NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Basics of …….. NMR phenomenonNMR phenomenon Chemical shiftChemical shift Spin-spin splittingSpin-spin splitting.

NUCLEAR MAGNETIC RESONANCE NUCLEAR MAGNETIC RESONANCE SPECTROSCOPYSPECTROSCOPY

Basics of ……..Basics of ……..•NMR phenomenonNMR phenomenon

•Chemical shiftChemical shift•Spin-spin splittingSpin-spin splitting

NUCLEAR MAGNETIC RESONANCE NUCLEAR MAGNETIC RESONANCE

SPECTROSCOPY concerns the SPECTROSCOPY concerns the interaction of nuclear spins with interaction of nuclear spins with radio frequency radiation in radio frequency radiation in presence of an applied magnetic presence of an applied magnetic fieldfield

Nuclear spinNuclear spin

• Subatomic particles (electrons, protons Subatomic particles (electrons, protons and neutrons) can be imagined as and neutrons) can be imagined as spinning on their axes. spinning on their axes.

• In many atoms (such as In many atoms (such as 1212C) these spins C) these spins are paired against each other, such that are paired against each other, such that the nucleus of the atom has no overall the nucleus of the atom has no overall spin. spin.

• However, in some atoms (suchHowever, in some atoms (such as as 11H and H and 1313C) the nucleus does possess an overall C) the nucleus does possess an overall spin. spin.

The nucleus…..The nucleus…..

The nucleus hasThe nucleus has

• SpinSpin

• ChargeCharge

• Magnetic momentMagnetic moment

The magnetic moment….The magnetic moment….

• The nucleus has a positive charge and The nucleus has a positive charge and is spinning. This generates a small is spinning. This generates a small magnetic field. The nucleus therefore magnetic field. The nucleus therefore possesses a magnetic moment, m, possesses a magnetic moment, m, which is proportional to its spin,which is proportional to its spin,II..

• The constant, g, is called the The constant, g, is called the magnetogyric ratio magnetogyric ratio and is a and is a fundamental nuclear constant which fundamental nuclear constant which has a different value for every has a different value for every nucleus.nucleus. hh is Plancks constant is Plancks constant

The rules for determining The rules for determining the net spin of a nucleus are the net spin of a nucleus are as followsas follows• If the number of neutrons and the If the number of neutrons and the

number of protons are both even, then number of protons are both even, then the nucleus has NO spin. the nucleus has NO spin.

• If the number of neutrons plus the If the number of neutrons plus the number of protons is odd, then the number of protons is odd, then the nucleus has a half-integer spin (i.e. 1/2, nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2) 3/2, 5/2)

• If the number of neutrons and the If the number of neutrons and the number of protons are both odd, then the number of protons are both odd, then the nucleus has an integer spin (i.e. 1, 2, 3) nucleus has an integer spin (i.e. 1, 2, 3)

The overall spin, The overall spin, II, is , is important….important….

• Quantum mechanics tells us that a Quantum mechanics tells us that a nucleusnucleus ofof spin spin II will have 2 will have 2II + 1 possible + 1 possible orientations.orientations.

• A nucleus with spin 1/2 will have 2 A nucleus with spin 1/2 will have 2 possible orientations. possible orientations.

• In the absence of an external magnetic In the absence of an external magnetic field, these orientations are of equal field, these orientations are of equal energy.energy.

• If a magnetic field is applied, then the If a magnetic field is applied, then the energy levels split. energy levels split.

• Each level is given aEach level is given a magnetic quantum magnetic quantum number, mnumber, m..

Initial populations ….the Initial populations ….the Boltzmann distribution…..Boltzmann distribution…..

• The lower energy level will contain The lower energy level will contain slightly more nuclei than the higher slightly more nuclei than the higher level – according to Boltzmann level – according to Boltzmann distributiondistribution

• It is possible to excite these nuclei into It is possible to excite these nuclei into the higher level with electromagnetic the higher level with electromagnetic radiation. radiation.

• The frequency of radiation needed is The frequency of radiation needed is determined by the difference in energy determined by the difference in energy between the energy levels.between the energy levels.

Energy …. Energy ….

• The energy of a particular energy level is The energy of a particular energy level is given by:given by:

• Where Where BB is the strength of the magnetic is the strength of the magnetic field field at the nucleusat the nucleus..

THE TRANSITION ENERGYTHE TRANSITION ENERGY• The difference in energy between The difference in energy between

levels (the transition energy) can be levels (the transition energy) can be found fromfound from

• This means that if the magnetic field, This means that if the magnetic field, BB, is increased, so is , is increased, so is ∆∆EE. .

• It also means that if a nucleus has a It also means that if a nucleus has a relatively large magnetogyric ratio, relatively large magnetogyric ratio, then then ∆∆EE is correspondingly large. is correspondingly large.

The absorption of radiation by The absorption of radiation by a nucleus in a magnetic fielda nucleus in a magnetic field

• Imagine a nucleus (of spin 1/2) in a Imagine a nucleus (of spin 1/2) in a magnetic field. magnetic field.

• This nucleus is in the lower energy This nucleus is in the lower energy level (i.e. its magnetic moment level (i.e. its magnetic moment does not oppose the applied field). does not oppose the applied field).

• The nucleus is spinning on its axis. The nucleus is spinning on its axis. • In the presence of a magnetic field, In the presence of a magnetic field,

it will it will precess.precess.

The frequency of precession is termed the Larmor frequency

Precession of nuclei….

Absorption of radiofrequency Absorption of radiofrequency radiationradiation

• For a nucleus of spin 1/2, For a nucleus of spin 1/2, absorption of radio frequency absorption of radio frequency radiation "flips" the magnetic radiation "flips" the magnetic moment so that it opposes the moment so that it opposes the applied field and the nuclear applied field and the nuclear spin goes to the higher energy spin goes to the higher energy levellevel

Flipping ….Flipping ….

NUCLEAR MAGNETIC NUCLEAR MAGNETIC RESONANCERESONANCE

• When the energy of the radiofrequency When the energy of the radiofrequency radiation matches the transitional energy radiation matches the transitional energy between the two energy states (lower between the two energy states (lower level and upper level), nuclear spins from level and upper level), nuclear spins from lower level absorb the radiation and jump lower level absorb the radiation and jump over to the upper level.over to the upper level.

• This is nuclear magnetic resonance This is nuclear magnetic resonance hhעע = ∆= ∆EE

At this condition a signal appears in the At this condition a signal appears in the NMR spectrumNMR spectrum

NMR phenomenon ………

E= ½ spinstate

Stage I Stage II Stage III

E= - ½ spinstate

Application of magnetic field

Under normal conditions Application of Rf

radiation

Spins Precessing

Spins alligningAnd opposing

Spins randomlyoriented

SaturationSaturation … …

• It is important to realize that only a It is important to realize that only a small proportion of "target" nuclei are small proportion of "target" nuclei are in the lower energy state (and can in the lower energy state (and can absorb radiation). absorb radiation).

• There is the possibility that by There is the possibility that by exciting these nuclei, the populations exciting these nuclei, the populations of the higher and lower energy levels of the higher and lower energy levels will become equal. will become equal.

• If this occurs, then there will be no If this occurs, then there will be no further absorption of radiation. further absorption of radiation.

• The spin system is The spin system is saturatedsaturated. .

Relaxation ….Relaxation ….

• In NMR energy is absorbed only when the In NMR energy is absorbed only when the lower energy state has excess of nucleilower energy state has excess of nuclei

• For this to be maintained nuclear spins For this to be maintained nuclear spins going to higher energy state must get going to higher energy state must get back to lower energy stateback to lower energy state

• This is called relaxationThis is called relaxation• There are two major relaxation There are two major relaxation

processes; processes; • Spin - lattice (longitudinal) relaxation Spin - lattice (longitudinal) relaxation • Spin - spin (transverse) relaxationSpin - spin (transverse) relaxation

Spin - lattice relaxationSpin - lattice relaxation•

Nuclei in an NMR experiment are in a sample. The Nuclei in an NMR experiment are in a sample. The sample in which the nuclei are held is called the sample in which the nuclei are held is called the latticelattice. .

• Nuclei in the lattice are in vibrational and rotational Nuclei in the lattice are in vibrational and rotational motion, which creates a complex magnetic field. motion, which creates a complex magnetic field.

• The magnetic field caused by motion of nuclei within The magnetic field caused by motion of nuclei within the lattice is called the the lattice is called the lattice fieldlattice field..

• This lattice field has many components. Some of This lattice field has many components. Some of these components will be equal in frequency and these components will be equal in frequency and phase to the Larmor frequency of the nuclei of phase to the Larmor frequency of the nuclei of interest. interest.

• These components of the lattice field can interact These components of the lattice field can interact with nuclei in the higher energy state, and with nuclei in the higher energy state, and causecause them to lose energy (returning to the lower state). them to lose energy (returning to the lower state).

Spin - spin relaxationSpin - spin relaxation

Spin - spin relaxation describes the interaction Spin - spin relaxation describes the interaction between neighboring nuclei with identical between neighboring nuclei with identical precessional frequencies but differing precessional frequencies but differing magnetic quantum states.magnetic quantum states.The nuclei can exchange quantum states; a The nuclei can exchange quantum states; a nucleus in the lower energy level will be nucleus in the lower energy level will be excited, while the excited nucleus relaxes to excited, while the excited nucleus relaxes to the lower energy state.the lower energy state.

There is no There is no netnet change in the populations of change in the populations of the energy states, but the average lifetime of the energy states, but the average lifetime of a nucleus in the excited state will decrease. a nucleus in the excited state will decrease. This can result in line-broadening. This can result in line-broadening.

Chemical shiftChemical shift

• The magnetic field at the nucleus The magnetic field at the nucleus is not equal to the applied is not equal to the applied magnetic field for every protonmagnetic field for every proton

• Electrons around the nucleus Electrons around the nucleus shield it from the applied field.shield it from the applied field.

Magnetic field induced by Magnetic field induced by circulating electroncirculating electron

• Chemical shift is a function of Chemical shift is a function of the nucleus and its environment.the nucleus and its environment.

• It is measured relative to a It is measured relative to a reference compound.reference compound.

• For For 11H NMR, the reference is H NMR, the reference is usually tetramethylsilane, Si usually tetramethylsilane, Si (CH3)4(CH3)4. .

Induced field opposes Induced field opposes external magnetic fieldexternal magnetic field• The induced magnetic field The induced magnetic field

produced by the circulating produced by the circulating electrons (Belectrons (Bii) opposes the external ) opposes the external magnetic field (Bmagnetic field (Boo))

• The actual magnetic field felt by The actual magnetic field felt by the nucleus (also called as effective the nucleus (also called as effective magnetic field Bmagnetic field Beffeff) is thus reduced) is thus reduced

BBeffeff = B = Boo – B – Bii

BBii ∞∞ B Beffeff

The effective field matters!The effective field matters!

• This means that the applied field This means that the applied field strength must be increased for the strength must be increased for the nucleus to absorb at its transition nucleus to absorb at its transition frequency. frequency.

• Greater the electron density around Greater the electron density around the nucleus, greater is the induced the nucleus, greater is the induced field.field.

• Greater the induced field, lesser will Greater the induced field, lesser will be the effective field felt by the be the effective field felt by the nucleusnucleus

• Lesser the effective field, greater Lesser the effective field, greater should be the applied field strengthshould be the applied field strength

BBeffeff = B = Boo – B – Bii

BBii ∞∞ B Beffeff

When BWhen Bii is greater, B is greater, Beffeff is reduced is reduced

When BWhen Beffeff is reduced, greater field is reduced, greater field strength is needed for the nucleus strength is needed for the nucleus

to to come to resonance.come to resonance.

Hence each nuclei will absorb at Hence each nuclei will absorb at different field strengths.different field strengths.

NMR SPECTRUMNMR SPECTRUM

Range of chemical shifts for PMR Range of chemical shifts for PMR spectrumspectrum

0-15 0-15 δδ

015

TMS

δ

Field B0

Upfield regiondownfield

Spin - spin couplingSpin - spin coupling

Consider the Consider the structurestructure

of ethanolof ethanol

• The 1H NMR spectrum of ethanol shows The 1H NMR spectrum of ethanol shows that the methyl peak has been split into that the methyl peak has been split into three peaks (a three peaks (a triplettriplet) and the ) and the methylene peak has been split into four methylene peak has been split into four peaks (a peaks (a quartetquartet). ).

• This occurs because there is a small This occurs because there is a small interaction through the electron spins interaction through the electron spins ((couplingcoupling) between the two groups of ) between the two groups of protons. protons.

• The spacing between the peaks of the The spacing between the peaks of the methyl triplet are equal to the spacing methyl triplet are equal to the spacing between the peaks of the methylene between the peaks of the methylene quartet. quartet.

• This spacing is measured in Hertz and is This spacing is measured in Hertz and is called the called the coupling constant,coupling constant,JJ..

• To see why the methyl peak is To see why the methyl peak is split into a triplet, let's look at the split into a triplet, let's look at the methylene protonsmethylene protons

• There are two of them, and each There are two of them, and each can have one of two possible can have one of two possible orientations (aligned with or orientations (aligned with or opposed against the applied field). opposed against the applied field).

• This gives a total of four possible This gives a total of four possible statesstates

i ii iii

• In the first possible combination, spins In the first possible combination, spins are paired and opposed to the field. are paired and opposed to the field.

• This has the effect of reducing the field This has the effect of reducing the field experienced by the methyl protons; experienced by the methyl protons;

• therefore a slightly higher field is therefore a slightly higher field is needed to bring them to resonance, needed to bring them to resonance, resulting in an upfield shift. resulting in an upfield shift.

• In the second combination, neither In the second combination, neither combination of spins opposed to each combination of spins opposed to each other has an effect on the methyl peak. other has an effect on the methyl peak.

• In the third combination, the spins In the third combination, the spins paired in the direction of the field paired in the direction of the field produce a downfield shift. produce a downfield shift.

• Hence, the methyl peak is split into Hence, the methyl peak is split into three, with the ratio of areas 1:2:1.three, with the ratio of areas 1:2:1.

Similarly, the effect of the methyl protons on Similarly, the effect of the methyl protons on the methylene protons is such that there are the methylene protons is such that there are eight possible spin combinations for the eight possible spin combinations for the three methyl protonsthree methyl protons

•Out of these eight groups, there are two groups of three magnetically equivalent combinations.

• The methylene peak is split into a quartet.

•The areas of the peaks in the quartet have the ratio 1:3:3:1.

• The multiplicity of a multiplet is given The multiplicity of a multiplet is given by the number of equivalent protons by the number of equivalent protons in neighbouring atoms plus one, i.e. in neighbouring atoms plus one, i.e. the n + 1 rulethe n + 1 rule

• Equivalent nuclei do not interact with Equivalent nuclei do not interact with each other. The three methyl protons each other. The three methyl protons in ethanol cause splitting of the in ethanol cause splitting of the neighbouring methylene protons; they neighbouring methylene protons; they do not cause splitting among do not cause splitting among themselves themselves

• The coupling constant is not The coupling constant is not dependant on the applied field. dependant on the applied field. Multiplets can be easily distinguished Multiplets can be easily distinguished from closely spaced chemical shift from closely spaced chemical shift peaks. peaks.

Question 1. How many possible orientations do spin 1/2 nuclei have when they are located in an applied magnetic field? Answer: Question 2. The frequency of precession, the transition frequency and the Larmor frequency are different terms for the same frequency.True or false? Answer: Question 3. When radiation energy is absorbed by a spin 1/2 nucleus in a magnetic field, what happens? a) The precessional frequency of the nucleus increasesb) The nucleus spins fasterc) The angle of precession " flips " so that the magnetic moment of the nucleus opposes the applied field Answer: Question 4. What is the name given to the relaxation process due to an interaction between an excited nucleus and the magnetic fields caused by nuclei in molecules moving around in the sample? a) Spin - lattice relaxationb) Spin - spin relaxation Answer: Your score:

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