NMR

Views:
 
     
 

Presentation Description

NMR

Comments

Presentation Transcript

:

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Presentation by, Mr.YASHWANTH G DEPT.OF PHARMACEUTICS VIPS,BENGALURU .

OUTLINE OF PRESENTATION:

2 OUTLINE OF PRESENTATION Fundamental principle. Magnetic properties of nuclei. Applied field and precession. Absorption and transition. Frequencies. Brief application.

PowerPoint Presentation:

HISTORY DISCOVERY OF NMR - 1946 NOBEL PRIZE IN PHYSICS- 1952 NUCLEAR PROPERTY 1 H SIGNALS FROM WATER AND PARAFFIN WAX Felix Bloch, Stanford Univ. Edward Purcell, Harvard Univ. 3

PowerPoint Presentation:

Definition : NMR spectrometry is a absorption spectrometry in which the radio frequency waves absorbed by the nucleus in a magnetic field resulting in spin changes at nuclear level. or The study of spin changes at the nuclear level when radio frequency energy is absorbed in the presence of external magnetic field. Measures the absorption of EM radiation in the radiofrequency region from 4MHz to 750MHz. (wavelength 0.4 m to 75 m) Most commonly done on 1 H and 13 C. 4

PowerPoint Presentation:

The energy associated with radio waves is given by E = h v where, h = plank’s constant v = frequency The energy associated with the radio frequency waves is very less. This energy is not sufficient to cause rotational, vibrational or exite a molecule or atom. But this energy is sufficient to affect the nuclear spin of the atom in a molecule. 5

PowerPoint Presentation:

Nuclear spin & Magnetic moment : Protons, neutrons and electrons are the elements of an atom. Protons are positively charged, neutrons are neutral elements constituting the nuclei of an atom and electrons are negatively charged revolving around the nucleus. This elements of an atom are having an intrinsic property called spin. i.e. they spin on their own axis. Each can possess an angular momentum ½ ( h/2 π ) in accordance to quantum theory. 6

PowerPoint Presentation:

Net result of the angular momentum of all element is spin. The spin angular momentum is given by S = I [( I + 1 ) 1/2 ] h/2 π . Where, I = spin quantum number. Magnetic momentum µ = γ × spin angular momentum. = γ × I [( I + 1 ) 1/2 ] h/2 π where, γ = gyromatic ratio or magnetogyric ratio For a nucleus having spin quantum number (I), there are (2 I + 1) spin states. 7

PowerPoint Presentation:

The net spin quantum number (I) of a particular nucleus can be obtained by adding spin number of individual proton and neutron of ½ each assuming- neutrons cancel neutrons only and protons cancel protons only. Pairing or spinning in opposite directions. The spin number or spin quantum number (I) have the values 0,1/2, 1, 3/2, 5/2 etc. 8

PowerPoint Presentation:

There are three broad principles for nuclear spin- If the sum of protons and neutrons is even, I is zero or integral (0.1.2.3.. ) If the spins of all the particles are paired, there will be no net spin and the nuclear spin quantum number I will be zero. E.g. 12 C, 16 O, 32 S etc, nuclei have even number of neutrons and even number of protons and so will show zero spin and consequently there will be no magnetic moment. If the sum of protons and neutrons is odd, I is half integral (1/2, 3/2, 5/2 etc). E.g. 1 H, 19 F, 13 C have I = 1/2, 11 B, 35 Cl, 79 Br have I = 3/2. 9

PowerPoint Presentation:

10 The spin quantum number of various nuclei : No. of protons No. of neutrons (I) values examples Even Even 0 12 C 16 O 32 S etc Odd Odd 1 2 H 14 N Odd Even 1/2, 3/2 1 H 11 B 79 Br Even Odd ½ 3/2 13 C 127 I

PowerPoint Presentation:

THEORY AND PRINCIPLE OF NMR. Nuclei of atoms with an odd atomic number or an odd mass number have a nuclear spin or angular momentum. The total angular momentum depends on the spin quantum number (I). Because nuclei are positively charged, their spin induce a magnetic field. When a magnetic field is applied to atomic nuclei, the magnetic fields of the nuclei align themselves either parallel or anti-parallel to the applied magnetic field. 11

PowerPoint Presentation:

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 B 0 , 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). 12

PowerPoint Presentation:

13

PowerPoint Presentation:

The NMR spectroscopy is most often concerned with I = ½ Best example is 1 H 1 with a spin of ½. Since hydrogen nuclei consists of only one proton and consequently it can have a spin of +1/2 or -1/2.depending up on the direction of spin. As mentioned earlier, the nucleus having the spin quantum number I the spin states is given by (2I+1). For Hydrogen I= ½ . Spin states = [2 (½ )+1] = 2. i.e., hydrogen is having two spin states +½ and -½ In absence of external magnetic field the energy levels corresponding to two spin states are degenerate i.e., have an equal energy values. 14

PowerPoint Presentation:

In presence of magnetic field. The spin levels are separated by an the magnetic field strength. In an applied magnetic field B 0 ,all the protons have their magnetic moment either aligned with the magnetic field or opposed to it. 15

PowerPoint Presentation:

16 Magnetic moments and energy states for a nucleus with a spin quantum number of + 1/2 .

PowerPoint Presentation:

17 N s S N N s N S + ½ Aligned with B 0 - ½ Opposes with B 0 Nuclear Magnetic moment when no applied magnetic field β β β β α α Applied Magnetic Field B 0 The energy difference between these two states is given by ∆E=E 1 -E 2 The absorption of radio frequency occurs only when the radio waves having energy equal to ∆E E= hv but ∆E = E 1 - E 2 Therefore hv = E 1 -E 2 v = E1 - E2 / h

PowerPoint Presentation:

POPULATION DENSITIES OF NUCLEAR SPIN STATE In the presence of the magnetic field , the nuclei tend to orient themselves so that the lower energy state (+1/2) predominates. Thermal agitation however tends to offset the magnetic effects, because thermal energies at room temperature is higher than these magnetic energy differences. Because of this energy, majority of the nuclei go to higher energy state (-1/2) and only a small excess of nuclei persists in the lower energy state at 25°C 18

PowerPoint Presentation:

Precessional motion: Because the proton is behaving as a spinning magnet, it can align itself either with or opposed to an external magnetic field. It can also move a characteristic way under the influence of the external magnet. Considering the behavior of a spinning top, the top has a spinning motion around its axis. It also performs a slower waltz like motion in which the spinning axis of top moves slowly around the vertical. Thus is called “precessional motion” and the top is said to be precessing around the vertical axis of the gravitational force of the earth. Precession arises due to the interaction of spin and the gravitational force acting downwards. This is the reason why only a spinning top will precess; where as a static top will topple over. 19

PowerPoint Presentation:

Since the proton is a spinning magnet, it will precess around the axis of an applied external magnetic field. It will precess in two main orientations. Aligned or parallel with the field-low energy Opposed or anti parallel to the field-high energy. 20 ∆E= hυ High energy Low energy E OPPOSED ALIGNED Bo (strength of magnetic field)

PowerPoint Presentation:

PRESESSIONAL FREQUENCY : It may be defined as the no. of revolutions per second mode by the magnetic moment vector of the nucleus around the external field ( Bo ). The precessional frequency ( ν ) is directly proportional to the strength of the external magnetic field ( Bo ) . ᴠ α Bo The exact frequency is given by ᴠ = γ.Bo/2π 21

PowerPoint Presentation:

Where γ= gyro magnetic ratio (ratio between nuclear magnetic moment (µ) and nuclear angular momentum. 22

PowerPoint Presentation:

23 v = E1 - E2 / h => ½( γ h/2 π )B 0 + ½( γ h/2 π )B 0 h h v V =( γ /2 π )B 0 V =( γ /2 π )B 0 “Larmor Equation” The nucleus in the magnetic field absorbs energy from the radio waves having the frequency equal to ( γ /2 π )B 0 becomes excited. Then it loses energy to return to the unexcited state. Again it absorbs energy and returns to the excited state. The nucleus which alternatively becomes excited and unexcited is said to be in a state of resonance . The transition from one energy state to the other is known as flipping of the proton.

PowerPoint Presentation:

ENERGY TRANSITIONS: 1) If a proton is precessing in an aligned orientation, it can absorb energy and pass into the opposed orientation. 2) It will then lose this extra energy and relax back to the aligned position. 3) If we irradiate the precessing nuclei with a beam of radiofrequency energy of the correct frequency, the low energy nuclei may absorb this energy and move to a high energy state. 4) The precessing proton will absorb energy from the radiofrequency source only if the precessional frequency of the proton is same as the frequency of radio frequency source. 5) When the two frequencies are similar, the nucleus and radiofrequency source are said to be in RESONANCE hence the term NMR. 24

PowerPoint Presentation:

6) A simple NMR experiment consists of exposing the protons in an organic molecule to a powerful magnetic field. The protons will precess , but all protons may not precess at the same frequency. 7) When these protons are irradiated with the appropriate radio frequency energy, the protons get promoted from low energy aligned state to the high energy opposed state the energy absorbed is recorded in the form of an NMR spectrum is plotted between absorption signal at detector and Bo. 25

PowerPoint Presentation:

26

PowerPoint Presentation:

In NMR spectrometry to bring nucleus in resonance : Radio frequency is kept constant and strength of B 0 is varied. Radio frequency is varied and strength of B 0 is kept constant. As the excited state is unstable, the proton loses its energy and comes back to unexcited state. This process is called relaxation . It occurs by two methods- 1. Spin lattice relaxation / Longitudinal relaxation. 2. Spin-spin relaxation / Transverse relaxation. 27

PowerPoint Presentation:

1. Spin lattice relaxation or Longitudinal relaxation It involves the transfer of energy from the excited protons to the surrounding protons that are tumbling at an appropriate frequency. The energy is transferred to the components of lattice as the additional, vibrational and rotational energy. 2. Spin-spin or Transverse relaxation It is due to mutual exchange of spins by two processing protons or the excited nucleus may transfer its energy to an unexcited nucleus of a similar molecule that is near by. 28

PowerPoint Presentation:

When energy in the form of Radiofrequency is applied and when, absorption of energy occurs and a NMR signal is recorded . 29 Applied frequency = Precessional frequency

PowerPoint Presentation:

30 D E = h n n = g B o / 2 p D E = g h B o / 2 p w = 2 pn w = g B o = g h I / 2 p n - Electromagnetic frequency in radio frequency w - Precessional frequency  - Magnetic dipole moment I - Spin quantum number D E - Energy Difference h - Planck’s constant  - Gyro magnetic ratio B o - Applied magnetic field

Different Types of NMR:

31 Different Types of NMR Electron Spin Resonance ( ESR) 1-10 GHz (frequency) used in analyzing free radicals (unpaired electrons) Magnetic Resonance Imaging ( MRI) 50-300 MHz (frequency) for diagnostic imaging of soft tissues (water detection) NMR Spectroscopy ( MRS) 300-900 MHz (frequency) primarily used for compound ID and characterization

PowerPoint Presentation:

Which Elements or Molecules are NMR Active? * Any atom or element with an odd number of neutrons and/or an odd number of protons * Any molecule with NMR active atoms 1 H - 1 proton, no neutrons, AW = 1 13 C - 6 protons, 7 neutrons, AW =13 15 N - 7 protons, 8 neutrons, AW = 15 19 F = 9 protons, 10 neutrons, AW = 19 32

PowerPoint Presentation:

NMR SPECTRA REVEALS, The presence of particular functional groups. Relative number of nuclei present in the functional group. Relative position of these groups. In quantitative analysis, as the number of protons corresponds to the concentration of reacting species. The hydrogen bonding in inorganic compounds as well as metal chelates can be studied using NMR. 33

PowerPoint Presentation:

REFERENCES Instrumental methods of chemical analysis 1st edition. GR Chatwal , SK Anand . Organic spectroscopy by William kemp , 3 rd edt , Pharmaceutical analysis By R.S.Ravishankar WEBSITES www.pharmacy.ualbert.co.in www.wikipedia.com www.google.com 34

PowerPoint Presentation:

THANK YOU ONE AND ALL … 35 35