Infrared Spectroscopy Chapter 7

Infrared spectroscopy Infrared radiation is lower in energy and of a longer wavelength than visible and ultraviolet light. The energy from infrared radiation is not enough to promote electrons to a higher energy level. It is enough to cause changes to the bonds in molecules.

What is a chemical bond? Ball and stick figure of an ethanol molecule. But exactly what is the ball, and for that matter, what is the stick? An atom doesn’t really look like a ball, nor does a chemical bond look like a stick, right?

Covalent Bonds Covalent bonds can be compared to springs that can undergo specific amounts of bending or stretching. As the atoms in the bonds change position due to bending or stretching the molecule vibrates.

Vibrating molecules Electrons can only occupy discrete electronic energy levels (the shells) Molecules are only able to occupy discrete vibrational energy levels. A molecule will absorb a discrete quantum of energy to move from one vibrational level to the next. As the molecule moves to a higher energy level the frequency of stretching vibration increases

Infrared A powerful analytical tool as: All molecules absorb infrared radiation. The only exceptions are O2 and N2. It exploits the fact that molecules are flexible structures that can bend and stretch.

How Infrared Works Infrared radiation looks at the amount of infrared radiation absorbed by a molecule, bond or functional group The range of energy absorbed depends on the strength of the bonds. A single C-C bond absorbs lower-energy infrared radiation than a triple CΞC bond

How Infrared Works The mass of the atoms attached by the bond also affects the energy fo the infrared radiation absorbed. The higher the mass, the lower the energy of the radiation absorbed These characteristics of the infrared spectrum make it a powerful tool for identifying the bonds in molecules.

Wavenumber Infrared spectroscopy frequency is measured in wavenumber. This is the reciprocal of wavelength (1/λ) Has the units of cm-1. Frequency and wavelength are related by the expression c= λv, where c is the speed of light, v is the frequency and λ is the wavelength.

Instrumentation The main features are: A source of infrared radiation Sample and reference cells or discs are made out of NaCl, KBr or similar. Cells cannot be made out of glass or plastic as they absorb IR radiation Wavelength selector An infrared detector We measure the transmittance which is the amount of light that passes through the sample. What does the transmittance tell us about the absorption?

Interpreting Infrared Spectrum – Qualitative analysis Infrared spectroscopy is most important for the information it can give us about the functional groups in a molecule Functional groups are groups of atoms that determine the properties and reactions of an organic molecule. This can be used to identify a known substance or clarify the structure of an unknown material

Qualitative analysis The energy of radiation absorbed and released depends on: The kind of bonds in the molecule. Each type of bond absorbs infrared radiation over a typical narrow range of wavelengths. If bonds are thought of as springs then the strength of the spring will determine the energy of the wavelength absorbed.

Qualitative analysis The energy of radiation absorbed and released depends on: The environment or other bonds in the molecule. The wavelength absorbed depends upon the entire molecular structure For example propanoic acid and methyl ethanoate have the same molecular formula (C3H6O2) but their structures are different. They produce different Infrared spectrums.

Qualitative Analysis a b

Fingerprint Region No two molecular compounds are identical in bonds and bonding environment, so infrared spectroscopy can give us a characteristic fingerprint of each compound. The infrared spectrum above 1000 cm-1 is used to identify functional groups. The spectrum at frequencies less than 1000cm-1 is characteristic of a particular compound. This region is largely a result of bending and stretching of the entire molecule and so is unique to a particular compound. This region below 1000cm-1 is known as the fingerprint region and no two compounds are the same.

Qualitative analysis Table 7.8 Characteristic infrared absorbance bands.

Worked Example Use table 7.8 on page 93. Identify the bond types corresponding to peaks A, B and C.

Quantitative Analysis If one peak in the spectrum is selected, the absorbance increases as the concentration of the molecule in the sample increases. A strong sharp peak in the spectrum is usually selected. As with chromatography a calibration curve is constructed using standards of known concentration. Unfortunately infrared spectrum calibration curves are often curved rather than linear.

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