Fran Adar

Vibrational spectra are of interest to analytical and materials scientists because of their potential to yield nondestructively detailed information on materials. This is because the vibrations quite sensitively reflect interatomic bonding. There are several spectroscopies in which vibrational transitions can be detected. The simplest is infrared (IR) absorption. In this case, when a molecule is irradiated with photons whose energy corresponds to the energy of one of its vibrations (which is in the mid- and far-IR part of the electromagnetic spectrum), those photons will be absorbed. If an instrument is built to monitor the transmission of IR light, there will be a dip in the transmission at the energy corresponding to the vibration. In the near IR (NIR) region, there are similar processes that correspond to absorption of overtones (multiples of a vibration) and combinations of vibrations. Absorption in the MIR detects vibrational "fundamentals" and some higher-order transitions, whereas the NIR detects second or higher-order processes in direct absorption. It is also possible to "scatter" off of a molecule. When a photon in the visible part of the electromagnetic spectrum is used to initiate a scattering event, it is called Raman scattering. By its nature, Raman is a second-order process — there are two photons, the incident laser photon, and the scattered photon. Second-order processes in general have much lower quantum yields than first-order processes. So even though it is easier to build spectrographs and detectors for the visible part of the spectrum, IR spectroscopy developed more rapidly during the middle and latter part of the last century. As a consequence of the physical origin of IR absorption and Raman scattering, the spectra appear quite different. What is intense in the IR is weak in the Raman, and vice versa, and when there is a center of symmetry in the molecule or crystal under study, no vibrational band will appear in both spectra. Consequently, the ability to detect both types of spectra produces a lot of information on a sample, with the potential to be more informative than either by itself, or, indeed, the sum of the two. As an example, we will discuss here some of what can be done with two simple polymers — polyethylene (PE) and polypropylene (PP).

In polyatomic systems, each pair of bonded atoms has three degrees of freedom to vibrate, but the atomic motion of one pair of atoms is mixed together with adjacent atoms. Not only are there stretches, but there are bending, rocking, and torsional motions as well. If you can imagine a stretching motion between two atoms in a complicated system, you can see quickly that those two atoms will pull and twist other surrounding atoms. The final result is that there are "normal modes" of motion; that is, the entire polyatomic system will exhibit vibrations where many atoms will move in concert, but each normal mode will be independent and will not mix with other normal modes.

If a molecule is systematically modified, for example, by replacing a proton with a methyl group, the vibrations will change. In this case, there will be new methyl group vibrations, but, in addition, the added mass attached to the modified carbon will tug more on that carbon, which will in turn affect its normal vibrations.

Over the years, it has been shown that it is possible to use this type of analysis to characterize regions of spectra. Functional groups such as >C=C<, >C=O<, CH (methine, methylene, methyl), SH, NH, and OH vibrate with frequencies that are well-separated from anything else (and each other) so their bands are easily recognizable in the spectra (1,2). Below 1500 cm^–1 is the "fingerprint" region, where the observed vibrations represent mixed motions of multiple atoms tugging at each other.