lecture 4 13 c nmr: dept ir spectroscopy: - how it works - interpretation of spectra due: lecture...
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Lecture 4
• 13C NMR: DEPT
• IR Spectroscopy:- How it works- Interpretation of spectra
Due:Lecture Problem 2
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Determine the structure of this unknown (MF is C8H9Cl)
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13C NMR Correlation Chart
5.0 0.01.02.03.04.06.07.08.09.010.011.012.0
Chemical Shift, δ (ppm)
CO2H
CHO
ArH C=CH
X-CH
O-CH
C CH
O-H
N-H
COCH
CH, CH2
CH3
1H NMR Correlation Chart
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DEPT-NMR (Distortionless Enhancement by Polarization Transfer)
• Distinguishes between CH, CH2, and CH3 carbons
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13C NMR:broadbanddecoupled(normal)
13C NMR:DEPT-90
13C NMR:DEPT-135
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MRI: A Medicinal Application of NMR
Magnetic Resonance Imaging:• MRI Scanner: large magnet; coils to excitenuclei, modify magnetic field, and receiveSignals• Different tissues yield different signals• Signals are separated into components byFourier transform analysis• Each component is a specific site of originin the patient a cross-sectional image ofthe patient’s body
How it works:• Most signals originate from hydrogens ofWater molecules• Water is bound to different organs in different way variation of signal amongorgans & variation between healthy and diseased tissue
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
MRI showing a verticalCross section through aHuman head. http://en.wikipedia.org/wiki/Magnetic_resonance_imaging
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MRI: A Medicinal Application of NMR
Some Magnetic Resonance Imaging Uses:
• Detailed images of blood vessels
• Examine the vascular tree
• Differentiate intracelluar and extracelluar edema stroke patients
• Detecting cancer, inflammation, tumors
Current research:
• 31P nuclei analysis: investigate celluarmetabolism (ATP and ADP)
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
MRI showing a verticalCross section through aHuman head. http://en.wikipedia.org/wiki/Magnetic_resonance_imaging
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Spectroscopy
1H NMR:Determine bond connectivities/pieces of a structure, whole structure
13C NMR:Types of carbons (DEPT)
IR:Determine the functional groups present in a structure:
-OH, C=O, C-O, NH2, C=C, CC, C=N, CN
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IR Spectroscopy
Main Use: To detect the presence or absence of a functional group (specific bonds) in a molecule
How It Works:1. Bonds vibrate freely at specific wavelengths (wavenumbers)2. Want to cause the bonds to increase the magnitude of this vibrational
frequency3. Subject compound to IR radiation, 4000-625 cm-1
cm-1 is the unit for wavenumber () is directly proportional to energy (unlike wavelength)
4. Bonds absorb energy equal to their natural vibrational energy - it is quantized. This absorption of energy causes a change in dipole moment
for the bond.5. Upon absorption, bonds stretch and/or bend; the IR measures this
absorption.
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Vibrational Modes of Bonds
Stretches are more noted thanbends
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Correlation Chart Specific bonds absorb specific IR radiation and signals will appear within certainwavenumber ranges (similar to NMR).
Note: O-H stretches are broader than N-H stretches
N-H Stretches:1° Amines (RNH2) has two peaks2° Amines (RNHR) has one peak3° Amines (NR3) has no peaks
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IR Correlation Chart Specific bonds absorb specific IR radiation and signals will appear within certainwavenumber ranges (similar to NMR).
Correlation of Bond Stretching and IR Absorption (See also Correlation Chart & Table in LabGuide)
Type of Bond Group Family of Compounds Wavenumber Range (cm-1)
Single Bonds —C—H Alkanes 2850-3300
=C—H Alkenes, aromatics 3000-3100
≡ —C H Alkynes 3300-3320
O—H Alcohols 3200-3600
N—H Amines 3300-3500
—C O Ethers, Ester , s AlcoholsCarboxyl icAcids
1330-1000
Doubl e Bonds =C C Alkene, s aromatics 1600-1680
=C O Carbonyls 1680-1750
Aldehyde , s ketones 1710-1750
Carboxyl icacids 1700-1725
Esters, amides 1680-1750
=C N Imines 1500-1650
Trip leBonds C≡C Alkynes 2100-2200
C≡N Nitriles 2200-2300
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A: O-H stretch (strong, broad)C: C-H stretch (strong, sharp)E: CC or CN stretch (sharp)F: C=O stretch (strong, medium to sharp)G: C=C stretch (sharp)J: C-O stretch (strong, medium)K: C-X stretch (sharp)
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IR spectrum of hexanoic acid
Functional Group Region:1550-4000 cm-1
Most useful portion
Fingerprint Region:400-1550 cm-1
More difficult to interpret
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An IR Spectrum
O-H stretches are broad due to H-bonding.
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Sample Problem 1
Indicate how the following pairs of compounds could be distinguished usingcharacteristic IR peaks:
(a) Benzaldehyde (C6H5O) and benzoic acid (C6H5COOH)
1. Consider each structure:
2. Determine the main differences that would be seen in IR. Use correlationchart.
H
O
OH
O
benzaldehyde Benzoic acid
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Sample Problem 2
An unknown oxygen-containing compound is suspected of being an alcohol,a ketone, or a carboxylic acid. Its IR spectrum shows a broad strong peakat 3100-3400 cm-1 and a strong, sharp peak at 1700 cm-1. What kind ofcompound is it?
Consider what type of bonds appear in the ranges given. Refer to correlation chart.
Broad peak at 3100-3400 cm-1 Strong, sharp peak at 1700 cm-1