Chapter 13 Spectroscopy. © 2013 Pearson Education, Inc.Chapter 122 Introduction Spectroscopy is a technique used to determine the structure of a compound

Chapter 13Spectroscopy

© 2013 Pearson Education, Inc. Chapter 12 2

Introduction

• Spectroscopy is a technique used to determine the structure of a compound.

• Most techniques are nondestructive (destroys little or no sample).

• Absorption spectroscopy measures the amount of light absorbed by the sample as a function of wavelength.


Types of Spectroscopy• Infrared (IR) spectroscopy measures the bond

vibration frequencies in a molecule and is used to determine the functional group.

• Mass spectrometry (MS) fragments the molecule and measures the mass. MS can give the molecular weight of the compound and functional groups.

• Nuclear magnetic resonance (NMR) spectroscopy analyzes the environment of the hydrogens in a compound. This gives useful clues as to the alkyl and other functional groups present.

• Ultraviolet (UV) spectroscopy uses electronic transitions to determine bonding patterns.

Is propagated at the speed of light

Has properties of particles and waves

The energy of a photon is proportional to its frequency

Electromagnetic Radiation

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

E = h


The Electromagnetic Spectrum

Introduction to Introduction to 11H NMR SpectroscopyH NMR Spectroscopy

1H and 13C

both have spin = ±1/2

1H is 99% at natural abundance

13C is 1.1% at natural abundance

The Nuclei that are Most Useful toOrganic Chemists are:

Nuclear Spin

A spinning charge, such as the nucleus of 1H or 13C, generates a magnetic field. The magnetic field generated by a nucleus of spin +1/2 is opposite in direction from that generated by a nucleus of spin –1/2.

+ +

++

+

+

+

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

+

+

+

+

+

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

B0

+

+

+

+

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

B0

No difference in absence of magnetic fieldProportional to strength of external magnetic field

Energy Differences Between Nuclear Spin States

+

+

E E '

increasing field strength

Some Important Relationships in NMR

The frequency of absorbedelectromagnetic radiationis proportional to

the energy difference betweentwo nuclear spin stateswhich is proportional to

the applied magnetic field.

Units

Hz

kJ/mol(kcal/mol)

tesla (T)


The frequency of absorbed electromagneticradiation is different for different elements, and for different isotopes of the same element.

For a field strength of 4.7 T:1H absorbs radiation having a frequencyof 200 MHz (200 x 106 s-1)13C absorbs radiation having a frequencyof 50.4 MHz (50.4 x 106 s-1)


The frequency of absorbed electromagneticradiation for a particular nucleus (such as 1H)depends on its molecular environment.

This is why NMR is such a useful toolfor structure determination.

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"?

Shielding

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

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

C H

B 0

Shielding

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

A stronger external field is needed in order for energy difference between spin states to match energy of rf radiation. B 0

C H

Chemical Shift

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

Protons in different environments are shielded to greater or lesser degrees; they have different chemical shifts. B 0

C H

Chemical Shift

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

Si CH3

CH3

CH3

H3C

=position of signal - position of TMS peak

spectrometer frequencyx 106

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Chemical shift (, ppm)

measured relative to TMS

UpfieldIncreased shielding

DownfieldDecreased shielding

(CH3)4Si (TMS)

Chemical Shift

Example: The signal for the proton in chloroform (HCCl3) appears 1456 Hz downfield from TMS at

a spectrometer frequency of 200 MHz.

=position of signal - position of TMS peak

spectrometer frequencyx 106

=1456 Hz - 0 Hz

200 x 106 Hx

x 106

= 7.28

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7.28 ppm

H C

Cl

Cl

Cl

Effects of Molecular Structureon

1H Chemical Shifts

Protons in different environments experience different degrees of shielding and have

different chemical shifts.

Electronegative Substituents Decreasethe Shielding of Methyl Groups

least shielded H most shielded H CH3F CH3OCH3 (CH3)3N CH3CH3 (CH3)4Si

4.3 3.2 2.2 0.9 0.0

Electronegative Substituents Decrease Shielding

H3C—CH2—CH3

O2N—CH2—CH2—CH3

0.9 0.9 1.3

1.0 4.3 2.0

Effect is Cumulative

CHCl3 7.3

CH2Cl2 5.3

CH3Cl 3.1

Methyl, Methylene, and Methine

CH3 more shielded than CH2 ; CH2

more shielded than CH

H3C C

CH3

CH3

H

0.9

1.6

0.8

H3C C

CH3

CH3

CH2

0.9

CH3

1.2

Protons Attached to sp2 Hybridized Carbonare Less Shielded than those Attached

to sp3 Hybridized Carbon H H

HH

H

H

C C

HH

H H

CH3CH3

7.3 5.3 0.9

But Protons Attached to sp Hybridized Carbonare More Shielded than those Attached

to sp2 Hybridized Carbon

2.4CH2OCH3C CHC C

HH

H H

5.3

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

1.5 0.8

H3C CH3

1.2

H3C CH2

2.6

H3C—CH2—CH3

0.9 0.9 1.3

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

2.4

9.7

1.1

C C

O

H

H

CH3

H3C

Table 13.1

Type of proton Chemical shift (),ppm


CH R 0.9-1.8

1.5-2.6CH CC

2.0-2.5CH C

O

2.1-2.3CH NC

CH Ar 2.3-2.8

Table 13.1



CH Br 2.7-4.1

9-10C

O

H

2.2-2.9CH NR

3.1-4.1CH Cl

6.5-8.5H Ar

C C

H

4.5-6.5

3.3-3.7CH O

Table 13.1


1-3H NR

0.5-5H OR

6-8H OAr

10-13C

O

HO

Interpreting Interpreting 11H NMR H NMR

SpectraSpectra

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1. Number of signals

2. Their intensity (as measured by area under peak)

3. Splitting pattern (multiplicity)

Information Contained in an NMRSpectrum Includes:

Number of Signals

Protons that have different chemical shifts are chemically nonequivalent.

They exist in different molecular environments.

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CCH2OCH3N

OCH3

NCCH2O

Figure 13.12

Are in identical environments

Have same chemical shift

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

H3CCH2CH3

chemically equivalent

Chemically Equivalent Protons

H3CCH2CH3

chemically equivalent

CH3CH2CH2ClClCH2CH2CH3

Chemically Equivalent Protons

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

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

Replacement by some arbitrary test group generates diastereomers.

Diastereotopic protons can have differentchemical shifts.

Diastereotopic Protons

C C

Br

H3C

H

H

5.3 ppm

5.5 ppm

Are in mirror-image environments.

Replacement by some arbitrary test group generates enantiomers.

Enantiotopic protons have the samechemical shift.

Enantiotopic Protons

C CH2OH

H3C

HH

EnantiotopicProtons

C CH2OH

H3C

ClH

C CH2OH

H3C

HCl

R S

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

inin11H NMR SpectroscopyH NMR Spectroscopy

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Cl2CHCH3Figure 13.13

4 lines;quartet

2 lines;doublet

CH3CH

Two-bond and Three-bond Coupling

C C

H

H

C C HH

protons separated bytwo bonds

(geminal relationship)

protons separated bythree bonds

(vicinal relationship)

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

Coupling constant (2J or 3J) is independent of field strength.

Two-bond and Three-bond Coupling

C C

H

H

C C HH

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Cl2CHCH3Figure 13.13

4 lines;quartet

2 lines;doublet

CH3CH

coupled protons are vicinal (three-bond coupling)

CH splits CH3 into a doublet

CH3 splits CH into a quartet

Why Do the Methyl Protons of1,1-Dichloroethane Appear as a Doublet?

C C HH

Cl

Cl

H

Hsignal for methyl protons is split into a doublet

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


signal for methyl protons is split into a doublet

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

C C HH

Cl

Cl

H

H


signal for methyl protons is split into a doublet

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

C C HH

Cl

Cl

H

H


"true" chemical

shift of methyl

protons (no coupling)

This line corresponds

to molecules in which

the nuclear spin of

the proton at C-1

reinforces

the applied field.

This line corresponds

to molecules in which

the nuclear spin of

the proton at C-1

opposes

the applied field.

C C HH

Cl

Cl

H

H

Why Does the Methine Proton of1,1-Dichloroethane Appear as a Quartet?

signal for methine proton is split into a quartet

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

C C HH

Cl

Cl

H

H

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

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

Why Does the Methine Proton of1,1-Dichloroethane Appear as a Quartet?

C C HH

Cl

Cl

H

H

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

The Splitting Rule for 1H NMR

Splitting Patterns:The 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).

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BrCH2CH3Figure 13.16

4 lines;quartet

3 lines;tripletCH3

CH2

Splitting Patterns of Common Multiplets

Number of equivalent Appearance Intensities of linesprotons to which H of multiplet in multipletis coupled

1 Doublet 1:1

2 Triplet 1:2:1

3 Quartet 1:3:3:1

4 Pentet 1:4:6:4:1

5 Sextet 1:5:10:10:5:1

6 Septet 1:6:15:20:15:6:1

Table 13.2

Splitting Patterns:The Isopropyl Group

(CH(CH33))22CHX is characterized by a doublet-septet CHX is characterized by a doublet-septet

pattern (septet at lower field than the doublet).pattern (septet at lower field than the doublet).

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ClCH(CH3)2Figure 13.18

7 lines;septet

2 lines;doublet

CH3

CH

Splitting Patterns:Pairs 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 Doublets

Consider coupling between two vicinal protons.

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

C CH H

Pairs of Doublets

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

Let J be the coupling constant between them in Hz.

C CH H

AX

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

C CH H

J J

AM

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

J J

C CH H

AB

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

J J

C CH H

A2

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

C CH H

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Figure 13.20

OCH3

skewed doublets

H H

HH

Cl OCH3

Complex Splitting Patterns

Multiplets of multipletsMultiplets of multiplets

m-Nitrostyrene

Consider the proton shown in red.

It is unequally coupled to the protons shown in blue and green.

Jcis = 12 Hz; Jtrans = 16 Hz

H

HO2N

H

m-Nitrostyrene

16 Hz16 Hz

12 Hz 12 Hz

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

H

HO2N

H

Figure 13.21 H

HO2N

H

doublet of doublets

doublet doublet

1H NMR Spectra of Alcohols

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

O—H

The chemical shift for O—H is variable ( 0.5-5 ppm) and depends on temperature and concentration.

Splitting of the O—H proton is sometimes observed, but often is not. It usually appears as a broad peak.

Adding D2O converts O—H to O—D. The O—H peak disappears.

C OH H

NMR and ConformationsNMR is “Slow”

Most conformational changes occur faster than NMR can detect them.

An NMR spectrum shows the weighted average of the conformations.

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

1313C NMR SpectroscopyC NMR Spectroscopy

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1H and 13C NMR Compared:

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

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

It is convenient to use FT-NMR techniques for 1H; it is standard practice for 13C NMR.


13C requires FT-NMR because the signal for a carbon atom is 10-4 times weaker than the signal for a hydrogen atom.

A signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei, and

at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C).


13C signals are spread over a much wider range than 1H signals, making it easier to identify and count individual nuclei.

Figure 13.26 (a) shows the 1H NMR spectrum of 1-chloropentane; Figure 13.26 (b) shows the 13C spectrum. It is much easier to identify the compound as 1-chloropentane by its 13C spectrum than by its 1H spectrum.

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ClCH2

Figure 13.26(a)

CH3ClCH2CH2CH2CH2CH3

1H


Figure 13.26(b)

ClCH2CH2CH2CH2CH3

020406080100120140160180200

13C

CDCl3

A separate, distinct peak appears for each of the 5 carbons.

1313C Chemical ShiftsC Chemical Shifts

are measured in ppm (are measured in ppm ())

from the carbons of TMSfrom the carbons of TMS

13C Chemical Shifts are Most Affected By:

• Electronegativity of groups attached to carbon • Hybridization state of carbon

Electronegativity Effects

Electronegativity has an even greater effect on 13C chemical shifts than it does on 1H chemical shifts.

Types of Carbons

(CH3)3CH

CH4

CH3CH3

CH3CH2CH3

(CH3)4C

primary

secondary

tertiary

quaternary

Classification Chemical shift, 1H 13C

0.2

0.9

1.3

1.7

-2

8

16

25

28

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

Electronegativity Effects on CH3

CH3F

CH4

CH3NH2

CH3OH

Chemical shift, 1H

0.2

2.5

3.4

4.3

13C

-2

27

50

75

Electronegativity Effects and Chain Length

Chemicalshift,

Cl CH2 CH2 CH2 CH2 CH3

45 33 29 22 14

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

13C Chemical Shifts are Most Affected By:

• Electronegativity of groups attached to carbon • Hybridization state of carbon

Hybridization Effects

sp3 hybridized carbon is more shielded than sp2.

114

138

36

36 126-142

sp hybridized carbon is more shielded than sp2, but less shielded than sp3.

CH3H C C CH2 CH2

68 84 22 20 13

Carbonyl Carbons are Especially Deshielded O

CH2 C O CH2 CH3

127-13441 1461171

Table 13.3

Type of carbon Chemical shift (),ppm


RCH3 0-35

CR2R2C

65-90CRRC

R2CH2 15-40

R3CH 25-50

R4C 30-40

100-150 110-175

Table 13.3



RCH2Br 20-40

RCH2Cl 25-50

35-50RCH2NH2

50-65RCH2OH

RCH2OR 50-65

RCOR

O

160-185

RCR

O

190-220

RC N 110-125

1313C 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.

CH3

OH

Figure 13.27


020406080100120140160180200

7 carbons give 7 signals, but intensities are not equal.

1313C—H CouplingC—H Coupling

13C—13C splitting is not seen because theprobability of two 13C nuclei being in the samemolecule is very small.

13C—1H splitting is not seen because spectrumis measured under conditions that suppress this splitting (broadband decoupling).

Peaks in a 13C NMR Spectrum are TypicallySinglets

Documents

Chapter 13 Spectroscopy. © 2013 Pearson Education, Inc.Chapter 122 Introduction Spectroscopy is a technique used to determine the structure of a compound