The differences between carbon and proton NMR

We introduced nuclear magnetic resonance (NMR) in Chapter 3 as part of a three-pronged attack on the problem of determining molecular structure. We showed that mass spectrometry weighs the molecules, Infrared spectroscopy tells us about functional groups, and and Iff NMR tell us about the hydrocarbon skeleton. We concentrated on NMR because it s simpler, and we were forced to admit that we were leaving the details of the most important technique of all—proton ( H) NMR—until a later chapter because it is more complicated than NMR. This is that chapter and we must now tackle those complications. We hope you will see NMR for the beautiful and powerful technique that it surely is. The difficulties are worth mastering for this is the chemist s primary weapon in the battle to solve structures. 

We shall examine each of these points in detail and build up a full understanding of proton NMR spectra. 

Integration tells us the number of hydrogen atoms in each peak 

Simply measuring the height of the steps with a ruler gives you the ratio of the numbers of protons represented by each peak. In many spectra this will be measured for you and reported as a number at the bottom of the spectrum. Knowing the atomic composition (from the mass spectrum) we also know the distribution of protons of various kinds, ffere the heights are 6 mm and 18 mm, a ratio of about 1 3. The compound is C2H4O2 so, since there are four H atoms altogether, the peaks must contain 1 x H and 3 x H, respectively. 

In the spectrum of 1,4-dimethoxybenzene there are just two signals in the ratio of 3 2. This time the compound is CgHio02 so the true ratio must be 6 4. The positions of the two signals are exactly where you would expect them to be from our discussion of the regions of the NMR spectrum in Chapter 3 the 4H aromatic signal is in the left-hand half of the spectrum, between 5 and 10 ppm, where we expect to see protons attached to sp C atoms, while the 6H signal is in the right-hand half of the spectrum, where we expect to see protons attached to sp C atoms. 

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The use of two-dimensional (2D) NMR techniques has become almost routine for detailed analysis of complex organic molecules containing carbon and hydrogen. In contrast, 2D 19F NMR methods are not nearly so commonly used in the analysis of fluorine-containing molecules. The reasons for this are generally a combination of instrumental requirements combined with intrinsic differences between fluorine and proton NMR, in particular the wide range of 19F chemical shifts, which to an extent negates the need for 2D, but also can create problems, for example with respect to uniform excitation of the entire 19F band width. 

Huperzine A 5-1 has a molecular formula of Ci5H18N20 m.p. 230°C [a]D-150.4°C (C, 0.5, MeOH). Its UV spectrum absorptions at A,max (log ) 231 (4.0), 313 (3.89) nm, IR spectrum absorptions at vmax 3180,1650,1550 cm-1 and H-NMR signals (Table 1) show the presence of a-pyridone ring in its molecule. The proton and carbon signals indicate existence of an endocyclic and an exocyclic double bond in the molecule. The final structure 5-1 was further determined by spectral analyses, including decoupling experiments, NOE measurements and comparison with the known structure of selagine 5-2. The difference between 5-1 and 5-2 is the situation of the olefinic proton at the endocyclic double bond in... 

Modern FT-NMR instruments produce the same type of NMR spectrum just described, even though they do it by a different method. See your lecture textbook for a discussion of the differences between classic CW instruments and modern FT-NMR instruments. Fourier transform spectrometers operating at magnetic field strengths of at least 7.1 tesla and at spectrometer frequencies of 300 MHz and above allow chemists to obtain both the proton and carbon NMR spectra on the same sample. 

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