Theory of NMR

THEORY of NMR CHE3100 Dr. Drotar

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Theory of NMR• The positively charged nuclei of certain elements (e.g.,

13C and 1H) behave as tiny magnets.

• In the presence of a strong external magnetic field (Bo), these nuclear magnets align either with ( ) the applied field or opposed to ( ) the applied field.

• The latter (opposed) is slightly higher in energy than aligned with the field.

Bo

E is very smallenergy


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• The small energy difference between the two alignments of magnetic spin corresponds to the energy of radio waves according to Einstein’s equation E=h.

• Application of just the right radiofrequency ()causes the nucleus to “flip” to the higher energy spin state

• Not all nuclei require the same amount of energy for the quantized spin ‘flip’ to take place.

• The exact amount of energy required depends on the chemical identity (H, C, or other element) and the chemical environment of the particular nucleus.

h


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4


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• At a field strength of 9.4 Tesla. 1H nuclei absorb (resonate) near a radiofrequency of 400 MHz; 13C nuclei absorb around 100 MHz.

• Nuclei are surrounded by electrons. The strong applied magnetic field (Bo) induces the electrons to circulate around the nucleus (left hand rule).

Bo

e-

(9.4 T)


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• The induced circulation of electrons sets up a secondary (induced) magnetic field (Bi) that opposes the applied field (Bo) at the nucleus (right hand rule).

• We say that nuclei are shielded from the full applied magnetic field by the surrounding electrons because the secondary field diminishes the field at the nuclei.

Bo

e-

Bi


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Theory of NMR• The electron density surrounding a given nucleus

depends on the electronegativity of the attached atoms.• The more electronegative the attached atoms, the less

the electron density around the nucleus in question.• We say that that nucleus is less shielded, or is

deshielded by the electronegative atoms.• Deshielding effects are generally additive. That is, two

highly electronegative atoms (2 Cl atoms, for example) would cause more deshielding than only 1 Cl atom.

C

H

HH

H

C

H

ClH

H

C

H

ClH

Cl

C and H are deshielded C and H are more deshielded


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Chemical Shift• We call the relative position of absorption in the NMR spectrum

(which is related to the amount of deshielding) the chemical shift.It is a unitless number (a ratio in which the units cancel), but we assign ‘units’ of ppm or (Greek letter delta).

• Different nuclei absorb EM radiation at different wavelength (energy required to cause resonance).

• Nuclei of a given type will resonate at different energies, depending on their chemical and electronic environment.

• The position (chemical shift, ) and pattern (splitting or multiplicity)of the NMR signals gives important information about the chemical environment of the nuclei.


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• For 1H, the usual scale of NMR spectra is 0 to 10 (or 12) ppm (or ).

• The usual 13C scale goes from 0 to about 220 ppm.• The zero point is defined as the position of absorption of

a standard, tetramethylsilane (TMS):• This standard has only one type

of C and only one type of H.Si

CH3

CH3

CH3

CH3


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13


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C13 Chemical Shift ( ) vs. Electronegativity

-10

0

10

20

30

40

50

60

70

80

90

1.5 2 2.5 3 3.5 4

Electronegativity

C13

Che

mic

al S

hift

Chemical Shifts

CH3 Si

CH3 C

CH3 N

CH3 O

CH3 F


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Chemical Shifts• Both 1H and 13C Chemical shifts are related to three

major factors:– The hybridization (of carbon)– Presence of electronegative atoms or electron attracting groups– The degree of substitution (1º, 2º or 3º). These latter effects

are most important in 13C NMR, and in that context are usually called ‘steric’ effects.

• First we’ll focus on Carbon NMR spectra (They are simpler.)


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CMR Spectra• Each unique C in a structure gives a single peak in the

spectrum. There is rarely any overlap.– The carbon spectrum spans over 200 ppm; chemical shifts only

0.001 ppm apart can be distinguished; this allows for over 2x105

possible chemical shifts for carbon.

• The intensity (size) of each peak is NOT directly related to the number of that type of carbon. Other factors contribute to the size of a peak:– Peaks from carbon atoms that have attached hydrogen atoms

are bigger than those that don’t have hydrogens attached.

• Carbon chemical shifts are usually reported as downfield from the carbon signal of tetramethylsilane (TMS).


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13C Chemical Shifts

downfield upfield

20406080100120140160180200220 0

CH3

CH2

CH

C X (halogen)C N

C O C C

C N

C CC O

13C Chemical shift ()

TMSAromatic C


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Predicting 13C Spectra• Predict the number of carbon resonance lines in the 13C

spectra of the following (= # unique Cs): plane of symmetryCH3

C CC

CC

CH3CH3

O

CH3

CH3

O

CH3

CH3

CC

cCH3

O

CH3

CH3C

CH3

CCH3

H


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Predicting 13C Spectra• Predict the number of carbon resonance lines in the 13C

spectra of the major product of the following reaction:

7 lines

5 lines

plane of symmetry

CH3

C

c c C

CC

CH3 CH2

C

C c C

CC

CH2CH2

CH3 CH2ClCH3

orKOH

ethanol, heat ???


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Predicting 13C SpectraCH3

CH3

H3C CH3

C CC

CCC

H3C CH3

4 lines

C CCH3

CH3

CH3

CH3

2 lines

Symmetry Simplifies Spectra!!!

C CCH3

CH3

CH3

CH3


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CH3

CDCl3 (solvent)

CH3CCH3

O

C

O

Acetone


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OCH3

CDCl3 (solvent)

CH3

CH3COCH3

O

C

O

Methyl Acetate


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CDCl3 (solvent)

CH3

CH3CH3COCH2CH3

O

C

O

OCH2

Ethyl Acetate


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CH2

CH3CH3

CDCl3 (solvent)

CH3

CH3CH2COCH2CH3

O

C

O

OCH2

Ethyl Propanoate


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C

O

CH2CH2CH3

CH3CCH2CH3

O

CH3

CDCl3 (solvent)

2-Butanone


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C

OCH3

CDCl3 (solvent)

CH3C

OHH

H

HH

CC

C

CC

C

H

H

H

H

H

C C

C

C

and

expanded below


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CH2 Br

CDCl3 (solvent)

CH3

CH3CH2CH2Br

CH2


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CH2 OH

CH3

CDCl3 (solvent)

CH3CH2CH2OH CH2


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CDCl3 (solvent)

CH2 OH

CH3

CH3CH2CH2CH2OH

CH2CH21-Butanol


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C

C

CDCl3 (solvent)

CC

CH3

H

1-methylcyclohexene


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EquivalenceAsk yourself: If you interchange the two groups or atoms, will the compound be the same as the original (or the enantiomer), or is it different ? If it’s the same, then the two groups (or atoms) are equivalent.


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1. The chemical shift gives the environment of a proton.2. The number of signals in a 1H NMR gives the number of sets of equivalent protons.3. Integration gives the relative rations of protons in the sets.4. Spin–Spin splitting of NMR signals results from coupling of the proton that gives the signal to protons separated by: a.) two bonds (geminal)

b.) three bonds (Vicinal) (most common)

5. The number of peaks in a split signal is equal to n+1 where n is the number of equivalent 1H.

CH

H

H H

CC


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1,1-dichloroethane


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5.(continued) The Ethyl Group has a distinct pattern. (CH3–CH2–) • Methyl protons of an ethyl group form a triplet with a 1:2:1 ratio. • Methylene protons of an ethyl group form a quartet, 1:3:3:1 ratio

Below: the NMR plot for Ethyl Bromide


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CH3

CH3

CH —

(ppm)

• Methyl protons of an isopropyl group form a doublet with a 1:1 ratio.

• Methine proton of an isopropyl group forms a heptet with a 1:6:15:26:15:6:1 ratio


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• A pair of nonequivalent protons spit each other’s signal into a pair of doublets.


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• Splitting from hydroxyl groups on alcohols is not observed because of rapid exchange of hydrogen with other alcohol molecules.

6. Many processes, such as conformational changes, occur faster than NMR can detect them. • Can’t detect difference between axial and equatorial protons in cyclohexane.

Theory of NMR

Documents

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