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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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Wednesday, January 04, 2012

IR Spectroscopy

Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer.

The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The higher-energy near-IR, approximately 14000–4000 cm−1 (0.8–2.5 μm wavelength) can excite overtone or harmonic vibrations. The mid-infrared, approximately 4000–400 cm−1 (2.5–25 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The far-infrared, approximately 400–10 cm−1 (25–1000 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The names and classifications of these subregions are conventions, and are only loosely based on the relative molecular or electromagnetic properties.

The atoms in a CH2 group, commonly found in organic compounds, because of Infrared, can vibrate in six different ways: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting. See the pictures below:

Symmetric
Asymmetric
Scissoring

Rocking

Wagging

Twisting

Gaseous samples require a sample cell with a long pathlength to compensate for the diluteness. The pathlength of the sample cell depends on the concentration of the compound of the interest. A simple glass tube with length of 5 to 10 cm equipped with infrared windows at the both ends of the tube can be used for concentrations down to several hundred ppm. Sample gas concentrations well below ppm can be measured with a White's cell in which the infrared light is guided with mirrors to travel through the gas. White's cells are available with optical pathlength starting from 0.5 m up to hundred meters.

Liquid samples can be sandwiched between two plates of a salt (commonly sodium chloride ). The plates are transparent to the infrared light and do not introduce any lines onto the spectra. In this case, we have to use a solvent that can't dilute the plates like a nonpolar aprotic solvent.

Solid samples can be prepared in a variety of ways. One common method is to crush the sample with an oily mulling agent (usually Nuiol) in a marble. A thin film of the mull is smeared onto salt plates and measured. The second method is to grind a quantity of the sample with a specially purified salt (usually KBr) finely (to remove scattering effects from large crystals). This powder mixture is then pressed in a mechanical press to form a translucent pellet through which the beam of the spectrometer can pass. A third technique is the "cast film" technique, which is used mainly for polymeric materials. The sample is first dissolved in a suitable, non hygroscopic solvent. A drop of this solution is deposited on surface of KBr or NaCl cell. The solution is then evaporated to dryness and the film formed on the cell is analysed directly. Care is important to ensure that the film is not too thick otherwise light cannot pass through. This technique is suitable for qualitative analysis. The final method is to use microtomy to cut a thin (20–100 µm) film from a solid sample. This is one of the most important ways of analysing failed plastic products for example because the integrity of the solid is preserved.


Usual Infrared Spectroscopy Tool Schema


The IR Spectrometry Examples


Alkane Example: n-octane

(Source: http://orgchem.colorado.edu/hndbksupport/irtutor/images/octane.gif)


Alkyl Halide Example: Chloroform
(Source: http://paultkay.info/sitebuilder/images/scanfig13-600x350.jpg)

Alcohol Example: Ethanol
(Source:http://orgchem.colorado.edu/hndbksupport/irtutor/images/ethanol.gif)

Alcohols and amines has almost the same absorption range. In liquid, the hydrogen bond makes a wider peak. To different the alcohols and amines, the finger print range is the one.


Summary:



Source and Further Reading. . .
Wikipedia
Fessenden & Fessenden. Organic Chemistry


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