Lecture 4
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Transcript of Lecture 4
Lecture 4
• 13C NMR: DEPT
• IR Spectroscopy:- How it works- Interpretation of spectra
Due:Lecture Problem 2
Determine the structure of this unknown (MF is C8H9Cl)
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
DEPT-NMR (Distortionless Enhancement by Polarization Transfer)
• Distinguishes between CH, CH2, and CH3 carbons
13C NMR:broadbanddecoupled(normal)
13C NMR:DEPT-90
13C NMR:DEPT-135
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
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MRI showing a verticalCross section through aHuman head. http://en.wikipedia.org/wiki/Magnetic_resonance_imaging
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
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MRI showing a verticalCross section through aHuman head. http://en.wikipedia.org/wiki/Magnetic_resonance_imaging
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
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.
Vibrational Modes of Bonds
Stretches are more noted thanbends
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
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
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)
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
An IR Spectrum
O-H stretches are broad due to H-bonding.
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
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