What Is Infrared Spectroscopy

In organic chemistry, we use a portion of the electromagnetic spectrum called the vibrational infrared region. This region extends from 2.5 X 10 to 25 X 10 m and corresponds to energies from 48-4.8 kj/mol (11—1.2 kcal/mol). We commonly refer to radiation in the vibrational infrared region by its wavenumber (v), the number of waves per centimeter  

Expressed in wavenumbers, the vibrational region of the infrared spectrum extends from 4000 to 400 cm (the unit cm is read reciprocal centimeter )  

An advantage of using wavenumbers is that they are direcdy proportional to energy the higher the wavenumber, the higher is the energy of radiation. 

For a molecule to absorb infrared radiation, the bond rmdergoing vibration must be polar, and its vibration must cause a periodic change in the bond dipole the greater the polarity of the bond, the more intense is the absorption. Any vibration that meets this criterion is said to be infrared active. Covalent bonds in homonuclear diatomic molecules, such as Hg and Br2, and some carbon-carbon double bonds in symmetrical alkenes and alkynes do not absorb infrared radiation because they are not polar bonds. The multiple bonds in the following two molecules, for example, do not have a dipole moment and, therefore, are not infrared active  

Neither of the unsaturated bonds in these molecules is infrared active because the vibrational motions shown do not result in a change in bond dipole (due to the symmetry about these bonds). 

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Figure 2.18 The infrared and Raman spectra of styrene/butadiene rubber illustrating the complementary nature of the two techniques (From What is Raman Spectroscopy , Hendra, P., Int J Vib Spect, [www.ijvs.com] 1 (5), pp. 6-16 (1998) f Copyright John Wiley Sons Limited. Reproduced with permission).

Hydrogenation of polybutadiene converts both cis and trans isomers to the same linear structure and vinyl groups to ethyl branches. A polybutadiene sample of molecular weight 168,000 was found by infrared spectroscopy to contain double bonds consisting of 47.2% cis, 44.9% trans, and 7.9% vinyl. After hydrogenation, what is the average number of backbone carbon atoms between ethyl side chains ... 

As already indicated above, what one may consider a surface depends on the property under consideration. Adhesion is very much an outer atomic layer issue, unless one is dealing with materials like fibreboard in which the polymer resin may also be involved in mechanical anchoring onto the wood particles. Gloss and other optical properties are related to the penetration depth of optical radiation. The latter depends on the optical properties of the material, but in general involves more than a few micrometer thickness and therewith much more than the outer atomic layers only. It is thus the penetration depth of the probing technique that needs to be suitably selected with respect to the surface problem under investigation. Examples selected for various depths (< 10 nm, 10 s of nm, 100 nm, micrometer scale) have been presented in Chapter 10 of the book by Garton on Infrared Spectroscopy of Polymer Blends, Composites and Surfaces... 

The spectroscopy, reaction kinetics, and photophysics of coordinatively unsaturated metal carbonyls generated in the gas phase via UV photolysis are probed via transient infrared spectroscopy. The parent compounds that have been used to generate coordinatively unsaturated species are Fe(CO)5, Cr(C0)5 and Mn2(CO)io- In contrast to what is observed in solution phase, photolysis of these compounds produces a variety of coordinatively unsaturated photoproducts. 

Obviously, there is an isotope effect on the vibrational frequency v . For het-eroatomic molecules (e.g. HC1 and DC1), infrared spectroscopy permits the experimental observation of the molecular frequencies for two isotopomers. What does one learn from the experimental observation of the diatomic molecule frequencies of HC1 and DC1 To the extent that the theoretical consequences of the Born-Oppenheimer Approximation have been correctly developed here, one can deduce the diatomic molecule force constant f from either observation and the force constant will be independent of whether HC1 or DC1 was employed and, for that matter, which isotope of chlorine corresponded to the measurement as long as the masses of the relevant isotopes are known. Thus, from the point of view of isotope effects, the study of vibrational frequencies of isotopic isomers of diatomic molecules is a study involving the confirmation of the Born-Oppenheimer Approximation. 

A 21.3-g sample of poly(hexamethylene adipamide) is found to contain 2.50 x 10-3 mol of carboxyl groups by both titration with base and infrared spectroscopy. From these data the polymer is calculated to have a number-average molecular weight of 8520. What assumption is made in the calculation How can one experimentally obtain the correct value of M ... 

Overall perspectives of the results from ethene and the higher alkenes have been attempted in Sections VI.B.6 and VI.G. What has become clear, particularly in the context of hydrocarbon adsorption, is that the study of spectra on single-crystal surfaces is of great assistance in finding the correct interpretation of the more complex multispecies spectra obtained from finely divided metal catalysts. This has only become possible by the development of VEELS and RAIRS, the latter allied with the Fourier-transform methods that have also transformed the quality of the spectra from metal-particle catalysts obtained by transmission infrared spectroscopy. The use of RAIRS in turn has emphasized the general significance of the MSSR. 

Infrared spectroscopy is mainly used to tell what types of bonds are present in a molecule (using the functional group region, 1500-5000 cm-1) and whether two substances are identical or different (using the fingerprint region, 700-1500 cm-1). 

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