Infrared Spectroscopy Features of infrared spectrophotometer: *a source of infrared rediation *sample and reference cell *a wavelength selector *an infrared detector

How Infrared Spectroscopy works… The infrared radiation is split into two beams. One beam passes through the sample cell and the other passes through the reference cell. The reference cell (like a control group) is used to measure the effect of the material of the sample cell, the solvent and the conditions of the atmosphere so that these effects can be discounted from information retrieved by the sample cell. The difference in transmittance (transmitted radiation) between the sample and the reference cell is because of the Absorption of certain frequencies by the molecules of the sample. These absorbtions then result in changes in the vibrational energy of the molecule under examination. This is picked up by the detector.

Now, getting down to the nitty gritty of it… Infrared radiation is lower in energy and has a lower wavelength than visible and ultraviolet light. This energy is just enough to cause changes to the bonds in the molecules. Imagine covalent bonds (bonds between two non-metal atoms) as springs which can bend and stretch. When the atoms change position due to this bending and stretching the molecules vibrate. Now molecules are only able to occupy distinct vibrational energy levels (similar the distinct energy levels which an atoms electrons are found in). A molecule will absorb a distinct quantum of energy to move from one energy level to the next, this amount of energy is provided by radiation in the infrared region of the electromagnetic spectrum, ie infrared radiation. Infrared spectroscopy therefore exploits the fact that molecules are flexible in structure and have the ability to bend and stretch. Apart from a few homonuclear molecules (eg O2 and N2) all molecules absorb infrared radiation.

To clarify, this is the section of radiation we are focussing on, you can see along the bottom and along the top the wavelength and frequency of the radiation is noted, what is not mentioned in the wavenumber. It is the wavenumber which is used for measurement in infrared spectroscopy. It is easy to find though wavenumber = 1 / wavelength and is written in units of cm-1

This diagram shows how a molecule in a sample will absorb a particular frequency of infrared light, the detector will then register how much of each infrared frequency passes through the sample. This is why the graph in the next slide looks a bit up side down, if there is a high concentration of a certain bond little of its corresponding IR frequency will get through, this is seen as a dip in the graph where that particular wavenumber is.

Infrared spectrum, just fyi, this is what they look like: For the qualitative analysis we have the Wavenumber along the horizontal axis to tell us when bonds are in the sample and for the quantitative analysis the percentage of transmittance is shown along the vertical axis, the transmittance decreases as the concentration increases.

The energy of the radiation absorbed and released by the sample depends on: Bond: Absorption frequency C-H 3000 cm-1 C-C 1200 cm-1 C-O 1100 cm-1 C-Cl 750 cm-1 C-Br 600 cm-1 The kind of bonds in the molecules of the sample (what is bonded to what). Each type of bond absorbs infrared over a narrow range of wavelengths. eg. It takes a different amount of energy, and so a different range of wavelengths of radiation to stretch a C-H bond compared to a C-C bond. The environment or other bonds in the molecule. The wavelength absorbed depends upon the entire molecular structure. eg. Propanoic acid and methyl ethanoate have the same molecular formula (C3H6O2) and both contain a C=O group but their structures are different. Therefore the infrared spectra of these compounds have some features in common and some different.

Important understanding. No two molecular compounds are identical in bonds and bonding environments, and so infrared spectroscopy can give us a characteristic fingerprint for each compound. Some peaks in the spectrum are narrow, this usually means that the peak corresponds to one particular type of vibration of a bond. Other peaks are broad, these are combinations of peaks due to a number of vibrational changes with overlapping energies. Peaks can be strong medium and weak, strong peaks are observed when a high proportion of molecules absorb at a given wavenumber Infrared spectrum above 1000cm-1 is used to identify functional groups (atoms or groups of atoms which determine an inorganic molecules properties and reactions). The spectrum at frequencies less than 1000cm-1 is unique to a particular compound (because in this region is largely the result of bending and stretching of the entire molecule) for this reason it is called the finger print region. If a known substance and an unknown sample have the same absorption spectrum below 1000cm they are almost certainly the same compound.

Advantages Disadvantages Infrared spectroscopy is a POWERFUL tool which is able to be applied to a wide range of samples. Infrared spectroscopy can be used for quantitative analysis by constructing a calibration curve. It can be used to test a wide range of organic samples which can be gas, liquid and solids. Small samples needed for testing. Can be used in conjunction with UV spectroscopy, chromatography, mass spectroscopy and nuclear magnetic resonance spectroscopy. Disadvantages The equipment necessary is moderately expensive and a trained technician is needed to operate it. Calibrations are slightly curved rather than linear and therefore slightly less accurate.

Usefulness and suitability Infrared spectroscopy is limited to compounds with covalent bond ie. organic molecules but any form of organic molecule can be used, gas, liquid or solid. Only small samples are needed and infrared spectroscopy is capable of both quantitative and qualitative analysis. Therefore IR is suitable for analysing and sample of organic molecules and giving quantitative and qualitative information. Infrared Spectroscopy can be used to: Analyse paint, dye, glass and fibre samples for forensic investigations. Find the concentration of toxic gases in the atmosphere such as sulfur dioxide, hydrogen cyanide and carbon disulfide. Accurately test the BAC of a person who has taken a breath test which indicated that they are over the limit.

Infrared spectroscopy, comparatively. Infrared spectroscopy is similar to NMR, UV-visible, HPLC and GC in that they are all used to analyse organic samples, however unlike IR and NMR, the samples which can be tested by UV-visible, HPLC and GC are limited by their molar masses. Unlike IR, NMR is not suitable for gasses and is extremely expensive, however, it is much more sensitive and precise. Both deal with the structure of molecules rather than routine chemical analysis. Bibliography: http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__4%3A_Structure_Determination_I/Section_4.2%3A__Infrared_spectroscopy http://homepage.mac.com/stray/ib/chem/Options/analytical.html