INTEGRATIONINTEGRATION

CH2 C

O

CH3

Each different type of proton comes at a different place .You can tell how many different types of hydrogen there are in the molecule.

NMR Spectrum of PhenylacetoneNMR Spectrum of Phenylacetone

RECALLfrom lasttime

The area under a peak is proportionalto the number of hydrogens 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

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.

DIAMAGNETIC ANISOTROPYDIAMAGNETIC ANISOTROPY

SHIELDING BY VALENCE ELECTRONS

valence electronsshield the nucleusfrom the full effectof the applied field

B induced (opposes Bo)

Bo applied

magnetic fieldlines

The applied fieldinduces circulationof the valenceelectrons - thisgenerates amagnetic fieldthat opposes theapplied field.

Diamagnetic AnisotropyDiamagnetic Anisotropy

fields subtract at nucleus

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.

CHEMICAL SHIFTCHEMICAL SHIFT

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

h = Bo

2

constants

frequency

fieldstrength

Stronger magnetic fields (Bo) causethe instrument to operate at higherfrequencies ().

REMEMBER FROM OUR EARLIER DISCUSSION

NMR Field Strength

1H Operating Frequency

60 Mhz

100 MHz

300 MHz7.05 T

2.35 T1.41 T

= ( K) Bo

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).

01234567 ppm

Hz Equivalent of 1 ppm

1H Operating Frequency

60 Mhz 60 Hz

100 MHz 100 Hz

300 MHz 300 Hz

HERZ EQUIVALENCE OF 1 PPMHERZ EQUIVALENCE OF 1 PPM

Each ppm unit represents either a 1 ppm change in Bo (magnetic field strength, Tesla) or a 1 ppm change in the precessional frequency (MHz).

1 part per millionof n MHz is n Hz

n MHz = n Hz 1

106( )

What does a ppm represent?

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

Ranges can be defined for different general types of protons.This chart is general, the next slide is more definite.

APPROXIMATE CHEMICAL SHIFT RANGES (ppm) FOR SELECTED TYPES OF PROTONS

R-CH3 0.7 - 1.3

R-C=C-C-H 1.6 - 2.6

R-C-C-H 2.1 - 2.4

O

O

RO-C-C-H 2.1 - 2.5O

HO-C-C-H 2.1 - 2.5

N C-C-H 2.1 - 3.0

R-C C-C-H 2.1 - 3.0

C-H 2.3 - 2.7

R-N-C-H 2.2 - 2.9

R-S-C-H 2.0 - 3.0

I-C-H 2.0 - 4.0

Br-C-H 2.7 - 4.1

Cl-C-H 3.1 - 4.1

RO-C-H 3.2 - 3.8

HO-C-H 3.2 - 3.8

R-C-O-C-H 3.5 - 4.8

O

R-C=C-H

H

6.5 - 8.0

R-C-H

O

9.0 - 10.0

R-C-O-H

O

11.0 - 12.0

O2N-C-H 4.1 - 4.3

F-C-H 4.2 - 4.8

R3CH 1.4 - 1.7R-CH2-R 1.2 - 1.4 4.5 - 6.5

R-N-H 0.5 - 4.0 Ar-N-H 3.0 - 5.0 R-S-H

R-O-H 0.5 - 5.0 Ar-O-H 4.0 - 7.0

R-C-N-H

O

5.0 - 9.0

1.0 - 4.0R-C C-H 1.7 - 2.7

YOU DO NOT NEED TO MEMORIZE THE PREVIOUS CHART

IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPESOF HYDROGENS COME IN SELECTED AREAS OFTHE NMR CHART

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

MOST SPECTRA CAN BE INTERPRETED WITH A KNOWLEDGE OF WHAT IS SHOWN HERE

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.

We will discuss these factors in the sections thatfollow.

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 pi bond or pi system 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