1

INSTRUMENTATION OF NMR SPECTROSCOPY

ByDr. Jaishree V.

2

INTRODUCTION• Nuclear magnetic resonance spectroscopy is a powerful analytical

technique used to characterize organic molecules by identifying carbon-hydrogen frameworks within molecules.

• It is the study of spin changes at the nuclear level when the radiofrequency energy is absorbed in the presence of magnetic field.

• It consist of measuring the energy that is required to change a spinning nucleus from stable (lower energy) orientation to less stable (higher energy) orientation in the magnetic field. Energy.pptx

•

3

• When a charged particle such as a proton spins on its axis, it creates a magnetic field. Thus, the nucleus can be considered to be a tiny bar magnet.

• Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a magnetic field B0, they are oriented with or against this applied field. More nuclei are oriented with the applied field because this arrangement is lower in energy.

• The energy difference between these two states is very small (<0.1 cal).

4

• Any proton or nucleus with odd mass number spin on its own axis.

• By the application of an external magnetic field, the nucleus spin its own axis and a magnetic moment is created, resulting in a precessional orbit, with a frequency.

• In this stage when energy in the form of radiofrequency is applied and when

• Applied frequency = Precessional frequency• Absorption of energy occurs and a NMR signal is

recorded.

5

• Two common types of NMR spectroscopy are used to characterize organic structure: 1H NMR is used to determine the type and number of H atoms in a molecule; 13C NMR is used to determine the type of carbon atoms in the molecule.

• The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency.

• Nuclei with odd mass number only give NMR spectra• E.g. 1H, 13C etc, because they have asymmetrical

charge distribution.

6

• The absorption depends upon the type of protons or certain nuclei contained in the sample.

• Chemical shift:• It is the difference between the absorption position of a sample

proton and reference position of the reference compound( Tetramethylsilane (TMS)).

• UNITS:• Chemical shift - Delta(δ)• Magnetic field strength - Tesla (T)• Frequency of electro - Cycles per second (cps)• magnetic radiation•

7

TETRAMETHYLSILANE, (TMS) Advantages of TMS, (CH3)4Si:• Sharp signal at low concentration• Clear of most organic protons• Chemically inert• low boiling point; remove from sample. easily(b.p.27 OC).• soluble in most organic solvents can be added as an internal standard at low concentration (≈ 1%)

8

Element 1H 2H 12C 13C 14N 16O 17O 19F

Nuclear SpinQuantum No 1/2 1 0 1/2 1 0 5/2 1/2

( I )No. of Spin 2 3 0 2 3 0 6 2States

Spin Quantum Numbers of Some Common NucleiThe most abundant isotopes of C and O do not have spin.

Elements with either odd mass or odd atomic numberhave the property of nuclear “spin”.

The number of spin states is 2I + 1, where I is the spin quantum number.

9

Spins Orientation in a Magnetic Field (Energy Levels)• Transition from the low energy to high energy spin state occurs through an

absorption of a photon of radio-frequency (RF) energy

Frequency of absorption: n = g Bo / 2p

10

INSTRUMENTATION• Instruments that are used to obtain NMR spectra are

usually named NMR spectrometer.• A high resolution spectrometer contain a complex

collection of electronic equipment. • NMR spectrometers are referred to as 300 MHz

instruments, 500 MHz instruments, and so forth, depending on the frequency of the RF radiation used for resonance.

• These spectrometers use very powerful magnets to create a small but measurable energy difference between two possible spin states.

11

• 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.

• Modern NMR spectrometers use a constant magnetic field strength B0, and then a narrow range of frequencies is applied to achieve the resonance of all protons.

12

Nuclear Magnetic Resonance Spectroscopy

13

COMPONENTS OF NMR SPECTROMETER• MAGNET• FIELD LOCK• SHIM COILS• PROBE UNIT• Sample holder• RF oscillator• Sweep generator• RF receiver• DETECTOR• AMPLIFIER & RECORDER

14

1) MAGNET:

• The NMR magnet is one of the most expensive components of the nuclear magnetic resonance spectrometer system.

• Sensitivity and resolution are critically dependent on quality of magnet.

Feature:• It should give homogenous magnetic field, i.e. the

strength of the magnetic field should not change from point to point.

• The magnet must be capable of producing a very strong magnetic field with strength at least 10,000 gauss.

N S

15

Types:-A. Permanent Magnet:• It is simple and inexpensive to operate but requires

extensive shielding and must be thermostated to ± O.OO1o .• They are operated up to 1.9 T. they provide field of

good homogeneity. Disadvantage:- • Field variation is not possible, as it is required,

because different nuclei resonate at different magnetic field.

N S

16

B) Electro Magnets:

• They are more effective than the permanent magnet

because of possibility of field variation.

• They require power supply to produce magnetic field and cooling system to counter the heat generated from the electric power.

• They are operated at 14090, 12140, or 23490 Gauss

17

C) Super conducting magnet:• A super conducting magnet has an electromagnet

made of superconducting wire.

• Superconducting wire has a resistance approximately equal to zero by immersing it in liquid helium (at 00).

• The length of superconducting wire in the magnet is typically several miles.

18

2) FIELD LOCK:• In order to produce a high resolution NMR spectrum

of a sample there is need to homogeneous magnetic field.

• The field strength might vary over time due to aging of the magnet, movement of metal object near the magnet, and temperature fluctuations

Types:a) External lock systemb) Internal lock system

19

a)External lock system: • In this an external reference nucleus is continuously

irradiated at its resonance frequency and resultant NMR dispersion signal is continuously monitored.

b)Internal lock system:• In this system a suitable reference such as TMS is added

to the sample.• The analytical sample nuclei and reference experience

the same field • An automatic shim coil control continuously compensate

for the several magnetic field gradient.

20

3) SHIM COILS:• The purpose of shim coils on a spectrometer is to

correct minor spatial inhomogeneities in the magnetic field.

• These inhomogeneities could be caused by the magnet design, materials in the probe, variations in the thickness of the sample tube, sample permeability, and ferromagnetic materials around the magnet.

• A shim coil is designed to create a small magnetic field, which will oppose and cancel out an inhomogeneity in the B0 magnetic field.

21

4)PROBE UNIT: It contain,a) Sample holder:• Sample should be held in holder, which should be

chemically inert, durable and transparent to RF radiation.

• Glass tubes of about 8.5 cm long and 0.3 cm diameter are employed.

• Glass tube are amberized in order to protect UV sensitive sample.

22

• Quartz of polyfluoroethylene sample cells are also available for photochemical studies.

• A spinner is provided to spin the tube at several hundred rotation/min in order to make the sample experience homogenous magnetic field.

Sample solvent: (sample preparation)• Few microgram and a few milligrams of sample is

dissolved in a solvent such as CCL4 or CS2 or a solvent that has had all the proton replaced by deuterium atoms.

• E.g. Chloroform –d, acetone-d6, benzene-d6 etc.

23

b) Radiofrequency oscillator:• To irradiate the sample with electromagnetic

radiation a highly stable crystal controlled oscillator coil is imposed at right angle to the applied magnetic field.

• To achieve maximum interaction of the RF radiation with the sample the oscillator coil is wound around the sample cell.

24

C) Sweep Generator:• For a nucleus to resonate, the precession frequency

should become equal to the frequency of the applied RF radiation.

This can be achieved by,I. Frequency Sweep method:• This method is used to resonate the nucleus • The frequency of the RF radiation is changed so that

it become equal to resonance frequency or precession frequency.

25

ii. Field sweep method:• In this method to resonate the nucleus the

frequency of the RF radiation is kept constant and the precession frequency is changed by changing the applied magnetic field.

26

d)Radiofrequency receiver:• A few turns wire is wound around the sample

tube tightly.• This receiver coil is perpendicular to both the

external magnetic field and radio frequency oscillator (transmitter).

• When RF radiation is passed through the magnetized sample, the resonance occurs, which cause the current voltage across the coil to drop.

27

28

5) DETECTOR:• When radiation is passed through the sample, the

two phenomena namely, absorption and dispersion may occur.

• The observation of dispersion or absorption will enable the resonance frequency to be determined.

There are two method to detect the resonance signals.

a)Single coil method (Radiofrequency bridge method)

b)Double coil method (Nuclear induction method)

29

a)Single coil method:(Radiofrequency bridge)

• In this method coil used for surrounding the sample serves as both transmitter and receiver coil.

• The applied signal is balanced against the received signal and the resonance signal is recorded as an out of balance e.m.f which may be amplified and recorded.

30

b)Double coil method (Nuclear induction method):• In this method there are separate transmitter and

receiver coils.• These two coils are fixed at right angle to each other

as well on to the direction of external magnetic field.• This energy transfer (induction) is influenced by

magnetic nuclei in a sample placed between the two coils, as long as the RF current in the first coil was in resonance with nuclear transition.

31

32

AMPLIFIER & RECORDER:

• The received signal is amplified by the help of amplifier.

• And it is recorded by using a computer device.