TYPES OF INFORMATION FROM NMR SPECTRUM

1. Each different type of hydrogen gives a peak or group of peaks (multiplet).

3. The integral gives the relative numbers of each type of hydrogen.

2. The chemical shift (δ, in ppm) gives a clue as to the type of hydrogen generating the peak (alkane, alkene, benzene, aldehyde, etc.)

4. Spin-spin splitting gives the number of hydrogens on adjacent carbons.5. The coupling constant J also gives information about the arrangement of the atoms involved.

CHEMICAL SHIFT

DIAMAGNETIC ANISOTROPY

SHIELDING BY VALENCE ELECTRONS

valence electronsshield the nucleusfrom the full effectof the applied field

B induced (opposes Bo)Bo applied

magnetic field lines

Applied magnetic field induces circulationof valenceelectrons - thisgenerates amagnetic fieldthat opposes theapplied field.

Diamagnetic Anisotropy

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

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

PROTONS DIFFER IN THEIR SHIELDING

UPFIELDDOWNFIELDshielded protons appear here.

deshielded protonsappear here.

SPECTRUM

PEAKS ARE MEASURED RELATIVE TO TMS

TMSshift in Hz

0

Si CH3CH3

CH3

CH3

tetramethylsilane “TMS”

reference compound

n

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

Highly shieldedprotons appearway upfield.

downfield

chemical shift

= δδδδ = shift in Hzspectrometer frequency in MHz

= ppm

This division gives a number independent of the instrument used.

parts permillion

THE 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 have the same chemical shift (constant value).

NMR Correlation Chart

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

-OH -NH

CH2FCH2ClCH2BrCH2ICH2OCH2NO2

CH2ArCH2NR2CH2SC C-HC=C-CH2CH2-C-

O

C-CH-CC

C-CH2-CC-CH3

RCOOH RCHO C=CH

TMS

HCHCl3 ,

δδδδ (ppm)

DOWNFIELD UPFIELDDESHIELDED SHIELDED

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

DESHIELDING 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 ELECTRONEGATIVE ELEMENTS

More shieldedprotons appearupfield

“deshielded”protons appear downfield

deshielding moves protonresonance to lower field

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

element

DESHIELDING BY AN ELECTRONEGATIVE ELEMENT

NMR CHART

δδδδ- δδδδ++++

δδδδ- δδδδ++++

Electronegativity Dependenceof Chemical Shift

Compound CH3X

Element X

Electronegativity of XChemical shift δδδδ

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 onChemical 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 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 fieldgenerated by circulating ππππelectrons deshields aromaticprotons

Circulating ππππ electrons

Ring Current in BenzeneRing Current in Benzene

Bo

Deshielded

H H fields add together

C=CHH

H H

Bo

ANISOTROPIC FIELD IN AN ALKENE

protons aredeshielded

shifteddownfield

secondarymagnetic(anisotropic)field lines

Deshielded

fields add

HYDROGEN BONDING

HYDROGEN 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 EXAMPLES

SPIN-SPIN SPLITTING

Often a group of hydrogens will appear as a multipletrather than as a single peak.

SPIN-SPIN SPLITTING

Multiplets are named as follows:

Singlet QuintetDoublet SeptetTriplet OctetQuartet Nonet

This happens because of interaction with neighboring hydrogens and is called SPIN-SPIN SPLITTING.

C CH

Cl

Cl H

H

Cl

integral = 2

integral = 1

triplet doublet

1,1,2-TrichloroethaneThe two kinds of hydrogens do not appear as single peaks,rather there is a “triplet” and a “doublet”.

The subpeaks are due tospin-spin splitting and are predicted by the n+1 rule.

n + 1 RULE

1,1,2-Trichloroethane

C CH

Cl

Cl H

H

Cl

integral = 2

integral = 1

Where do these multiplets come from ? Ö.. interaction with neighbors

C C

H H

H

C C

H H

H

two neighborsn+1 = 3triplet

one neighborn+1 = 2doublet

singletdoublettripletquartetquintetsextetseptet

MULTIPLETSthis hydrogen’s peakis split by its two neighbors

these hydrogens aresplit by their singleneighbor

EXCEPTIONS TO THE N+1 RULEIMPORTANT !

Protons that are equivalent by symmetryusually do not split one another

CH CHX Y CH2 CH2X Y

no splitting if x=y no splitting if x=y

1)

2) Protons in the same group usually do not split one another

C

H

H

H or C

H

H

moredetaillater

SOME COMMON PATTERNS

SOME COMMON SPLITTING PATTERNS

CH2 CH2X Y

CH CHX Y

CH2 CH

CH3 CH

CH3 CH2

CH3

CHCH3

( x = y )

( x = y )

SOME EXAMPLE SPECTRA WITH SPLITTING

NMR Spectrum of Bromoethane

CH2CH3Br

NMR Spectrum of 2-Nitropropane

CCH3 CH3

N

H

O O+

-

1:6:15:20:16:6:1 in higher multiplets the outer peaksare often nearly lost in the baseline

NMR Spectrum of Acetaldehyde

offset = 2.0 ppm

CCH3

O

H

INTENSITIES OF MULTIPLET PEAKS

PASCAL’S TRIANGLE

1 2 1

PASCALíS TRIANGLE

11 1

1 3 3 11 4 6 4 1

1 5 10 10 5 11 6 15 20 15 6 1

1 7 21 35 35 21 7 1

singlet

doublet

triplet

quartet

quintet

sextet

septet

octet

The interiorentries arethe sums ofthe two numbersimmediatelyabove.

Intensities ofmultiplet peaks

THE COUPLING CONSTANT

J J

J

J J

THE COUPLING CONSTANT

The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet.

J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet.

C

H

H

C H

H

H

J

NOTATION FOR COUPLING CONSTANTSThe most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms.

C C

HH This is sometimes called vicinal coupling.It is designated 3J since three bondsintervene between the two hydrogens.

Another type of coupling that can also occur in special cases is

C H

H2J or geminal coupling

Geminal coupling does not occur whenthe two hydrogens are equivalent due torotations around the other two bonds.

( most often 2J = 0 )

3J

2J

C C

H H

C CH

H

C CHH

CH

H

6 to 8 Hz

11 to 18 Hz

6 to 15 Hz

0 to 5 Hz

three bond 3J

two bond 2J

three bond 3J

three bond 3J

SOME REPRESENTATIVE COUPLING CONSTANTS

Hax

Hax

Heq

Heq

Hax,Hax = 8 to 14Hax,Heq = 0 to 7Heq,Heq = 0 to 5

three bond 3J

trans

cis

geminal

vicinal

OVERVIEW

TYPES OF INFORMATION FROM THE NMR SPECTRUM

1. Each different type of hydrogen gives a peak or group of peaks (multiplet).

3. The integral gives the relative numbers of each type of hydrogen.

2. The chemical shift (δ, in ppm) gives a clue as to the type of hydrogen generating the peak (alkane, alkene, benzene, aldehyde, etc.)

4. Spin-spin splitting gives the number of hydrogens on adjacent carbons.

5. The coupling constant J also gives information about the arrangement of the atoms involved.