Chapter 13 Spectroscopy Nuclear Magnetic Resonance Spectroscopy Infrared Spectroscopy Ultraviolet-Visible Spectroscopy Mass Spectrometry

Chapter 13Chapter 13SpectroscopySpectroscopy

Nuclear Magnetic Resonance Spectroscopy Nuclear Magnetic Resonance Spectroscopy

Infrared SpectroscopyInfrared Spectroscopy

Ultraviolet-Visible SpectroscopyUltraviolet-Visible Spectroscopy

Mass SpectrometryMass Spectrometry

13.113.1

Principles of Molecular Principles of Molecular

Spectroscopy:Spectroscopy:

Electromagnetic RadiationElectromagnetic Radiation

Propagated at the speed of light.Propagated at the speed of light.

Has properties of particles and waves.Has properties of particles and waves.

The energy of a photon is proportional The energy of a photon is proportional to its frequency.to its frequency.

Electromagnetic RadiationElectromagnetic Radiation

The Electromagnetic SpectrumThe Electromagnetic Spectrum

400 nm400 nm 750 nm750 nm

Visible Light

Longer Wavelength (Longer Wavelength ())Shorter Wavelength (Shorter Wavelength ())

Higher Frequency (Higher Frequency ()) Lower Frequency (Lower Frequency ())

Higher Energy (Higher Energy (EE)) Lower Energy (Lower Energy (EE))


UltravioletUltraviolet InfraredInfrared

Longer Wavelength (Longer Wavelength ())Shorter Wavelength (Shorter Wavelength ())

Higher Frequency (Higher Frequency ()) Lower Frequency (Lower Frequency ())

Higher Energy (Higher Energy (EE)) Lower Energy (Lower Energy (EE))

Cosmic raysCosmic rays

RaysRays

X-raysX-rays

Ultraviolet (UV) Ultraviolet (UV) lightlight

Visible lightVisible light

Infrared (IR) Infrared (IR) radiationradiation

MicrowavesMicrowaves

Radio wavesRadio waves

Cosmic raysCosmic rays

RaysRays

X-raysX-rays

Ultraviolet (UV) Ultraviolet (UV) lightlight

Visible lightVisible light

Infrared (IR) Infrared (IR) radiationradiation

MicrowavesMicrowaves

Radio wavesRadio waves


EnergyEnergy

13.213.2Principles of Molecular Spectroscopy: Principles of Molecular Spectroscopy:

Quantized Energy StatesQuantized Energy States

Electromagnetic radiation is absorbed when theElectromagnetic radiation is absorbed when theenergy of the photon corresponds to the energy of the photon corresponds to the difference in energy between two states.difference in energy between two states.

EE = = hh

ElectronicElectronic

VibrationalVibrational

RotationalRotational

Nuclear spinNuclear spin

UV-VisibleUV-Visible

InfraredInfrared

MicrowaveMicrowave

RadiofrequencyRadiofrequency

What Kind of States?What Kind of States?

13.313.3Introduction to Introduction to

11H NMR SpectroscopyH NMR Spectroscopy

11H and H and 1313C:C:

Both have spin = ±1/2.Both have spin = ±1/2.

11H is 99.985% at natural abundance.H is 99.985% at natural abundance.

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

The nuclei that are most useful toThe nuclei that are most useful toorganic chemists are:organic chemists are:

Nuclear SpinNuclear Spin

A spinning charge, such as the nucleus of A spinning charge, such as the nucleus of 11H H or or 1313C, generates a C, generates a magnetic fieldmagnetic field. The . The magnetic field magnetic field generated by a nucleus of spin generated 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 –1/2.generated by a nucleus of spin –1/2.

+ +

++

+

+

+

The distribution of The distribution of nuclear spins is nuclear spins is random in the random in the absence of an absence of an external magnetic external magnetic field.field.

++

+

+

+

An external magnetic An external magnetic field causes nuclear field causes nuclear magnetic moments to magnetic moments to align parallel and align parallel and antiparallel to applied antiparallel to applied field.field.

HH00

++

+

+

+There is a slight There is a slight excess of nuclear excess of nuclear magnetic moments magnetic moments aligned parallel to aligned parallel to the applied field.the applied field.

HH00

No difference in absence of magnetic field.No 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 strengthIncreasing field strength

Some Important Relationships in NMRSome Important Relationships in NMR

The frequency of absorbedThe frequency of absorbedelectromagnetic radiationelectromagnetic radiationis proportional to:is proportional to:

the energy difference the energy difference betweenbetweentwo nuclear spin states,two nuclear spin states,which is proportional to:which is proportional to:

the applied magnetic field.the applied magnetic field.

UnitsUnits

HzHz

kJ/molkJ/mol(kcal/mol)(kcal/mol)

tesla (T)tesla (T)


The frequency of absorbed electromagneticThe frequency of absorbed electromagneticradiation 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 frequencyof 200 MHz (200 x 10of 200 MHz (200 x 1066 s s-1-1))1313C absorbs radiation having a frequencyC absorbs radiation having a frequencyof 50.4 MHz (50.4 x 10of 50.4 MHz (50.4 x 1066 s s-1-1))


The frequency of absorbed electromagneticThe frequency of absorbed electromagneticradiation for a particular nucleus (such as radiation 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 toolfor structure determination.for structure determination.

13.413.4Nuclear ShieldingNuclear Shielding

andand11H Chemical ShiftsH 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 field An external magnetic field affects the motion of the affects the motion of the electrons in a molecule, electrons in a molecule, inducing a magnetic field inducing a magnetic field within the molecule.within the molecule.

The direction of the The direction of the induced magnetic field is induced magnetic field is opposite to that of the opposite to that of the applied field.applied field.

CC HH

HH 00

ShieldingShielding

The induced field shields The induced field shields the nuclei (in this case, C the nuclei (in this case, C and H) from the applied and H) from the applied field.field.

A stronger external field is A stronger external field is needed in order for energy needed in order for energy difference between spin difference between spin states to match energy of states to match energy of rf radiation.rf radiation.

CC HH

HH 00

Chemical shift is a Chemical shift is a measure of the degree to measure of the degree to which a nucleus in a which a nucleus in a molecule is shielded.molecule is shielded.

Protons in different Protons in different environments are shielded environments are shielded to greater or lesser to greater or lesser degrees; they have degrees; they have different chemical shifts.different chemical shifts.

CC HH

HH 00

Chemical ShiftChemical Shift

Chemical shifts (Chemical shifts () are ) are measured relative to the measured relative to the protons in protons in tetramethylsilane (TMS) tetramethylsilane (TMS) as a standard.as a standard.

SiSi CHCH33

CHCH33

CHCH33

HH33CC

==position of signal - position of TMS peakposition of signal - position of TMS peak

spectrometer frequencyspectrometer frequencyx 10x 1066


NMR SpectrometersNMR Spectrometers

01.02.03.04.05.06.07.08.09.010.0

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

measured relative to TMSmeasured relative to TMS

UpfieldUpfieldIncreased shieldingIncreased shielding

DownfieldDownfieldDecreased shieldingDecreased shielding

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

Example: The signal for the proton in chloroform Example: The signal for the proton in chloroform (HCCl(HCCl33) appears 1456 Hz downfield from TMS at ) appears 1456 Hz downfield from TMS at

a spectrometer frequency of 200 MHz.a spectrometer frequency of 200 MHz.

==position of signal - position of TMS peakposition of signal - position of TMS peak

spectrometer frequencyspectrometer frequencyx 10x 1066

==1456 Hz - 0 Hz1456 Hz - 0 Hz

200 x 10200 x 1066 Hz Hzx 10x 1066

= 7.28= 7.28


01.02.03.04.05.06.07.08.09.010.0


7.28 ppm7.28 ppm

HH CC

ClCl

ClCl

ClClChloroformChloroform

13.513.5Effects of Molecular StructureEffects of Molecular Structure

onon11H Chemical ShiftsH Chemical Shifts

Protons in different environments experience Protons in different environments experience different degrees of shielding and have different degrees of shielding and have

different chemical shifts.different chemical shifts.

Electronegative Substituents DecreaseElectronegative Substituents Decreasethe Shielding of Methyl Groupsthe Shielding of Methyl Groups

Least shielded HLeast shielded H Most shielded HMost shielded H

CHCH33FF CHCH33OOCHCH33 (CH(CH33))33NN CHCH33CHCH33 (CH(CH33))44SiSi

4.34.3 3.23.2 2.22.2 0.90.9 0.00.0

Electronegative Substituents Decrease ShieldingElectronegative Substituents Decrease Shielding

HH33C—CC—CHH22—C—CHH33

OO22N—CN—CHH22—C—CHH22—C—CHH33

0.90.9 0.90.9 1.31.3

1.01.0 4.34.3 2.02.0

Effect is CumulativeEffect is Cumulative

CHCH33ClCl 3.1 3.1

CHCH22ClCl22 5.3 5.3

CHCHClCl33 7.3 7.3

Methyl, Methylene and MethineMethyl, Methylene and Methine

CCHH33 more shielded than CH more shielded than CH22..CCHH22 more shielded than C more shielded than CHH..

HH33CC CC

CCHH33

CHCH33

HH

0.90.9

1.61.6

0.80.8

HH33CC CC

CCHH33

CHCH33

CCHH22

0.90.9

CHCH33

1.21.2

Protons Attached to Protons Attached to spsp22-hybridized Carbon-hybridized Carbonare Less Shielded than Those Attachedare Less Shielded than Those Attached

to to spsp33-hybridized Carbon-hybridized Carbon

HH HH

HHHH

HH

HH

CC CC

HHHH

HH HH

CHCH33CHCH33

7.37.3 5.35.3 0.90.9

But Protons Attached to But Protons Attached to spsp-hybridized Carbon-hybridized Carbonare More Shielded than Those Attachedare More Shielded than Those Attached

to to spsp22-hybridized Carbon-hybridized Carbon

CC CC

HHHH

HH HH

5.35.3

2.42.4CHCH22OCHOCH33CC CCHH

Protons Attached to Benzylic and AllylicProtons Attached to Benzylic and AllylicCarbons are Somewhat Less Shielded than UsualCarbons are Somewhat Less Shielded than Usual

1.51.5 0.80.8

HH33CC CHCH33

1.21.2

HH33CC CHCH22

2.62.6

HH33C—CHC—CH22—CH—CH33

0.90.9 0.90.9 1.31.3

Proton Attached to C=O of AldehydeProton Attached to C=O of Aldehydeis Most Deshielded C—His Most Deshielded C—H

2.42.4

9.79.7

1.11.1

CC CC

OO

HH

HH

CHCH33

HH33CC

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


CCHH RR 0.9-1.80.9-1.8

1.5-2.61.5-2.6CCHH CCCC

2.0-2.52.0-2.5CCHH CC

OO

2.1-2.32.1-2.3CCHH NNCC

CCHH ArAr 2.3-2.82.3-2.8

2.52.5CCHH CCCC

11H Chemical Shifts of Some Common GroupsH Chemical Shifts of Some Common Groups

CCHH BrBr 2.7-4.12.7-4.1

9-109-10CC

OO

HH

2.2-2.92.2-2.9CCHH NRNR

3.1-4.13.1-4.1CCHH ClCl

6.5-8.56.5-8.5HH ArAr

CC CC

HH

4.5-6.54.5-6.5

3.3-3.73.3-3.7CCHH OO




1-31-3HH NRNR

0.5-50.5-5HH OROR

6-86-8HH OArOAr

10-1310-13CC

OO

HHOO



13.613.6

Interpreting Proton NMR Interpreting Proton NMR

SpectraSpectra

1. Number of signals.1. 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 NMRspectrum includes:spectrum includes:

Number of SignalsNumber of Signals

Protons that have different chemical shifts Protons that have different chemical shifts are chemically nonequivalent.are chemically nonequivalent.

Exist in different molecular environment.Exist in different molecular environment.

01.02.03.04.05.06.07.08.09.010.0


CCCCHH22OCOCHH33NN

OCOCHH33

NCCNCCHH22OO

MethoxyacetonitrileMethoxyacetonitrile

Are in identical environments.Are in identical environments.

Have same chemical shift.Have same chemical shift.

Replacement test: replacement by some Replacement test: replacement by some arbitrary "test group" generates same compound.arbitrary "test group" generates same compound.

HH33CCHCCH22CCHH33

chemically equivalentchemically equivalent

Chemically Equivalent ProtonsChemically Equivalent Protons

HH33CCHCCH22CCHH33

Chemically equivalentChemically equivalent

CCHH33CHCH22CCHH22ClClClClCCHH22CHCH22CCHH33

Chemically Equivalent ProtonsChemically Equivalent Protons

Replacing protons at C-1 and C-3 gives same Replacing protons at C-1 and C-3 gives same compound (1-chloropropane).compound (1-chloropropane).

C-1 and C-3 protons are chemically C-1 and C-3 protons are chemically equivalent and have the same chemical shift.equivalent and have the same chemical shift.

Replacement by some arbitrary test group Replacement by some arbitrary test group generates diastereomers.generates diastereomers.

Diastereotopic protons can have differentDiastereotopic protons can have differentchemical shifts.chemical shifts.

Diastereotopic ProtonsDiastereotopic Protons

CC CC

BrBr

HH33CC

HH

HH

5.3 ppm5.3 ppm

5.5 ppm5.5 ppm

Are in mirror-image environments.Are in mirror-image environments.

Replacement by some arbitrary test group Replacement by some arbitrary test group generates enantiomers.generates enantiomers.

Enantiotopic protons have the sameEnantiotopic protons have the samechemical shift.chemical shift.

Enantiotopic ProtonsEnantiotopic Protons

CC CHCH22OHOH

HH33CC

HHHH

EnantiotopicEnantiotopicProtonsProtons

CC CHCH22OHOH

HH33CC

ClClHH

CC CHCH22OHOH

HH33CC

HHClCl

RR SS

Not all peaks are singlets.Not all peaks are singlets.

Signals can be split by coupling of Signals can be split by coupling of nuclear spins.nuclear spins.

13.713.7Spin-Spin SplittingSpin-Spin Splitting

ininNMR SpectroscopyNMR Spectroscopy

01.02.03.04.05.06.07.08.09.010.0


ClCl22CCHHCCHH331,1-Dicholoroethane1,1-Dicholoroethane

4 lines;4 lines;quartetquartet

2 lines;2 lines;doubletdoublet

CCHH33CCHH

Two-Bond and Three-Bond CouplingTwo-Bond and Three-Bond Coupling

CC CC

HH

HH

CC CC HHHH

Protons separated byProtons separated bytwo bondstwo bonds

(geminal relationship).(geminal relationship).

Protons separated byProtons separated bythree bondsthree bonds

(vicinal relationship).(vicinal relationship).

In order to observe splitting, protons cannot In order to observe splitting, protons cannot have same chemical shift.have same chemical shift.

Coupling constant (Coupling constant (22JJ or or 33JJ) is independent ) is independent of field strength and are measured in Hz.of field strength and are measured in Hz.

Two-Bond and Three-Bond CouplingTwo-Bond and Three-Bond Coupling

CC CC

HH

HH

CC CC HHHH

01.02.03.04.05.06.07.08.09.010.0


ClCl22CCHHCCHH331,1-Dicholoroethane1,1-Dicholoroethane



CCHH33CCHH

Coupled protons are vicinal (three-bond coupling).Coupled protons are vicinal (three-bond coupling).

CCHH splits C splits CHH33 into a doublet. into a doublet.

CCHH33 splits C splits CHH into a quartet. into a quartet.

Why do the methyl protons ofWhy do the methyl protons of1,1-dichloroethane appear as a doublet?1,1-dichloroethane appear as a doublet?

CC CC HHHH

ClCl

ClCl

HH

HHSignal for Signal for methylmethyl protons is split into protons is split into a doublet.a doublet.

To explain the splitting of the To explain the splitting of the protonsprotons at C-2, at C-2, we first focus on the two possible spin we first focus on the two possible spin orientations of the orientations of the protonproton at C-1. at C-1.


CC CC HHHH

ClCl

ClCl

HH


There are two orientations of the nuclear spin There are two orientations of the nuclear spin for the proton at C-1. One orientation shields for the proton at C-1. One orientation shields the protons at C-2; the other deshields the C-the protons at C-2; the other deshields the C-2 protons.2 protons.


CC CC HHHH

ClCl

ClCl

HH


The protons at C-2 “feel” the effect of both the The protons at C-2 “feel” the effect of both the applied magnetic field and the local field applied magnetic field and the local field resulting from the spin of the C-1 proton.resulting from the spin of the C-1 proton.


CC CC HHHH

ClCl

ClCl

HH

HH““True” chemicalTrue” chemical

shift of methylshift of methyl

protons (no coupling).protons (no coupling).

This line correspondsThis line corresponds

to molecules in which to molecules in which

the nuclear spin of the nuclear spin of

the proton at C-1 the proton at C-1

reinforcesreinforces

the applied field.the applied field.

This line correspondsThis line corresponds

to molecules in which to molecules in which

the nuclear spin of the nuclear spin of

the proton at C-1 the proton at C-1

opposesopposes

the applied field.the applied field.

Why does the methine proton ofWhy does the methine proton of1,1-dichloroethane appear as a quartet?1,1-dichloroethane appear as a quartet?

CC CC HHHH

ClCl

ClCl

HH

HHSignal for Signal for methinemethine proton is split into proton is split into a quartet.a quartet.

The The protonproton at C-1 “feels” the effect of the at C-1 “feels” the effect of the applied magnetic field and the local fields applied magnetic field and the local fields resulting from the spin states of the three resulting from the spin states of the three methyl protons. The possible combinations methyl protons. The possible combinations are shown on the next slide.are shown on the next slide.

CC CC HHHH

ClCl

ClCl

HH

HHThere are eight combinations of There are eight combinations of nuclear spins for the three methyl nuclear spins for the three methyl protons.protons.

These 8 combinations split the These 8 combinations split the signal into a 1:3:3:1 quartet.signal into a 1:3:3:1 quartet.

Why does the methine proton ofWhy does the methine proton of1,1-dichloroethane appear as a quartet?1,1-dichloroethane appear as a quartet?

For simple cases, the multiplicity of a signalFor simple cases, the multiplicity of a signalfor a particular proton is equal to the number for a particular proton is equal to the number of equivalent vicinal protons + 1.of equivalent vicinal protons + 1.

The Splitting Rule for The Splitting Rule for 11H NMRH NMR

13.813.8Splitting Patterns:Splitting Patterns:The Ethyl GroupThe Ethyl Group

CHCH33CHCH22X is characterized by a triplet-quartet X is characterized by a triplet-quartet

pattern (quartet at lower field than the triplet).pattern (quartet at lower field than the triplet).

01.02.03.04.05.06.07.08.09.010.0


BrCBrCHH22CCHH33Ethyl bromideEthyl bromide


3 lines;3 lines;triplettriplet

CCHH33

CCHH22

Splitting Patterns of Common MultipletsSplitting Patterns of Common Multiplets

Splitting Patterns of Common MultipletsSplitting Patterns of Common Multiplets

Number of equivalentNumber of equivalent AppearanceAppearance Intensities of linesIntensities of linesprotons to which H protons to which H of multipletof multiplet in multipletin multipletis coupledis coupled

11 DoubletDoublet 1:11:1

22 TripletTriplet 1:2:11:2:1

33 QuartetQuartet 1:3:3:11:3:3:1

44 PentetPentet 1:4:6:4:11:4:6:4:1

55 SextetSextet 1:5:10:10:5:11:5:10:10:5:1

66 SeptetSeptet 1:6:15:20:15:6:11:6:15:20:15:6:1

13.913.9Splitting Patterns:Splitting Patterns:

The Isopropyl GroupThe Isopropyl Group

(CH(CH33))22CHX is characterized by a doublet-CHX is characterized by a doublet-septet pattern (septet at lower field than the septet pattern (septet at lower field than the doublet).doublet).

01.02.03.04.05.06.07.08.09.010.0


ClCClCHH(C(CHH33))22Isopropyl chlorideIsopropyl chloride

7 lines;7 lines;septetseptet


CCHH33

CCHH

13.1013.10Splitting Patterns:Splitting Patterns:Pairs of DoubletsPairs of Doublets

Splitting patterns are not always symmetrical, Splitting patterns are not always symmetrical, but lean in one direction or the other.but lean in one direction or the other.

Pairs of DoubletsPairs of Doublets

Consider coupling between two vicinal Consider coupling between two vicinal protons.protons.

If the protons have different chemical shifts, If the protons have different chemical shifts, each will split the signal of the other into a each will split the signal of the other into a doublet.doublet.

CC CCHH HH

Pairs of DoubletsPairs of Doublets

Let Let be the difference in chemical shift in be the difference in chemical shift in Hz between the two protons.Hz between the two protons.

Let Let JJ be the coupling constant between peaks be the coupling constant between peaks for each proton in Hz.for each proton in Hz.

CC CCHH HH

AXAX

When When is much larger than is much larger than JJ the signal for the signal for each proton is a doublet, the doublet is each proton is a doublet, the doublet is symmetrical, and the spin system is called symmetrical, and the spin system is called AX.AX.

CC CCHH HH

JJ JJ

AMAM

As As //JJ decreases the signal for each proton decreases the signal for each proton remains a doublet, but becomes skewed. The outer remains a doublet, but becomes skewed. The outer lines decrease while the inner lines increase, lines decrease while the inner lines increase, causing the doublets to "lean" toward each other.causing the doublets to "lean" toward each other.

CC CCHH HH

JJ JJ

ABAB

When When and and JJ are similar, the spin system is are similar, the spin system is called AB. Skewing is quite pronounced. It is called AB. Skewing is quite pronounced. It is easy to mistake an AB system of two doublets easy to mistake an AB system of two doublets for a quartet.for a quartet.

CC CCHH HH

JJ JJ

AA22

When When = 0, the two protons have the same = 0, the two protons have the same chemical shift and don't split each other. A chemical shift and don't split each other. A single line is observed. The two doublets have single line is observed. The two doublets have collapsed to a singlet.collapsed to a singlet.

CC CCHH HH

01.02.03.04.05.06.07.08.09.010.0


2,3,4-Trichloroanisole2,3,4-Trichloroanisole(1,2,3-Trichloro-4-methoxybenzene)(1,2,3-Trichloro-4-methoxybenzene)

OCOCHH33

Skewed doubletsSkewed doublets

HH HH

ClClClCl

ClCl OCOCHH33

13.1113.11Complex Splitting PatternsComplex Splitting Patterns

Multiplets of multipletsMultiplets of multiplets

mm-Nitrostyrene-Nitrostyrene

Consider the proton shown in Consider the proton shown in redred..

It is unequally coupled to the protons shown It is unequally coupled to the protons shown in in blueblue and and whitewhite..

JJciscis = 12 Hz; = 12 Hz; JJtranstrans = 16 Hz = 16 Hz

HH

HHOO22NN

HH


16 Hz16 Hz

12 Hz12 Hz 12 Hz12 Hz

The signal for The signal for the proton the proton shown in red shown in red appears as a appears as a doublet of doublet of doublets.doublets.

HH

HHOO22NN

HH


HH

HHOO22NN

HH

Doublet of doubletsDoublet of doublets

13.1213.1211H NMR Spectra of AlcoholsH NMR Spectra of Alcohols

What about H bonded to O?What about H bonded to O?

O—HO—H

The chemical shift for O—The chemical shift for O—HH is variable and is variable and depends on temperature and concentration.depends on temperature and concentration.

Splitting of the O—Splitting of the O—HH proton is sometimes proton is sometimes observed but usually is not. It usually appears observed but usually is not. It usually appears as a broad singlet peak. as a broad singlet peak.

Adding Adding DD22O converts O—O converts O—HH to O— to O—DD..

The O—The O—HH peak disappears. peak disappears.

CC OOHH HH

13.1313.13NMR and ConformationsNMR and Conformations

NMR is “Slow”NMR is “Slow”

Most conformational changes occur faster Most conformational changes occur faster than NMR can detect them.than NMR can detect them.

An NMR spectrum is the weighted average of An NMR spectrum is the weighted average of the conformations.the conformations.

For example, cyclohexane gives a single peak For example, cyclohexane gives a single peak for its H atoms in NMR. Half of the time a for its H atoms in NMR. Half of the time a single proton is axial and half of the time it is single proton is axial and half of the time it is equatorial. The observed chemical shift is half equatorial. The observed chemical shift is half way between the axial chemical shift and the way between the axial chemical shift and the equatorial chemical shift.equatorial chemical shift.

13.1413.141313C NMR SpectroscopyC NMR Spectroscopy

11H and H and 1313C NMR ComparedC NMR Compared

Both give us information about the number of Both give us information about the number of chemically nonequivalent nuclei chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent (nonequivalent hydrogens or nonequivalent carbons).carbons).

Both give us information about the Both give us information about the environment of the nuclei (hybridization state, environment of the nuclei (hybridization state, attached atoms, attached atoms, etcetc.)..).

It is convenient to use FT-NMR techniques for It is convenient to use FT-NMR techniques for 11H; it is standard practice for H; it is standard practice for 1313C NMR.C NMR.


1313C NMR requires FT-NMR because the C NMR requires FT-NMR because the signal for a carbon atom is 10signal for a carbon atom is 10-4-4 times weaker times weaker than the signal for a hydrogen atom,than the signal for a hydrogen atom,

because of differences in the magnetic because of differences in the magnetic properties of the two nuclei and,properties of the two nuclei and,

at the “natural abundance” level, only 1.1% of at the “natural abundance” level, only 1.1% of all the C atoms in a sample are all the C atoms in a sample are 1313C (most are C (most are 1212C).C).


1313C signals are spread over a much wider C signals are spread over a much wider range than range than 11H signals making it easier to H signals making it easier to identify and count individual nucleiidentify and count individual nuclei

For 1-chloropentane, it is much easier to For 1-chloropentane, it is much easier to identify the compound by its identify the compound by its 1313C spectrum C spectrum than by its than by its 11H spectrum.H spectrum.

01.02.03.04.05.06.07.08.09.010.0


ClClCCHH22

1-Chloropentane1-Chloropentane

CCHH33ClClCCHH22CHCH22CHCH22CHCH22CCHH33

11HH


1-Chloropentane1-Chloropentane

ClClCHCH22CHCH22CHCH22CHCH22CHCH33

020406080100120140160180200

1313CC

CDClCDCl33

a separate, distinct peak appears for each of the 5 carbons

13.1513.151313C Chemical ShiftsC Chemical Shifts

Measured in ppm (Measured in ppm ())

from the carbons of TMS.from the carbons of TMS.

Factors AffectingFactors Affecting 13 13C Chemical ShiftsC Chemical Shifts

• Electronegativity of groups attached to Electronegativity of groups attached to carbon.carbon.

• Hybridization state of carbon.Hybridization state of carbon.

Electronegativity EffectsElectronegativity Effects

Electronegativity has an even greater effect Electronegativity has an even greater effect on on 1313C chemical shifts than it does on C chemical shifts than it does on 11H H chemical shifts.chemical shifts.

Types of CarbonsTypes of Carbons

(CH(CH33))33CCHH

CCHH44

CCHH33CCHH33

CHCH33CCHH22CHCH33

(CH(CH33))44CC

primaryprimary

secondarysecondary

tertiarytertiary

quaternaryquaternary

ClassificationClassification Chemical shift, Chemical shift, 11HH 1313CC

0.20.2

0.90.9

1.31.3

1.71.7

-2-2

88

1616

2525

2828

Replacing H by C (more electronegative) deshieldsReplacing H by C (more electronegative) deshieldsC to which it is attached.C to which it is attached.

Electronegativity Effects on CHElectronegativity Effects on CH33

CCHH33FF

CCHH44

CCHH33NHNH22

CCHH33OHOH

Chemical shift, Chemical shift, 11HH

0.20.2

2.52.5

3.43.4

4.34.3

1313CC

-2-2

2727

5050

7575

Electronegativity Effects and Chain LengthElectronegativity Effects and Chain Length

ChemicalChemicalshift, shift,

ClCl CHCH22 CHCH22 CHCH22 CHCH22 CHCH33

4545 3333 2929 2222 1414

Deshielding effect of Deshielding effect of ClCl decreases as decreases as number of bonds between number of bonds between ClCl and C increases. and C increases.

Factors AffectingFactors Affecting 13 13C Chemical ShiftsC Chemical Shifts

• Electronegativity of groups attached to Electronegativity of groups attached to carbon.carbon.

• Hybridization state of carbon.Hybridization state of carbon.

Hybridization EffectsHybridization Effects

spsp33-Hybridized -Hybridized carbon is more carbon is more shielded than shielded than spsp22.. 114114

138138

3636

3636 126-142126-142spsp-Hybridized -Hybridized carbon is more carbon is more shielded than shielded than spsp22, but less , but less shielded than shielded than spsp33..

CHCH33HH CC CC CHCH22 CHCH22

6868 8484 2222 2020 1313

Carbonyl Carbons Are Especially DeshieldedCarbonyl Carbons Are Especially Deshielded

OO

CHCH22 CC OO CHCH22 CHCH33

127-134127-1344141 14146161171171

1313C Chemical Shifts for Some Common GroupsC Chemical Shifts for Some Common Groups

Type of carbonType of carbon Chemical shift (Chemical shift (),),ppmppm


RRCCHH33 0-350-35

CCRR22RR22CC

65-9065-90CCRRRRCC

RR22CCHH22 15-4015-40

RR33CCHH 25-5025-50

RR44CC 30-4030-40

100-150100-150

110-175110-175

RRCCHH22BrBr 20-4020-40

RRCCHH22ClCl 25-5025-50

35-5035-50RRCCHH22NHNH22

50-6550-65RRCCHH22OHOH

RRCCHH22OROR 50-6550-65

RRCCOROR

OO

160-185160-185

RRCCRR

OO

190-220190-220

RRCC NN 110-125110-125

1313C Chemical Shifts for Some Common GroupsC Chemical Shifts for Some Common Groups



13.1613.161313C NMR and Peak IntensitiesC NMR and Peak Intensities

Pulse FT-NMR distorts intensities of signals. Pulse FT-NMR distorts intensities of signals. Therefore, peak heights and areas can be Therefore, peak heights and areas can be deceptive.deceptive.

mm-Cresol-Cresol


020406080100120140160180200

7 carbons give 7 signals, but intensities are not equal

CHCH33

OHOH

13.2013.20


Gives information about the functional groups Gives information about the functional groups in a molecule.in a molecule.

Characteristic functional groups usually found Characteristic functional groups usually found

between 4000-1600 cmbetween 4000-1600 cm-1-1..

From 1300-625 cmFrom 1300-625 cm-1-1 called “fingerprint region.” called “fingerprint region.”

Depends on transitions between vibrational Depends on transitions between vibrational energy states:energy states:

Stretching.Stretching.

Bending.Bending.


Stretching Vibrations of a CHStretching Vibrations of a CH22 Group Group

SymmetricSymmetric AntisymmetricAntisymmetric

Bending Vibrations of a CHBending Vibrations of a CH22 Group Group

In planeIn plane““scissoring”scissoring”

In planeIn plane““rocking”rocking”

Bending Vibrations of a CHBending Vibrations of a CH22 Group Group

Out of planeOut of plane““wagging”wagging”

Out of planeOut of plane““twisting”twisting”

Structural unitStructural unit Frequency, cmFrequency, cm-1-1

Stretching vibrations (single bonds)Stretching vibrations (single bonds)

sp C—Hsp C—H 3310-33203310-3320

spsp22 C—H C—H 3000-31003000-3100

spsp33 C—H C—H 2850-29502850-2950

spsp22 C—O C—O 12001200

spsp33 C—O C—O 1025-12001025-1200

Infrared Absorption FrequenciesInfrared Absorption Frequencies

Francis A. Carey, Organic Chemistry, Fifth Edition. Copyright © 2003 The McGraw-Hill Companies, Inc. All rights reserved.

Infrared Spectrum of HexaneInfrared Spectrum of HexaneInfrared Spectrum of HexaneInfrared Spectrum of Hexane

Infrared Spectrum of BenzeneInfrared Spectrum of BenzeneInfrared Spectrum of BenzeneInfrared Spectrum of Benzene


Infrared Spectrum of Dihexyl EtherInfrared Spectrum of Dihexyl EtherInfrared Spectrum of Dihexyl EtherInfrared Spectrum of Dihexyl Ether



Stretching vibrations (multiple bonds)Stretching vibrations (multiple bonds)

CC CC 1620-16801620-1680

——CC NN

——CC C—C— 2100-22002100-2200

2240-22802240-2280


Infrared Spectrum of 1-HexeneInfrared Spectrum of 1-HexeneInfrared Spectrum of 1-HexeneInfrared Spectrum of 1-Hexene


Infrared Spectrum of HexanenitrileInfrared Spectrum of HexanenitrileInfrared Spectrum of HexanenitrileInfrared Spectrum of Hexanenitrile



Stretching vibrations (carbonyl groups)Stretching vibrations (carbonyl groups)

Aldehydes and ketonesAldehydes and ketones 1710-17501710-1750

Carboxylic acidsCarboxylic acids 1700-17251700-1725

Acid anhydridesAcid anhydrides 1800-1850 and 1740-17901800-1850 and 1740-1790

EstersEsters 1730-17501730-1750

AmidesAmides 1680-17001680-1700

CC OO


Infrared Spectrum of 2-HexanoneInfrared Spectrum of 2-HexanoneInfrared Spectrum of 2-HexanoneInfrared Spectrum of 2-Hexanone


Infrared Spectrum of Hexanoic AcidInfrared Spectrum of Hexanoic AcidInfrared Spectrum of Hexanoic AcidInfrared Spectrum of Hexanoic Acid


Infrared Spectrum of Methyl HexanoateInfrared Spectrum of Methyl HexanoateInfrared Spectrum of Methyl HexanoateInfrared Spectrum of Methyl Hexanoate



Bending vibrations of alkenesBending vibrations of alkenes

CHCH22RCHRCH

CHCH22RR22CC

CHR'CHR'ciscis-RCH-RCH

CHR'CHR'transtrans-RCH-RCH

CHR'CHR'RR22CC

910-990910-990

890890

665-730665-730

960-980960-980

790-840790-840


Infrared Spectrum of 1-HexeneInfrared Spectrum of 1-HexeneInfrared Spectrum of 1-HexeneInfrared Spectrum of 1-Hexene



Bending vibrations of derivatives of benzeneBending vibrations of derivatives of benzene

MonosubstitutedMonosubstituted 730-770 and 690-710730-770 and 690-710

orthoortho-Disubstituted-Disubstituted 735-770735-770

metameta-Disubstituted-Disubstituted 750-810 and 680-730750-810 and 680-730

parapara-Disubstituted-Disubstituted 790-840790-840


Infrared Spectrum of HexylbenzeneInfrared Spectrum of HexylbenzeneInfrared Spectrum of HexylbenzeneInfrared Spectrum of Hexylbenzene


Structural unitStructural unit Frequency, Frequency, cmcm-1-1

Stretching vibrations (single bonds)Stretching vibrations (single bonds)

O—H (alcohols)O—H (alcohols) 3200-36003200-3600

O—H (carboxylic acids) O—H (carboxylic acids) 3000-31003000-3100

N—HN—H 3350-35003350-3500


Infrared Spectrum of 1-HexanolInfrared Spectrum of 1-HexanolInfrared Spectrum of 1-HexanolInfrared Spectrum of 1-Hexanol


Infrared Spectrum of HexylamineInfrared Spectrum of HexylamineInfrared Spectrum of HexylamineInfrared Spectrum of Hexylamine


Infrared Spectrum of HexanamideInfrared Spectrum of HexanamideInfrared Spectrum of HexanamideInfrared Spectrum of Hexanamide


13.2113.21

Ultraviolet-Visible (UV-VIS) Ultraviolet-Visible (UV-VIS)

SpectroscopySpectroscopy

Gives information about conjugated Gives information about conjugated electron electron systemssystems

Gaps between electron energy Gaps between electron energy levels are greater than thoselevels are greater than thosebetween vibrational levels.between vibrational levels.

Gap corresponds to wavelengthsGap corresponds to wavelengthsbetween 200 and 800 nm.between 200 and 800 nm.

Transitions between Electron Energy StatesTransitions between Electron Energy States

EE = = hh

X-axis is wavelength in nm (high energy at left, X-axis is wavelength in nm (high energy at left, low energy at right).low energy at right).

maxmax is the wavelength of maximum absorption is the wavelength of maximum absorption and is related to electronic makeup of molecule— and is related to electronic makeup of molecule— especially especially electron system. electron system.

Y axis is a measure of absorption of electromagnetic Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity (radiation expressed as molar absorptivity ().).

Conventions in UV-VISConventions in UV-VIS

200200 220220 240240 260260 280280

10001000

20002000

Wavelength, nmWavelength, nm

maxmax 230 nm 230 nm

maxmax 2630 2630

MolarMolar

absorptivity (absorptivity ())

UV Spectrum of UV Spectrum of ciscis,,transtrans-1,3-Cyclooctadiene-1,3-Cyclooctadiene

Most stable Most stable -electron -electron

configurationconfiguration

-Electron -Electron configuration of configuration of

excited stateexcited state

* Transition in * Transition in ciscis,,transtrans-1,3-Cyclooctadiene-1,3-Cyclooctadiene

HOMOHOMO

LUMOLUMO

EE = = hh

* Transition in Alkenes* Transition in Alkenes

HOMO-LUMO energy gap is affected by HOMO-LUMO energy gap is affected by substituents on double bond.substituents on double bond.

As HOMO-LUMO energy difference As HOMO-LUMO energy difference decreases (smaller decreases (smaller EE), ), maxmax shifts to longer shifts to longer

wavelengths.wavelengths.

Effect of SubstitutionEffect of Substitution

Methyl groups on double bond cause Methyl groups on double bond cause maxmax

to shift to longer wavelengthsto shift to longer wavelengths

CC CC

HH

HH

HH

HH

CC CC

HH

HH CHCH33


CHCH33


Effect of ConjugationEffect of Conjugation

Extending conjugation has a larger effect Extending conjugation has a larger effect on on maxmax; shift is again to longer ; shift is again to longer

wavelengths.wavelengths.

CC CC

HH

HH

HH

HH

CC CC

HH

HH

maxmax 170 nm 170 nm maxmax 217 nm 217 nm

HH

CC CC

HH

HHHH

Effect of ConjugationEffect of Conjugation


for conjugated for conjugated dienediene

HH

CC CC

HH

HH CC CC

HH

HHHH

CC CC

HH CHCH33

HH

HH

CC CC

HH33CC

HH CC CC

HH

HH


for conjugated for conjugated trienetriene plus two methyl plus two methyl

groupsgroups

LycopeneLycopene


Orange-red pigment in tomatoes.Orange-red pigment in tomatoes.

13.2213.22

Mass SpectrometryMass Spectrometry

Atom or molecule is hit by high-energy electron.Atom or molecule is hit by high-energy electron.

Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry

ee––


Electron is deflected but transfers much of Electron is deflected but transfers much of its energy to the molecule.its energy to the molecule.

ee––



Electron is deflected but transfers much of Electron is deflected but transfers much of its energy to the molecule.its energy to the molecule.

ee––


This energy-rich species ejects an electron.This energy-rich species ejects an electron.


This energy-rich species ejects an electron.This energy-rich species ejects an electron.


Forming a positively charged, odd-electron Forming a positively charged, odd-electron species called the molecular ion.species called the molecular ion.

ee––++••

Molecular ion passes between poles of a Molecular ion passes between poles of a magnet and is deflected by magnetic field.magnet and is deflected by magnetic field.

Amount of Amount of deflection depends deflection depends on mass-to-charge on mass-to-charge ratio (ratio (m/zm/z).).

Highest Highest m/zm/z deflected least.deflected least.

Lowest Lowest m/zm/z deflected most.deflected most.


++••


If the only ion that is present is the molecular If the only ion that is present is the molecular ion, mass spectrometry provides a way to ion, mass spectrometry provides a way to measure the molecular weight of a compound measure the molecular weight of a compound and is often used for this purpose.and is often used for this purpose.

However, the molecular ion often fragments to However, the molecular ion often fragments to a mixture of species of lower a mixture of species of lower m/zm/z..

The molecular ion dissociates to a cationThe molecular ion dissociates to a cationand a radical.and a radical.


++••

The molecular ion dissociates to a cationThe molecular ion dissociates to a cationand a radical.and a radical.


++ ••

Usually several fragmentation pathways are Usually several fragmentation pathways are available and a mixture of ions is produced.available and a mixture of ions is produced.

Mixture of ions of Mixture of ions of different mass different mass gives separate peak gives separate peak for each for each m/zm/z..

Intensity of peak Intensity of peak proportional to proportional to percentage of each percentage of each ion of different ion of different mass in mixture.mass in mixture.

Separation of peaks Separation of peaks depends on relative depends on relative mass.mass.


++

++++

++

+

+

Mixture of ions of Mixture of ions of different mass different mass gives separate peak gives separate peak for each for each m/zm/z..

Intensity of peak Intensity of peak proportional to proportional to percentage of each percentage of each atom of different atom of different mass in mixture.mass in mixture.

Separation of peaks Separation of peaks depends on relative depends on relative mass.mass.

++ ++ ++ ++

+ +


2020 4040 6060 8080 100100 120 120m/zm/z

m/z m/z = 78= 78

100100

8080

6060

4040

2020

00

Relative Relative intensityintensity

Some Molecules Undergo Very Little FragmentationSome Molecules Undergo Very Little Fragmentation

Benzene is an example. The major peak Benzene is an example. The major peak corresponds to the molecular ion.corresponds to the molecular ion.

HH

HH

HH

HHHH

HH

HH

HH

HH

HHHH

HH

HH

HH

HH

HHHH

HH

All H are All H are 11H and all H and all C are C are 1212C.C.

One C is One C is 1313CC.. One H is One H is 22HH..

Isotopic ClustersIsotopic Clusters

7878 7979 7979

93.4%93.4% 6.5%6.5% 0.1%0.1%

2020 4040 6060 8080 100100 120 120

m/zm/z

100100

8080

6060

4040

2020

00


112112

114114

Isotopic ClustersIsotopic Clustersin Chlorobenzenein Chlorobenzene

Visible in peaks Visible in peaks for molecular ion.for molecular ion.

3535ClCl 3737ClCl

2020 4040 6060 8080 100100 120 120

m/zm/z


7777

Isotopic ClustersIsotopic Clustersin Chlorobenzenein Chlorobenzene

No No m/zm/z 77, 79 77, 79 pair; therefore, ion pair; therefore, ion responsible forresponsible form/zm/z 77 peak does 77 peak does not contain Cl.not contain Cl.

HH

HH

HH

HH

HH ++

100100

8080

6060

4040

2020

00

Alkanes Undergo Extensive FragmentationAlkanes Undergo Extensive Fragmentation

m/zm/z

DecaneDecane

142142

7171

8585

9999

CHCH33—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH22—CH—CH33


100100

8080

6060

4040

2020

00 2020 4040 6060 8080 100100 120 120

4343 5757

Propylbenzene Fragments MostlyPropylbenzene Fragments Mostlyat the Benzylic Positionat the Benzylic Position

2020 4040 6060 8080 100100 120 120

m/zm/z


120120

9191

CHCH22—CH—CH22CHCH33

100100

8080

6060

4040

2020

00

13.2313.23

Molecular FormulaMolecular Formula

as aas a

Clue to StructureClue to Structure

Molecular WeightsMolecular Weights

One of the first pieces of information we try to One of the first pieces of information we try to obtain when determining a molecular obtain when determining a molecular structure is the molecular formula.structure is the molecular formula.

However, we can gain some information even However, we can gain some information even from the molecular weight. Mass spectrometry from the molecular weight. Mass spectrometry makes it relatively easy to determine makes it relatively easy to determine molecular weights.molecular weights.

The Nitrogen RuleThe Nitrogen Rule

A molecule with an A molecule with an odd number of odd number of nitrogens has an odd nitrogens has an odd molecular weight.molecular weight.

A molecule that A molecule that contains only C, H, contains only C, H, and O or which has and O or which has an even number of an even number of nitrogens has an nitrogens has an even molecular even molecular weight.weight.

NNHH22 9393

138138

NNHH22OO22NN

183183

NNHH22OO22NN

NNOO22

Exact Molecular WeightsExact Molecular Weights

CHCH33(CH(CH22))55CHCH33

HeptaneHeptane

CHCH33COCO

OO

Cyclopropyl acetateCyclopropyl acetate

Molecular formulaMolecular formula

Molecular weightMolecular weight

CC77HH1616 CC55HH88OO22

100100 100100

Exact massExact mass 100.1253100.1253 100.0524100.0524

Mass spectrometry can measure exact Mass spectrometry can measure exact masses. Therefore, mass spectrometry can masses. Therefore, mass spectrometry can give molecular formulas.give molecular formulas.

Molecular FormulasMolecular Formulas

Knowing that the molecular formula of a Knowing that the molecular formula of a substance is Csubstance is C77HH1616 tells us immediately that it is tells us immediately that it is an alkane because it corresponds to Can alkane because it corresponds to CnnHH2n+22n+2..

ButBut CC77HH1414 lacks two hydrogens of an alkane; lacks two hydrogens of an alkane; therefore, it contains either a ring or a double therefore, it contains either a ring or a double bond.bond.

Index of Hydrogen DeficiencyIndex of Hydrogen Deficiency

Relates molecular formulas to multiple bonds Relates molecular formulas to multiple bonds and rings.and rings.

Index of hydrogen deficiency = Index of hydrogen deficiency =

1122

(molecular formula of alkane –(molecular formula of alkane – molecular formula of compound) molecular formula of compound)

Example 1Example 1

Index of hydrogen deficiency Index of hydrogen deficiency

C7H14C7H14

1122

(molecular formula of alkane –(molecular formula of alkane – molecular formula of compound) molecular formula of compound)

==

1122

(C(C77HH1616 – C – C77HH1414))==

1122

(2) = 1(2) = 1==

Therefore, one ring or one double bond.Therefore, one ring or one double bond.

Example 2Example 2

C7H12C7H12

1122

(C(C77HH1616 – C – C77HH1212))==

1122

(4) = 2(4) = 2==

Therefore, two rings, one triple bond,Therefore, two rings, one triple bond,two double bonds or one double bond + one ring.two double bonds or one double bond + one ring.

Oxygen Has no EffectOxygen Has no Effect

CHCH33(CH(CH22))55CHCH22OH (1-heptanol, COH (1-heptanol, C77HH1616O) has O) has

same number of H atoms as heptane.same number of H atoms as heptane.


1122

((CC77HH1616 – – CC77HH1616OO)) = 0= 0

No rings or double bonds.No rings or double bonds.

Oxygen Has no EffectOxygen Has no Effect


1122

((CC55HH1212 – – CC55HH88OO22)) = 2= 2

One ring plus one double bond.One ring plus one double bond.

CHCH33COCO

OO

Cyclopropyl acetateCyclopropyl acetate

If Halogen is PresentIf Halogen is Present

Treat a halogen as if it were hydrogen.Treat a halogen as if it were hydrogen.

CC CC

CHCH33

ClClHH

HH

CC33HH55ClCl

Same index of hydrogenSame index of hydrogendeficiency as for Cdeficiency as for C33HH66..

Rings Rings versusversus Multiple Bonds Multiple Bonds

Index of hydrogen deficiency tells us the sum ofIndex of hydrogen deficiency tells us the sum ofrings plus multiple bonds.rings plus multiple bonds.

Catalytic hydrogenation tells us how many Catalytic hydrogenation tells us how many multiple bonds there are.multiple bonds there are.

Chapter 13 Spectroscopy Nuclear Magnetic Resonance Spectroscopy Infrared Spectroscopy Ultraviolet-Visible Spectroscopy Mass Spectrometry

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