FT NMR WORKSHOP/===/ S.A.I.F./===/ NEHU/==/ Shillong http://www.worldofteaching.com/ CLICK HERE for more details about this link above FT NMR WORKSHOP/===/ S.A.I.F./===/ NEHU/==/ Shillong INTRODUCTORY LECTURE S.ARAVAMUDHAN

Electrons circulate in Orbits 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. In a Scheme of an Atom Electrons circulate in Orbits 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 NUCLEUS is stationery at the center 2

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 3

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? In magnetic resonance, the nuclei are stimulated in such a way that the electronic dispositions are influenced by the perturbation in such a way that the resulting stimulated response is indicative of the specifics about equilibrium free electron dispositions in the molecular structures

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

The spinning object has an angular momentum Spin and Magnetic moment: Illustration: Nucleus rotates about an axis within itself, which is referred to as the spinning of nuclei. Spin axis The spinning object has an angular momentum Nucleus has electrical Charge. Thus a rotating charge has associated magnetic moment. Spin Angular momentum 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 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. NOTE: that the single spin is invariably associated with an XY component perpendicular to the magnetic field direction I h +1/2ħ ±1/2 I h -1/2ħ Magnetic Field

Vector diagramatic description Spin=1/2 Energy Field +1/2 -1/2 A single spin description cannot be complete without the associated XY component The Larmor precession frequency depends on the strength of external field hν=gβH +1/2 ħ For proton spin of ½, there are two allowed orientations so that the component along z-axis is either +1/2 or -1/2 hν=gβH Quantized Energy level description 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. +1/2 ħ Lower energy Vector diagramatic description -1/2 ħ Upper energy Photon energy absorbed; transition occurs hν=gβH -1/2 ħ +1/2 ħ Radiation Induced Transition or stimulaed transition

This NMR phenomenon is due to the fact that nuclei, placed in a strong external magnetic field, can resonate with externally applied electro magnetic radiations in the radio frequency range of the electro magnetic spectrum. Such of those nuclei which have nuclear magnetic moments are the ones which can be responding by this resonance phenomenon. The frequency of the electro magnetic radiation at which the resonance can occur is governed by a specific equation which relates the frequency to the strength of the external magnetic field. NUCLEAR MAGNETIC MOMENTS

A definition: 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 Single line NMR spectrum When the experimental conditions are set for the NUCLEI to resonate, then it is the Nuclear Magnetic Resonance. NMR spectrum of a sample of spin ensemble…….. 0 ppm TMS 10ppm PMR spectrum

On application of external magnetic field, nuclei start precessing Even before applying magnetic field electrons are in constant circulation in atoms and molecules protons electrons The application of the external field can alter the motional characteristics of the atomic molecular electrons. Thus the precessional frequency when placed in the electronic surrounding would not be the same for the nucleus as it is in a bare nucleus. These can give rise to magnetic moments and fields at the atomic nuclei and such orbital magnetic moments and their interaction with spin magnetic moments are the ones which are studied under the topic of SPIN-ORBIT couplings, Lande g-factor, effective moments and so on.. At this juncture these would be granted as what happens in the absence of magnetic field; and consider only the additional consequences due to the application of external magnetic fields. SPIN angular momentum & Magnetic Moments A reference molecule (TMS) is also placed along with the test molecule in the sample. The changes in the test molecule with reference to (the differences from the reference) are termed as “chemical shifts”. On application of external magnetic field, nuclei start precessing chemical shifts.

SPIN-SPIN Interaction Dipole-Dipole interaction Through space protons electrons Not encountered in solution spectra C, P, N, H j θij rij i A B Electron pair shared J x ( I1 • I2 ) J coupling constant expressed in Hz CONTACT Interaction Through bond Scalar interaction

Nucleus at the Centre of the circulating charge cloud Representing circulating charge cloud and associated induced field (moment) + This Secondary field influence depends on the internuclear distance Rij and an angle θij to be defined Rij This nucleus can come under the influence of the neighboring charge cloud due to the secondary fields j θij i Nucleus at the Centre of the circulating charge cloud (under the influence of primary moment)

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