Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance Spectroscopy

INTRODUCTION

NMR is defined as the measurement and interpretation of the radiofrequency which induces the transitions in the nuclei by absorbing the radiofrequency waves and the spectra are known as the nuclear magnetic resonance spectra. The atoms which contain the nuclei with the property are called the spin. The number of spins of the active spin nucleus is denoted by the spin quantum number, I. The spin is the property of the elements containing the odd mass or odd atomic number. The following Table shows the spin quantum numbers of the nuclei:

Element	Spin quantum number (I)	Number of the spin states
1H	1/2	2
2H	1	3
13C	1/2	2
14C	1	3

Table for the elements with the spin quantum number and spin states

The resonance is defined as the change in the nuclear spin of the nucleus from low energy state to high energy state by absorbing the energy. This is possible by the creation of magnetic field by placing the nuclei in the magnetic field. If the molecule has no magnetic moment, then only isotope molecules should posses the NMR spectra. Example: ¹H,¹¹ B, ¹³C, ¹⁵N, ¹⁷O, ¹⁹F. etc. Nuclei of these isotopes produces the spin or angular momentum depending upon the spin number I and contains the values 0, 1/2, 1, 3/2 … depending on the nucleus. NMR is mainly used for the compound identification.

PRINCIPLE AND THEORY

• A spinning charge generates a magnetic field. The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.
• In the presence of an external magnetic field (B₀), two spin states exist, +1/2 and −1/2. The magnetic moment of the lower energy +1/2 state is created by the external magnetic field and the magnetic moment of the higher energy −1/2 spin state is opposed to the external field.
  
  
  
  

  
  

  Spin energy states
  
  
• The difference in energy between the two spin states depends on the external magnetic field strength and is always very small. The diagram illustrates that the two spin states have the same energy when the external field is zero, but changes as the field increases. At a field equal to B_x, a formula for the energy difference is given (remember I = 1/2 and μ is the magnetic moment of the nucleus in the field).
  
  
  
  

  
  

  Excitation process in the NMR spectroscopy
  
  
• The magnetic flux is denoted by the Tesla (T). NMR is useful for the measurement of the magnetic fields of 1–20 T. The sample irradiated with the radiofrequency that must be equal to the spin rate. This produces two spin states: one is +1/2 to higher energy level −1/2. In this aspect, NMR is mostly used in the determination of the structure of the molecules.
• The frequency of the NMR is derived by the following equation:
  The spin angular momentum = [I (I +1)]^1/2h/2Π
  where I is the spin quantum number; h is plank's constant.
  The magnetic momentum is given by the following eqaution:
  
  
  μ = γ × spin quantum number
  = γ × [I (I+1)]^1/2h/2Π
  
  
  where γ is the gyro magnetic ratio.
  The frequency v at which it is emitted is given by Bohr's equation:
  
  
  v = E₂−E₁/h
  where
  E₁ = −1/2 (γh/2Π)H₀
  E₂ = +1/2 (γh/2Π)H₀
  
  
  H₀ is magnetic field which separates the energy into two: one is −1/2 which is anti-parallel to the magnetic field and the other is +1/2 which is parallel to the magnetic field.

There are two major relaxation processes:

• Spin–lattice (longitudinal) relaxation.
• Spin–spin (transverse) relaxation.

Spin–lattice relaxation: The sample nuclei held in the frame work is generally called as lattice. The magnetic field created by the ration or vibration of the nuclei is called as lattice field. The spin lattice relaxation is defined as the magnetic field equilibrates to the ground state energy field. The spin lattice relaxation is effected by the viscosity of the sample and elevated temperatures. If the relaxation time is long then it implies that sensitivity is less.

The relaxation time, T1, is the average lifetime of nuclei in the higher energy state which depends on the gyro magnetic ratio of the nucleus and the mobility of the lattice. As mobility increases, the vibrational and rotational frequencies increase the lattice field and it is able to interact with excited nuclei.

Spin–spin relaxation: Spin–spin relaxation describes the interaction between neighbouring nuclei with identical frequencies but differing magnetic quantum states. In this situation, the nuclei can exchange quantum states that a nucleus in the lower energy level is excited and the excited nucleus relaxes to the lower energy state. There is no change in the energy states, but the average lifetime of a nucleus in the excited state will decrease. This can result in line broadening.

CHEMICAL SHIFT

The magnetic field at the nucleus is not equal to the applied magnetic field and electrons around the nucleus shield it from the applied field. The difference between the applied magnetic field and the field at the nucleus is termed the nuclear shielding. This means that the applied field strength must be increased for the nucleus to absorb at its transition frequency. This up field shift is also termed diamagnetic shift.

Chemical shift = nuclear shielding/applied magnetic field.

Chemical shift is a function of the nucleus and its environment.

The causes for the chemical shift are the effective field which is produced by the nucleus is less than the applied field and negative shielding.

There are two types of shielding:

• Local shielding: The nucleus field is modified by the fields created by the local electrons on that nucleus.
• Low range shielding: In aromatic compounds, the field is created by the Π-electrons which are not associated with the nucleus.

Spin–Spin coupling: The splitting of the lines in the NMR spectra because of the interaction between the spins of the neighbouring nuclei in a molecule is known as spin–spin coupling. The spacing of the adjacent lines is the measure of spin–spin coupling and is known as spin–spin coupling constant (J). It is expressed in cycles per second which depends on the structural relations between the nucleuses.

INSTRUMENTATION

The following are the major components of the NMR instrument:

• The magnet.
• Sweep generator.
• Radiofrequency oscillator.
• Sample holder.
• Radiofrequency receiver.
• Recorder and integrator.

NMR spectrophotometer

The magnet: The magnetic field in NMR is generated by the super conducting magnet. At first a low temperature is needed for stainless steel or aluminium Dewar which contains liquid nitrogen. An inner Dewar contains a super conducting coil immersed in liquid helium. Then a bore is fitted with the shim coils and a spinner assembly to spin the NMR sample tube.

Sweep generator: It is mainly used to resonate the nucleus and thus producing the equal frequency of the applied radiofrequency radiation. This can be achieved by passing the current through the coils around the magnet pole pieces or through a HELMHOLZ coils holding the sample.

Slow sweep leads to saturation effects and a fast sweep results in ringing.

Radiofrequency oscillator: This is mainly used to induce the transitions in the nuclei which is present in the ground state to the excited state. This can be achieved by the coil of oscillator wound around the sample so that the maximum interaction between the radiofrequency radiations with the sample is achieved.

Sample holder: Generally glass tubes are used as sample holders. They should posses the following characteristics:

• Chemically inert
• Durable
• Transparent.

Radiofrequency receiver: This is mainly used for the detection of the radiofrequency signal by two methods. They are absorption and dispersion.

In absorption method of detection, Wheatstone bridge is used. The main principle is the absorption of the applied radiofrequency is detected by using the Wheatstone bridge.

In other method, a receiver coil is used. These coils are set at right angles to each other to the sample.

Recorder and integrator: The signals obtained from the receiver are recorded and integrated by the recorders. Generally electronic integrator is used for this purpose.

SOLVENTS REQUIREMENTS

The ideal characters of solvents are the following:

• Chemical inertness.
• Magnetic isotropy.
• Volatility.
• Absence of hydrogen atoms.

Generally used solvents are carbon tetra chloride, cadmium chloride, deuterium etc.

There are two general types of NMR instruments:

1. Continuous wave.
2. Fourier transforms.

Continuous wave NMR instruments: These are of low cost and low maintenance when compared to other NMR spectrometers. It consists of console, magnet and two orthogonal coils of wire that receives the radiofrequency waves.

Fourier transform NMR instruments: The sensitivity of the NMR is less, so to increase that it is combined with Fourier transform principle. FTNMR spectrometer consists of console, magnet and a coil of wire. This coil of wire acts as transmitter and receiver for the radiofrequency. But the main disadvantage of this is it is time consuming which takes 2–8 min. The advantages are high sensitivity, higher resolution and minimized noise ratio.

ADVANTAGES

• High resolution.
• Chemical kinetics are determined.
• Very fine structures are determined.
• Highly sensitive.
• High flexibility.
• Non-destructive method.

DISADVANTAGES

• Less accurate.
• High cost equipment.
• Not able to differentiate the same compounds.
• Time consuming.

APPLICATIONS

• Used to generate metabolic fingerprints from biological fluids to obtain information about disease states or toxic insults.
• Used in the detection of tumours
• Used for the structural determinations.
• Used for the molar ratio of the components in a mixture.
• Used for the detection of hydrogen bonding in metal chelates.
• Used in the non-destructive analysis of aminoacids, proteins, RNA and DNA.
• Used in the petroleum industry.
• Used in the determination of the number of carbon atoms present in the sample.
• Used in the determination of the position of the carbon atoms in the carbon chain.
• Used in the purity determinations.
• Used in the determination of the phase changes.
• Used in the determination of the atomic resolution structure of the compounds.
• Used in the determination of the protein hydration.
• Used in the determination of the bonding of the molecules in the compounds.
• Used in the screening of the drugs.
• Used in the metabolite analysis.
• Used in the polymer science.

REVIEW QUESTIONS

1. What is chemical shift?
2. Explain the coupling constant.
3. What are the causes for the chemical shifts?
4. What are the main components of the NMR instrument?
5. Give two examples for the solvents used in the NMR spectroscopy?
6. What is spin–spin coupling?
7. What are the applications of the NMR spectroscopy?
8. What are the advantages and disadvantages of the NMR spectroscopy?
9.