Structure Identification by NMR

Generally die NMR spectrum of a compound is used in conjunction witii otiier available information for identification purposes. The reactants and die reagents and reaction conditions can serve as a guide to die types of products diat might be expected. Structure identification often merely confirms die structures of products that were predicted from die chemistry employed in die syndiesis. In odier cases products are obtained whose spectta do not match die predicted products. In such cases more information is usually required to solve die structure. Thus while NMR is an extraordinarily powerful tool, it is not sufficient to solve all structural problems. This latter fact must be kept in mind. 

The methyl singlet is indicative of the -NHCH3 group, and the aromatic signal has increased to 10H, indicating that two phenyl rings are present in the product. 

The signal at 4.45 is proper for the benzyl group, but the splitting pattern is problematic until it is recognized that because there is a chiral center at C-2, the benzyl protons are diastereotopic and thus nonequivalent. They are part of an ABX spin system and thus give the complex splitting pattern seen—actually a two-proton multiplet that looks like a doublet or a very close AB quartet. 

When the proton does not exchange rapidly as on the N-H of the amide group, normal coupling is observed. Since the rates of proton exchange are often critically dependent on the solution conditions, coupling to acidic protons is variable and thus may or may not be observed. 

The above examples illustrate how NMR spectra are routinely used to answer questions about reactions and products. Spectra are usually examined in conjunction with other information that permits a broad-based structure identification to be earned out. Outside of structure questions in texts and on exams, one is almost never handed an NMR spectrum and asked to identify die compound in die absence of odier supporting information. 

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When analyzing any compound obtained as a result of subsequent chemical reactions, chemists always address three major issues identity (Did we synthesize what we intended ), quality (How pure is our compound What side product(s) do we have in our sample ), and quantity (How much did we synthesize What is the yield of the reactions ). In the process of analysis of organic compounds, the focus has always been on obtaining comprehensive information with a variety of analytical methods. The traditional scheme of analysis includes structure identification by NMR ( 11 and 13C) and MS (including high-resolution mass spectrometry), with additional confirmation of structure provided by IR spectroscopy, X-ray, etc. [5]. 

Structure identification by NMR ( H and nC) and MS (including high-reso-lution mass spectrometry), with additional confirmation of structure provided by IR spectroscopy, X-ray, etc. [5]... 

The oxidation mixtures are analyzed by the combination gas chromatography-mass spectrometry. Figure 33 displays a typical analysis. The identification is given in Table 6. Most components have been separated by preparative GC to collect sufficient material for structural identification by NMR. Also, several compounds have been synthesized, allowing GC identification by standard addition. 

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