation process involves a radiationless transfer of protons back from the opposed stage to the aligned state.

Relaxation processes in nmr spectroscopy are of two types

Spin lattice relaxation

i)  In this phenomenon the proton loses its excess energy to the lattice framework or the solvent system thus the total energy of the system remains constant and there are two distinct advantages.

ii)  Number of protons in the aligned stage is always greater than the number of protons in the opposed stage.

iii)  Time spent by protons in the opposed stage is limited to 10^-4 to 10^-2 seconds for solid and ’10^-2 to 10^-4’ seconds for liquid.

Spin-Spin Relaxation

In this phenomenon, the proton in the opposed state loses the excess energy to another proton to the proton in align stage which gets excited while the first proton returns back to the aligned state this phenomenon is useful for limiting the time spent by a proton in the opposed state.

Principle and working of an NMR spectroscopy

When the nucleus (i.e. protons) is subjected to a magnetic field, its orientation is governed by quantum rules, and the nucleus occupies different permitted energy levels. When the transition occurs between two energy levers, the energy absorbed by the nucleus, originally in the ground state, just exceeds that liberated by the nucleus in the excited state, so that there is a net absorption of energy. Thus, NMR spectroscopy is basically another form of absorption spectroscopy.

This absorbed quantum energy can be balanced (i.e. can be brought into resonance) when that nucleus is subjected to low-frequency radiation (i.e. low energy – radiation – radio wave region). With this background, the definition for NMR can be given as follows:

(a) It is the situation, in which the frequency of the processing nucleus is equal to the frequency of a rotating magnetic field.

(b) There is a transition between two energy levels in the nucleus. When the energy at the ground level is greater than the energy at the excited level, there will be a net absorption of energy. This type of absorption is a resonance phenomenon and it is called Nuclear Magnetic Resonance(NMR).

(c)NMR is a form of spectroscopy, in which due to nuclear magnetic properties, radiofrequency energy is absorbed in a magnetic field.

Construction and  Working of NMR instrument

Experimental Arrangements

Lets know the construction and working of NMR instrument. The most important feature of an NMR spectrometer is the magnet itself. The magnet should produce the condition necessary for the absorption of radiofrequency radiation. For obtaining high-resolution spectra, the field produced by the magnet must be homogeneous over a considerable area between the pole faces. In case, the magnetic field is non – homogeneous, nuclei in different parts of the sample will experience different magnetic fields. This will process over a range of frequencies, thus resulting in broadened absorption signals. NMR spectrometer consists of the following essential components.

(i) Magnet :-

The blocks labeled N and S represent the North and South poles of a large magnet of field strength H0 and it is an electromagnet operated through a stabilized power supply. A field of  14000 gausses and a pole of about 1.75 – 1.80 inches in diameter is necessary for high-resolution spectra. The frequency and field strength are related to each other by the Larmor equation,

D.C field sweep source is used to vary the field strength throughout the range of resonance.

(ii) Radiofrequency source :- 

1. The signal from a radio–frequency (R – F) oscillator is fed into a pair of coils mounted at the right angle to the path of the field.
2.  This is connected through a large resistance ‘R’ to ensure a constant current and capacitor ‘C’ to achieve the balance.
3. The R-F current flows through the coil perpendicular to H0 surrounding the sample.
4. 3-to 4 drops of liquid sample, (if it is solid, it is to be dissolved in 0.5 ml of non-polar, low boiling solvent like CCl4) are taken in a glass tube with a diameter of about 3 mm and 1 cm in length.
5. The most widely used solvent in deuterated chloroform (CHCl3). If CHCl3 impurity is present, it rarely interferes.
6. CS2 is also a useful solvent. A drip of tetra methylsilaneSi(CH3)4 i.e. TMS is added as an internal standard.
7. TMS is a chemically inert low boiling liquid (b.pt 260C) that is miscible with organic solvents.
8.  Its protons absorb R.F. radiation at a very high magnetic field where other types of protons do not show absorption.
9. All the 12 protons in TMS are equivalent and absorb R.F. energy at the same magnetic field producing a single peak.

(iii) Director:-

The R-F energy is absorbed by certain nuclei of the sample and R-F signals are picked up by the detector. A nuclear resonance spectrum is thus a plot of detector signal against the magnetic field at the constant oscillator frequency.

(iv) Recorder:-

The signals obtained by the detector are recorded on graph paper of a recorder, where the applied magnetic field is plotted against the absorption of radiation.

Units for NMR

The NMR values are expressed in any of three following ways,

(a) The reference compound is quoted ( denotes chemical shift, which is independent of oscillating frequency).

(b) Cps (cycles per second) – Reference compound must be quoted and the oscillator frequency is given.

(c) TMS (Tetramethyl silane) is assumed to be independent of both oscillator frequency and reference compound. It is represented by the symbol  ‘𝜏 ’

Where,    γ  = 10 – δ

Chemical Shift in NMR spectroscopy

Chemical shift is defined as the relative position of absorption shown by the different protons with respect to TMS (tetramethylsilane) which is selected as an internal reference.  It is measured or expressed in terms of ‘ δ ’.

‘ δ ’ for TMS (reference) is assigned a ‘ 0 ’ (Zero) value.

TMS is selected as the internal reference due to following reasons.

1) It is miscible with all organic compounds.

2) It is volatile and can be easily removed.

3) It does not interact chemically with any organic compounds.

• All the 12 protons of TMS are chemically equivalent (same chemical environment) and will resonate at the same frequency.
• Due to the low electronegativity of ‘Si’ the protons are shielded from the effect of the external magnetic field.
• i.e. the protons experience a less effective magnetic field, H’ = H (1 – σ). Where σ = Shielding parameter.

• Therefore the protons will require a much higher external magnetic field to come into resonance.
• Thus shielding shifts the absorption upfield or towards a lower δ value [ δ for TMS = 0].
• If the protons are not surrounded by an electron cloud or if the electron spins add to the external magnetic field strength then the protons will resonate at a lower external magnetic field the protons are said to be de shielded.
• Thus deshielding shifts the absorption downfield or towards a higher δ value.

Thus different protons of a molecule in the different chemical environments (different external shielding and deshielding) will resonate at different d values.  (different field strength) and a plot of absorption intensity (resonance current) versus ‘δ’ gives the ‘NMR Spectrum’.

i)  The magnetic field can be kept constant the radio frequency is constantly changed till it matches the precessional frequency resulting in the resonance (flipping).

ii)   It is better to keep the radio frequency constant and change the magnetic field strength (precessional frequency changes).  At some value of magnetic field strength proton undergo resonance).

Instead of a d scale another parameter called ‘ γ(tau)’ can express the chemical shift where γ = 10- δ for TMS γ = 10.

More the shielding higher the value of ‘γ’ and more the deshielding lower the value of ‘γ’.

Flipping of Protons

• If the nucleus of H (proton) with I=1/2  is placed in uniform external magnetic field H₀.
• Then nucleons orient themselves into energy levels out of these parallel orientations has less energy and hence more stable.
• The antiparallel orientation has more energy and is hence unstable.
• At room temperature, the ratio of the number of proton in Higher energy level to low energy level is given by,

• Thus there is a slight excess of protons in low energy levels.
• It is this small excess of proton in the low energy level which are responsible for NMR.
• They absorbed radiofrequency radiation from a source and undergo a transition from low energy level to high energy level, such transition caused by absorbance of radiofrequency radiation is called “ Flipping of Protons”

Please also read Gas Chromatography

Low-resolution N.M.R. Spectrum

Methanol

The underline should be only for H in all cases.

1. The three hydrogens of the methyl group are chemically equivalent.
2. The hydrogen of the –OH group is deshielded. Hence, it will resonate first and the methyl protons being shielded will resonate at a higher field strength.
3. This will result in two peaks.
4. The peak heights will be in the ratio of 1:3.
5. The peak at lower field strength will be due to proton of –OH and the one at the higher field strength will be due to methyl protons.

Ethanol

1. Three different groups of hydrogens that are chemically equivalent within the group are identified. The methyl, methylene, and the OH proton from the three groups.
2. The spectrum will have three peaks with three different groups of chemically equivalent protons.
3. The least shielded being –OH hydrogen. It will resonate downfield, next will be the methylene group and the last will be the methyl group that is most shielded.
4. The peak heights will be in the ratio of 1:2:3.