A short presentation in the group by Prem Basnet 09/29/04

 A phenomenon which occurs when the nuclei of certain atoms are immersed in a static magnetic field and exposed to a second oscillating magnetic field.  Some nuclei experience this phenomenon, and others do not.  Dependent upon whether they possess a property called spin.  Most of the matter you can examine with NMR is composed of molecules.  Molecules are composed of atoms. NMR (Nuclear Magnetic Resonance) In Medical, Magnetic Resonance Imaging(MRI)

Consider a water molecules ¤ In one of the hydrogen atoms, past the electron cloud, we see a nucleus composed of a single proton. ¤ The proton possesses a property called spin which: a) Can be thought of as a small magnetic field, b) Will cause the nucleus to produce an NMR signal

Proton as a tiny magnet

Not all nuclei possess the property called spin  If the number of neutrons and the number of protons are both even, then the nucleus has NO spin.  If the number of neutrons plus the number of protons is odd, then the nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2)  If the number of neutrons and the number of protons are both odd, then the nucleus has an integer spin (i.e. 1, 2, 3)  The overall spin, I, is important.  Quantum mechanics tells us that a nucleus of spin I will have 2 I + 1 possible orientations.  A nucleus with spin 1/2 will have 2 possible orientations.  In the absence of an external magnetic field, these orientations are of equal energy.  If a magnetic field is applied, then the energy levels split. Each level is given a magnetic quantum number

Isotope Magnetogyric Ratio  (radians/Tesla) # of Spin States Resonance Frequency (MHz) 1H1H H2H C N O F P Cl 45.9 Important values for spin active nuclei (field strength = 1.41 Tesla).

Two energy States (a) Low energy (b) High energy

Difference in Energy of two states -1/2 +1/2

Two spin states for Hydrogen nuclei No magnetic field External magnetic field B o

Systems of Spins in a magnetic field

Larmor Precession

Transitions

Absorption of photon by spinning proton

Spin flip after photon absorption

Magnetic field and the Energy gap

Relaxation Process: 1. Spin-lattice, 2.Spin-spin (1) Spin – lattice(longitudnal)  Nulei in the lattice are in vibrational and rotational motion and creates a complex magnetic field, called Lattice field  Some components of the lattice field have frequency and phase equal to the Larmor frequency of the nulei of interest.  These componets interact with nulei in higher energy state  Nuclei lose energy to the lattace.  The relexation time T 1 depends on magnetogyric ratio of the nulcleus and the mobility of the lattice.

(1) Spin – spin (transverse)  Interaction between neighboring nuclei with same Larmor frequencies  But differing magnetic quantum states.  A nucleus in the lower energy state will be excited  A nucleus in the higher energy state relaxes  There is no net change in the population of the energy states.  Average life-time of a nucleus in the excited state will decrease.

Diamagnetic shielding of a nucleus by valence electrons

Chemical shift

Tetramethylsilane (TMS) A organic molecule

We summarize our observations of the simple spectrum in terms of five rules: Rule 1: Each unique proton environment generates one signal. Rule 2: The intensity (size) of an NMR signal gives a measure of the number of equivalent atoms responsible for the signal. A two-proton signal is twice as large as a one-proton signal. Rule 3: The greater the number of electronegative atoms near a proton, the further to the left of TMS the proton signal will be. Rule 4: Protons on neighboring atoms interact with each other, and split each other’s signals. Rule 5: The number of subsignals in a multiplet is one more than the number of neighbor protons.

a magnet surrounding the sample compartment, a RF generator, a sample container (glass tube), a RF detector. An NMR spectrometer is composed of Instrumentation

Schematic diagram of NMR spectrometer.

Conclusion 1. A nuclear phenomenon coupled with magnetic fields. 2. A spin dependent process. 3. Two spin states are the key to this process. 4. A powerful tool to determine and analyze the physical, chemical, and biological properties of matter.

Ph.D. Dissertation – S.V.Primak References :-