13.9 Spin-Spin Splitting13.9 Spin-Spin Splitting

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

SPIN-SPIN SPLITTINGSPIN-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-Trichloroethane1,1,2-Trichloroethane

nn + 1 RULE + 1 RULE

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

Some Common PatternsSome Common Patterns

SOME COMMON SPLITTING PATTERNSSOME COMMON SPLITTING PATTERNS

CH2 CH2X Y

CH CHX Y( x = y )

( x = y )CH3

CH

CH3

-CH2-CH3

CH-CH3

CH-CH2

tert-butyl group

CH3

C Cl

CH3

H3C 9 equivalent protons = singlet

EXCEPTIONS TO THE N+1 RULEEXCEPTIONS 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

SOME EXAMPLE SPECTRASOME EXAMPLE SPECTRA WITH SPLITTINGWITH 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

The propyl group

CH3-CH2-CH2-X

Can you predict the splitting patterns for this compound?

INTENSITIES OF INTENSITIES OF MULTIPLET PEAKSMULTIPLET PEAKS

PASCAL’S TRIANGLE

1 2 1

PASCAL’S TRIANGLEPASCAL’S 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 ORIGIN OF THE ORIGIN OF SPIN-SPIN SPLITTINGSPIN-SPIN SPLITTING

HOW IT HAPPENS

C C

H H

C C

H HA A

upfielddownfield

Bo

THE CHEMICAL SHIFT OF PROTON HTHE CHEMICAL SHIFT OF PROTON HAA IS IS

AFFECTED BY THE SPIN OF ITS NEIGHBORSAFFECTED BY THE SPIN OF ITS NEIGHBORS

50 % ofmolecules

50 % ofmolecules

At any given time about half of the molecules in solution willhave spin +1/2 and the other half will have spin -1/2.

aligned with Bo opposed to Bo

neighbor aligned neighbor opposed

+1/2 -1/2

C C

H H

C C

H H

one neighborn+1 = 2doublet

one neighborn+1 = 2doublet

SPIN ARRANGEMENTSSPIN ARRANGEMENTS

yellow spins

blue spins

The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.

C C

H H

H

C C

H H

H

two neighborsn+1 = 3triplet

one neighborn+1 = 2doublet

SPIN ARRANGEMENTSSPIN ARRANGEMENTS

methylene spins

methine spins

three neighborsn+1 = 4quartet

two neighborsn+1 = 3triplet

SPIN ARRANGEMENTSSPIN ARRANGEMENTS

C C

H H

H

H

H

C C

H H

H

H

H

methyl spins

methylene spins

13.10 The Coupling Constant13.10 The Coupling Constant

J J

J

J J

THE COUPLING CONSTANTTHE 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

100 MHz

200 MHz

123456

123

100 Hz

200 Hz

200 Hz

400 Hz

J = 7.5 Hz

J = 7.5 Hz

7.5 Hz

7.5 Hz

Coupling constants areconstant - they do not change at differentfield strengths

The shift isdependanton the field

ppm

FIELD COMPARISON

Separationis larger

123

123

100 MHz

200 MHz

Why buy a higherfield instrument?

Spectra aresimplified!

Overlapping multiplets areseparated.

Second-ordereffects are minimized.

123

50 MHz

J = 7.5 Hz

J = 7.5 Hz

J = 7.5 Hz

NOTATION FOR COUPLING CONSTANTSNOTATION 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

Couplings larger than 2J or 3J also exist, but operate only in special situations, especially in unsaturatedsystems.

Couplings larger than 3J (e.g., 4J, 5J, etc) are usually called “long-range coupling.”

LONG RANGE COUPLINGSLONG RANGE COUPLINGS

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 CONSTANTSSOME REPRESENTATIVE COUPLING CONSTANTS

trans

cis

geminal

vicinal

CH

C H4 to 10 Hz

H C C CH

0 to 3 Hz four bond 4J

three bond 3J

C CC H

H0 to 3 Hz four bond 4J

H

H

cis

trans

6 to 12 Hz

4 to 8 Hzthree bond 3J

Couplings that occur at distances greater than three bonds arecalled long-range couplings and they are usually small (<3 Hz)

13.11 NMR Spectra of Carbonyl Compounds

• Anisotropy in carbonyl compounds• Anisotropy deshields C-H on aldehydes:

9-10 ppm• Anisotropy also deshields methylene

and methyl groups next to C=O: 2.0 - 2.5 ppm

• Methylene groups directly attached to oxygen appear near 4.0 ppm

CH3 C

O

CH2CH3

2-Butanone (Methyl Ethyl Ketone)60 MHz Spectrum

1

WWU Chemistry

2-butanone, 300 MHz spectrum

Ethyl Acetate2

CH3 C

O

O CH2CH3

Compare the methylene shift to that of Methyl Ethyl Ketone (previous slide).

t-Butyl Methyl Ketone3

C

O

CCH3 CH3

CH3

CH3

(3,3-dimethyl-2-butanone)

Phenylethyl Acetate4

CH2CH2 O C

O

CH3

Ethyl Succinate5

O C

O

CH2CH2 C

O

OCH3CH2 CH2CH3

-Chloropropionic Acid6

CH C

O

OH

Cl

CH3

13.12 and 13.1313.12 and 13.13Alkenes, Alkynes Alkenes, Alkynes and Aromatic and Aromatic CompoundsCompounds

• vinyl protons appear between 5 to 6.5 ppm (anisotropy)

• methylene and methyl groups next to a double bond appear at about 1.5 to 2.0 ppm

• for terminal alkynes, proton appears near 2 ppm

CHEMICAL SHIFTS

Alkenes and alkynes

Ring current causes protons attached to the ring to appear in the range of 7 to 8 ppm.

Protons in a methyl or methylene group attached to the ring appear in the range of 2 to 2.5 ppm.

BENZENE RING HYDROGENSBENZENE RING HYDROGENS

NMR Spectrum of Toluene

CH35

3

C

HH

ROC

RO

HH

Only the o- protons are in range for this effect.

When a carbonyl group is attached to the ring theo- and p- protons are deshielded by the anisotropicfield of C=O

THE EFFECT OF CARBONYL SUBSTITUENTSTHE EFFECT OF CARBONYL SUBSTITUENTS

CCH3O

HH

Acetophenone (90 MHz)

2 3

3

deshielded

NMR Spectrum of 1-iodo-4-methoxybenzene

OCH3I

CHCl3 impurity

2 2

3

NMR Spectrum of 1-bromo-4-ethoxybenzene

OCH2CH3Br

4

2

3

X

Y

X

X'

X

X

X = Y X ~ X’ X = X

THE p-DISUBSTITUTED PATTERN CHANGES AS THETHE p-DISUBSTITUTED PATTERN CHANGES AS THE TWO GROUPS BECOME MORE AND MORE SIMILARTWO GROUPS BECOME MORE AND MORE SIMILAR

all Hequivalent

All peaks move closer.Outer peaks get smaller …………………..… and finally disappear.Inner peaks get taller…………………………. and finally merge.

same groups

NMR Spectrum of 1-amino-4-ethoxybenzene

OCH2CH3H2N4

2 2

3

NMR Spectrum of p-Xylene

(1,4-dimethylbenzene)

CH3CH3

4

6

13.14 Hydroxyl 13.14 Hydroxyl and Amino and Amino ProtonsProtons

Hydroxyl and Amino Protons

Carboxylic acid protons generally appear fardownfield near 11 to 12 ppm.

Hydroxyl and amino protons can appear almost anywhere in the spectrum (H-bonding).

These absorptions are usually broader than other proton peaks and can often be identified because of this fact.

SPIN-SPIN DECOUPLING BY EXCHANGESPIN-SPIN DECOUPLING BY EXCHANGE

In alcohols coupling between the O-H hydrogen andthose on adjacent carbon atoms is usually not seen.

C O

H H

This is due to rapid exchange ofOH protons between the various alcohol molecules in the solution. The OH peak is usually broad.

In ultrapure alcohols, however,coupling will sometimes be seen.

NMR Spectrum of Ethanol

CH3CH2 OH

2 1

3

1-propanol

CH3CH2CH2 OH

13.16 Unequal Couplings13.16 Unequal Couplings Tree DiagramsTree Diagrams

WHERE DOES THE N+1 RULE WORK ?WHERE DOES THE N+1 RULE WORK ?The n+1 rule works only for protons in aliphatic chainsand rings, and then under special conditions.

1) All 3J values must be the same all along the chain.

There are two requirements for the n+1 rule to work:

2) There must be free rotation or inversion (rings) to make all of the hydrogens on a single carbon be nearly equivalent.

CH

HCH

HCH

H3Ja = 3Jb

The typical situationwhere the n+1 ruleapplies.

Hydrogens can interchange theirpositions byrotations aboutthe C-C bonds.

WHAT HAPPENS WHEN THE J VALUES ARE NOT EQUAL ?

CH

HCH

HCH

H3Ja

3Jb

3Ja = 3Jb

In this situation each coupling must be consideredindependently of the other.

A “splitting tree” is constructed as shown on thenext slide.

CH

HCH

HCH

H3Ja = 7

-CH2-CH2-CH2-

CONSTRUCTING A TREE DIAGRAMCONSTRUCTING A TREE DIAGRAM( SUPPOSE 3Ja = 7 Hz and 3Jb = 3 Hz )

The largest J value is usually used first.

CH

HCH

HCH

H3Jb = 3

triplet of triplets

WHEN BOTH 3J VALUES ARE THE SAME

-CH2-CH2-CH2-

….. because of overlapping legsyou get the quintet predicted bythe n+1 rule.

The n+1 rule is followed

n+1 = (4 + 1) = 5

2-PHENYLPROPANAL2-PHENYLPROPANAL

A case where there are unequal J values.

Spectrum of 2-Phenylpropanal

J = 2 Hz

J = 7 Hz

a

b

c

d

CHCH3 CHO

a b d

c TMS

CHCH3 CHO

3J1 = 7 Hz

7 Hz 2 Hz

3J2 = 2 Hz

the methine hydrogen is split by two different3J values.

Rather than the expectedquintet …..

ANALYSISANALYSISOF METHINEOF METHINEHYDROGEN’SHYDROGEN’SSPLITTINGSPLITTING

quartet by -CH3

doubletby -CHO

quartet of doublets

• proton b is a quartet of doublets

• Adjacent protons are three bonds away from each other: 3J, often = 7 Hz

• The aldehyde proton d has a 3J = 2 Hz coupling to the single proton b

• the methyl protons a have a 3J = 7 Hz coupling to proton b

2-PHENYLPROPANAL2-PHENYLPROPANAL

VINYL ACETATEVINYL ACETATE

• In alkenes, 3J-cis = 8 Hz • In alkenes, 3J-trans = 16 Hz

• In alkenes, when protons are on the same carbon, 2J-geminal = 0-2 Hz

PROTONS ON C=C DOUBLE BONDS

H

H

HH

H

H

PROTONS ON C=C DOUBLE BONDSPROTONS ON C=C DOUBLE BONDSCOUPLING CONSTANTS

NMR Spectrum of Vinyl Acetate

CH3 C

O

O CH CH2

Analysis of Vinyl AcetateAnalysis of Vinyl Acetate

HC HB HA

CCHH33 CC

OO

OOCC

HHCC

CCHHAA

HHBB

3JBC

3JAC

3JAC3JBC

2JAB2JAB

trans trans

cis

cis

gem gem

3J-trans > 3J-cis > 2J-gem

OVERVIEW

TYPES OF INFORMATION TYPES OF INFORMATION FROM THE NMR SPECTRUMFROM 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 (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.

Generally, with only three pieces of data

1) empirical formula (or % composition)

2) infrared spectrum

3) NMR spectrum

a chemist can often figure out the completestructure of an unknown molecule.

SPECTROSCOPY IS A POWERFUL TOOLSPECTROSCOPY IS A POWERFUL TOOL

FORMULA

Gives the relative numbers of C and H and other atoms

INFRARED SPECTRUM

Reveals the types of bonds that are present.

NMR SPECTRUM

Reveals the environment of each hydrogenand the relative numbers of each type.

EACH TECHNIQUE YIELDS VALUABLE DATAEACH TECHNIQUE YIELDS VALUABLE DATA