Introduction to Spectroscopy: Infrared Spectroscopy

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Introduction to

SpectroscopyInfrared Spectroscopy

CHM 243 Organic Chemistry I

Dr. Frank Yepez CastilloMisericordia University

Fall 2013

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Introduction to Spectroscopy

Organic chemists use several spectroscopictechniques to determine the structure of acompound

Most techniques are nondestructive.

All of the spectrometric techniques rely on the

interaction of matter (your sample) and differentsources of energy

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Light has wave-like and particle-like properties

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The frequency of a wave is the

number of complete cyclesthat pass a fixed point in asecond

Wavelength is the distance betweenany two peaks (or any two troughs) ofthe wave

Energy Sources: Light (Electromagnetic

radiation)

=c


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Light has wave-like and particle-like properties When viewed as a particle, electromagnetic radiationconsists of packets of energy, called photons.The energy of each photon is directly proportional to itsfrequency.

h is Plancks constant (6.62 x 10-37 kJ.s)

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radiation)

E= h


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radiation) and

These two equations can be combined into thefollowing:

High frequencies and short wavelengths areassociated with high energy

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E= h =c

E=hc


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Spectroscopy

Spectroscopy involves an interaction between matter andlight (electromagnetic radiation).

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IR region ranges from 10,000 100 cm-1


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Types of Spectroscopy

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Different regions of the electromagnetic spectrum are usedto probe different aspects of molecular structure


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Molecules can store energy in a variety of ways: Rotational motion Bond vibrations Electronic transitions

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All these forms or energy are quantized

Electromagnetic radiation


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The IR Region

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From right below the visible region to just above thehighest microwave and radar frequencies

Wavelengths are usually 2.5 x 10-4 to 25 x 10-4 cm More common units are wavenumbers (), or cm-1, the

reciprocal of the wavelength in centimeters Wavenumbers are proportional to frequency and energy


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The IR Region

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The IR region that is of particular interest to organicchemists is the limited portion between 4000 cm-1 and400 cm-1

Absorption in the IR region corresponds to energychanges in the order of 8 to 40 kJ/mol.

These energy levels encompass the stretching andbending vibrational frequencies of the bonds in mostcovalent molecules.


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IR SpectrumIntensity of the signals are normally expressed as either transmittance (T) orabsorbance (A)

Transmittance is the ratio of the radiant power transmitted by a sample to theradiant power incident on the sampleAbsorbance is the logarithm, to the base 10, of the reciprocal of thetransmittance:

Each signal on the IR spectrum has three important characteristics:wavenumber, intensity, and shape

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A = log10(1T)


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IR SpectrumPeaks in the IR correspond to a particular kind of bond, and each bond type(such as O-H and C-H) occurs at a characteristic frequency

The IR spectrum is divided into two regions: a) The functional group region ( 1500 cm-1)b)

The fingerprint region (

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Introduction to Spectroscopy: Infrared Spectroscopy