13C NMR Spectroscopy

2

Introduction

• Nuclear magnetic resonance spectroscopy (NMR) is the most

powerful tool available for structural determination.

• A nucleus with an odd number of protons, an odd number of

neutrons, or both, has a nuclear spin that can be observed by the

NMR spectrometer.

• NMR active nuclei include: 1H, 13C, 19F, and 31P.

• Remember a spinning nucleus generates a magnetic field (magnetic

moment).

• In the absence of an external magnetic field, proton magnetic

moments have random orientations.

• However, in the presence of an external magnetic field, the

magnetic moment is aligned either with or against the external field.

3

Introduction

• The stronger the magnetic field, the greater the energy difference

between the two spin states, resulting in a greater population

difference between the two states greater sensitivity.

4

Nuclear Spin Energy Levels

• A photon of light with the right amount of energy (radiofrequency, rf)

can be absorbed and cause the spinning proton to flip.

• The nuclei undergo a “spin flip”, and the nuclei are said to be “in

resonance”. This absorption of energy leads to the NMR signal

Exte

rnal m

ag

neti

c f

ield

E1

E2

E1

E2

habsorption

of energy

5

Nuclear Spin Energy Levels

• If the two states become equally populated, then no net spin transitions occur and no signal is produced. This is called saturation.

• The frequency of EM radiation necessary for resonance depends on the strength of the magnetic field and on the chemical environment of the nucleus.

• Fortunately, protons (in 1H NMR) in molecules usually experience different chemical environments (i.e. are shielded to varying extents).

6

1H NMR Spectroscopy

• Therefore, different frequencies are required to bring

different protons into resonance.

• Consider CH3OH:

O C H

H

H

H

Deshielded, senses higher

effective magnetic field so

comes into resonance at a

higher frequency.

Shielded, senses a smaller

effective magnetic field so

comes into resonance at a

lower frequency.

7

1H NMR Spectroscopy

• A 1H NMR spectrum provides the following information:

1. The # of different types of H – number of basic groups of

signals.

2. The relative numbers of different types of H –

3. The electronic environment of the different types of H –

4. The number of hydrogen “neighbors” a proton has –

8

Simple Correlation Table of 1H

Chemical Shifts

* See text and Lab manual for more extensive tables

9

Why Carbon (13C) NMR Spectroscopy

• Some organic compounds have few C-H bonds:

• Others have very similar 1H NMR spectra:

C

CC

C

C

O

O

O

HO

HO

2,6-dimethylbenzoquinone

O

O

CH3H3C

HH

2,5-dimethylbenzoquinone

O

O

HH3C

CH3H

10

Carbon (13C) NMR vs 1H NMR

• The 13C nucleus can also undergo nuclear magnetic resonance.

• 13C NMR vs 1H NMR :

– 12C, the most abundant isotope of carbon, does NOT exhibit NMR behavior. Why?

– 13C, only _____ natural abundance, does exhibit NMR behavior.

– Due to low abundance, 13C-13C coupling is usually not observed.

– Chemical shift ranges are much larger –

– Integration in 13C NMR is NOT reliable due to variable relaxation times from C to C. Also Nuclear Overhauser Effect - the intensity of the C signal increases as the number of attached protons increases. Not uniform however.

11

Fourier Transform (FT) spectroscopy

• The magnetic moment of the 13C nucleus is about 1/4 that of the H nucleus resulting in lower sensitivity.

• The low natural abudance and small magnetic moment of the 13C isotope results in the 13C nucleus being about ______ less sensitive than the 1H nucleus to NMR phenomena.

• Consequently, much longer acquisition times were required.

• The development of Fourier transform (FT) spectroscopy has made 13C NMR acquisition routine.

• The old way of acquiring NMR was to apply a constant magnetic field to the sample and scan the range of frequencies = continuous wave (CW) NMR.

• With FT-NMR the data is collected all at once by exciting the sample with an RF pulse (typically only a few microseconds long) which covers all the resonance frequencies, and thus changes the orientation of all the protons.

12

Fourier Transform (FT) spectroscopy

• After the pulse has stopped, the decay of the signal from the sample is measured. The decaying sine wave called a free induction decay (FID):

• A Fourier transform converts the intensity vs time data into intensity vs frequency information.

Time (s) Frequency

Fourier

Transform

CH3C CH3

O

Inte

nsity o

f sig

nal

13

Fourier Transform (FT) spectroscopy

Fourier

Transform

14

Chemical Shifts in 13C NMR

• Two simple ideas will make interpretation of 13C

NMR spectra easier:

1. Hybridization of the C atom determines the

chemical shift:

sp3 hybridized carbons have chemical shift

values _________.

sp2 hybridized carbons have chemical shift

values _________.

2. The presence of an EN element near a C atom will

cause its chemical shift to move _____________.

15

Simple Correlation Table of 13C

chemical shifts

See text (p. 593) and Lab manual (p. 60) for more extensive tables

16

Coupling in Carbon NMR

• The low abundance of 13C makes C-C coupling very rare.

• However, 13C-H coupling is common. N+1 rule still applies:

Coupling constants are large ~100-200 Hz for directly attached H’s.

17

Coupling in Carbon NMR

• Spectra which show 13C-H coupling are called proton-coupled spectra.

• However, extensive 13C-H coupling often produces splitting patterns that are difficult to interpret.

• To simply 13C NMR spectra, often recorded using broad band proton decoupling.

• Therefore each carbon signal appears as a singlet, because C-H splitting has been eliminated.

• Spectra recorded in the broad band proton decoupling mode give the number of unique carbon atoms in a molecule.

18

Proton-coupled vs Proton-decoupled 13C NMR Spectra

19

Interpreting 13C NMR Spectra

H3C C CH2 CH3

CH3

CH3

TMS

CDCl3solvent

20

Interpreting 13C NMR Spectra

CCH3 O

O

CH2 CH3

21

Interpreting 13C NMR Spectra

CH3

O

22

Interpreting 13C NMR SpectraO

O

CH3H3C

HH

O

O

HH3C

CH3H

Mass Spectrometry

24

Basic Principles

• Mass Spectroscopy (MS) is a destructive analytical technique for

measuring the ______________ (____) of ions in the gas phase.

This allows accurate determination of the _______________ of a

molecule.

Structural information is also gained.

Molecular Formula determination is sometimes possible.

• While the method is destructive, only very small amounts (1 mg or

less) is required.

25

• MS does not involve the absorption or emission of light.

• A mass spectrometer is designed to do 3 things:

1. Convert a neutral molecule, M, into positive (or negative) ions

usually by bombardment with a beam of high energy

electrons.

2. Separate the ions based on mass (mass-to-charge ratio, ___).

3. Measure the relative abundance of each ion.

Basic Principles

M + e M + 2e

10-70 eVin energy

1 eV = 23 kcal/mol

26

Schematic of Mass Spectrometer

• First the sample is vaporized under vacuum.

• A beam of electrons bombards the molecules in the gas phase

causing ionization and formation of radical cations.

~70 Volts

+

_

+

_

e- e-e-

++ ++

++

_

Electron Collector (Trap)

Repeller

ExtractionPlate

Filament

toAnalyzer

Inlet

Electrons

NeutralMolecules

PositiveIons

Electron ImpactIonization Source

Electron impact

Ionization source

27

Schematic of Mass Spectrometer

• The radical cations fragment further after ionization owing to the

large amount of energy transferred by the electron beam.

• Some fragments carry a positive charge, others are neutral:

• Only the positively charged fragments are accelerated into the

analyzer tube.

CH4- e

C

H

HH

H

m/z =

[CH3] + H

m/z =

[CH2] + 2H

m/z =Molecular ion

28

Schematic of Mass Spectrometer

• The analyzer tube is surrounded by a magnet whose magnetic field

deflects the positively charge fragments in a curved path.

• The amount of deflection depends on m/z.

ion trajectory not in register(too heavy)

IonSource

Detector

ion trajectory not in register

(too light)

ion trajectory in register

S

N

Magnetic Sector Mass Analyzer

Electromagnet

29

Basis of Fragment Separation

• Fragments with smaller m/z value are deflected ______ than a

larger m/z value.

• Since z is usually _____, the fragments are sorted by mass.

• By varying the magnetic field, cations of different masses are sorted

and counted by a detector.

• The more stable the fragment the more likely it will make it to the

detector.

• The masses are graphed or tabulated according to their relative

abundance = The Mass Spectrum.

30

The Mass Spectrum of Methane

m/z

12 13 14 15 16 17

M+ = 15

C12H3+

M+ = 16

Molecular ion

[C12H4]+.

[C12H2]+.

[C12]+.

C12H+

m/z Intensity

1 3.4

2 0.2

12 2.8

13 8.0

14 16.0

15 86.0

16 100.0

17 1.11

Base peak

31

Isotopes

• Most elements common to organic compounds are mixtures of isotopes.

• The existence of atomic isotopes in nature accounts for the appearance of

M+1 and M+2 peaks in a mass spectrum.

• Organic compounds containing only C, H, O, and N usually have relatively

small M+1 and M+2 peaks.

C6H12

Rela

tive a

bundance, % M+

M+1+

m/z

M+

M+1+

M+2+

m/z

C20H42

32

Isotopes

Element Most abundant isotope Less abundant isotope Relative abundance

Hydrogen 1H 2H 0.016

Carbon 12C 13C 1.08

Nitrogen 14N 15N 0.38

Oxygen 16O 18O 0.20

Sulfur 32S 34S 4.4

Chlorine 35Cl 37Cl 32.5

Bromine 79Br 81Br 98.0

33

Isotopes

• MS is particularly valuable for compounds which contain Cl and Br:

If one S atom is present, M + 2 is ~ 4% of M+.

If one Cl atom is present, M + 2 is ~ 33% of M+.

If one Br atom is present, M + 2 is ~ to M+.

m/z

Rela

tive a

bundance, % M+

M+2+

M

M+224

1

M

M+23

1

M

M+21

1

M+

M+2+

M+ M+2+

m/z m/z

34

Mass Spectrum with Chlorine

M+

M+2+

CHH3C CH3

Cl

35

Mass Spectrum with Bromine

M+ M+2+

CH2H3C CH2 Br

36

Isotopes

• Carbon Rule – For compounds containing only C, H, and O, the

following formula can be used to determine the number of carbons

in the molecule:

1.1

peak 1+ Mofintensity relative = sC no.

Determine the molecular formula of the unknown organic compound

whose mass spectral data is given in the table below:

Peak Mass

(m/z)

Relative

intensity

M 86 100.0

M+1 87 5.6

M+2 88 0.4

37

Isotopes

• Nitrogen Rule: if a compound has:

– An odd number of nitrogen atoms, its molecular ion, M+, will be

odd.

– Zero or an even number of nitrogen atoms, its molecular ion, M+,

will be even.

38

Resolution

• Resolution: a measure of how well a mass spectrometer separates

ions of different mass.

Low resolution – capable of distinguishing among ions of different

nominal mass, i.e. different by at least one or more amu.

High resolution – capable of distinguishing among ions that differ in

mass by as little as 0.0001 amu.

• For example: CO, N2, and ethene all have a nominal mass of 28

amu. High resolution MS can distinguish these molecules.

CO 27.9949 amu

N2 28.0061 amu

CH2=CH2 28.0314 amu

39

Fragmentation Pathways

• Structural information is available from analysis of fragments formed

by bond cleavages in the molecular ion, M+.

• In general, the molecular ion, M+, will fragment so as to form the

most stable cationic fragment (usually a carbocation).

• In some cases, the M+ peak is very small or absent. Occurs if the

fragments are considerably more stable M+.

40

Mass Spectrum-Fragmentation• Consider the mass spectrum of pentane: p. 516-517 text

• Fragmentation of the molecular ion often results:

41

Mass Spectrum of Pentane

42

Fragmentation of Alkanes

CH3CHCH2CH2CH3

CH3

CH2CH2CH3 CH3HC

CH3

+

CHCH2CH2CH3

CH3

+ CH3

CH2CH3 + CH3CHH2C

CH3

1

2

3m/z 86

m/z 43

m/z 71

m/z 57

86 M+

71

57

CH3CHCH2CH2CH3

CH343

43

Compounds with Heteroatoms

• Molecules containing O, N, halogens, or other heteroatoms often

undergo ___________ (adjacent to heteroatom).

• Driving force is resonance stabilized cations.

44

Fragmentation of Alcohols

• Alcohols common fragmentation is -cleavage and loss of “H2O” to

give an M-18 peak.

M+ = 88 (not observed) M-15

M-18

M-29

M-18 -15

C

OH

H3C CH2 CH3

CH3

45

Fragmentation of Amines

Mass spectrum of isobutylamine

M+ = 73

CH2=NH2

+

m/z=30

NH2CH2CH

H3C

H3C

46

Fragmentation of Ketones

Mass spectrum of 2-butanone

M+ = 72

57

43

C

O

H3C CH2 CH3

47

McLafferty Rearrangement

• If one of the alkyl groups attached to the carbonyl carbon of an

aldehyde or ketone has a hydrogen, a cleavage known as a

McLafferty rearrangement can occur.Mass spectrum of butyraldehyde

M+=72

M–28

O

H

48

Aromatic Compounds

• Usually strong M+ peak.

• m/z =91 for tropylium ion and methylene spacings above 91 (105,

119, etc. for alkyl chains) often observed.

• m/z = 65 (C5H5+), 77 (C6H5

+) are sometimes observed.

-R

m/z 91

CH2 R CH2

m/z 91

49

Aromatic Compounds

50

Common Fragments

CH2

CH3 CH3CH2

O

CH

O

CH3C

O

CHO

C

O

m/z = 15 29 29 43

m/z = 45 91 105