Spectroscopy and Structure Determination

An aspiring young chemist thinks he has just synthesized 2-phenylethanol, but how does he know for sure In the early years of organic chemistry, determining the structure of a new compound was often a formidable task. The first step, of course, was an elemental analysis. Knowing the percentage of each element present allowed the empirical formula to be calculated the molecular formula was then either the same as or a multiple of that formula. Elemental analysis is still an important criterion of the purity of a compound. 

But how are the atoms arranged What functional groups are present And what about the carbon skeleton Is it acyclic or cyclic Are there branches and where are they located Are benzene rings present All of these questions and more had to be answered by chemical means. Reactions, such as ozonolysis (Sec. 3.17.b) or saponification (Sec. 10.13), could be used to convert complex molecules to simpler ones whose structures were easier to determine. To identify functional groups, various chemical tests could be applied (such as the bromine or permanganate tests for unsaturation or the Tollens silver mirror test for an aldehyde group). 

Once the functionality was known, reactions whose chemistry was well understood could be used to convert the unknown compound to a compound whose structure was already known. For example, if the compound was an aldehyde suspected to have the same R group as a known acid, it could be oxidized (eq. 12.1). If the physical properties (bp, mp, specific rotation if chiral, and so on) 

These methods—which often required weeks, months, even years—are still used in appropriate situations. But since the 1940s, various types of spectroscopy have simplified and speeded up the process of structure determination greatly. Automated instruments have been developed that permit us to determine and record spectroscopic properties often with little more effort than pushing a button. And these spectra, if properly interpreted, yield a great deal of structural information. For example, 2-phenylethanol can easily be identified from its NMR spectrum alone. 

Spectroscopic methods have many advantages. Usually only a very small sample of material is required, and it can often be recovered if necessary. The methods are rapid, sometimes requiring only a few minutes. And usually we obtain more detailed structural information from spectra than from ordinary laboratory methods. 

Spectroscopic methods provide rapid, nondestructive ways to determine molecular structures. One of the most powerful of these methods is nuclear magnetic resonance (NMR) spectroscopy, which involves the excitation of nuclei from lower to higher energy spin states while they are placed between the poles of a powerful magnet. In organic chemistry, the most important nuclei measured are 1H and 13C. 

Protons in different chemical environments have different chemical shifts, measured in 8 (delta) units from the reference peak of tetramethylsilane [TMS, (CH3)4Si]. Peak areas are proportional to the number of protons. Peaks may be split (spin-spin coupling) depending on the number of protons. Proton NMR gives at least three types of structural information (1) the number of signals and their chemical shifts can be used to identify the kinds of chemically different protons in the molecules (2) peak areas tell how many protons of each kind are present (3) spin-spin coupling patterns identify the number of near-neighbor protons. 

13C NMR spectroscopy can tell how many different kinds of carbon atoms are present, and 1H-13C splitting can be used to determine the number of hydrogens on a given carbon. 

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). 

Mass spectra are used to determine molecular weights and molecular composition (from the parent or molecular ion) and to obtain structural information from the fragmentation of the molecular ion into daughter ions. Electrospray ionization (ESI-MS) and matrix-assisted laser desorption ionization (MALDI-MS) mass spectroscopy can be used to obtain structural information about macromolecules, including proteins, polymers, and drug-DNA complexes. 

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In these compounds, xenon is in one of the following oxidation states Xe(II) in XeA2E3 types (three lone pairs) in the terminology of Gillespie and Nyholm s VSEPR theory of molecular structure, Xe(IV) in XeA4E2 types, Xe(VI) in XeA E or 0 = XeA4, and Xe(VIII) in XeO. Several reviews of noble gas chemistry, spectroscopy, and structure determination discuss the early work soon after the first discovery of a xenon compound in 1962. " Two comprehensive reviews published in 1970 and 1973, and the 1982 review by Seppelt and Lentz cover the later developments. ... 

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