Infrared Spectroscopy. Vibration-Rotation Spectra

Infrared spectroscopy was the province of physicists and physical chemists until about 1940. At that time, the potential of infrared spectroscopy as an analytical tool began to be recognized by organic chemists. The change was due largely to the production of small, quite rugged infrared spectrophotometers and instruments of this kind now are virtually indispensable for chemical analysis. A brief description of the principles and practice of this spectroscopic method is the topic of this section. 

Absorption of infrared radiation causes transitions between vibrational energy states of a molecule. A simple diatomic molecule, such as H—Cl, has only one vibrational mode available to it, a stretching vibration somewhat like balls on the ends of a spring  

Molecules with three or more atoms can vibrate by stretching and also by bending of the chemical bonds, as indicated below for carbon dioxide  

Diatomic molecules such as HC1 have one vibrational mode, but it is important to note that symmetrical diatomic molecules, such as 02, N2, Cl2, F2, and H2, do not absorb in the infrared region of the spectrum. This is 

In practice, infrared spectra can be obtained with gaseous, liquid, or solid samples. The sample containers (cells) and the optical parts of the instrument are made of rock salt (NaCl) or similar material that transmits infrared radiation (glass is opaque). 

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However, the first experimental achievement of a narrow Doppler width in fast-beam spectroscopy had already been reported independently by Wing et They performed the first precision measurements in the infrared vibrational-rotational spectrum of the simplest molecule by crossing the ion beam with the beam from a single-mode CO laser at a very small intersection angle. 

Figure 23.9 The Fundamental Band of the Vibration-Rotation Spectrum of HCI. The resolution of the spectrum is sufficient to show the lines for H CI and H CI. The CI is the more abundant isotope of chlorine, and produces the more intense lines. From N. L. Alpert, W. E. Keiser, and H. A. Szymanski, IR Theory and Practice of Infrared Spectroscopy, 2nd ed., Plenum Press, New York, 1970.

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

The dipole and polarization selection rules of microwave and infrared spectroscopy place a restriction on the utility of these techniques in the study of molecular structure. However, there are complementary techniques that can be used to obtain rotational and vibrational spectrum for many other molecules as well. The most useful is Raman spectroscopy. 

Infrared spectroscopy has broad appHcations for sensitive molecular speciation. Infrared frequencies depend on the masses of the atoms involved in the various vibrational motions, and on the force constants and geometry of the bonds connecting them band shapes are determined by the rotational stmcture and hence by the molecular symmetry and moments of inertia. The rovibrational spectmm of a gas thus provides direct molecular stmctural information, resulting in very high specificity. The vibrational spectrum of any molecule is unique, except for those of optical isomers. Every molecule, except homonudear diatomics such as O2, N2, and the halogens, has at least one vibrational absorption in the infrared. Several texts treat infrared instrumentation and techniques (22,36—38) and their appHcations (39—42). 

Raman spectroscopy gives results similar to those from infrared spectroscopy. This is why Raman spectroscopy is often used together with infrared spectroscopy in order to receive additional information about the sample analyzed. The motions of the molecule involved in the analysis of the sample in Raman spectroscopy are similar to those by infrared spectroscopy. These include rotational and vibrational motions. However, the physical causes of the resulting spectrum are different. 

Infrared and Raman spectroscopy are important analytical tools used to investigate a wide variety of molecules in the solid, liquid, and gas states, and yielding complementary information about molecular structure and molecular bonds. Both methods supply information about resonances caused by vibration, vibration-rotation, or rotation of the molecular framework, but because the interaction mechanism between radiation and the molecule differs in the two types, the quantum-mechanical selection rules differ as well. Therefore, not all of the molecular motions recorded by one type of spectroscopy will necessarily be recorded by the other. The geometrical configuration of the molecule and the distribution of electrical charge within that configuration determine which molecular motions may appear in each type of spectrum. 

The ideas fundamental to an understanding of infrared spectroscopy were introduced in this chapter. The electromagnetic spectrum was considered in terms of various atomic and molecular processes and classical and quantum ideas were introduced. The vibrations of molecules and how they produce infrared spectra were then examined. The various factors that are responsible for the position and intensity of infrared modes were described. Factors such as combination and overtone bands, Fermi resonance, coupling and vibration-rotation bands can lead to changes in infrared spectra. An appreciation of these issues is important when... 

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