introduction to spectroscopy - pccpl-kmkm.pccpl.ac.th/files/1205091111045234_12072316160831.pdf ·...

Introduction to Spectroscopy for Organic Compounds

Dr. Theeraphan Machan

Program of Natural Products Chemistry

School of Science

Mae Fah Luang University

What is spectroscopy?

2

Electromagnetic radiation

3

Electromagnetic componentsElectric component

magnetic component

nl

= c

El

= hc

4

time distance

1 cycle/sec = 1 Hz 1 cycle/cm = 1 cm-11 complete cycle

hv

absorption

matter transmitting

emission scattering

5

Types of Spectroscopy

Ultraviolet (UV) spectroscopy uses electron

transitions to determine bonding patterns.

Infrared (IR) spectroscopy measures the bond

vibration frequencies in a molecule and is used to

determine the functional groups.

Nuclear magnetic resonance (NMR)

spectroscopy detects signals from hydrogen

atoms and can be used to distinguish isomers.

Mass spectrometry (MS) fragments the

molecules and measures the masses.

6

7

Ultraviolet and visible spectroscopy

Theeraphan Machan, Ph.D


School of Science


8

Ultraviolet and visible radiation

The nature of electronic excitations

When continuous radiation passes

through a transparent material, a portion

of the radiation may be absorbed.

As a result of energy absorption, atom or

molecules pass from a state of low energy

(ground state) to a state of higherenergy (excited state)

9

The excitation process.

10

Atomic Orbitals, Molecular Orbitals, and Nonbonding orbitals

The way to construct such molecular orbitals

(MOs) is to combine the atomic orbitals

(AOs) of the atoms that make up the molecules.

Linear Combination of Atomic Orbitals

(LCAO)

11

Sigma bonds,

12

13

Pi bonds,

Electronic energy levels

16

Electronic transitions

17

*

*

nE

n * **n

(> 270 nm)< 105 kcal (> 185 nm)

< 150 kcal

(> 165 nm)< 170 kcal

(< 165 nm)> 170 kcal

*

antibonding

bonding

Principles of absorption spectroscopy

19

Vibrational sublevels

E E1

E2

sublevels

sublevels

Rotational sublevels

UV-Vis spectrum

20

Wavelength (nm)

Abso

rbance

200 400 600

lmax

21

* Transitions

An electron in a bonding s orbital is excitedto the corresponding antibonding orbital.The energy required is large.

For example, methane (which has only C-Hbonds, and can only undergo *transitions) shows an absorbance maximumat 125 nm.

Absorption maxima due to *transitions are not seen in typical UV-VISspectra (200 - 700 nm)

22

* Transitions in an alkane

23

Saturated compounds containing atoms withlone pairs (non-bonding electrons) arecapable of n * transitions.

These transitions usually need less energythan * transitions.

They can be initiated by light whosewavelength is in the range 150 - 250 nm.

The number of organic functional groupswith n * peaks in the UV region is small,such as C-N, C-O, C-S, and C-X.

n * Transitions

n * Transitions in an amine

24

25

n * and * Transitions

Most absorption spectroscopy of organic

compounds is based on transitions of n or

electrons to the * excited state.

These transitions fall in an experimentally

convenient region of the spectrum (200 -

700 nm).

These transitions need an unsaturated

group in the molecule to provide the

electrons such as alkene (170 nm), Alkyne

(175 nm) and carbonyl (188 and 280 to

290).

n * and * Transition in carbonyl

26

* Transition in alkene

27

Auxochromes

Substituents that increase the intensity of the absorption,

and possibly the wavelength are call auxochromes.

CH3, OH, OR, NH2, and X (halogens).

Auxochromes may have any of four kinds of effects on the

absorption of chromophores.

◦ Bathochromic shift (red shift) – a shift to lower

energy or longer wavelength.

◦ Hypsochromic shift (blue shift) – a shift to high

energy or shorter wavelength.

◦ Hyperchromic effect – an increase in intensity.

◦ Hypochromic effect – a decrease in intensity.

29

Effects of auxochromes to chromophores

30

ความยาวคลื่น (nm)

Absorbance

blue red

Hyperchromic effect

Hypochromic effect

Hypsochromic shift Bathochromic shift

The effect of conjugation on alkenes

31

32

A comparison of the * energy gap in a series of polyenes of increasing chain length.

HOMO

LUMO

HOMO

LUMO

34

Infrared spectroscopy



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35

The IR Region

Just below red in the visible region.

Wavelengths usually 2.5-25 mm.

More common units are wavenumbers, or

cm-1, the reciprocal of the wavelength in

centimeters.

Wave numbers are proportional to

frequency and energy.

36

Molecular Vibrations

Covalent bonds vibrate at only certain

allowable frequencies.

37

Stretching Frequencies

Frequency decreases with increasing atomic weight.

Frequency increases with increasing bond

energy.

38

Vibrational Modes

Nonlinear molecule with n atoms usually has

3n-6 fundamental vibrational modes.

39

Fingerprint of Molecule

Whole-molecule vibrations and bending

vibrations are also quantitized.

No two molecules will give exactly the same

IR spectrum (except enantiomers).

Simple stretching: 1600-3500 cm-1.

Complex vibrations: 600-1400 cm-1, called

the “fingerprint region.”

=>

40

IR-Active and Inactive

A polar bond is usually IR-active.

A nonpolar bond in a symmetrical molecule

will absorb weakly or not at all.

=>

41

An Infrared Spectrometer

42

FT-IR Spectrometer

Uses an interferometer.

Has better sensitivity.

Less energy is needed from source.

Completes a scan in 1-2 seconds.

Takes several scans and averages them.

Has a laser beam that keeps the

instrument accurately calibrated.

43

Carbon-Carbon Bond Stretching

Stronger bonds absorb at higher frequencies:◦ C-C 1200 cm-1

◦ C=C 1660 cm-1

◦ CC 2200 cm-1 (weak or absent if internal)

Conjugation lowers the frequency:◦ isolated C=C 1640-1680 cm-1

◦ conjugated C=C 1620-1640 cm-1

◦ aromatic C=C approx. 1600 cm-1

=>

44

Carbon-Hydrogen Stretching

Bonds with more s character absorb at a

higher frequency.

◦ sp3 C-H, just below 3000 cm-1 (to the right)

◦ sp2 C-H, just above 3000 cm-1 (to the left)

◦ sp C-H, at 3300 cm-1

=>

45

An Alkane IR Spectrum

=>

46

An Alkene IR Spectrum

=>

47

An Alkyne IR Spectrum

=>

48

O-H and N-H Stretching

Both of these occur around 3300 cm-1,

but they look different.

◦ Alcohol O-H, broad with rounded tip.

◦ Secondary amine (R2NH), broad with one

sharp spike.

◦ Primary amine (RNH2), broad with two sharp

spikes.

◦ No signal for a tertiary amine (R3N) =>

49

An Alcohol IR Spectrum

=>

50

An Amine IR Spectrum

=>

51

Carbonyl Stretching

The C=O bond of simple ketones,

aldehydes, and carboxylic acids absorb

around 1710 cm-1.

Usually, it’s the strongest IR signal.

Carboxylic acids will have O-H also.

Aldehydes have two C-H signals around

2700 and 2800 cm-1.

=>

52

A Ketone IR Spectrum

=>

53

An Aldehyde IR Spectrum

=>

54

O-H Stretch of a Carboxylic Acid

This O-H absorbs broadly, 2500-3500 cm-1, due

to strong hydrogen bonding.

55

Variations in C=O Absorption

Conjugation of C=O with C=C lowers the

stretching frequency to ~1680 cm-1.

The C=O group of an amide absorbs at an

even lower frequency, 1640-1680 cm-1.

The C=O of an ester absorbs at a higher

frequency, ~1730-1740 cm-1.

Carbonyl groups in small rings (5 C’s or

less) absorb at an even higher frequency.

56

An Amide IR Spectrum

57

Carbon – Nitrogen stretching

C - N absorbs around 1200 cm-1.

C = N absorbs around 1660 cm-1 and is

much stronger than the C = C absorption in

the same region.

C N absorbs strongly just above 2200 cm-1.

The alkyne C C signal is much weaker and

is just below 2200 cm-1 .

58

A Nitrile IR Spectrum

=>

59

Summary of IR Absorptions

60

Strengths and Limitations

IR alone cannot determine a structure.

Some signals may be ambiguous.

The functional group is usually indicated.

The absence of a signal is definite proof that

the functional group is absent.

Correspondence with a known sample’s IR

spectrum confirms the identity of the

compound. =>

61

Nuclear Magnetic Resonance Spectroscopy



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62

Introduction

NMR is the most powerful tool available for

organic structure determination.

It is used to study a wide variety of nuclei:

◦ 1H

◦ 13C

◦ 15N

◦ 19F

◦ 31P

63

Nuclear Spin

A nucleus with an odd atomic number or an

odd mass number has a nuclear spin.

The spinning charged nucleus generates a

magnetic field.

64

External Magnetic Field

When placed in an external field, spinning protons act like bar magnets.

Spin +1/2Aligned

Spin -1/2Opposed

65

The spin state energy separation as a function of the

strength of the applied magnetic field B0

66

Two Energy States

The magnetic fields of the spinning nuclei will aligneither with the external field, or against the field.

A photon with the right amount of energy can be

absorbed and cause the spinning proton to flip.

67

E and Magnet Strength

Energy difference is proportional to the

magnetic field strength.

E = hn = h B0

2

Gyromagnetic ratio, , is a constant for each

nucleus (26,753 s-1gauss-1 for H).

In a 14,092 gauss field, a 60 MHz photon is

required to flip a proton.

Low energy, radio frequency.

Nuclear magnetic resonance process ; absorption occurs when n =

B0B0

+1/2 -1/2

69

The NMR Spectrometer

=>

70

The NMR Spectrum

=>

TMS

0 ppm

71

Magnetic Shielding

If all protons absorbed the same amount of

energy in a given magnetic field, not much

information could be obtained.

But protons are surrounded by electrons

that shield them from the external field.

Circulating electrons create an induced

magnetic field that opposes the external

magnetic field.

72

Shielded Protons

Magnetic field strength must be increased for a shielded proton to flip at the same frequency.

73

Protons in a Molecule

Depending on their chemical environment,

protons in a molecule are shielded by

different amounts.

Electronic structure of ethanol

74

75

NMR Signals

The number of signals shows how many

different kinds of protons are present.

The location of the signals shows how

shielded or deshielded the proton is.

The intensity of the signal shows the

number of protons of that type.

Signal splitting shows the number of

protons on adjacent atoms. =>

76

Number of Signals

77

Tetramethylsilane

TMS is added to the sample.

Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero.

Organic protons absorb downfield (to the left) of the TMS signal.

Si

CH3

CH3

CH3

H3C

How many signal number are in the given compound?

78

2 3

How many signal number are in the given compound?

79

82

•Equivalent = proton magnetic environments

identical in every way

•Nonequivalent = proton magnetic environments

not identical in one or more ways

•Easier to test for nonequivalency than for

equivalency

•Models:

Build two copies; label protons in question

Superpose protons in question

If rest of molecule superposable then protons in

question are equivalent

How to test for proton equivalency?

83

Equivalent proton;

enantiomers

Non-equivalent proton;

diastereomers

84

Chemical Shift ()

Measured in parts per million (ppm.).

Ratio of shift downfield from TMS (Hz) to

total spectrometer frequency (Hz).

Same value for 60, 100, or 300 MHz

machine.

Called the delta scale.

Chapter 13 85

Delta Scale

86

Location of Signals

More electronegative atoms

deshield more and give larger

shift values.

Effect decreases with

distance.

Additional electronegative

atoms cause increase in

chemical shift.

87

Typical Values

89

Aromatic Protons, 7-8

90

Vinyl Protons, 5-6

91

Acetylenic Protons, 2.5

92

Aldehyde Proton, 9-10

Electronegative

oxygen atom

93

O-H and N-H Signals

Chemical shift depends on concentration.

Hydrogen bonding in concentrated solutions

deshield the protons, so signal is around 3.5

for N-H and 4.5 for O-H.

Proton exchanges between the molecules

broaden the peak.

94

Carboxylic Acid Proton, 10+

Chapter 13 95

Number of Signals

Equivalent hydrogens have the same chemical

shift.

96

Intensity of Signals

The area under each peak is proportional to the

number of protons.

Shown by integral trace (integration line).

97

How Many Hydrogens?

When the molecular formula is known, each integral rise can be assigned to a particular number of hydrogens.

98

Spin-Spin Splitting

Nonequivalent protons on adjacent carbons

have magnetic fields that may align with or

oppose the external field.

This magnetic coupling causes the proton

to absorb slightly downfield when the

external field is reinforced and slightly

upfield when the external field is opposed.

All possibilities exist, so signal is split.

99

1,1,2-Tribromoethane

Nonequivalent protons on adjacent carbons.

100

Doublet: 1 Adjacent Proton

101

Triplet: 2 Adjacent Protons

102

The N + 1 Rule

If a signal is split by N equivalent protons,

it is split into N + 1 peaks.

103

Range of Magnetic Coupling

Equivalent protons do not split each other.

Protons bonded to the same carbon will split each other only if they are not equivalent.

Protons on adjacent carbons normally will couple.

Protons separated by four or more bonds will not couple.

104

Splitting for Ethyl Groups

105

Splitting for Isopropyl Groups

106

Coupling Constants ; J

Distance between the peaks of multiplet

Measured in Hz

Not dependent on strength of the external

field

Multiplets with the same coupling constants

may come from adjacent groups of protons

that split each other.

107

Coupling Constants

108

Complex Splitting

Signals may be split by adjacent protons, different from each other, with different coupling constants.

Example: Ha of styrene which is split by an adjacent Hb trans to its (J = 17 Hz) and an adjacent Hc cis to its (J = 11 Hz).

C C

H

H

Ha

b

c

109

Splitting Tree C C

H

H

Ha

b

c

110

Spectrum for Styrene

111

Stereochemical Nonequivalence

Usually, two protons on the same C are

equivalent and do not split each other.

If the replacement of each of the protons of a

-CH2 group with an imaginary “Z” gives

stereoisomers, then the protons are non-

equivalent and will split each other.

112

Some Nonequivalent Protons

C C

H

H

Ha

b

cOH

H

H

H

a

b

c

d

CH3

H Cl

H H

Cl

a b

113

Time Dependence

Molecules are tumbling relative to the

magnetic field, so NMR is an averaged

spectrum of all the orientations.

Axial and equatorial protons on cyclohexane

interconvert so rapidly that they give a single

signal.

Proton transfers for OH and NH may occur

so quickly that the proton is not split by

adjacent protons in the molecule.

114

Hydroxyl Proton

Ultrapure samples of ethanol show splitting.

Ethanol with a small amount of acidic or basic

impurities will not show splitting.

115

N-H Proton

Moderate rate of exchange.

Peak may be broad.

116

Identifying the O-H or N-H Peak

Chemical shift will depend on concentration and solvent.

To verify that a particular peak is due to O-H or N-H, shake the sample with D2O

Deuterium will exchange with the O-H or N-H protons.

On a second NMR spectrum the peak will be absent, or much less intense.

117

Carbon-13 NMR

12C has no magnetic spin.

13C has a magnetic spin, but is only 1% of the

carbon in a sample.

The gyromagnetic ratio of 13C is one-fourth

of that of 1H.

Signals are weak, getting lost in noise.

Hundreds of spectra are taken, averaged.

118

Fourier Transform NMR

Nuclei in a magnetic field are given a radio-

frequency pulse close to their resonance

frequency.

The nuclei absorb energy and precess

(spin) like little tops.

A complex signal is produced, then decays

as the nuclei lose energy.

Free induction decay is converted to

spectrum.

Chapter 13 119

Hydrogen and Carbon Chemical

Shifts

120

Combined 13C and 1H Spectra

121

Differences in 13C Technique

Resonance frequency is ~ one-fourth, 15.1

MHz instead of 60 MHz.

Peak areas are not proportional to number

of carbons.

Carbon atoms with more hydrogens absorb

more strongly.

122

Spin-Spin Splitting

It is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible.

13C will magnetically couple with attached protons and adjacent protons.

These complex splitting patterns are difficult to interpret.

123

Proton Spin Decoupling

To simplify the spectrum, protons are continuously irradiated with “noise,” so they are rapidly flipping.

The carbon nuclei see an average of all the possible proton spin states.

Thus, each different kind of carbon gives a single, unsplit peak.

124

Off-Resonance Decoupling

13C nuclei are split only by the protons

attached directly to them.

The N + 1 rule applies: a carbon with N

number of protons gives a signal with

N + 1 peaks.

125

Interpreting 13C NMR

The number of different signals indicates the

number of different kinds of carbon.

The location (chemical shift) indicates the

type of functional group.

The peak area indicates the numbers of

carbons (if integrated).

The splitting pattern of off-resonance

decoupled spectrum indicates the number of

protons attached to the carbon.

126

Two 13C NMR Spectra

127

MRI

Magnetic resonance imaging, noninvasive

“Nuclear” is omitted because of public’s fear that it would be radioactive.

Only protons in one plane can be in resonance at one time.

Computer puts together “slices” to get 3D.

Tumors readily detected.

128

Mass Spectrometry



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129

Mass Spectrometry

Molecular weight can be obtained from a very small sample.

It does not involve the absorption or emission of light.

A beam of high-energy electrons breaks the molecule apart.

The masses of the fragments and their relative abundance reveal information about the structure of the molecule.

130

Electron Impact Ionization

A high-energy electron can dislodge an

electron from a bond, creating a radical

cation (a positive ion with an unpaired e-).

e- + H C

H

H

C

H

H

H

H C

H

H

C

H

H

H

H C

H

H

C

H

H

+ H

H C

H

H

C

H

H

H

+

131

Separation of Ions

Only the cations are deflected by the

magnetic field.

Amount of deflection depends on m/z.

The detector signal is proportional to the

number of ions hitting it.

By varying the magnetic field, ions of all

masses are collected and counted.

132

Mass Spectrometer

133

The Mass Spectrum

Masses are graphed or tabulated according to

their relative abundance.

134

The GC-MS

A mixture of compounds is separated

by gas chromatography, then identified

by mass spectrometry.

135

High Resolution MS

Masses measured to 1 part in 20,000.

A molecule with mass of 44 could be C3H8,

C2H4O, CO2, or CN2H4.

If a more exact mass is 44.029, pick the

correct structure from the table:

C3H8 C2H4O CO2 CN2H4

44.06260 44.02620 43.98983 44.03740

136

Molecules with Heteroatoms

Isotopes: present in their usual abundance.

Hydrocarbons contain 1.1% C-13, so there

will be a small M+1 peak.

If Br is present, M+2 is equal to M+.

If Cl is present, M+2 is one-third of M+.

If iodine is present, peak at 127, large gap.

If N is present, M+ will be an odd number.

If S is present, M+2 will be 4% of M+. =>

137

Isotopic Abundance

81Br

138

Mass Spectrum with Sulfur

139

Mass Spectrum with Chlorine

140

Mass Spectrum with Bromine

141

Mass Spectra of Alkanes

More stable carbocations will be more

abundant.

142

Mass Spectra of Alkenes

Resonance-stabilized cations favored.

143

Mass Spectra of Alcohols

Alcohols usually lose a water molecule.

M+ may not be visible.

References

1. Pavia, D.L.; Lampma, G.M.; Kriz, G.S.; and

Vyvyan, J.R., Introduction to spectroscopy,

2009, 4th ed, USA, Brooks/Cole, 2009.

2. Clayden, J.; Greeves, N.; Warren, S.; and

Wothers, P., Organic Chemistry, 1st ed.,UK

Oxford University Press, 2001.

3. Field, L.D.; Sternhell, S.; and Kalman, J.R.,

Organic Structures from Spectra, 3rd ed., UK,

John Wiley & Sons, 2002.

144

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