pvrsk nmr presentation m.sc(previous) chemistry

8/7/2019 PVRSK NMR PRESENTATION M.SC(PREVIOUS) CHEMISTRY

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Nuclear Magnetic

Resonance Spectroscopy-A Review

P.V.R.SATISH KUMAR

ASSISTANT PROFESSOR OF ORGANIC CHEMISTRY

DEPARTMENT OF CHEMISTRY

MRPG COLLEGE, VIZIANAGARAM

M.Sc.(Previous)Chemistry,

Second semester-Organic Chemistry


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Introduction NMR is the most powerful tool available for

organic structure determination.

It is used to study a wide variety of nuclei: 1H

13C

15N

19F 31P

=>


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Nuclear Spin A nucleus with an odd atomic number or

an odd mass number has a nuclear spin.

The spinning charged nucleus generates

a magnetic field.

=>


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The nuclei of some atoms have a property called SPIN.

NUCLEAR SPINNUCLEAR SPIN

These nuclei behave as if

they were spinning.

This is like the spin property

of an electron, which can have

two spins: +1/2 and -1/2 .

Each spin-active nucleus has a number of spins defined by

its spin quantum number, I.

.. we dont know if they actually do spin!

The spin quantum numbers of some common nuclei follow .

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Element 1H 2H 12C 13C 14N 16O 17O 19F

Nuclear Spin

Quantum No 1/2 1 0 1/2 1 0 5/2 1/2( I )

No. of Spin 2 3 0 2 3 0 6 2

States

Spin Quantum Numbers of Some Common NucleiSpin Quantum Numbers of Some Common Nuclei

Elements with either odd mass or odd atomic number

have the property of nuclear spin.

The number of spin states is 2I + 1,

where I is the spin quantum number.

The most abundant isotopes of C and O do not have spin.

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NUCLEAR SPIN STATES - HYDROGEN NUCLEUS

+ 1/2 - 1/2

The two states

are equivalent

in energy in theabsence of a

magnetic or an

electric field.

+ +

The spin of the positively

charged nucleus generates

a magnetic moment vector, Q.Q

Q

TWO SPIN STATES

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When a charged particle such as a proton spins on its axis, itcreates 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 B0, they

are oriented with or against this applied field.

The energy difference between these two states is very small

(


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External Magnetic FieldWhen placed in an external field, spinning

protons act like bar magnets.

=>


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THE RESONANCE PHENOMENONTHE RESONANCE PHENOMENON

absorption of energy by the

spinning nucleus

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Nuclear Spin Energy LevelsNuclear Spin Energy Levels

Bo

+1/2

-1/2

In a strongmagnetic field (Bo)

the two spin states

differ in energy.

aligned

unaligned

N

S 10PVRSK


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Absorption of EnergyAbsorption of Energy

Bo

+1/2

-1/2

+1/2

-1/2

(E = hR

(E

quantized

Radiofrequency

Applied

Field

Aligned

Opposed

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Bo

(E

+ 1/2

- 1/2

= kBo = hR

degenerate

at Bo = 0

increasing magnetic field strength

THE ENERGY SEPARATIONTHE ENERGY SEPARATION

DEPENDS ON BDEPENDS ON Boo

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TheThe LarmorLarmor Equation!!!Equation!!!

B0

R!

T

KR !

T

Bo

K is a constant which is different foreach atomic nucleus (H, C, N, etc)

(E = kBo = hR can be transformed into

gyromagnetic

ratio K

strength of the

magnetic field

frequency of

the incoming

radiation that

will cause a

transition

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WHEN A SPIN-ACTIVE HYDROGEN ATOM IS

PLACED IN A STRONG MAGNETIC FIELD

.. IT BEGINS TO PRECESS

A SECOND EFFECT OF A STRONG MAGNETIC FIELDA SECOND EFFECT OF A STRONG MAGNETIC FIELD

OPERATION OF AN NMR SPECTROMETER DEPENDS

ON THIS RESULT

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N

S

[ Nuclei precess at

frequency[ when

placed in a strong

magnetic field.

hR

IfR = [ thenenergy will be

absorbed and

the spin will

invert.

NUCLEAR

MAGNETIC

RESONANCE

NMR

RADIOFREQUENCY

40 - 600 MHz

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1H 99.98% 1.00 42.6 267.53

1.41 60.0

2.35 100.0

7.05 300.0

2H 0.0156% 1.00 6.5 41.1

7.05 45.8

13C 1.108% 1.00 10.7 67.282.35 25.0

7.05 75.0

19F 100.0% 1.00 40.0 251.7

Resonance Frequencies of Selected NucleiResonance Frequencies of Selected Nuclei

Isotope Abundance Bo (Tesla) Frequency(MHz) K(radians/Tesla)

4:1

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The strength of the NMR signal depends on the

Population Difference of the two spin states

resonanceinduced

emission

excess

population

Radiation

induces both

upward and

downward

transitions.

For a net positive signalthere must be an excess

of spins in the lower state.

Saturation = equal populations = no signal

POPULATION AND SIGNAL STRENGTHPOPULATION AND SIGNAL STRENGTH

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Two Energy States

The magnetic fields ofthe spinning nucleiwill align eitherwith

the external field, oragainstthe field.

A photon with the rightamount of energy

can be absorbedand cause thespinning proton toflip. =>


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(Eand Magnet Strength Energy difference is proportional to the

magnetic field strength.

(E= hR = K h B0

2T

Gyromagnetic ratio, K, is a constant for

each nucleus (26,753 s-1gauss-1 for H).

In a 14,092 gauss field, a 60 MHzphoton is required to flip a proton.

Low energy, radio frequency. =>


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Magnetic Shielding

If all protons absorbed the same

amount of energy in a given magnetic

field, not much information could be

obtained.

But protons are surrounded by electrons

that shield them from the external field.

Circulating electrons create an inducedmagnetic field that opposes the external

magnetic field. =>


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Shielded Protons

Magnetic field strength must be increased

for a shielded proton to flip at the same

frequency.

=>


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Protons in a MoleculeDepending on their chemical

environment, protons in a molecule are

shielded by different amounts.

=>


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NMR Signals The numberof signals shows how many

different kinds of protons are present.

The location of the signals shows howshielded or deshielded the proton is.

The intensityof the signal shows the

number of protons of that type. Signal splittingshows the number of

protons on adjacent atoms. =>


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The NMR Spectrometer

=>


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The NMR Graph

=>


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Internal standards:

Tetramethylsilane(TMS)

TMS is added to the sample.

Since silicon is less electronegative than carbon, TMS protonsare highly shielded. Signal defined as zero.

It gives a single sharp line from twelve magnetically equivalentprotons.

It is chemically inert and miscible with a large range of solvents.

It is highly volatile and hence can be easily be removed if thesample has to be recovered.

It does not involve in intermolecular association with the sample. Organic protons absorb downfield (to the left) of the TMS signal.

=>

Si

CH3

CH3

CH3

H3C


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Internal standards

Sodium salt of 3-(trimethyl silyl)

propane sulphonate.

Used as an internal standard for water

soluble organic compounds in D2O

solvent.

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O

S

O

-O

Si

sodium 3-(trimethylsilyl) propane sulphonate

Na+


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NMR Solvents

CCl4 - Ideal Solvent (Aprotic And Cheap)

CDCl3 - Useful To Dissolve Varying Polarityi.e., From Non Polar To Considerable Polar

DMSO-d6 Dissolving Relatively InsolubleHeterocyclic Compounds . But Have Disadvantages

(Viscous, Mild Oxidising , Nonvolatile(140c), Affinity

With Water.)

CH3OH-d4 - Polar solvent for high polar compounds& salts .Its disadvantage is susceptibility to exchange

ionizable protons like -OH, -NH2 and CONH2, -CH2-CO- etc.

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NMR Solvents

D2O- high polar solvent limited in practice to salts only.Disadvantage : it exchanges all acidic protons readily.

C6D6- anisotropic specialised solvent .used in routinework.it is often added with CDCl3.

CF3COOH- for amines and heterocyclic compounds.

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Chemical Shift Measured in parts per million.

Ratio of shift downfield from TMS (Hz)

to total spectrometer frequency (Hz).

Same value for 60, 100, or 300 MHz

machine.

Called the delta scale.

=>


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Delta Scale

=>


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Location of Signals

More electronegative

atoms deshield more and

give larger shift values.

Effect decreases withdistance.

Additional electronegative

atoms cause increase in

chemical shift.

=>


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Typical Values

=>


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Aromatic Protons, H7-H8

=>


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Vinyl Protons, H5-H6

=>


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Acetylenic Protons, H2.5

=>


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Aldehyde Proton, H9-H10

=>

Electronegative

oxygen atom


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O-H and N-H Signals Chemical shift depends on concentration.

Hydrogen bonding in concentrated

solutions deshield the protons, so signal

is around H3.5 for N-H and H4.5 for O-H.

Proton exchanges between the molecules

broaden the peak.=>


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Carboxylic Acid

Proton, H10+

=>


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Number of Signals

Equivalent hydrogens have the samechemical shift.

=>


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Intensity of Signals

The area under each peak isproportional to the number of protons.

Shown by integral trace.

=>


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How Many Hydrogens?

When the molecular formula is known,each integral rise can be assigned to aparticular number of hydrogens.

=>


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Spin-Spin Splitting Nonequivalent protons on adjacent carbons

have magnetic fields that may align with or

oppose the external field.

This magnetic coupling causes the proton

to absorb slightly downfield when the

external field is reinforced and slightly

upfield when the external field is opposed.

All possibilities exist, so signal is split. =>


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1,1,2-Tribromoethane

Nonequivalent protons on adjacent carbons.

=>


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Doublet: 1 Adjacent Proton

=>


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Triplet: 2 Adjacent Protons

=>


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The N+ 1 Rule

If a signal is split by Nequivalent protons,

it is split into N+ 1 peaks.

=>


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Range of Magnetic

Coupling Equivalent protons do not split each other.

Protons bonded to the same carbon will

split each other only if they are notequivalent.

Protons on adjacent carbons normally willcouple.

Protons separated by four or more bondswill not couple.

=>


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Splitting for Ethyl Groups

=>


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Splitting for

Isopropyl Groups

=>


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Coupling Constants

When protons couple to each other, they do sowith a certain intensity. This is called the

coupling constant. Coupling constants can

vary from 0 Hz to 16 Hz.

Distance between the peaks of multiplet

Measured in Hz

Not dependent on strength of the external field

Multiplets with the same coupling constants maycome from adjacent groups of protons that split

each other.

=>

V l f


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Values for

Coupling Constants

=>


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Complex Splitting

Signals may be split by adjacent

protons, different from each other, withdifferent coupling constants.

Example: Ha of styrene which is split by

an adjacent H trans to it (J = 17 Hz) andan adjacent H cis to it (J = 11 Hz).

=>

C C

H

H

Ha

b

c


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Splitting TreeC C

H

H

Ha

b

c

=>


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Spectrum for Styrene

=>


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Stereochemical

Nonequivalence Usually, two protons on the same C are

equivalent and do not split each other.

If the replacement of each of the protons of

a -CH2 group with an imaginary Z gives

stereoisomers, then the protons are non-

equivalent and will split each other.=>


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Some Nonequivalent

Protons

C C

H

H

Ha

b

cOH

H

H

H

a

b

c

d

CH3

H Cl

H H

Cl

a b=>


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Time Dependence

Molecules are tumbling relative to themagnetic field, so NMR is an averaged

spectrum of all the orientations.

Axial and equatorial protons oncyclohexane interconvert so rapidly that

they give a single signal.

Proton transfers for OH and NH may occur

so quickly that the proton is not split by

adjacent protons in the molecule.

=>


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Hydroxyl

Proton Ultrapure samples

of ethanol show

splitting. Ethanol with a small

amount of acidic or

basic impurities will

not show splitting.

=>


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N-H Proton

Moderate rate of exchange.

Peak may be broad.

=>


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Identifying the O-H

or N-H Peak Chemical shift will depend onconcentration and solvent.

To verify that a particular peak is due toO-H or N-H, shake the sample with D2O

Deuterium will exchange with the O-H

or N-H protons.

On a second NMR spectrum the peak

will be absent, or much less intense.

=>


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Fourier Transform NMR

Nuclei in a magnetic field are given aradio-frequency pulse close to their

resonance frequency.

The nuclei absorb energy and precess(spin) like little tops.

A complex signal is produced, then

decays as the nuclei lose energy.

Free induction decay is converted to

spectrum. =>


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Hydrogen and Carbon

Chemical Shifts

=>


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MRI

Magnetic resonance imaging, noninvasive

Nuclear is omitted because of publics

fear that it would be radioactive. Only protons in one plane can be in

resonance at one time.

Computer puts together slices to get 3D. Tumors readily detected.

=>


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THANK YOU

AND PLESE REFER TEXTBOOKS

1)PAVIA ,LAMPMAN &KRIZ2)SILVERSTEIN

3)WILLAM KEMP

4)JAGMOHAN5)P.S.KALSI

pvrsk nmr presentation m.sc(previous) chemistry

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