Infrared Spectroscopy of Organic Molecules

Practice Problem 12A Whichishigherinenergy,FMradiowaveswithafrequencyofl.015 x 10 Hz (101.5 MHz) or visible green light with a frequency of 5 x 10 Hz Strategy Remember the equations e = hv and e = hc/, which say that energy increases as frequency increases and as wavelength decreases. 

Solution Since visible light has a higher frequency than a radio wave, it is higher in energy. 

Problem 12.6 Which has the higher energy, radiation with v = 4.0 X 109 H2 or radiation with 

Problem 12.7 It s useful to develop a feeling for the amounts of energy that correspond to different parts of the electromagnetic spectrum. Use the relationships 

The infrared (IR) region of the electromagnetic spectrum covers the range from just above the visible (7.8 x 10 7 m) to approximately 10-4 m, but only the midportion from 2.5 x IQ-6 m to 2.5 x 10 5 m is used by organic 

Why does an organic molecule absorb some wavelengths of IR radiation but not others All molecules have a certain amount of energy di H trihuted throughout their structure, causing bonds to stretch and contract, atoms to wag back and forth, and other molecular vibrations to occur. Some of the kinds of allowed vibrations are shown below  

The amount of energy a molecule contains is not continuously variable but is quantized. That is, a molecule can stretch or bend only at specific fire-quencies. Take bond stretching, for example. Although we usually speak of 1 

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These same types of molecular motions occm even when other atoms or functional groups are involved in the vibrational transitions. However, depending on the types of atoms involved and their enviromnents in the molecule, each transition will have a specific energy associated with it. Each of these vibrational modes will give rise to the absorption of infrared spectrum. This forms the basis for qualitative analysis and structmal determinations by infrared spectroscopy. For organic molecules, hydrogen... 

Many techniques are available for the determination of the geometry of organic molecules in their ground states, such as microwave spectroscopy, infrared and Raman spectroscopy, and electron and x-ray diffraction. 

INFRARED SPECTROSCOPY OF HIGHLY REACTIVE ORGANIC SPECIES THE IDENTIFICATION OF UNSTABLE MOLECULES AND REACTIVE INTERMEDIATES USING AB /N/7/0 CALCULATED INFRARED SPECTRA... 

When the reaction product(s) is obtained in pure form, modern instrumental methods of structure determination, rather than traditional wet methods, provide file fastest way to determine the functionality and connectivity present. Today s chemist has a large number of tools available with which to probe the structure of molecules, but for determining the structures of organic molecules the big three are nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and mass spectrometry (MS). The frequency of use of these techniques generally falls in the same order (NMR > IR > MS). 

In contrast to the minimal activity in infrared reflection studies the technique of inelastic electron tunneling spectroscopy (IETS) recently has contributed a large amount of information on monolayer adsorption of organic molecules on smooth metal oxide surfaces,Q),aluminum oxide layers on evaporated aluminum. These results indicate that a variety of organic molecules with acidic hydrogens, such as carboxylic acids and phenols chemisorb on aluminum Oxide overlayers by proton dissociation - 1 — and that monolayer coverage can be attained quite repro-ducibly by solution doping techniques. - The IETS technique is sensitive to both infrared and Raman modes. — However, almost no examples exist in which Raman il and or infrared spectra have been taken for an adsorbate/substrate system for which IETS spectra have been observed. 

Bonino brought forward a further contribution to the theory of infrared spectra of organic liquids by incorporating the Bohr-Sommerfeld quantum conditions, including the correspondence principle of Bohr as well. This paved the way toward establishing a correlation between the physical and chemical image of molecules in the study of infrared spectra. From this series of papers on infrared spectroscopy, one can already observe the interdisciplinary character of Bonino s thought. In a lecture delivered some years later, Bonino offered these reflections on his chosen field of research ... 

Inelastic effects are exploited in the rapidly developing technique of high resolution electron energy loss spectroscopy (ELS or EELS) which permits identification of adsorbed molecules or molecular fragments by their vibrational spectra. Thus the method has much in common with the infrared spectroscopy of surfaces and, not surprisingly, the classic case of CO adsorption has received attention on Ni(lOO) and on stepped Ni and Pt surfaces. Other recent investigations of interest include H2 on organic species on Ni and Pt, and the observation of... 

A variety of spectroscopic methods have been employed to characterize the functional groups within the humic molecule. Traditionally, the method of choice for studying fimctional groups of organic molecules would be infrared spectroscopy. An... 

Hemmer, M.C., Steinhauer, V., and Gasteiger, J., The Prediction of the 3D Structure of Organic Molecules from Their Infrared Spectra, Vibrat. Spectroscopy, 19, 151, 1999. 

Methods for micro-measurement of surface areas include the Brunauer, Emmett, and Teller (BET) method (2), which relies on the adsorption of monolayers of gas, commonly nitrogen or argon, the adsorption of organic molecules such as ethylene glycol and ethylene glycol monoethyl ether (EGME) (10). and the use of infrared internal reflectance spectroscopy (11) which characterizes bonding of sorbed water. These last two techniques have been confined principally to surface areas of clay minerals. 

Table 2.1 summarizes the regions of the spectmm and the types of energy transitions observed there. Several of these regions, including the infrared, give vital information about the stmctures of organic molecules. Nuclear magnetic resonance, which occurs in the radiofrequency part of the spectrum, is discussed in Chapters 3,4,5,6 and 10 whereas ultraviolet and visible spectroscopy are described in Chapter 7. 

The spectra in Figs 20.17-20.25 showthe potential of infrared spectroscopy for distinguishing between common types of organic molecule, particularly if combined with information from other techniques such as mass spectrometry, nuclear magnetic resonance (NMR) and with the results of chemical tests. 

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