lecture 4 13 c nmr: dept ir spectroscopy: - how it works - interpretation of spectra due: lecture...

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

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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)

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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