MC 13.3 Spectroscopy, Pt III 1 Interpreting Carbon NMR Spectra Both spectra give us information about the number of chemically nonequivalent nuclei (nonequivalent

MC 13.3 Spectroscopy, Pt III1

Interpreting Carbon NMR Spectra

Both spectra give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogens or nonequivalent carbons)

Both spectra give us information about the environment of the nuclei (hybridization state, attached atoms, etc.)

It is convenient to use FT-NMR techniques for 1H

It is standard practice to use FT-NMR for 13C NMR

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1H and 13C NMR compared:


Interpreting Carbon NMR Spectra (cont)

13C requires FT-NMR because the signal for a carbon atom is 10-4 times weaker than the signal for a hydrogen atom

A signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei

In addition, at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C)

13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei

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1H and 13C NMR compared (cont):



13C requires FT-NMR because the signal for a carbon atom is 10-4 times weaker than the signal for a hydrogen atom

A signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei, and

In addition, at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C)

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1H and 13C NMR compared (cont):

4


01.02.03.04.05.06.07.08.09.010.0

Chemical shift (, ppm)

1H NMR Spectrum:

Cl CH2 CH2

CH2 CH3

2

2

4

3

Cl CH2 CH2 CH2 CH2 CH3

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13C NMR Spectrum: Cl CH2 CH2 CH2 CH2 CH3

Chemical shift (, ppm)

020406080100120140160180200

CDCl3

A separate, distinct peak appears for each of the 5 carbons

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Just as in 1H NMR spectroscopy, chemical shifts in 13C NMR depend on the electron density around the carbon nucleus

Decreased electron density causes the signal to move downfield (deshielding)

Increased electron density causes the signal to move upfield (shielding)

Because of the wide range of chemical shifts, it is rare to have two 13C peaks coincidentally overlap

A group of 3 peaks at 77 comes from the common NMR solvent deuteriochloroform and can be ignored


13C Chemical Shifts:

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13C Chemical Shifts (cont):

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DEPT (distortionless enhanced polarization transfer) spectra are created by mathematically combining several individual spectra taken under special conditions

The final DEPT spectra explicitly show C, CH, CH2 , and CH3 carbons

To simplify the presentation of DEPT data, the broadband decoupled spectrum is annotated with the results of the DEPT experiments using the labels C, CH, CH2 and CH3 above the appropriate peaks


DEPT 13C NMR:

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9


DEPT 13C NMR:

(a) The 13C spectrum and (b) a set of DEPT spectra showing the separate CH, CH2, and CH3 signals

Cl CH2 CH CH3

OH

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Interpreting Infrared Spectroscopy

Infrared spectroscopy gives information about the functional groups in a molecule

The region of infrared that is most useful lies between

2.5-16 m (4000-625 cm-1)

The infrared absorption depends on transitions between vibrational energy states

Stretching

Bending

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Interpreting Infrared Spectroscopy (cont)

Stretching Vibrations of a CH2 Group:

Symmetric Antisymmetric

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Bending Vibrations of a CH2 Group:

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In plane

In plane



Bending Vibrations of a CH2 Group (cont):

Out of plane Out of plane

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Infrared Spectrum of Hexane:

20003500 3000 2500 10001500 500Wave number, cm-1

CH3CH2CH2CH2CH2CH3

C—H stretching

bending bending

bending

CH3CH2CH2CH2CH2CH3

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15


Infrared Spectrum of 1-Hexene: H2C=CHCH2CH2CH2CH3

20003500 3000 2500 10001500 500

Wave number, cm-1

H2C=CHCH2CH2CH2CH3

C=C

H2C=CC=C H H C

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Infrared Absorption Frequencies:

Structural unit Frequency, cm-1

Stretching Vibrations (single bonds):

sp C — H 3310-3320

sp2 C — H 3000-3100

sp3 C — H 2850-2950

sp2 C — O 1200

sp3 C — O 1025-1200

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Infrared Absorption Frequencies (cont):


Stretching Vibrations (multiple bonds):

C C 1620-1680

— C C — 2100-2200

— C N 2240-2280

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Stretching Vibrations (carbonyl groups):

Aldehydes and ketones 1710-1750

Carboxylic acids 1700-1725

Acid anhydrides 1800-1850 and 1740-1790

Esters 1730-1750

Amides 1680-1700

C O

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Bending Vibrations of Alkenes:

CH2RCH 910-990

CH2R2C 890

CHR'cis-RCH 665-730

CHR'trans-RCH 960-980

CHR'R2C 790-840continue…..




Structural Unit Frequency, cm-1

Bending Vibrations of Derivatives of Benzene:

Monosubstituted 730-770 and 690-710

ortho-Disubstituted 735-770

meta-Disubstituted 750-810 and 680-730

para-disubstituted 790-840

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Infrared Spectrum of tert-butylbenzene: C6H5C(CH3)3

20003500 3000 2500 10001500 500

Wave number, cm-1

MonsubstitutedBenzene

C6H5C(CH3)3 H C

Ar H

Aromatic Double Bond

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Structural Unit Frequency, cm-1

Stretching Vibrations (single bonds):

O — H (alcohols) 3200-3600

O — H (carboxylic acids) 3000-3100

N — H 3350-3500

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Infrared Spectrum of 2-Hexanol:

20003500 3000 2500 10001500 500

Wave number, cm-1

OH

CH3CH2CH2CH2CHCH3

O H

H C

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Infrared Spectrum of 2-Hexanone:

20003500 3000 2500 10001500 500Wave number, cm-1

C = O

O

CH3CH2CH2CH2CCH3

H C

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MC 13.3 Spectroscopy, Pt III 1 Interpreting Carbon NMR Spectra Both spectra give us information about the number of chemically nonequivalent nuclei (nonequivalent