nmr-i-1h-05 organic chemistry

8/3/2019 nmr-I-1H-05 Organic Chemistry

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Spectroscopy:Spectroscopy:

Nuclear Magnetic ResonanceNuclear Magnetic Resonance


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electronicelectronic

vibrationalvibrational

rotationalrotational

nuclear spinnuclear spin

UV-VisUV-Vis

InfraredInfrared

microwavemicrowave

Radiofrequency (NMR)Radiofrequency (NMR)

Spectroscopic Energy StatesSpectroscopic Energy States


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An NMR Spectrometer


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11H andH and 1313CC

both have spin = 1/2both have spin = 1/2

11H is 99% at natural abundanceH is 99% at natural abundance

1313

C is 1.1% at natural abundanceC is 1.1% at natural abundance

The nuclei that are most useful toThe nuclei that are most useful to

organic chemists are:organic chemists are:


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Nuclear SpinNuclear Spin

A spinning charge, such as the nucleus ofA spinning charge, such as the nucleus of11H orH or1313C,C,

generates agenerates a magnetic fieldmagnetic field. The. The magnetic fieldmagnetic fieldgenerated by a nucleus of spin +1/2 is opposite ingenerated by a nucleus of spin +1/2 is opposite in

direction from that generated by a nucleus of spin direction from that generated by a nucleus of spin

1/2.1/2.

+ +


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++

+

+

+

The distribution ofThe distribution of

nuclear spins isnuclear spins is

random in therandom in theabsence of anabsence of an

external magneticexternal magnetic

field.field.


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++

+

+

+

An external magneticAn external magnetic

field causes nuclearfield causes nuclear

magnetic moments tomagnetic moments to

align parallel andalign parallel and

antiparallel to appliedantiparallel to applied

field.field.

HH00


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++

+

+

+

There is a slightThere is a slight

excess of nuclearexcess of nuclear

magnetic momentsmagnetic momentsaligned parallel toaligned parallel to

the applied field.the applied field.

HH00


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no difference in absence of magnetic fieldno difference in absence of magnetic field

proportional to strength of external magnetic fieldproportional to strength of external magnetic field

Energy Differences Between Nuclear Spin StatesEnergy Differences Between Nuclear Spin States

+

+

EE EE''

increasing field strengthincreasing field strength


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The energy difference between the two spin states

depends on the strength of the magnetic field


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-spin states -spin states

absorb E

release E

Signals detected by NMR


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Some important relationships in NMRSome important relationships in NMR

The frequency of absorbedThe frequency of absorbed

electromagnetic radiationelectromagnetic radiation

is proportional tois proportional to

the energy difference betweenthe energy difference between

two nuclear spin statestwo nuclear spin states

which is proportional towhich is proportional to

the applied magnetic fieldthe applied magnetic field


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The frequency of absorbedThe frequency of absorbed

electromagnetic radiationelectromagnetic radiation

is proportional tois proportional to

the energy difference betweenthe energy difference between

two nuclear spin statestwo nuclear spin states

which is proportional towhich is proportional to

the applied magnetic fieldthe applied magnetic field

UnitsUnits

HzHz

kJ/molkJ/mol

(kcal/mol)(kcal/mol)

tesla (T)tesla (T)


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The frequency of absorbed electromagneticThe frequency of absorbed electromagnetic

radiation is different for different elements,radiation is different for different elements,

and for different isotopes of the same element.and for different isotopes of the same element.

For a field strength of 4.7 T:For a field strength of 4.7 T:11H absorbs radiation having a frequencyH absorbs radiation having a frequency

of 200 MHz (200 x 10of 200 MHz (200 x 1066 ss-1-1))1313C absorbs radiation having a frequencyC absorbs radiation having a frequency

of 50.4 MHz (50.4 x 10of 50.4 MHz (50.4 x 1066 ss-1-1))


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The frequency of absorbed electromagneticThe frequency of absorbed electromagnetic

radiation for a particular nucleus (such asradiation for a particular nucleus (such as 11H)H)

depends on its molecular environment.depends on its molecular environment.

This is why NMR is such a useful toolThis is why NMR is such a useful tool

for structure determination and medical diagnoses.for structure determination and medical diagnoses.

QuickTime and aGIF decompressor

are needed to see this picture.


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Nuclear Shieldingand

1H Chemical Shifts

What do we mean by "shielding?"What do we mean by "shielding?"

What do we mean by "chemical shift?"What do we mean by "chemical shift?"


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The electrons surrounding a nucleus affect the effective

magnetic field sensed by the nucleus


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ShieldingShielding

An external magnetic fieldAn external magnetic field

affects the motion of theaffects the motion of the

electrons in a molecule,electrons in a molecule,

inducing a magnetic fieldinducing a magnetic fieldwithin the molecule.within the molecule.

CC HH

HH00


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ShieldingShielding

An external magnetic fieldAn external magnetic field

affects the motion of theaffects the motion of the

electrons in a molecule,electrons in a molecule,

inducing a magnetic field withininducing a magnetic field within

the molecule.the molecule.

The direction of the inducedThe direction of the induced

magnetic field is opposite tomagnetic field is opposite to

that of the applied field.that of the applied field.

CC HH

HH00


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ShieldingShielding

The induced field shields theThe induced field shields the

nuclei (in this case, C and H)nuclei (in this case, C and H)

from the applied field.from the applied field.

A stronger external field isA stronger external field isneeded in order for energyneeded in order for energy

difference between spin statesdifference between spin states

to match energy of rf radiation.to match energy of rf radiation.

CC HH

HH00


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Chemical ShiftChemical Shift

Chemical shift is a measure ofChemical shift is a measure of

the degree to which a nucleusthe degree to which a nucleus

in a molecule is shielded.in a molecule is shielded.

Protons in differentProtons in differentenvironments are shielded toenvironments are shielded to

greater or lesser degrees;greater or lesser degrees;

they have different chemicalthey have different chemical

shifts.shifts.

CC HH

HH00


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01.02.03.04.05.06.07.08.09.010.0

Chemical shift (Chemical shift ( , ppm), ppm)

measured relative to TMSmeasured relative to TMS

UpfieldUpfield

Increased shieldingIncreased shielding

DownfieldDownfield

Decreased shieldingDecreased shielding

(CH(CH33))44Si (TMS)Si (TMS)


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Chemical Shift

The reference point of an NMR spectrum is defined by

the position of TMS (zero ppm)

The chemical shift is a measure of how far the signal is

from the reference signal

he common scale for chemical shifts = (ppm)

=distance downfield from TMS (Hz)

operating frequency of the spectrometer (MHz)

Si CH3H3C

H3C

H3C

CH3


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01.02.03.04.05.06.07.08.09.010.0

Chemical shift (Chemical shift ( , ppm), ppm)

7.28 ppm7.28 ppmHH CC

ClCl

ClCl

ClCl


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Effects of Molecular Structure

on1

H Chemical Shifts

protons in different environments experience different

degrees of shielding and have different chemical

shifts


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The chemical shift is independent of the operating

frequency of the spectrometer


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Electron withdrawal produces NMR signals downfield

athigher frequency (at larger values)


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Electronegative substituents decreaseElectronegative substituents decrease

the shielding of methyl groupsthe shielding of methyl groups

CHCH33FF 4.3 ppm4.3 ppm

CHCH33OOCHCH33 3.2 ppm3.2 ppmCHCH33NN(CH(CH33))22 2.2 ppm2.2 ppm

CHCH33CHCH33 0.9 ppm0.9 ppm

CHCH33SiSi(CH(CH33))33 0.0 ppm0.0 ppm


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Electronegative substituents decreaseElectronegative substituents decrease

the shielding of methyl groupsthe shielding of methyl groups

CHCH33FF 4.3 ppm4.3 ppm least shielded Hleast shielded H

CHCH33OOCHCH33 3.2 ppm3.2 ppmCHCH33NN(CH(CH33))22 2.2 ppm2.2 ppm

CHCH33CHCH33 0.9 ppm0.9 ppm

CHCH33SiSi(CH(CH33))33 0.0 ppm0.0 ppm most shielded Hmost shielded H


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Effect is cumulativeEffect is cumulative

CHCHClCl33

7.3 ppm7.3 ppm

CHCH22ClCl22 5.3 ppm5.3 ppm

CHCH33ClCl 3.1 ppm3.1 ppm


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Protons attached to spProtons attached to sp22 hybridized carbonhybridized carbon

are less shielded than those attachedare less shielded than those attached

to spto sp33 hybridized carbonhybridized carbon

HH HH

HHHH

HH

HH

CC CC

HHHH

HH HH

CHCH33CHCH33

7.3 ppm7.3 ppm 5.3 ppm5.3 ppm 0.9 ppm0.9 ppm

Di ti A i t


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Diamagnetic Anisotropy

The electrons are less tightly held by the nuclei than

are electrons; they are more free to move in responseto a magnetic field

Causes unusual chemical shifts for hydrogen bonded to

carbons that form bonds


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Chemical Shift TableChemical Shift Table

Type of protonType of proton Chemical shift (Chemical shift ( ),),

ppmppm


ppmppm

CCHH RR 0.9-1.80.9-1.8

1.6-2.61.6-2.6CCHH CCCC

2.1-2.52.1-2.5CCHH CC

OO

2.52.5CCHH CCCC

CCHH ArAr 2.3-2.82.3-2.8

CC CC

HH

4.5-6.54.5-6.5


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ppmppm


ppmppm

6.5-8.56.5-8.5

9-109-10

2.2-2.92.2-2.9

3.1-4.13.1-4.1

CCHH BrBr 2.7-4.12.7-4.1

3.3-3.73.3-3.7

HH ArAr

CC

OO

HH

CCHH NRNR

CCHH ClCl

CCHH OO


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ppmppm

1-31-3HH NRNR

0.5-50.5-5HH OROR

6-86-8HH OArOAr

10-1310-13CC

OO

HHOO


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Characteristic Values of

Chemical Shifts


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1H NMR spectrum / Integration


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1H NMR spectrum / Integration1-bromo-2,2-dimethylpropane


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The area under each signal is proportional to the numberof protons that give rise to that signal.

The height of each integration step is proportional to thearea under a specific signal.

The integration tells us the relative number of protons

that give rise to each signal, not absolute number.

Integration

1H NMR spectrum / Integration


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1H NMR spectrum / Integration1-bromo-2,2-dimethylpropane

11

4.54.5

(2)(2)

(9)(9)

nmr-i-1h-05 organic chemistry

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