How to Interpret Proton NMR Spectra

Now that we have had an introduction to key aspects of NMR spectra (chanical shift, peak area, and signal splitting), we can start to apply NMR spectroscopy to elucidating the structure of unknown compounds. The following steps sununarize the process  

Count the number of signals to determine how many distinct proton environments are in the molecule (neglecting, for the time being, the possibility of overlapping signals). 

Use chemical shift tables or charts, such as Table 9.1 or Fig. 9.2 (or your own experience over time), to correlate chemical shifts with possible structural enviromnents. 

Determine the relative area of each signal, as compared with the area of other signals, as an indication of the relative number of protons producing the signal. 

Interpret the splitting pattern for each signal to determine how many hydrogen atoms are present on carbon atoms adjacent to those producing the signal and sketch possible molecular fragments. 

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At first glance splitting may seem to complicate the interpretation of NMR spectra In fact It makes structure determination easier because it provides additional information It tells us how many protons are vicinal to a proton responsible for a particular signal With practice we learn to pick out characteristic patterns of peaks associating them with particular structural types One of the most common of these patterns is that of the ethyl group represented m the NMR spectrum of ethyl bromide m Figure 13 15... 

Figure 15.1. (A) COSY, (B) TOCSY, (C) 1H-1T HSQC or HMQC, (D) dl- Y HMBC, for 4-oxopentanal. For clarity, only key assignments have been given as an example. Note that the double-ended arrows indicate how to interpret the spectra. In the case of COSY and TOCSY the information is represented as cross-peaks that are symmetrically oriented with respect to the central diagonal. In the single-bond correlation (HSQC/HMQC) a cross-peak represents in one dimension the carbon chemical shift and in the other dimension the proton chemical shift. Note there is no diagonal in heteronuclear NMR experiments. In the HMBC, lines are drawn vertically to connect the cross-peaks. In HMBC 2-4 bonds, H-13C correlations are often observed. Note that the 4-bond correlation is less common in NMR but has been included here as an example, and 1-bond correlation is commonly filtered from the HMBC experiment to improve detection limits for the weaker 2-4 bond correlations.

This course is aimed at those who are already familiar with using NMR on a day-to-day basis, but who wish to deepen their understanding of how NMR experiments work and the theory behind them. It will be assumed that you are familiar with the concepts of chemical shifts and couplings, and are used to interpreting proton and 13C spectra. It will also be assumed that you have at least come across simple two-dimensional spectra such as COSY and HMQC and perhaps may have used such spectra in the course of your work. Similarly, some familiarity with the nuclear Overhauser effect (NOE) will be assumed. That NMR is a useful for chemists will be taken as self evident. 

The hrst step in learning to interpret NMR spectra is to learn how to predict them from a known chemical structure. An example of a 1H (proton) NMR spectrum is shown for... 

Prepare a sample of your final product for analysis by gas chromatography. Determine the infrared spectrum and both proton and NMR spectrum of your product. Use these spectra to determine the structure of your product. In your laboratory report, include an interpretation of each spectrum, identifying the principal absorption bands and demonstrating how the spectrum corresponds to the structure of your compound. Submit your sample in a labeled vial with your laboratory report. 

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