| Infrared Spectroscopy |
Article Index for Infrared |
Website Links For Infrared Spectroscopy |
Information AboutInfrared Spectroscopy |
| CATEGORIES ABOUT INFRARED SPECTROSCOPY | |
| spectroscopy | |
|
THEORY The infrared portion of the electromagnetic spectrum is divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The far-infrared, approximately 400-10 Cm-1 (1000–30 μm), lying adjacent to the Microwave region, has low energy and may be used for Rotational Spectroscopy . The mid-infrared, approximately 4000-400 cm-1 (30–1.4 μm) may be used to study the fundamental vibrations and associated Rotational-vibrational structure. The higher energy near-IR, approximately 14000-4000 cm-1 (1.4–0.8 μm) can excite Overtone or Harmonic vibrations. The names and classifications of these subregions are merely conventions. They are neither strict divisions nor based on exact molecular or electromagnetic properties. Infrared spectroscopy works because Chemical Bond s have specific frequencies at which they vibrate corresponding to Energy Level s. The Resonant Frequencies or vibrational frequencies are determined by the shape of the molecular Potential Energy Surface s, the masses of the atoms and, eventually by the associated Vibronic Coupling . In order for a vibrational mode in a molecule to be IR active, it must be associated with changes in the permanent dipole. In particular, in the Born-Oppenheimer and harmonic approximations, i.e. when the Molecular Hamiltonian corresponding to the electronic Ground State can be approximated by a Harmonic Oscillator in the neighborhood of the equilibrium Molecular Geometry , the resonant frequencies are determined by the Normal Modes corresponding to the molecular electronic ground state potential energy surface. Nevertheless, the resonant frequencies can be in a first approach related to the strength of the bond, and the Mass Of The Atoms at either end of it. Thus, the frequency of the vibrations can be associated with a particular bond type. Simple diatomic molecules have only one bond, which may stretch. More complex molecules may have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups. The atoms in a CH2 group, commonly found in Organic Compound s can vibrate in six different ways, symmetrical and antisymmetrical stretching, '''scissoring''', '''rocking''', '''wagging''' and '''twisting'''; as shown below: In order to measure a sample, a beam of infrared light is passed through the sample, and the amount of energy absorbed at each wavelength is recorded. This may be done by scanning through the spectrum with a Monochromatic beam, which changes in wavelength over time, or by using a Fourier Transform instrument to measure all wavelengths at once. From this, a Transmittance or Absorbance spectrum may be plotted, which shows at which wavelengths the sample absorbs the IR, and allows an interpretation of which bonds are present. This technique works almost exclusively on Covalent Bond s. Clear spectra are obtained from samples with few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra. The technique has been used for the characterization of very complex mixtures however. SAMPLE PREPARATION Gaseous samples require little preparation beyond purification, but a sample cell with a long Pathlength (typically 5-10 cm) is used as gases show relatively weak absorbances. Liquid samples can be sandwiched between two plates of a high purity salt (commonly Sodium Chloride , or common salt, although a number of other salts such as Potassium Bromide or Calcium Fluoride are also used). The plates are transparent to the infrared light and will not introduce any lines onto the spectra. Some salt plates are highly soluble in water, and so the sample, washing reagents and the like must be Anhydrous (without water). Solid samples can be prepared in two major ways. The first is to crush the sample with a Mulling Agent (usually Nujol ) in a Marble or Agate mortar, with a pestle. A thin film of the mull is applied onto salt plates and measured. The second method is to grind a quantity of the sample with a specially purified salt (usually Potassium Bromide ) finely (to remove scattering effects from large crystals). This powder mixture is then crushed in a mechanical Die Press to form a translucent pellet through which the beam of the spectrometer can pass. It is important to note that spectra obtained from different sample preparation methods will look slightly different from each other due to the different physical states the sample is in. TYPICAL METHOD A beam of infrared light is produced and split into two separate beams. One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in. The beams are both reflected back towards a detector, however first they pass through a splitter which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained. A reference is used for two reasons:
SUMMARY OF ABSORPTIONS OF BONDS IN ORGANIC MOLECULES Wavenumbers listed in Cm-1 . USES AND APPLICATIONS Infrared spectroscopy is widely used in both research and industry as a simple and reliable technique for measurement, quality control and dynamic measurement. The instruments are now small, and can be transported, even for use in field trials. With increasing technology in computer filtering and manipulation of the results, samples in solution can now be measured accurately (water produces a broad absorbance across the range of interest, and thus renders the spectra unreadable without this computer treatment). Some machines will also automatically tell you what substance is being measured from a store of thousands of reference spectra held in storage. By measuring at a specific frequency over time, changes in the character or quantity of a particular bond can be measured. This is especially useful in measuring the degree of polymerization in Polymer manufacture. Modern research machines can take infrared measurements across the whole range of interest as frequently as 32 times a second. This can be done whilst simultaneous measurements are made using other techniques. This makes the observations of chemical reactions and processes quicker and more accurate.
|
|
|