Chapter 13Chapter 13 Nuclear Magnetic Resonance Nuclear Magnetic Resonance

Spectroscopy Spectroscopy

Assignment for Chapter 13

• 13.1 through 13.1413.16 and 13.18

• SKIP 13.15 and 13.17

Problem Assignment• In text problems

2 - 16 22 a, b, d, g24, 25

• End of chapter problems 27, 28, 29, 30, 31, 35, 39a, b40 a, c, d, f41 a, b, c, d

Element 1H 2H 12C 13C 14N 16O 17O 19F

Nuclear SpinQuantum No 1/2 1 0 1/2 1 0 5/2 1/2 ( I )No. of Spin 2 3 0 2 3 0 6 2States

13.1 Spin Quantum Numbers of Some Nuclei13.1 Spin Quantum Numbers of Some Nuclei

Elements with either odd mass or odd atomic numberhave the property of nuclear “spin”.

The number of spin states is 2I + 1, where I is the spin quantum number.

The most abundant isotopes of C and O do not have spin.

13.2 Nuclear Resonance13.2 Nuclear Resonance

absorption of energy by the spinning nucleus

Nuclear Spin Energy Nuclear Spin Energy LevelsLevels

Bo

+1/2

-1/2

In a strong magnetic field (Bo) the two spin states differ in energy.

aligned

unaligned

N

S

Bo

E

+ 1/2

- 1/2

= kBo = hdegenerate at Bo = 0

increasing magnetic field strength

The Energy Separation Depends on The Energy Separation Depends on BBoo

The Larmor Equation!!!The Larmor Equation!!!

B0

Bo

is a constant which is different for each atomic nucleus (H, C, N, etc)

E = kBo = hcan be transformed into

gyromagnetic

ratio

strength of themagnetic field

frequency ofthe incomingradiation thatwill cause atransition

1H 99.98% 1.00 42.6 267.531.41 60.02.35 100.07.05 300.0

13C 1.108% 1.00 10.7 67.282.35 25.07.05 75.0

Resonance Frequencies of Selected NucleiResonance Frequencies of Selected NucleiIsotope Abundance Bo (Tesla) Frequency(MHz)

(radians/Tesla)

13.3 Classical 13.3 Classical Instrumentation:Instrumentation: The Continuous-Wave NMR The Continuous-Wave NMR

typical before 1965;field is scanned

A Simplified 60 MHzA Simplified 60 MHzNMR SpectrometerNMR Spectrometer

TransmitterReceiver

Probe

h

SN

RF Detector Recorder

RF (60 MHz)Oscillator

~ 1.41 Tesla (+/-) a few ppm

absorption signal

MAGNETMAGNET

CH2 C

O

CH3

EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF PROTONS THERE ARE BY INTEGRATION.

NMR Spectrum of PhenylacetoneNMR Spectrum of Phenylacetone

13.4 Modern 13.4 Modern Instrumentation: Instrumentation: the Fourier-Transform NMRthe Fourier-Transform NMR

requires a computer

FT-NMR

PULSED EXCITATIONPULSED EXCITATION

CH2 C

O

CH3BROADBANDRF PULSE

All types of hydrogen are excitedsimultaneously with the single RF pulse.

contains a range of frequencies

N

S

1

2

3

1 ..... n

13.5 Proton NMR Spectrum13.5 Proton NMR Spectrum

CH2 C

O

CH3

EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF PROTONS THERE ARE BY INTEGRATION.

NMR Spectrum of PhenylacetoneNMR Spectrum of Phenylacetone

Some Generalizations

• NMR solvents contain deuterium

• Tetramethylsilane (TMS) is the reference

• Spectrum of 1,2,2-trichloropropane• Chemical shift in Hz from TMS vary

according to frequency of spectrometer!• Delta values ( are independent of

frequency of spectrometer (ppm)

PEAKS ARE MEASURED RELATIVE TO TMSPEAKS ARE MEASURED RELATIVE TO TMS

TMS

shift in Hz

0

Si CH3CH3

CH3

CH3

tetramethylsilane “TMS”

reference compound

n

Rather than measure the exact resonance position of a peak, we measure how far downfield it is shifted from TMS.

Highly shieldedprotons appearway upfield.

Chemists originallythought no other compound wouldcome at a higherfield than TMS.

downfield

TMS

shift in Hz

0n

downfield

The shift observed for a given protonin Hz also depends on the frequencyof the instrument used.

Higher frequencies= larger shifts in Hz.

HIGHER FREQUENCIES GIVE LARGER SHIFTSHIGHER FREQUENCIES GIVE LARGER SHIFTS

chemical shift

= = shift in Hz

spectrometer frequency in MHz= ppm

This division gives a number independent of the instrument used.

parts permillion

THE CHEMICAL SHIFTTHE CHEMICAL SHIFTThe shifts from TMS in Hz are bigger in higher field instruments (300 MHz, 500 MHz) than they are in the lower field instruments (100 MHz, 60 MHz).

We can adjust the shift to a field-independent value,the “chemical shift” in the following way:

A particular proton in a given molecule will always come at the same chemical shift (constant value).

13.6 and 13.7 13.6 and 13.7 The Chemical ShiftThe Chemical Shift

Generalizations

• electrons shield nucleus• electronegativity: withdraws electrons

to deshield nucleus • downfield (deshielding) = left side of

spectrum• upfield (shielding) = right side of

spectrum• delta values increase from right to left!

All different types of protons in a moleculehave a different amounts of shielding.

This is why an NMR spectrum contains useful information(different types of protons appear in predictable places).

They all respond differently to the applied magnetic field and appear at different places in the spectrum.

PROTONS DIFFER IN THEIR SHIELDINGPROTONS DIFFER IN THEIR SHIELDING

UPFIELDDOWNFIELD

Highly shielded protons appear here.

Less shielded protonsappear here.

SPECTRUM

It takes a higher fieldto cause resonance.

Overview of where protons appear in an NMR spectrum

aliphatic C-H

CH on Cnext to pi bonds

C-H where C is attached to anelectronega-tive atom

alkene=C-H

benzene CH

aldehyde CHO

acidCOOH

2346791012 0

X-C-HX=C-C-H

NMR Correlation ChartNMR Correlation Chart

12 11 10 9 8 7 6 5 4 3 2 1 0

-OH -NH

CH2FCH2ClCH2BrCH2ICH2OCH2NO2

CH2ArCH2NR2

CH2SC C-HC=C-CH2

CH2-C-O

C-CH-C

C

C-CH2-CC-CH3

RCOOH RCHO C=C

H

TMS

HCHCl3 ,

(ppm)

DOWNFIELD UPFIELD

DESHIELDED SHIELDED

Some approximate NMR Some approximate NMR values (part 1)values (part 1)

R

C O

H-O

11-12 ppm

R

C O 9-10 ppm

7-8 ppm

vinyl protons (alkene)

C C

H

5-7 ppm

carboxylic acid

H

aldehyde

benzene ring protons

H

Some approximate NMR Some approximate NMR values (part 2)values (part 2)

-CH2-O- about 4ppm

-CH2-X about 3.5 ppm

X = Cl, Br, I

If two chlorine atoms are attached, shifts to 5.3 ppm

-CH2-CH2-O- about 1.2 ppmfor boldfaced CH2

about 1 to 1.5 ppmCH3CH2CH

long way from electronegative atoms

CH2

about 2 ppm

andR CH2-

O

DESHIELDING AND ANISOTROPYDESHIELDING AND ANISOTROPY

Three major factors account for the resonance positions (on the ppm scale) of most protons.

1. Deshielding by electronegative elements.

2. Anisotropic fields usually due to pi-bonded electrons in the molecule.

3. Deshielding due to hydrogen bonding.

DESHIELDING BY DESHIELDING BY ELECTRONEGATIVE ELEMENTSELECTRONEGATIVE ELEMENTS

highly shieldedprotons appearat high field

“deshielded“protons appear at low field

deshielding moves protonresonance to lower field

C HClChlorine “deshields” the proton,that is, it takes valence electron density away from carbon, whichin turn takes more density fromhydrogen deshielding the proton. electronegative

element

DESHIELDING BY AN ELECTRONEGATIVE ELEMENTDESHIELDING BY AN ELECTRONEGATIVE ELEMENT

NMR CHART

-

-

Electronegativity Dependence Electronegativity Dependence

of Chemical Shiftof Chemical Shift

Compound CH3X

Element X

Electronegativity of X

Chemical shift

CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si

F O Cl Br I H Si

4.0 3.5 3.1 2.8 2.5 2.1 1.8

4.26 3.40 3.05 2.68 2.16 0.23 0

Dependence of the Chemical Shift of CH3X on the Element X

deshielding increases with theelectronegativity of atom X

TMSmostdeshielded

Substitution Effects on Substitution Effects on Chemical ShiftChemical Shift

CHCl3 CH2Cl2 CH3Cl

7.27 5.30 3.05 ppm

-CH2-Br -CH2-CH2Br -CH2-CH2CH2Br

3.30 1.69 1.25 ppm

mostdeshielded

mostdeshielded

The effect decreaseswith incresing distance.

The effect increases withgreater numbersof electronegativeatoms.

ANISOTROPIC FIELDSANISOTROPIC FIELDSDUE TO THE PRESENCE OF PI BONDS

The presence of a nearby bond greatly affects the chemical shift.

Benzene rings have the greatest effect.

Secondary magnetic field

generated by circulating electrons deshields aromaticprotons

Circulating electrons

Ring Current in BenzeneRing Current in Benzene

Bo

Deshielded

H H fields add together

C=CHH

H H

Bo

ANISOTROPIC FIELD IN AN ALKENEANISOTROPIC FIELD IN AN ALKENE

protons aredeshielded

shifteddownfield

secondarymagnetic(anisotropic)field lines

Deshielded

fields add

Bo

secondarymagnetic(anisotropic)field

H

HC

C

ANISOTROPIC FIELD FOR AN ALKYNEANISOTROPIC FIELD FOR AN ALKYNE

hydrogensare shielded

Shielded

fields subtract

HYDROGEN BONDINGHYDROGEN BONDING

HYDROGEN BONDING DESHIELDS PROTONSHYDROGEN BONDING DESHIELDS PROTONS

O H

R

O R

HHO

RThe chemical shift dependson how much hydrogen bondingis taking place.

Alcohols vary in chemical shiftfrom 0.5 ppm (free OH) to about5.0 ppm (lots of H bonding).

Hydrogen bonding lengthens theO-H bond and reduces the valence electron density around the proton- it is deshielded and shifted downfield in the NMR spectrum.

O

CO

RH

HC

O

OR

Carboxylic acids have stronghydrogen bonding - theyform dimers.

With carboxylic acids the O-Habsorptions are found between10 and 12 ppm very far downfield.

O

OO

H

CH3

In methyl salicylate, which has stronginternal hydrogen bonding, the NMRabsortion for O-H is at about 14 ppm,way, way downfield.

Notice that a 6-membered ring is formed.

SOME MORE EXTREME EXAMPLESSOME MORE EXTREME EXAMPLES

13.8 Integration13.8 Integration

The area under a peak is proportionalto the number of protons thatgenerate the peak.

Integration = determination of the area under a peak

INTEGRATION OF A PEAKINTEGRATION OF A PEAKNot only does each different type of hydrogen give a distinct peak in the NMR spectrum, but we can also tell the relative numbers of each type of hydrogen by aprocess called integration.

55 : 22 : 33 = 5 : 2 : 3

The integral line rises an amount proportional to the number of H in each peak

METHOD 1integral line

integralline

simplest ratioof the heights

Benzyl AcetateBenzyl Acetate

Benzyl Acetate (FT-NMR)Benzyl Acetate (FT-NMR)

assume CH3

33.929 / 3 = 11.3 per H

33.929 / 11.3 = 3.00

21.215 / 11.3 = 1.90

58.117 / 11.3 = 5.14

Actually : 5 2 3

METHOD 2

digital integration

Modern instruments report the integral as a number.

CH2 O C

O

CH3

Integrals aregood to about10% accuracy.