Principles of Molecular Spectroscopy: Electromagnetic ... · Molecular structure Nuclear Magnetic Resonance (NMR) Electromagnetic radiation is absorbed when the energy of photon corresponds

Principles of MolecularPrinciples of Molecular

Spectroscopy:Spectroscopy:

Electromagnetic Radiation andElectromagnetic Radiation and

Molecular structureMolecular structure

Nuclear Magnetic Resonance (NMR)Nuclear Magnetic Resonance (NMR)

Electromagnetic radiation is absorbed when theElectromagnetic radiation is absorbed when the

energy of photon corresponds to difference in energy of photon corresponds to difference in

energy between two states.energy between two states.

!!E = E = hh""

electronicelectronic

vibrationalvibrational

rotationalrotational

nuclear spinnuclear spin

UV-VisUV-Vis

infraredinfrared

microwavemicrowave

radiofrequencyradiofrequency

What Kind of States?What Kind of States?

NMR is concerned with change in the direction ofNMR is concerned with change in the direction of

spin orientation as the result of the absorption ofspin orientation as the result of the absorption of

radiofrequency radiation.radiofrequency radiation.

11H and H and 1313CC

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

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

1313C is 1.1% at natural abundanceC is 1.1% at natural abundance

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

organic chemists are:organic chemists are:

Nuclear SpinNuclear Spin

A spinning charge, such as the nucleus of A spinning charge, such as the nucleus of 11HH

or or 1313C, generates a C, generates a magnetic fieldmagnetic field. The. The

magnetic field magnetic field generated by a nucleus of spingenerated by a nucleus of spin

+1/2 is opposite in direction from that+1/2 is opposite in direction from that

generated by a nucleus of spin generated by a nucleus of spin ––1/2.1/2.

+ +

++

+

+

+

The distribution ofThe distribution of

nuclear spins isnuclear spins is

random in therandom in the

absence of anabsence of an

external magneticexternal magnetic

field.field.

++

+

+

+

An external magneticAn external magnetic

field causes nuclearfield causes nuclear

magnetic moments tomagnetic moments to

align parallel andalign parallel and

antiparallel antiparallel to appliedto applied

field.field.

HH00

++

+

+

+

There is a slightThere is a slight

excess of nuclearexcess of nuclear

magnetic momentsmagnetic moments

aligned parallel toaligned parallel to

the applied field.the applied field.

HH00

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

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

Energy Differences Between Nuclear Spin StatesEnergy Differences Between Nuclear Spin States

+

+

!!EE !!E E ''

increasing field strength, Hincreasing field strength, HZZ

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


The frequency (The frequency ("") of absorbed) of absorbed

electromagnetic radiationelectromagnetic radiation

is proportional tois proportional to

the energy difference (the energy difference (!!E) betweenE) between

two nuclear spin statestwo nuclear spin states

which is proportional towhich is proportional to

the applied magnetic field (Hthe applied magnetic field (H00))

UnitsUnits

Hz (sHz (s-1-1))

kJ/molkJ/mol

(kcal/mol)(kcal/mol)

tesla tesla (T)(T)


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 HFor a field strength of H00 = 4.7 T: = 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 s s-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 s s-1-1))

Compare to 10Compare to 101515 s s-1-1 for electrons; 10for electrons; 101313 s s-1-1 for vibrations for vibrations


The frequency of absorbed electromagneticThe frequency of absorbed electromagnetic

radiation for a particular nucleus (such as radiation for a particular nucleus (such as 11H orH or1313C) depends on the C) depends on the molecularmolecular environment of the environment of the

nucleus (the electronic environment).nucleus (the electronic environment).

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

for structure determination. The signals of differentfor structure determination. The signals of different

protons and carbon atoms in a molecule showprotons and carbon atoms in a molecule show

different signals, just like different functional groupsdifferent signals, just like different functional groups

showshow different signals in the IR.different signals in the IR.

Nuclear Shielding

and1H 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?"

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 field

within the molecule.within the molecule.CC HH

HH 00

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 field

within the molecule.within the molecule.

The direction of theThe direction of the

induced magnetic field isinduced magnetic field is

opposite to that of theopposite to that of the

applied field.applied field.

CC HH

HH 00

ShieldingShielding

The induced field shieldsThe induced field shields

the nuclei (in this case,the nuclei (in this case,1313C and C and 11H) from theH) from the

applied field.applied field.

A stronger external field isA stronger external field is

needed in order for energyneeded in order for energy

difference between spindifference between spin

states to match energy ofstates to match energy of

rf rf radiation.radiation.

CC HH

HH 00

Chemical ShiftChemical Shift

Chemical shift is aChemical shift is a

measure of the degree tomeasure of the degree to

which a nucleus in awhich a nucleus in a

molecule is shielded.molecule is shielded.

Protons in differentProtons in different

environments are shieldedenvironments are shielded

to greater or lesserto greater or lesser

degrees; they havedegrees; they have

different chemical shifts.different chemical shifts.

CC HH

HH 00

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)

HH00

01.02.03.04.05.06.07.08.09.010.0


## 7.28 ppm 7.28 ppm

HH CC

ClCl

ClCl

ClCl

HH00

Effects of Molecular Structure

on1H Chemical Shifts

protons in different environments experience

different degrees of shielding and have

different chemical shifts

Electronegative Electronegative substituents substituents decreasedecrease

the shielding of methyl groupsthe shielding of methyl groups

CHCH33FF ## 4.3 ppm 4.3 ppm

CHCH33OOCHCH33 ## 3.2 ppm 3.2 ppm

CHCH33NN(CH(CH33))22 ## 2.2 ppm 2.2 ppm

CHCH33CHCH33 ## 0.9 ppm 0.9 ppm

CHCH33SiSi(CH(CH33))33 ## 0.0 ppm 0.0 ppm

012345678910

CHCH33SiSi(CH(CH

33))33

CHCH33CHCH

33

CHCH33NN(CH(CH

33))22CHCH33FF

CHCH33OOCHCH

33

Effect is cumulativeEffect is cumulative

012345678910

CHCHClCl33 ## 7.3 ppm 7.3 ppm

CHCH22ClCl22 ## 5.3 ppm 5.3 ppm

CHCH33ClCl ## 3.1 ppm 3.1 ppm

CHCH33ClCl

CHCH22ClCl22

CHCHClCl33

Protons attached to spProtons attached to sp22 hybridized carbon hybridized carbon

are less shielded than those attachedare less shielded than those attached

to spto sp33 hybridized carbon hybridized carbon

012345678910

HH HH

HHHH

HH

HH

## 7.3 ppm 7.3 ppm

CC CC

HHHH

HH HH

## 5.3 ppm 5.3 ppm

CHCH33CHCH33

## 0.9 ppm 0.9 ppm

1. number of signals1. number of signals

2. their intensity (as measured by area 2. their intensity (as measured by area

under peak)under peak)

3. splitting pattern (multiplicity)3. splitting pattern (multiplicity)

Information contained in an NMRInformation contained in an NMR

spectrum includes:spectrum includes:

We shall not consider spin-spin splittingWe shall not consider spin-spin splitting

01.02.03.04.05.06.07.08.09.010.0


CCCCHH22OCOCHH33NN

OCOCHH33

NCCNCCHH22OO

Number of SignalsNumber of Signals

protons that have differentprotons that have different

chemical shifts are chemicallychemical shifts are chemically

nonequivalent and existnonequivalent and exist inin

chemically different molecularchemically different molecular

environmentsenvironments

HH00

11H and H and 1313C NMR compared:C NMR compared:

both give us information about the environment of theboth give us information about the environment of the

nuclei (hybridization state, attached atoms, etc.)nuclei (hybridization state, attached atoms, etc.)

both give us information about the number ofboth give us information about the number of

chemically nonequivalent nuclei (nonequivalentchemically nonequivalent nuclei (nonequivalent

hydrogens or nonequivalent carbons)hydrogens or nonequivalent carbons)

ClClCCHH22CHCH22CHCH22CHCH22CCHH33

CCHH33ClClCCHH22


01.02.03.04.05.06.07.08.09.010.0

11HH HH00

??

1H NMR cannot1H NMR cannot

distinguish two of the CHdistinguish two of the CH22

groups (Cgroups (C22 and C and C44))


ClClCHCH22CHCH22CHCH22CHCH22CHCH33

020406080100120140160180200

1313CC

CDClCDCl33

a separate, distinct

peak appears for

each of the 5 carbons

SolventSolvent

HH00

1313C Chemical shifts are most affected by:C Chemical shifts are most affected by:

•• hybridization state of carbonhybridization state of carbon

•• electronegativity of groups attached to carbonelectronegativity of groups attached to carbon

020406080100120140160180200220

Increasing electronegativityIncreasing electronegativity

HH00

020406080100120140160180200220

OOHH6161

2323

138138

spsp22 hybridized carbon is hybridized carbon is atat lower field than lower field than spsp33

As the atom attached to a carbon becomes moreAs the atom attached to a carbon becomes more

electronegative, the carbon atom signalelectronegative, the carbon atom signal isis observedobserved

at lower fieldat lower field

2323

OO

202202

HH00

020406080100120140160180200220

Principles of Molecular Spectroscopy: Electromagnetic ... · Molecular structure Nuclear Magnetic Resonance (NMR) Electromagnetic radiation is absorbed when the energy of photon corresponds

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