IR Spectroscopy for structural determination

As seen above, IR spectroscopy is most commonly used to identify functional groups and bonding patterns in molecules from the higher energy portion of the spectrum (1200-4000 cm ) where absorptions are primarily due to bondstretching vibrations. Some information on atom connectivity in the molecule can also be deduced from the frequency shifts caused by structural factors. In general, however, it is not possible to completely deduce the structure of a molecule by examination of its IR spectrum. However, IR spectroscopy is a powerful complement to NMR spectroscopy for structure determination. 

Good images indicating nearly uniform clusters of other metals are lacking, but evidence from EXAFS spectroscopy, combined with IR spectroscopy and extraction of clusters into solution, has provided a basis for structure determination of a number of small metal carbonyl clusters and clusters formed by their decarbonylation. Compilations of these are reported elsewhere [6,12,26]. 

Already a considerable number of transient organometallic species have been characterized by IR kinetic spectroscopy (see Table I). Like most other sporting techniques for structure determination, IR kinetic spectroscopy will not always provide a complete solution to every problem. What it can do is to provide more structural information, about metal carbonyl species at least, than conventional uv-visible flash photolysis. This structural information is obtained without loss of kinetic data, which can even be more precise than data from the corresponding uv-visible... 

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

We will concentrate upon the most commonly used techniques in organic structure determination nuclear magnetic resonance (NMR), infrared (IR) and ultraviolet-visible (UV-Vis) spectroscopy, and mass spectrometry (MS). The amount of space devoted to each technique in this text is meant to be representative of their current usage for structure determination. 

In the preceding example several types of spectroscopy are brought to bear. While the product structure could probably be deduced from IR spectroscopy or NMR (either 1II or 13C), the use of all three methods confirms the assignment. It is often prudent to use more than a single technique for structure determination so that the results reinforce each other. If a structure assignment is not consistent with all the data, the structure is probably incorrect. 

NMR spectroscopy of acid derivatives is complementary to IR spectroscopy. For the most part, IR gives information about the functional groups, and NMR gives information about the alkyl groups. In many cases, the combination of IR and NMR provides enough information to determine the structure. 

The benefits of using recycle in the analytical mode can be illustrated by referring to the chromatogram in Figure 6-8. The reacted mixture is separated into the parent compound (rubrene), its oxide, and the ozone complex. It was desired that both the ozone complex and the oxide be isolated as individual peaks for structure determination. Therefore, before each peak was collected and identified by an analytical technique such as IR, NMR, or mass spectroscopy, the peaks were recycled to attain high peak purity. 

This technique is complementary to IR spectroscopy and well suited for dark colored samples. As for IR, it has been employed for structural determination... 

The information derived from NMR spectroscopy is extraordinarily useful for structure determination. Not only can we count the number of nonec]uiva-lent carbon atoms in a molecule, we can also get information about tbe electronic environment of each carbon and can even find how many protons each is attached to. As a result, we can answ er mani structural questions that go unanswered by IR spectroscopy or mass spectrometry. 

Use MS and IR for Structure Determination 484 Nuclear Magnetic Resonance Spectroscopy... 

Chemists rely almost exclusively on instrumental methods of analysis for structure determination. We begin this chapter with a treatment of infrared (IR) spectroscopy, followed by a treatment of nuclear magnetic resonance (NMR) spectroscopy. These two commonly used techniques involve the interaction of molecules with electromagnetic radiation. Thus, in order to understand the fundamentals of spectroscopy, we must first review some of the fundamentals of electromagnetic radiation. 

Determine possible structures for the same spectrum (above) for a compound with molecular formula CsHiqO. What does example 11.7 and this problem tell you about the effectiveness of IR spectroscopy for determining the structure of an unknown compound ... 

X-ray diffractometry are also widely used for structure determination. With the notable exception of X-ray diffractometry, unequivocal definition of a structure is seldom possible by performing only a single type of spectroscopic analysis rather, a combination of different analyses is generally required. We emphasize IR and NMR spectroscopy (Sec. 8.2 and 8.3, respectively) in this textbook because the necessary instrumentation is more commonly available to students in the introductory organic laboratory course. However, in addition to these spectroscopic techniques, UV-Vis spectroscopy (Sec. 8.4) and MS (Sec. 8.5) will be discussed because of their importance to the practice of organic chemistry. You can find a number of invaluable resources and spectral databases on the Web. 

Nudear mi netic resonance (NMR) spectroscopy is arguably the most powerful and broadly applicable technique for structure determination available to organic chemists. It provides the most information about molecular structure, and in some cases, the structure of a compound can be determined using only NMRspectroscopy. In practice, the structures of compUcated molecules are determined through a combination of techniques that include NMR and IR spectroscopy and mass spectrometry. 

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