Infrared Spectroscopy

Dr. MilkevitchOrganic Chem II Lab

Spring 2010Feb 11 & 13, 2010

Introduction

The purpose of this experiment

To introduce the student to spectroscopy

Discuss the specific technique of Infrared Spectroscopy Which is used to acquire

structural information on organic molecules

Use this technique in the laboratory

First: In order to Understand Spectroscopy Must understand electromagnetic

radiation (EMR)

EMR is a form of energy

Has a particle and wave nature

Examples: Light, microwaves, radiowaves

We Use Symbols to Designate Properties of

Waves λ is the wavelength of the waves

ν is the frequency of the waves

c is the speed of light of all EMR actually

Relationships Between These Variables

Speed = wavelength x frequency

Therefore: c = λν λ = c/ν ν = c/λ

For electromagnetic waves, the speed (c) is constant 3 x 108 m/s

What This Means

Wavelength has a direct, inverse relationship with frequency: λ ∝ 1/ν The higher the frequency, the

shorter the wavelength The longer the wavelength, the

smaller the frequency

Summary of Relationships

Wavenumbers

When the wavelength is measured in centimeters: the reciprocal of the wavelength (1/cm)

Is directly proportional to the frequency

1/cm Is called the wavenumber and is a commonly used term in spectroscopy

The Electromagnetic Spectrum

Spectroscopy Is the Study of the

Interaction of Matter andElectromagnetic Radiation

In Organic Chemistry, the common techniques include: Infrared Spectroscopy Nuclear Magnetic Resonance

Spectroscopy UV/Visible Spectroscopy

What Spectroscopy Tells Us

Specific information on the structural features of the molecules being studied

The presence or absence of specific patterns of chemical bonding in a molecule

Infrared Spectroscopy: The use of infrared radiation to determine the presence or absence of specific patterns of bonding in a molecule (i.e., functional groups)

The Infrared Region

When IR Radiation is Applied to a Molecule

Some passes through it, but some does not

Some of it is absorbed

All bonds in a molecule have a vibrational frequency

If the frequency of the IR energy matches the specific vibrational frequency of a bond in a molecule The molecule will absorb the IR radiation at that

frequency

The bond is excited from a lower to a higher vibrational state Amplitude of vibration increases dramatically

We can measure this absorbance of IR radiation

We can come up with a graph of absorbance intensity vs. Wavelength

What an IR Spectrum Looks Like

E = hv = hc

λ

Graph of absorption intensity vs. radiation frequencyGiven as % transmittanceUnits are in wavenumbers (cm-1), (sometimes microns)

Now, Organic Molecules are Quite Diverse

Millions of organic compounds exist Remember the 12 families of organic compounds?

Structurally different molecules can have different functional groups Do not absorb exactly the same frequencies of IR

radiation Therefore, give different patterns of absorption Specific bonds and functional groups in a molecule

Have specific vibrational frequencies Therefore, will absorb characteristic frequency

ranges of IR radiation

This means: IR spectroscopy is a valuable tool for identifying

different functional groups Also, a valuable tool for helping identify the

structure of an organic compound

Ways Molecules Vibrate: Vibrational

Modes Vibrational Modes: Fancy way to describe the ways a molecule can vibrate 2 most important vibrational modes in IR spectroscopy:

Stretching: involves a change in interatomic distance Bending: involves a change in bond angles

Change in bond anglesChange in interatomic distance

IR-Active and Inactive Bonds

Stretching and bending must: Change the molecule’s dipole moment in

order to be IR active Large changes in dipole moment: very

intense IR absorption This is really important

Polar bonds will absorb strongly Does a polar bond have a dipole moment?

A nonpolar bond will absorb weakly or not at all Does a nonpolar bond have a dipole

moment?

Some Trends in Vibrational Frequency

The smaller the atoms in a bond, frequency increases

Larger the atoms in a bond, frequency decreases

Bond strength also effects frequency of absorption Stronger bonds, higher frequency of

absorption

Alkyne 2250 – 2100 cm-1

Alkene 1680 – 1600 cm-1

Alkane 1200 – 800 cm-1

Trends: Carbon-Hydrogen Stretching

Bonds with more s character absorb at a higher frequency

More s character, shorter and stronger bond

sp3 C-H, just below 3000 cm-1 (to the right)

sp2 C-H, just above 3000 cm-1 (to the left) sp C-H, at 3300 cm-1

The Three Most Important Regions of the IR Spectrum

3600 – 3100 cm-1 Where OH and NH stretching occur

Region around 1700 cm-1

Where C=O stretching occurs

Region around 1650 cm-1

Where C=C stretching occurs

Many of the important functional classes are identified by the presence (or absence) of absorptions in these regions

An Infrared Spectrophotometer

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FT-IR Spectrometer “The modern IR spectrometer”

Small and compact Computer controlled

Has better sensitivity than dispersive instruments

Irradiate the sample with all IR frequencies at the same time

Does multiple scans quickly Averages the results

An Alkane IR Spectrum: Notable Peaks (or absorbances)

An Alkene IR Spectrum: Notable Peaks

An Alkyne IR Spectrum: Notable Peaks

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CorrelationTable:Summary of Notable IR absorbances for the functional Groups

Very importantFor the organicChemist

Procedure

Each group should choose one of the following compounds:

Chlorooctane Dodecane ChlorodecaneCyclohexaneCyclohexene3,3-dimethyl-1-buteneHeptaneHexane1-heptyne1-hexene

Procedure (2)

Obtain an IR spectrum See Dr. M in the instrument lab

Interpret major absorption frequencies Using tables in this handout Annotate the spectrum with your

interpretations

Your Report

Your introduction should include a discussion of IR spectroscopy Your textbook also has a chapter on IR

Spectroscopy, use it if necessary No reaction mechanism or balanced equation in this

experiment Physical properties section should be the relevant

physical properties of your chosen compound

Your results section should include the spectrum of your chosen compound Annotated

What do the peaks correspond to? If it’s not annotated, it is meaningless Make a table of relevant absorbances, along with their identity

Conclusions: Things to think about Did you successfully obtain your IR spectrum?

Is the spectrum clean? Crappy? Easy to obtain? What does your IR spectrum look like?

What major peaks do you have? What stretches do these peaks correspond to?

Do the peaks correspond to what type of compound you have (alkane, alkene or alkyne)?

Prove this: correlate your peaks to what peaks these compounds should have in an IR spectrum

Prove it further: find an IR spectrum of the compound and compare it to your spectrum

Overall, what did you learn about IR spectroscopy?

Additional Questions to answer:

1. Which absorbs at a higher frequency: a C-H bond or a C-D bond? Explain.

2. Why does H2 not have an IR spectrum?

3. Explain why the C=C stretch for a trans-disubstituted alkene is weaker than for a cis-disubstituted alkene.