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13C-NMR, 2D-NMR, and MRI

Lecture Supplement:Take one handout from the stage

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Midterm Exam 2Date: Monday May 21Time: 5:00-6:50 PMTopics: All of spectroscopy (mass spectrometry today)Location: last name A-La in Haines 39 last name Le-Z in Moore 100

Calculators not allowed

Question and Answer Session•Lecture time, Monday May 21•Submit questions to harding@chem.ucla.edu•Label as “Question for Q&A”•Deadline for possible inclusion: noon Sunday May 20

Extra Office Hours•Saturday 3-5 PM, Young Hall 3077F (Steve Joiner)•Sunday ???

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13C-NMRIs NMR limited to 1H?•Any nucleus with I 0 can be observed• I 0 when nucleus has odd number of protons or odd number of neutrons•Includes 1H, 2H, 13C, 19F, 29Si, 31P, 127I, etc.

Examples•19F: 9 protons, 10 neutrons; 100% natural abundance•31P: 15 protons, 16 neutrons; 100% natural abundance}

•Easily observed by NMR•Limited value for organic structure analysis

13C-NMR•Carbon is backbone of organic molecules so 13C-NMR has high potential, but...•Low natural abundance: 13C = 1.1% (12C = 98.9% but has 6 protons and 6 neutrons)•Low probability that photon absorption causes spin flip: 1.6% compared to 1H•Result: 13C spin flip much harder to observe than 1H spin flip•Modern NMR spectrometers have overcome these problems; 13C-NMR now routine

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13C-NMR

What can we deduce about molecular structure from 13C-NMR spectrum?

•NMR fundamentals are the same regardless of nucleus

Information from carbon NMR spectrum

•Number of signals: equivalent carbons and molecular symmetry

•Chemical shift: presence of high EN atoms or pi electron clouds

•Integration: ratios of equivalent carbons

•Coupling: number of neighbors

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13C-NMR: Number of SignalsNumber of 13C-NMR signals reveals equivalent carbons•One signal per unique carbon type•Reveals molecular symmetryExamples

CH3CH2CH2CH2OH CH3CH2OCH2CH3

Two 13C-NMR signals

2 x CH3 equivalent2 x CH2 equivalent

No equivalent carbonsFour 13C-NMR signals

Symmetry exists when # of 13C-NMR signals < # of carbons in formula

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13C-NMR: Position of Signals•Position of signal relative to reference = chemical shift•13C-NMR reference = TMS = 0.00 ppm•13C-NMR chemical shift range = 0 - 250 ppm•Downfield shifts caused by electronegative atoms and pi electron clouds

OH does not have carbon

no 13C-NMR OH signal

Example: HOCH2CH2CH2CH3

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13C-NMR: Position of Signals

It is not necessary to memorize this table.It will be given on an exam if necessary.

Trends•RCH3 < R2CH2 < R3CH•EN atoms cause downfield shift•Pi bonds cause downfield shift•C=O 160-210 ppm

Characteristic Carbon NMR Chemical Shifts (ppm)

(CH3)4Si = TMS = 0.00 ppm (singlet) CDCl3 (solvent) = 77.0 ppm (triplet)

RCH3 0 – 40 RCH2Cl 35 – 80 benzene ring 110 – 160

RCH2R 15 – 55 R3COH 40 – 80 C=O ester 160 – 180

R3CH 20 – 60 R3COR 40 - 80 C=O amide 165 – 180

RCH2I 0 – 40 RCCR 65 – 85 C=O carboxylic acid 175 – 185

RCH2Br 25 - 65 R2C=CR2 100 - 150 C=O aldehyde, ketone 180 – 210

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13C-NMR: Integration1H-NMR: Integration reveals relative number of hydrogens per signal

13C-NMR: Integration reveals relative number of carbons per signal

•Rarely useful due to slow relaxation time for 13C

•Relaxation time important phenomenon for MRI

time for nucleus to relax fromexcited spin state to ground state

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13C-NMR: Spin-Spin Coupling•Spin-spin coupling of nuclei causes splitting of NMR signal•Only nuclei with I 0 can couple•Examples: 1H with 1H, 1H with 13C, 13C with 13C•1H NMR: splitting reveals number of H neighbors•13C-NMR: limited to nuclei separated by just one sigma bond; no pi bond “free spacers”

Conclusions•Carbon signal split by attached hydrogens (one bond coupling)•No other coupling important

1H

13C

13C

12C

Coupling observed

Coupling occurs but signal very weak:low probability for two adjacent 13C

1.1% x 1.1% = 0.012%

No coupling: too far apart

No coupling: 12C has I = 0

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13C-NMR: Spin-Spin Coupling

•Carbon signal split by attached hydrogens•N+1 splitting rule obeyed

Quartet Triplet Doublet Singlet

C

H

H

H

C

H

H

C

H

C

O

Example

1H-13C Splitting Patterns

How can we simply this?

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O

Proton decoupled

13C-NMR: Spin-Spin Coupling

•Broadband decoupling: all C-H coupling is suppressed•All split signals become singlets•Signal intensity increases; less time required to obtain spectrum

Simplification of Complex Splitting Patterns

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13C-NMR: Spin-Spin CouplingDistortionless Enhancement by Polarization Transfer (DEPT)

CH3

O

Example

All carbons

•Assigns each 13C-NMR signal as CH3, CH2, CH, or C

CH3 onlyCH2 onlyCH only

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Two-Dimensional NMR (2D-NMR)•Basis: interaction of nuclear spins (1H with 1H, 1H with 13C, etc.) plotted in two dimensions

•Applications:

Simplifies analysis of more complex or ambiguous cases such as proteins

Obtain structural information not accessible by one-dimensional NMR methods

•Techniques include:Correlation Spectroscopy (COSY)

Heteronuclear Correlation Spectroscopy (HETCOR)

Heteronuclear Multiple-Quantum Coherence (HMQC)

Nuclear Overhauser Effect Spectroscopy (NOESY)

Incredible Natural Abundance Double Quantum Transfer Experiment (INADEQUATE)

Many others

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2D-NMRCOSY: Correlation of 1H-1H coupling

O

O

H

CH2OH

O

H

CH2OH

HO

H

OH

H

H

HO

HOHO

H

HCH2

H

OH

1

2

3

4

5

6

7

8

9

10

11

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•Dots = correlations•Ignore dots on diagonal

Sucrose 1H-NMR

Sucr

ose

1 H-N

MR

Examples•H6 and H5 are coupled•Identify H8 by its coupling with H9

H8

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2D-NMRHMQC: Correlation of spin-spin coupling between 1H and nuclei other than 1H such as 13C

O

O

H

CH2OH

O

H

CH2OH

HO

H

OH

H

H

HO

HOHO

H

HCH2

H

OH

1

2

3

4

5

6

7

8

9

10

11

12

Sucr

o se

13C-

NMR

Sucrose 1H-NMR

•No diagonal•Example Which carbon bears H6?

92 ppm

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Magnetic Resonance Imaging (MRI)Basis: Spin-excited nuclei relax at a rate dependent on their environment•Environmental factors = bonding to other atoms, solvent viscosity, etc.•Photons released upon excitation are detected•1H relaxation times varies with tissue type (brain, bone, etc.)•Therefore tissues may be differentiated by NMR

Timeline

1971: First MRI publication: “Tumor Detection by Nuclear Magnetic Resonance”

Science, 1971, 171, 1151

2002: 22,000 MRI instruments in use; 6 x 107 MRI exams performed

2003: Nobel Prize in Physiology or Medicine: to Paul Lauterbur and Peter Mansfield

for “their discoveries concerning magnetic resonance imaging”

http://nobelprize.org

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Magnetic Resonance Imaging (MRI)NMR and MRI Use Similar Instruments

Powerful magnets

An NMR spectrometer An MRI instrument

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Magnetic Resonance Imaging (MRI)MRI Images: Quite Different from NMR Spectra!

MRI image: a foot MRI image: a head