Spectroscopy

•It is the interaction between the waves of electromagnetic radiation and matter.

•It is classified according to:

I- Shape of the spectrum (line, discrete or continuous).

II- Level of application (atomic or molecular).

III- Nature of usage (absorption, emission or reflection).

Electromagnetic spectrum:

Advantages of IR spectroscopy:

• Speed and accuracy of analysis.

• Small sample requirement.

• IR spectra are information rich; the peak position, intensities, widths, and shapes in a spectrum all provide useful information.

• Can be applied to solids, liquids, gases and polymers.

Disadvantages of IR spectroscopy:

• Can’t be applied to single atomic entities as they don’t contain chemical bonds and hence don’t absorb IR radiation.

• Nobel gases such as helium and argon don’t have infrared spectra.

• Monoatomic ions such as Pb+2 dissolved in water aren’t chemically bonded to anything and don’t have an infrared spectrum.

• Difficult to be applied to samples of complex composition because the spectrum will also be complex to be interpreted and it will be very hard to know which infrared bands are due to which molecules.

• Aqueous solutions are difficult to analyze using infrared spectroscopy.

What is an infrared spectrum?

• When infrared radiation interacts with matter, it can be absorbed, causing the chemical bonds in the material to vibrate.

• Functional groups in a given sample tend to absorb infrared radiation in the same wavenumber range.

• A plot of measured infrared radiation intensity versus wavenumber is known as infrared spectrum.

)/log( 0 IIA Where

A = AbsorbanceI = light intensity with a sample in the infrared beam (sample spectrum)I0 = light intensity measured with no sample in the

infrared beam (background spectrum)

0/ IIT Where

T = transmittanceI = light intensity with a sample in the infrared beamI0 = light intensity with no sample in the infrared

beam

How molecules absorb infrared radiation?There are two necessary conditions:

I- Total dipole moment of the functional group must not be equal to zero.

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Dipole moment:

qr•The first necessary condition for a molecule to absorb infrared light is that the molecule must have a vibration during which the change in dipole moment with respect to distance is non-zero.

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II- The energy of infrared light photon absorbed must be equal to the energy difference between energy levels between which the molecule is excited.

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Origin of IR peak position, intensity and width:I-Peak position:

• Suppose the atoms of a diatomic molecule are 2 masses and the bond between them is a spring.

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Mass effect:

• A shift in band position caused by a change in reduced mass is called a mass effect.

• As the reduced mass of a typical diatomic molecule increases, the stretching vibration decreases.

Electronic effect:

• Changes in force constant are typically caused by changes in the electronic structure of a molecule, and the shifts in infrared band positions thus caused are called electronic effects.

Reduce mass and force constant determine the position of the peak; at which frequency and hence which wavelength

II-Peak intensity:

• In IR spectroscopy, many peaks corresponding to different bonds can be obtained.

• Intensity of each peak differs according to,

• Therefore, the greater the change in dipole moment with respect to distance, the more intense the peak is.

x

• Concentration of the molecules in the sample is also a factor that determines the peak heights in infrared spectra.

• The equation that relates the concentration to absorbance is Beer’s law, which has the following form:

lcA

WhereA = absorbance

ε = absorptivity

l = path length

c = sample concentration

III-Peak width:

• Samples with strong intermolecular interactions have more chemical environments than samples with weak ones. This is because the electronic effects are greater for the stronger interactions.

• Broad IR bands are observed for samples with many chemical environment and vice versa.

• Also, overlap between different bands in a mixture causes broadening of IR band.

IV- The origin of group wavenumbers:

• Many functional groups absorb IR at about the same wavenumber, giving result to what is known as group wavenumbers.

• To have a good group wavenumber, it must:

1. Be intense

2. Appear in a unique wavenumber range; no overlap.

3. Appear in a narrow wavenumber range and have a band position that is insensitive to changes in molecular structure and sample condition.

Dealing with mixtures:

• It takes place by one of two ways:

1. Spectral subtraction: This process can remove the bands of unwanted

components from a spectrum. Subtraction involves taking the spectrum of a

mixture and subtracting from it the spectrum of a pure compound that is present in the mixture.

2. Library searching: In this technique, the mixture spectrum is

mathematically compared to a collection of known spectra kept in a library.

A number called the hit quality index (HQI) describes how similar or different the spectra are to each other.

Properly performing identities:

• In this part we make use of a reference sample in addition to the unknown sample we are working on.

• We obtain the spectra of the sample and the reference and then compare them.

• Plot the two spectra using the same scale and then compare the two spectra with each others.

Thank you