proton nuclear magnetic resonance ( 1 h-nmr) spectroscopy - part 1 lecture supplement page 133

Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 1

Lecture Supplement page 133

Organic Structure AnalysisCrews, Rodriguez, Jaspers1998 p.5-6

1H-NMR SpectroscopyBackground and Theory

Fundamental principleThe energy required to cause nuclear spin flip is a function of the magnetic environment

of the nucleus.

•Protons, electrons, neutrons have “spin” (I)•Motion of charged particle creates magnetic field•In absence of external influence, magnetic poles (spin axis) randomly oriented

No external magnetic fieldSpin alignment random

With external magnetic fieldSpins aligned

Bo

•Add external magnetic field (Bo) spins align

add magnetic field

Background and TheoryNuclear Spin Flip

•I = +1/2 parallel to Bo (lower energy); I = -1/2 antiparallel to Bo (higher energy)

•Addition of energy results in nuclear spin flip

Excited stateNuclear spin antiparallel to Bo

Higher energy

I = +1/2

I = -1/2

Ground stateNuclear spin parallel to Bo

Lower energy

Incr

easi

ng e

nerg

y

Absorb energy(excitation)

Release energy(relaxation)

E ~ 0.02 cal mol-1 = radio wave photons

Contrast this with absorption of infrared light (p. 114 of lecture supplement)

Background and TheoryMagnetic Field Controls E

• E influenced by magnetic field strength at nucleus

Information about magnetic field strength at nucleus

Energy required for spin flip (E)

Information about chemical structure

I = -1/2

I = +1/2

Spin

sta

te e

nerg

y

Magnetic field strength at nucleus

Large magnetic field large E

Small magnetic field small EE E

Background and TheoryThe NMR Spectrum

•Spectrum = plot of photon energy versus photon quantityIn

tens

ity o

f sig

nal

(pho

ton

quan

tity)

Spin flip energy (photon energy)

NMR signal

Background and TheoryThe NMR Spectrum

Nuclear: Manipulation of nuclear spin

Magnetic: Magnetic field strength influences E

X

Resonance: Tendency of a system to oscillate at maximumamplitude at a certain frequency

1H nucleus = a proton 1H-NMR = proton NMR

C O C O

Background and TheorySpectrum Structure

Information from NMR spectrum:

•Number of signals number of nonequivalent proton groups in molecule

•Position of signals (chemical shift) magnetic environment of protons

•Relative intensity of signals (integration) ratio of equivalent proton types

•Splitting of signals (spin-spin coupling) proton neighbors

How do we deduce structure from NMR spectrum?

Number of SignalsProton Equivalency

•Photon energy controlled by magnetic environment of nucleus•Nuclei in same magnetic environment = equivalent•Multiple magnetic environments multiple signals•Number of signals = number of equivalent proton sets

Protons equivalentOne NMR signal

OH H

Protons not equivalentTwo NMR signals

Cl CN

HH

•NMR signal due to photon absorption


•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

How to test for equivalency?

Useful vocabulary

CH3 = methyl CH2 = methylene CH = methine

H C

H

H

C

H

H

C H

Cl

Cl

H C

H

H

C

H

H

C H

Cl

Cl

H C

H

H

C

H

H

C H

Cl

Cl


Proton equivalency examples

One signal

CH

H

Cl

H

CC

H

Cl

H

H

H

H

Two signals ?

•NMR = camera with slow shutter speed•NMR detects only average when rotation is fast•Thousands of 360o bond rotations per second•Therefore Ha, Hb, Hc appear equivalent

Hb, Hc equivalent

Ha, Hc not equivalent

rapid equilibrium

Ha, Hc equivalent

Hb, Ha not equivalent

Single bond rotation in acyclic molecules often allows equivalency


Proton equivalency examples

One signal

CH

H

Cl

H

CC

H

Cl

H

H

H

H

Two signals ? Hb, Hc equivalent

Ha, Hc not equivalent

rapid equilibrium

Ha, Hc equivalent

Hb, Ha not equivalent

vs.

C

C C

C

CC

H

HH

H

H H


More proton equivalency examples

Three signals

CC

H

OH

H

H

H

H

CC

Cl

H

Cl

F

H

F

Two signals

CC

F

Cl

Cl

H

H

F

One signal

C

C C

C

CC

C

HH

H

H H

H

H

H

Four signals

mirror plane

One signal


•Verify what we have learned about equivalent protons•Which spectrum belongs to this molecule?

Sample spectra

Three proton sets three signals

C C

O

H3C

H3C H

H

H3C C

H3C

H3C

C OH

H

H


Which spectrum belongs to this molecule?

Two proton sets two signals

HH

H H

OHHO

HO OH H3C CH3



CH3C

OCH3

OCH3

CH3

1H-NMR Spectroscopy Part 1 Summary

•Nuclear spin axis can be parallel or antiparallel to external magnetic field (Bo)•Spin parallel to Bo (I = +1/2) lower energy than spin antiparallel to Bo (I = -1/2)•Energy difference between spin states (E) controlled by magnetic field at nucleus•Absorption of radio wave photon with energy = E causes nuclear spin flip•NMR spectrum = plot of photon energy (spin flip energy) versus photon quantity

Information from NMR spectrum•Number of signals reveals number of sets of equivalent protons

Equivalency: Protons must be identical in all ways to be equivalentNonequivalency: Protons can be different in just one wayExample: 1H-NMR spectrum of CH3CH2OH has three signals

•Position of signal (chemical shift)•Relative intensity of signals (integration)•Splitting of signals (spin-spin coupling)

•Atomic nucleus has spin, and therefore generates a magnetic field

Vollhardt, 10-2

Position of SignalsThe Chemical Shift

How does spin flip energy relate to molecular structure?

•Magnetic field strength varies between NMR spectrometers•High magnetic field = higher spectral resolution (more spectral detail)•Need a spin flip energy scale that is independent of magnetic field strength•Chemical shift: Spin flip energy scale normalized to be independent of field strength

I = -1/2

I = +1/2

Spin

sta

te e

nerg

y

Magnetic field strength at nucleus

Large magnetic field large E

Small magnetic field small E

•Spin flip energy depends on magnetic field strength:

E E


How does molecular structure influence chemical shift?

•Chemical shift controlled by E which is controlled by magnetic field at nucleus

What contributes to magnetic field at nucleus?

•Electron cloud shields atomic nucleus from external magnetic fields

Shielded: Nucleus feels weaker magnetic field

Deshielded: Nucleus feels stronger magnetic field

Spectrometer’s magnetic fieldStrong; typically 94 kilogauss

Earth’s magnetic fieldWeak; 0.3-0.6 gauss

Other electrons andnuclei in the molecule

HO CH2 CH2 CH3


Inte

nsity

of s

igna

l(p

hoto

n qu

antit

y)

Spin flip energy (photon energy)

Deshielded Shielded

15 ppm 0 ppmChemical shift scale(ppm)

Reference point?

0.00 ppm (CH3)4SiTetramethylsilane (TMS)

High Magnetic Field StrengthLow Magnetic Field Strength


•How does molecular structure influence chemical shift? Chemical shifts for H3C-X:0.01.02.03.04.05.0 ppm

(CH3 )4 Si 0.00 ppm

CH4 0.23 ppm

CH3 I 2. 16 ppm

CH3 Br 2.68 ppm

CH3 Cl 3.05 ppm

CH3 O

H 3.42 ppm

CH3 F 4.26 ppm

1.0

2.0

3.0

4.0

EN o

f X in

CH

3-X

Conclusion: EN of atoms near H chemical shift

Si1.8H

2.1I

2.5Br2.8Cl

3.0O

3.5F

4.0

CH3 CH

3 0.86 ppm

C2.5

Vollhardt, Fig 10-9

HO CH2 CH2 CH3

•Electron cloud shields atomic nucleus from external magnetic fields

Shielded: Nucleus feels weaker magnetic field

Deshielded: Nucleus feels stronger magnetic field

deshielded shielded


How does electronegativity influence chemical shift?

H I H Br H Cl H F

Iodine has low EN Fluorine has high EN

Electron density at H is high Electron density at H is low

H more shielded H is less shielded

H has higher chemical shiftH has lower chemical shift

Metaphor: Ozone layer shields Earth

•Chemical shift related to magnetic field strength at nucleus

•Electron cloud shields nucleus from effects of Bo

Position of Signals

Do not memorize chemical shifts. Table given on exams.

Position of SignalsNotes On Characteristic Chemical Shifts Table

•Characteristic shifts are typical proton averages. Actual shifts may lie outside given range.

CH3

O

Typically 2.0-2.6 ppm CH3O CH3

O

2.01 ppm CH3

O

2.59 ppm

Useful chemical shift trends•RCH3 < RCH2R < R3CH

•EN effects decrease with distance:

EN of C (in R) > EN of H

CH4 CH3OH CH3CH2OH CH3CH2CH2OH

0.23 ppm 3.39 ppm 1.18 ppm 0.93 ppm 3.49 ppm

3.59 ppm 1.53 ppm

Position of Signals

•Example:

3.8 ppm not always ROCH3

2.3 ppm not always ArCH3

Common exception

benzene ring protons6.5-8.0 ppm usually

C=O stretch

Avoid this common misconception:“NMR peaks can be assigned based on chemical shift alone”

Relative Intensity of PeaksIntegration

Information from NMR spectrum

•Number of signals number of nonequivalent proton groups in molecule

•Position of signals (chemical shift) magnetic environment of protons

•Relative intensity of signals (integration) ratio of equivalent proton types

•Splitting of signals (spin-spin coupling) proton neighbors

Relative Intensity of PeaksIntegration

•Beer’s Law: Amount of energy absorbed or released proportional to moles of stuff present

∫ir I∫aac Newton Gottfried LeibnizInventor∫ of calculu∫

•Relative intensities of NMR signals proportional to relative number of equivalent protons

•Integrals do not always correspond to exact number of protons

Example: Integrals of 2:1 might be 2H:1H or 4H:2H or...

•NMR: Amount of radio wave energy proportional to peak area

•Measurement of peak areas = integration

Peakheight

Peakarea

Sample Spectra

•Verify what we have learned about equivalent protons, chemical shifts, and integration

4.19 ppm: integral = 1.03.41 ppm: integral = 3.0

(1 H)(3 H)

CH3OH has 4 H

H C

H

OH

H

•Assign peaks to corresponding hydrogens:

Sample Spectra

Assign peaks to corresponding hydrogens:


(6 H)(6 H)

C5H12O2 has 12 HTwo equal integrals

Two groups of equivalent H

CH3C

OCH3

OCH3

CH3

Smallest integral often set = 1Integration gives proton ratio

Sample Spectra

Assign peaks to corresponding hydrogens:


(4 H)(6 H)

Two groups of equivalent HTwo unequal integrals

C4H10O2 has 10 H10 H / (1.0 + 1.5) = 4 H per unit

CH3OCH2CH2OCH3

C C

O

H3C

H3C H

H

H3C C

H3C

H3C

C OH

H

H

Lecture Supplement p. 138, 139, 146

HO OH

H3C CH3

CH3C

OCH3

OCH3

CH3

CH3OCH2CH2OCH3

CH3CH2Br

1

2

3

1

2

2

2

2

2

Sample Spectra•Assign peaks to corresponding hydrogens:

CH3CH2Br

•Why the extra peaks? Hint: Think about spin and magnetic fields

Four peaks!

Three peaks!



CH3CH2Br


1. Number of signals how many sets of equivalent protons2. Position of signals (chemical shift) magnetic environment of nucleus

Deshielding by electronegative atoms higher chemical shift3. Relative intensity of signals (integration) how many hydrogens per signal

Integrals give proton ratio; not always equal to absolute proton count (i.e., 1.5:1)4. Splitting of signals (spin-spin coupling)Example: 3.55 ppm: integral = 1.0

3.39 ppm: integral = 1.54 H6 H

Two groups of equivalent HTwo unequal integrals

C4H10O2 has 10 H10 H / (1.0 + 1.5) = 4 H per unit

CH3OCH2CH2OCH3

Information from NMR Spectrum

Signal Splitting


Two unequal integrals5 H / (1.0 + 1.5) = 2 H per unit

2 H3 H

Four linesA quartet

Three linesA triplet

Signals are split

1H-NMR spectrum of CH3CH2Br has more details...

CH3CH2Br

Signal Splitting

What is the origin of signal splitting?

A nucleus with only one magnetic environment causes a singlet

A nucleus with two magnetic environments causes a doublet

Each line in signal... ...has slightly different chemical shift ...represents slightly different spin flip energy ...represents nucleus with slightly different magnetic environment

Signal Splitting

How can one nucleus have different magnetic environments?

Ha feels Bo + Hb

•Ha feels two different magnetic environments

•Ha has two different spin flip E

Ha feels Bo - Hb

Bo

Ha C C Hb

•Caused by spin direction of adjacent nuclei

Larger E

Smaller E

•Ha has two different (but very similar) chemical shifts

•Ha signal is split into a doublet

NMR signal for Ha

Signal SplittingSome Useful Terms

J

Spin-spin coupling: One nuclear spin influences spin of another nucleus

Splitting: Effect on NMR signal caused by spin-spin coupling

Coupling constant (J): Spacing between lines in a splitting pattern

Signal SplittingMore Than One Neighbor

What is splitting when there is more than one neighbor?

Ha feels Bo + Hb + Hc

•Ha has three different (but very similar) chemical shifts

•Ha signal is split into a triplet

Ha C C Hc

Hb

Bo

Ha feels Bo - Hb + Hc = Bo

Ha feels Bo + Hb - Hc = Bo

Ha feels Bo - Hb - Hc

} equal energy

1:2:1 because of energy state population probabilities

NMR signal for Ha

Signal SplittingRules and Restrictions

General rule: The signal for a proton with n neighbors is split into n+1 lines

Rules and Restrictions for Proton-Proton Spin-Spin Coupling1. Only nonequivalent protons couple.

•Hc and Hd do not couple because they are equivalent

•Hb couples with Hc

•Hb and Ha do not couple because they are equivalentHa C

Hb

H

C

Hc

H

C

Hd

H

C

H

H

H

XX


2. Protons separated by more than three single bonds usually do not couple.

•Ha and Hb _____ bonds apart Can couple Cannot couple

•Ha and Hc _____ bonds apart Can couple Cannot couple

•Ha and Hd _____ bonds apart Can couple Cannot couple

Pi bonds do not count toward this bond limit, but J may be too small to observe.

CC

Ha

Hd

C

HcHb •Ha and Hb _____ bonds apart Can couple Cannot couple

•Ha and Hc _____ bonds apart Can couple Cannot couple

•Ha and Hd _____ bonds apart Can couple Cannot couple

Free spacer

CC

Ha

Hd

C

HcHb

X

*But Jad may be very small

*

*Assuming Ha and Hb are not equivalent

*

2

3

4

2

2 (+1)

3 (+1)


2. Protons separated by more than three single bonds usually do not couple.

•Benzene ring = one big free spacer

•All benzene ring protons may couple with each other but J may be small

•Ha, Hb, Hc, and Hd all couple with each other

•Jad may be too small to observe

F

Hd

Ha

Cl

Hc

Hb

Benzene ring is a “gated community”; it blocks some coupling that we expect to observe.

CH3

H

CH2CH3

X

X


3. Signals for O-H and N-H are usually singlets

•Splitting of O-H or N-H protons may be observed in rare circumstances

singletsinglet

triplettriplet

H2N C

H

H

C

H

H

OH

Sample Spectra•Verify what we have learned about equivalency, chemical shifts, integration, and splitting

3.39 ppm (triplet; integral = 1.0)

1.87 ppm (sextet; integral = 1.0)

1.03 ppm (triplet; integral = 1.5)

7 H / (1.0 + 1.0 + 1.5) = 2 H per unit

2 H

2 H

3 H

•Assign peaks to corresponding hydrogens in structure

BrCH2CH2CH3

Sample Spectra•Assign peaks to corresponding hydrogens in structure

3.78 ppm (septet; integral = 1.0)

1.31 ppm (doublet; integral = 6.0)7 H / (1.0 + 6.0) = 1 H per unit

1 H

6 H

H3C C

Br

CH3

H

C C

O

H3C

H3C H

H

H3C C

H3C

H3C

C OH

H

H

Lecture Supplement p. 138, 139, 146

HO OH

H3C CH3

CH3C

OCH3

OCH3

CH3

CH3OCH2CH2OCH3

CH3CH2Br

Sample SpectraFor next lecture:

5.66 ppm (multiplet; integral = 1.0)



CH2C

H2CC

C

C

H

H

H H

HH

•Assign peaks to corresponding hydrogens in structure

•Explain splitting patterns



CH3


•Number of signals how many sets of equivalent protons

•Position of signals (chemical shift) magnetic environment of nucleus

Deshielding by electronegative atoms higher chemical shift

•Relative intensity of signals (integration) how many hydrogens per signal

Integrals give proton ratio; not always equal to absolute proton count (i.e., 1.5:1)

•Splitting of signals (spin-spin coupling)

The signal of a proton with n neighbors is split into n+1 lines (first order coupling)

Example: CH3CH2Br CH3 is a triplet, CH2 is a quartet

More complex patterns (non first-order coupling) are common

Information from NMR Spectrum


•Only nonequivalent hydrogens couple with each other

•Hydrogens can be at most three single bonds distant

•Pi bonds and benzene rings are “free spacers”

•Benzene ring “gated community”

•OH, NH usually do not couple, and usually are not split

Splitting rules

C HH

C HH

C HH

OH

H

H

H

H

H

Sample Spectra•Assign peaks to corresponding hydrogens in structure




10 H / (1.0 + 2.0 + 2.0) = 2 H per unit

2 H

4 H

4 H

CH2C

H2CC

C

C

H

H

H H

HH

Multiplet: A splitting pattern that is too complex to decipher

Non-First Order SplittingWhy Is Cyclohexene Splitting Not Simple?

•n+1 rule obeyed; “normal” doublets, triplets, etc. result

Non-first order splitting: J values in a splitting pattern are unequal•More complex splitting patterns result

Example:

For Chem 14C we predict J values equal and “normal” coupling results. Exceptions are plentiful.

Ha C C

Hb

Hc Jab = Jac

“normal” tripletJab ≠ Jac

doublet of doublets

First order splitting: All J values in a splitting pattern are equal

Sample SpectraEffects of Pi Electron Clouds: Magnetic Induction

CH2C

H2CC

C

C

H

H

H H

HH

•Vinyl protons deshielded by magnetic induction

BoDeshielded

H C H

•May also cause shielding

Sample SpectraBenzene Ring Protons

•Magnetic induction causes benzene ring proton chemical shifts ~6.5-8.0 ppm

10.00 ppm (singlet)

7.87-7.56 ppm (multiplet)

•Magnetic induction by C=O causes aldehyde proton chemical shift ~9.5-11 ppm

6.5-8.0 ppm

C

O

H

HH

H H

H

•Due to long range coupling, benzene ring proton signals often multiplets

9.5-11 ppm

Sample SpectraBenzene Ring Protons

Benzene ring proton signals can be deceptively simple...

•7.2 ppm = singlet?•Benzene ring protons not equivalent

When chemical shifts very similarJ 0; splitting disappears

CH3

proton nuclear magnetic resonance ( 1 h-nmr) spectroscopy - part 1 lecture supplement page 133

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