nuclear magnetic resonance principle


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NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Presented by A.Mounika M.Pharm 1 st year Pharmaceutical analysis Ssrcp,mahabubnagar 1


INTRODUCTION Most of the nuclei studied by NMR spectroscopy. Ex: hydrogen, carbon etc The number of each of the distinct types of magnetically induced nuclei. It reveals the different chemical environments of the various forms of hydrogen present in molecule. 2


NMR NMR involves interaction between nuclei and radio frequency radiation. The energy observed by the sample, and the absorption can be observed as a change in the signal developed by a radio frequency detector and amplifier. The energy absorption can be related to the magnetic dipolar nature of spinning nuclei. 3

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The study of radio frequency radiation by nuclei is called nuclear magnetic resonance. NMR mainly involves the magnetic energy of nuclei when they are placed in magnetic field and the transitions occur in the wave region of the spectrum. The amount of energy available in radio frequency region is just sufficient to affect the nuclear spin of the atom in a molecule. 4

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Three important factors– Frequency of spectral lines or bands. Intensity of spectral lines or bands . Shape of spectral lines or bands. 5


NUCLEAR SPIN Many atomic nuclei carry a charge. Nuclei of atoms are composed of protons and neutrons. This nuclei having property called spin. In nuclei this charge spins on the nuclear axis and this circulation of nuclear charge generates a magnetic dipole along the axis. 6

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=> 7

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All charged particles in a nucleus will cause the nucleus behave like a small bar magnet, with its magnetic moment along the axis of rotation. 8

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Nucleus have the property to spin their own axis and angular momentum is ½(h/2 π ). The net resultant of the angular momentum of nuclear particles is called nuclear spin . The nucleus having nuclear spin quantum number I, there are (2I+1) spin states. Nuclear particles in NMR having two properties. Net spin associated with protons and neutrons. The distribution of positive charge. 9

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The net spin obtained by adding spin numbers of individual protons and neutrons. The spin numbers I have values 0,1/2,3/2,5/2 and so forth. (I=0 represents no spin). 10


PRINCIPLES FOR THE NUCLEAR SPIN If the sum of protons and neutrons is even, I is 0 or integral (0,1,2,3….). If the sum of the protons and neutrons is odd, I is half integral (1/2, 3/2, 5/2….). If both protons and neutrons are even numbered, I is zero. 11


SPIN QUANTUM OF VARIOUS NUCLEI Number of protons Number of neutrons Spin quantum number Examples Even Even 0 12 C, 16 O, 32 S Odd Even ½, 3/2 1 H, 19 F, 31 P, 11 B , 79 Br Even Odd ½ ,3/2 13 C, 127 I Odd Odd 1 2 H, 14 N 12

Magnetic properties occur with those nuclei which have-:

Magnetic properties occur with those nuclei which have- Odd atomic number and odd mass number Ex: Odd atomic number and even mass number Ex: Even atomic number and odd mass number Ex: 13

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The number of allowed spin states I may adopt is quantized and is determined by its nuclear spin quantum number I. The number I is a physical constant, and that are 2I+1 allowed spin states with integral differences ranging +I to –I. The individual spin states fit into the sequence +I, (I-1),-----(I+1), -I 14

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A proton has the spin quantum number I=1/2, two allowed spin states (2(1/2)+1=2), for its nucleus -1/2 and +1/2. For Chlorine nucleus I=3/2, there are four allowed spin stated (2(3/2)+1=4), for its -3/2, -1/2, +1/2, +3/2. 15

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MAGNETIC MOMENT A hydrogen nucleus may have a clockwise (+1/2) or counter clockwise (-1/2) spin, and the nuclear magnetic moment (µ) in the two cases are pointed in opposite directions. In an applied magnetic field, all protons have their magnetic moment either aligned with the field or opposed to it. 17

The two allowed spin states for a proton:

The two allowed spin states for a proton 18

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The aligned configuration of magnets is stable having lower energy. The magnets are opposed (not aligned) the center magnet is repelled out of its current having high energy orientation. 19

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Protons and neutrons posses magnetic properties, they are associated with a magnetic moment µ for the nuclei. The relation between the magnetic moment and nuclear spin quantum number is µ=k(I(I+1)) ½ K= constant 20

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Spin quantum number I determines the no of orientations a nucleus may assume in an external uniform magnetic field in accordance with the formula 2I+1. 1H nucleus having spin quantum number I=1/2 , There are two spin states given by (2I+1) = 2*1/2+1=2 Which have the values +1/2 and -1/2 21

Two energy levels for a proton in a magnetic field Bo:

Two energy levels for a proton in a magnetic field B o 22

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In the case of chlorine nucleus there are four energy levels +3/2 and -3/2 spin states are aligned with the applied field and opposed to the applied field respectively. +1/2 ad -1/2 spin states have intermediate orientations. 23

Absorption of energy:

Absorption of energy When the nuclei aligned with an applied field are induced to absorb energy and change their spin orientation with respect to the applied field. 24

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The energy absorption is a quantized process, and the energy absorbed must equal the energy difference between the two states involved. h = plank’s constant V= frequency 25

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The energy difference is a function of the strength of the applied magnetic field Bo. 26

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This energy difference depends upon the applied magnetic field. ∆E = µ β N H o / I µ = nuclear magnetic moment Ho= applied magnetic field β N = nuclear magneton I= spin quantum number 27

The origin of nmr spectra:

The origin of nmr spectra According to the quantum mechanics a proton having two orientations +1/2,-1/2. The energies characterized by the energies E 1 = +1/2 µHo and E 2 = -1/2 µHo. The energy difference between them ∆E = E 1 -E 2 = µHo µ= magnetic moment of the spinning nucleus 28

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Applying Bohr frequency relation V=µHo/h This is basic NMR equation and would imply that all protons would resonate at the same field and frequency. The fundamental equation for NMR correlating electromagnetic frequency with magnetic field strength– V= γ Ho/2 π γ = gyro magnetic ratio 29

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Gyro magnetic ratio or magnetogyric ratio is characteristic of any nuclear species that has a finite value I. Each nucleus has a different ratio of magnetic moment to angular momentum, since each has different charge and mass. This ratio called gyromagnetic ratio. It is regarded as the proportionality constant between the magnetic moment µ and spin number I. γ =2 πµ / hI 30

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Theory of nmr spectroscopy:

Theory of nmr spectroscopy Spinning nuclei-magnetic moments : the spinning of a charged particle generates magnetic field. Nuclear magnetic moments applied magnetic field Ho with no magnetic moment 32

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Magnetic moment and magnetic field : E= - µ H .Ho µ H= component of magnetic moment in the direction of the field. Ho=the strength of external field. A particle with a spin quantum number I and magnetic quantum number m, the energy of quantum level is given by E=- mµB.Ho /I B= nuclear magneton (5.049X10 -24 erg-1/gauss ) 33

Nuclear Magnetic Resonance:

Nuclear Magnetic Resonance 34

The mechanism of absorption:

The mechanism of absorption Protons absorbs energy because they begin to precess in an applied magnetic field. Owing to the earth gravitational field the top begins to wobble or precess about its axis. A top precessing in the earth’s gravitational field 35

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The spinning nucleus behaves in a similar fashion under the influence of an applied magnetic field. The nucleus begins to precess about its own axis of spin with angular frequency ω . ω= larmor frequency ω directly proportional to applied magnetic energy. 36

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If the applied field 1.41 tesla , the frequency of precession is approx 60Mhz. Precession oscillating electric field. If the radiofrequency waves of this frequency are supplied to the precession proton , the energy can be absorbed. Frequency of oscillating electric field = frequency of electric field generated by the precessing nucleus. This condition is called resonance. 37

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conclusion Nuclei having protons, this charge spins on the nuclear axis and generates the magnetic field, when applied magnetic field, the protons will precess . The precessing protons absorb energy from radio frequency source, and the absorption can be observed as a change in signal developed by the radio frequency detector. 39


Reference: Organic spectroscopy– william kemp third edition. Instrumental methods of chemical analysis– B.K. Sharma. Introduction to spectroscopy— pavia . Instrumental methods of chemical analysis— gurudeep R.chatwal . 40


QUESTION: What is the spin quantum number of this molecule &how? 41

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