Anal Chem Report 2- NMR

8/3/2019 Anal Chem Report 2- NMR

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NUCLEAR MAGNETIC

RESONANCE


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the most powerful tool for determining thestructure of organic compounds

it is called nuclear for the instrument workson stimulating the nuclei of the atoms to

absorb radio waves.

relies on the ability of atomic nuclei tobehave like a small magnet and to align withexternal magnetic field.


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The frequency of radiation

necessary for adsorption of

energy depends on three things:

1) characteristic of the type of

nucleus (e.g., 1H or 13C)

2) chemical environment of the

nucleus

3) spatial location in the magnetic

field


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- resultant of the spin and orbital angular

momenta of the neutrons and protons that

compose the nucleus

- characterized by a quantum number I, which

may be integral, half-integral or 0

Themagnetic quantum numbermI has

values of I,-I+1, ..+I


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1. A nucleus with an even mass A and even

charge Z -> nuclear spin I is zeroEx: 12C, 16O, 32S -> No NMR signal

2. A nucleus with an even mass A and oddcharge Z -> integer value I

Ex: 2H, 10B, 14N -> NMR detectable

3. A nucleus with odd mass A -> I=n/2, where nis an odd integer

Ex: 1H, 13C, 15N, 31P ->NMR detectab


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NUCLEUS %

ABUNDANCE

SPINI gN

1H 99.99 1/2 5.585

2H 0.01 1 0.857

7Li 92.5 3/2 2.171

13C 1.11 1.405

14N 99.6 1 0.403

15N 0.4 -0.567

17

O 0.04 5/2 -0.757


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Where: gn=nuclear g factor

The magnitude of the nuclear spin

angular momentum (I) isI = [ I ( I + 1 ) ] h

The z component of nuclear spin (Iz) is

given by

Iz = mih where mi = -I, -I+1,... I-1, I

total of (2I+1)

The magnetic dipole moment therefore

of the nucleus (n) is:

n = gnNI

Where: gn=nuclear g factor


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Boltzmann distribution

Quantum mechanics tells us that, for net

absorption of radiation to occur, there must be

more particles in the lower-energy state than in

the higher one. If no net absorption is possible, acondition called saturation occurs.

The rate of absorption is proportional to the

number of nuclei in lower state (Nl); rate of

emission is proportional to the number in upperstate (Nu).


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Nu = e-E/KT = e-gnNB/Kt

Nl= 1-gnNB

kT

It follows that

Nl Nu = gnNB

Nl+Nu 2kT


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Sample ProblemWhat is the ratio of the number of proton spins in the

lower state to the number in the higher state in the

magnetic field of1T at room temperature? What is the

excess population in the lower state?

Nl/Nu = 1 + (gnNB)= 1 + [(5.585)(5.05x10-27 J T-1)(1T)]/[(1.38x10-23J K1)

(298K)]

= 1 + 6.86x10-6

(N1- Nu)/(N1+ Nu) = 3.43 x 10-6


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Deriving the Boltzmann equation:

Taking the ideal gas equation

PV = nRT

Rearranging gives:

PV = (nNa)(R/NA)T

PV = (nNA)(kT)

Rearranging again gives:

PV = (nNA/V)(kT)


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- A nucleus with a magnetic moment may

interact with other nuclei spins resulting in

mutual splitting of NMR signal from each

nucleus into multiplets

- Splitting of resonances into peaks

Reference: http://bouman.che

m.georgetown.edu/nmr/scalar/scalar.htme


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A nuclear magnetic moment will precess about

the axis of an externally applied field at a frequency

proportional to the strength of the applied field, Bo.= Bo(rad/s)

= Bo/2 Larmor Frequency

The direction of motion can be clockwise orcounterclockwise and is determined by the sign of .

By convention, the field is applied along the z axis of a

Cartesian co-ordinate frame.


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Derivation ofLarmor Frequency Formula

Fromn=gnNI which is the maximum

observable component in any particular

allowed direction, and

E= - nH which is the potential

energy,


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By substituting the 1st equation to the 2nd, we

will get

E = -gnNHI

The difference in energy, E, between two levels

I1 and I2 follows:

E = E2 E1 = -gnNH(I2 I1)

But, selection rules limit transitions only to

adjacent levels, i.e., I = I2 I1 = +- 1.

Hence,


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But, selection rules limit transitions only toadjacent levels, i.e., I = I2 I1 = +- 1.

Hence, E = +-gnNH

Since E = hv, the frequency which

corresponds to a transition is

v = E /h = +- gnNH/h


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Chemical Shift

Describes the dependence of nuclear

magnetic energy levels on the electronic

environment in a molecule.

The variations of nuclear magnetic

resonance frequencies of the same kind of

nucleus, due to variations in the electron

distribution


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The study of chemical shifts has

produced a large store of information

about the chemical bonds and thestructure of molecules.

the chemical shift is usually indicatedby a symbol which is defined in

terms of a standard reference.

Vr

VrVs610)( v

!H


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The signal shift is very small, parts per million,

but the great precision with which frequenciescan bemeasured permits the determination of

chemical shift. The reference material is often

tetramethylsilane,Si(CH3)4, abbreviated TMS.

Since the signal frequency is related to the

shielding by:


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Chemical shift can be

expressed by:


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It consists of a magnet that can produce auniform, intense field and the appropriate

sources of radio freqeuncy electromagnetic

radiation.

The magnetic field is provided by a

permanent magnet


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a superconductingmagnet capable

of producing fields of the order of

2T andmore is used

sample is usually operated at roomtemperature eventhough a

superconductingmagnet operates at

the temperature liquid helium (4K)


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Why can't I lock the spectrometer?

Locking problems can be caused by

incorrectly set lock parameters (Z0, loc

power, lock phase, lock gain) and/or by very

badly adjusted shims.

Make sure that you are using a deuterated

solvent and have selected the correct solventunder the setupmenu.


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C R f I bilit t L k


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Common Reasons for Inability to Lock

Cause Result Solution

No deuteratedsolvent

No deuterium signal to lock on.Use a deuterated solventlike CDCl3.

Shimming isvery poor

The signal is so broad that it is not wellobserved.

Type fixshims to get a goodstart.

Suspendedparticles

This causes line broadening, whichmakes the signal intensity weaker.

Filter your sample.

Too

concentrated

There is little deuterated solvent to get

a lock. Especially problematic when usingCDCl3.

Dilute your sample or use a

solvent with moredeuteriums (e.g. C6D6).

Improperlygauged

When injected, the sample is not in thecoil.

Eject sample and ensurethat the sample resides

between the white lines on

the gauge.

Lockpower istoo low

The sample is not receiving enough rfsignal to flip the deuterium spins.

Increase lockpower to 80%of max before trying to

lock.


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This is usually a result of a failure in the acquisition

computer.

Look at the ACQUISITION STATUS window and see if the

Status is Idle, Acquiring, or Inactive.

If it is Acquiring, type aa. If you get the message similar

to,'no experiment is active', you will need to reboot the

acquisition computer.If it is Inactive, you will need to reboot the acquisition

computer.


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Poor Shimming:Common Reasons for Poor Shimming

Cause Result Solution

No deuterated

solvent

No deuterium signal to lock on and, thus

nothing to shim on.

Use a deuterated solvent like

CDCl3.

Initial shimming is

poorYour peaks are very broad.

Type fixshims to get a good

start.

Suspendedparticles This causes line broadening, which makesthe signal intensity weaker. Filter your sample.

Too concentratedConcentrated samples can be viscous,

which will cause line broadening.Dilute your sample.

Too little solvent

When injected, the sample is not properly

positioned in the coil and the liquid/air

interface is 'seen' by the coil. This will makeshimming very difficult.

Add more solvent.

Improperly

gauged

When injected, the sample is not properly

positioned in the coil and the liquid/air

interface is 'seen' by the coil. This will make

shimming very difficult.

Eject sample and ensure that

the sample resides between

the white lines on the gauge.


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My Sample is not Spinning

The VNMR Screen Colors are Terrible

The odd colors for your VNMR interface are

usually a result of having too many

windows/applications open.


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CHEMISTRY

DETERMINING THE STRUCTURE OF COMPOUNDS

NUCLEAR MAGNETIC RESONANCE ISEXTREMELYUSEFUL FOR ANALYZINGSAMPLE NON DESTRUCTIVELY


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APPLICATIONSOF NMR IN

MEDICINE CLINICAL APPLICATION OF PROTON IMAGINGIN DIAGNOSIS

BRAIN

Distinguishing gray matter & white matter

Imaging posterior fossae, brain stem, spinalcord

Detect demyelinating lesions, tumors,hemorrhages, infarctions


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Anal Chem Report 2- NMR