Chem 325 Chemical Shift

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The NMR Graph

Peak positions (x-axis scale):

1. Field strength: 1.500000000 versus 1.500002085 T

cumbersome and depends on resonance frequency!

2. Resonance frequency at constant field strength:

60000000 versus 60000089 Hz

cumbersome and depends on magnetic field strength!

The NMR Graph

Use a reference and quote all field strengths or

frequencies relative to the field or frequency of the

reference peak.

The δδδδ Scale: 6-

ref

sampleref10 1.39

01.50000000

50.00000208

B

B -B δ ×===

6-

6

ref

sampleref10 1.39

Hz 10 63.87

Hz 88.8

- δ ×=

×==

ν

νν

i.e. The sample signal is shifted by 1.39 ppm relative to the reference.

The chemical shift is 1.39 ppm.

Sample signal is at δδδδ = 1.39 relative to the reference (δδδδ = 0).

OR

δδδδ values INDEPENDENT of applied field or frequency

Tetramethylsilane

“TMS”

• TMS is added to the sample (internal standard).

Soluble in most organic solvents.

• Since silicon is less electronegative than carbon,

TMS protons are highly shielded. Signal defined

as zero.

• Organic protons absorb downfield (to the left) of

the TMS signal.

• All 12 H’s identical, strong signal.

• Also used for 13C spectra.

Si

CH3

CH3

CH3

H3C

Chemical Shift

• Measured in parts per million.

• Ratio of shift downfield from TMS (Hz) to total

spectrometer frequency (Hz).

• Same value for 60, 100, 300 MHz instrument.

• Called the delta (δδδδ) scale.

Chem 325 Chemical Shift

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Delta ScaleThe NMR Spectrum

If a proton is shielded the signal is near the high field or upfield region of the spectrum (right).

If the proton is deshielded the signal is near the low field or downfield region of the spectrum (left).

A typical 1H NMR is recorded from -2 to 15 δδδδ (ppm); what is typically reported is the region from 0 to 10 δδδδ. Exceptions exist!

δ or ppm 010

low ∆Eshielded 1H reduces B0upfield

high ∆Edeshielded 1H sees full B0downfield

Chemical Shift

The chemical shift (δδδδ) of a nucleus is dependent on the

electronic environment around the nucleus – chemical shift is

a result of local diamagnetic shielding.

There are five main effects that contribute to local

diamagnetic shielding and signal positions:

1) Hybridization

2) ππππ-electron delocalization

3) Inductive Effects (Electronegativity)

4) Resonance

5) Proton acidity/exchange

1) Hybridization

The hybridization of the carbon the proton is bound exerts a strong

electronic effect.

The greater the s-character, the more tightly bound the electrons are to

carbon, raising the effective electronegativity of that atom (sp = 50% s,

sp2, 33% s and sp3 25% s).

6.5-8.5aromaticAr-H

9.5-10.1aldehydicO=C-H

4.6-5.7VinylicC=C-H

2.0-3.0AcetylenicC≡≡≡≡C-H

1.6-2.6Allylic/benzylicC=C-CH3

0.8-1.7alkylR-CH3, R2CH2, R3CH

Chemical

Shift, δδδδ

Name of HType of H

Something odd is

happening here

Chem 325 Chemical Shift

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2) π2) π2) π2) π-Systems

There is an additional effect of circulating electrons, observed in ππππ-systems.

In benzene, the 6 p-orbitals overlap to allow full circulation of electrons; as

these electrons circulate in the applied magnetic field they oppose the

applied magnetic field. Extensive electron delocalization/circulation.

B0

Ring Current

Inside the ring, the induced magnetic field opposes the applied field

and outside the ring, it reinforces the field. As a result, H’s on the

periphery feel a stronger magnetic field than normal, signals

appear downfield (lower ∆∆∆∆E).

Aromatic Rings

For this reason, hydrogens attached to the outside of an aromatic

ring are shifted very far downfield, usually in the 7-9 ppm range.

Hydrogens on the inside of aromatic rings feel the opposite effect

and often show up upfield of TMS (negative δδδδ values).

HH

−−−−1.8 δ1.8 δ1.8 δ1.8 δ

8.9 δ8.9 δ8.9 δ8.9 δ

Alkynes

An ring current is also established in alkynes, however in this case,

the terminal hydrogen is in the inside of the ring current and so is

shifted upfield.

Chem 325 Chemical Shift

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Alkynes

Thus alkyne hydrogens are upfield from alkene hydrogens.

6.5-8.5aromaticAr-H

4.6-5.7VinylicC=C-H

2.0-3.0AcetylenicC≡≡≡≡C-H

Chemical Shift, δδδδ

Name of HType of H

H H

H

ππππ-Electron Groups

• H’s attached, or close, to ππππ-electron groups are shielded or

deshielded according to spatial location.

ANISOTROPIC effect

ππππ-Electron Systems

CC CH3

CH3

CH3

O

H

9.48δδδδ

1.08δδδδ

3) Inductive Effects

Electronegative groups comprise most organic functionalities:

-F -Cl -Br -I -OH -OR -NH2

-NHR -NR2 -NH3+ -C=O -NO2 -NO -SO3H

-PO3H2 -SH -Ph -C=C and most others

In all cases, the inductive withdrawing effect of these groups

on electrons decreases the electron density in the C-H covalent

bond – proton is deshielded – downfield shift

Chem 325 Chemical Shift

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Halomethanes

CH3Cl

CH3Br

CH3I

Electronegativity

Protons bound to carbons bearing electron withdrawing

groups are deshielded based on the magnitude of the

withdrawing effect – Pauling electronegativity:

0.0

1.8

(CH3)4Si

0.232.162.683.053.404.26δδδδ of H

Pauling

Electronegativity

2.12.52.83.13.54.0

CH4CH3ICH3BrCH3ClCH3O-CH3F

Effect is Cumulative

CH3Cl

CH2Cl2

CHCl3

3.1 δδδδ

5.3 δδδδ

7.3 δδδδ

Electronegativity

As the electronegative atom or group becomes more distant

from a particular hydrogen, its effect becomes smaller.

1.251.693.30δδδδ of H

-CH2CH2CH2Br-CH2CH2Br-CH2Br

Chem 325 Chemical Shift

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4) Resonance

Conjugated and aromatic systems: donation and withdrawal of

electron density is not just a simple matter of electronegativity.

In fact, very electronegative atoms (such as oxygen and nitrogen) are

the best donors of electron density into ππππ-systems (σσσσ-acids vs. ππππ-bases)

In general,

1) Groups or atoms that possess a lone pair are good electron donating

groups by resonance when attached to a ππππ-system.

Examples: -OH, -OR, -NR2, -SR, etc.

2) Groups attached to ππππ-systems that are sp or sp2 hybridized are good

electron withdrawing groups.

Examples: -COR, CHO, COOH, NO2, CN

3) Alkyl groups are weak EDG groups due to an effect called

hyperconjugation.

Resonance

NH2 NH2+

-

OCH3 OCH3+

-

NOO +

-N

O O

+

+ --

C

N

C

N

+

-

Resonance

OCH3

OCH3

NO2

NO2

CN

CN

7.808.46

6.837.34

5) Exchangable Protons

If sample molecule possesses hydrogen atoms of low pKA (< 20) and

is dissolved in a deuterated solvent that also has a low pKA, the

acidic protons will rapidly exchange with deuterium from solvent

and become “invisible” to the NMR spectrometer.

Such studies are useful, if it is desired to see which H-atoms in a

molecule are acidic.

The positions of OH protons are also highly solvent dependent.

OH

D2O

OD

Chem 325 Chemical Shift

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Peak Correlation Table Number of peaks

The number of peaks observed in an NMR spectrum is the

same as the number of chemically equivalent protons in the

spectrum.

OCH3

OCH3

Peak Area - Integration

One especially useful feature of 1H NMR spectra is that the

area under the curve for each peak is proportional to the

number of hydrogens that make up that peak. Peak areas can

be done electronically: “integration”.

Peak Area - Integration