FT NMR WORKSHOP/===/ S.A.I.F./===/ NEHU/==/ Shillong INTRODUCTORY LECTURE S.ARAVAMUDHAN Distinct Spectroscopic context for the NMR Spectroscopy Slides # 2- 4 A quick exposition of the Magnetic Resonance Phenomena Slide # 5 An animated elaboration of the Single spin Interactions Slides #6-7 Concluding Slide with a Definition of N.M.R. Slide # 8

In a Scheme of an Atom Electrons circulate in Orbits NUCLEUS is stationery at the center Chemical Molecular structure depends on the electronic structural changes due to bonding between atoms. Nucleus plays no role in determining the optimum geometry except that they get an assigned place as they occupy in the molecule. Molecular spectroscopic studies involve studies assuming an equilibrium structure, but the molecular phenomenon responsible for spectroscopic absorptions require changes in the electron dispositions around the equilibrium geometries

3 One of the aspect to be reckoned with is the fact that all the chemical consequences are because of the electrons present in the elemental atom or ion because of which these elements exhibit chemically binding characteristics. It is known that the atoms and ions of such elements have their characteristic nucleus around which the electrons of the system revolve in orbits. For the chemical consequences there is not any significant role assigned to the nuclear characteristics unless it is a radio active element and the nuclear radiations can make it possible to be tracked by radio active tracer techniques. The radio activity itself can be hazardous besides the toxic effects of such elements by chemical reactivities

In magnetic resonance, the nuclei are stimulated in such a way that the electronic dispositions are influenced by the perturbation so that the resulting stimulated response is indicative of the specifics about equilibrium free electron dispositions in the molecular structures When all the extra nuclear electrons are in such continuous motion and participate in the bonding, if there can be a stimulation of nucleus, which does not in any way affect the electronic dispositions, then, would that be useful studies of molecular electronic structure?

The central nucleus of some of the elements posses intrinsic spin and hence also can be associated with a magnetic moment SPIN Magnetic moment These tiny nuclear magnetic moments are similar to the bar magnets which are influenced by Externally applied magnetic fields Applied magnetic field Only Discrete orientations of the spin are possible due to the quantization criteria at atomic regimes or Electromagnetic radiation with frequency ν can cause transition between these levels and this is the resonance phenomenon 2π ν = γ H Similar effects are possible with electrons also but only in PARAMGNETIC IONS or Molecules when there are unpaired electrons present. Here the reference is only to Diamagnetic Samples (compounds) to introduce NMR exclusive of any other effects. The value of γ differs from one nucleus to the other. This unique value of ‘ γ’ for each element’s nucleus different from every other element is what makes multi nuclear NMR possible SPIN Magnetic moment Discrete orientations RESULTS in discrete energy levels Illustration is a case of SPIN=3/2 results in 4 equally spaced energy levels Spin Quantum number value= 3/2 Spin=1/2 Energy Field +1/2 -1/2 +1/2 -1/2

Spin and Magnetic moment: Nucleus rotates about an axis within itself, which is referred to as the spinning of nuclei. The spinning object has an angular momentum The NUCLEI thus possess angular momentum; and the angular momentum in atomic system are quantized. Due to this quantization, the angular momentum component in any chosen direction can take only specified discrete values. Spin Angular momentum Nucleus has electrical Charge. Thus a rotating charge has associated magnetic moment. For protons: Spin quantum number=1/2 The angular momentum and magnetic moment are in the same direction because of the positive charge of the nucleus Illustration: Magnetic Field +1/2ħ -1/2ħ ±1/2 I h NOTE: that the single spin is invariably associated with an XY component perpendicular to the magnetic field direction Spin axis In the quantized orientations the spins experience a torque due to which they must be precessing (Larmor precession) Electromagnetic radiation causes the spin flipping

The Larmor precession frequency depends on the strength of external field hν=gβH If a rotating magnetic field of relatively small magnitude is present in the perpendicular plane at frequency ν, then the resonance occurs and the spin undergoes a flipping transition to another orientation. For proton spin of ½, there are two allowed orientations so that the component along z-axis is either +1/2 or -1/2 +1/2 ħ -1/2 ħ Lower energy Upper energy +1/2 ħ -1/2 ħ Photon energy absorbed; transition occurs Induced Transition or stimulaed transition Radiation hν=gβH Spin=1/2 Energy Field +1/2 -1/2 +1/2 -1/2 hν=gβH +1/2 ħ A single spin description cannot be complete without the associated XY component Quantized Energy level descriptio n Vector diagramatic description

Magnetic Resonance Phenomenon is a manifestation due to the presence of INTRINSIC SPIN angular momentum and the associated Magnetic Moment characteristically in electrons and Nuclei When the experimental conditions are set for the NUCLEI to resonate, then it is the Nuclear Magnetic Resonance. A definition: 0 ppm TMS 10ppm Single line NMR spectrum PMR spectrum NMR spectrum of a sample of spin ensemble……..