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 ¹³C and ¹H NMR tell us about the hydrocarbon skeleton. We concentrated on ¹³C 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 (¹H) NMR—until a later chapter because it is more complicated than ¹³C NMR. This is that chapter and we must now tackle those complications. We hope you will see 1H 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 will make use of ¹H and ¹³C NMR evidence for structure throughout this book, and it is essential that you are familiar with the explanations in this chapter before you read further. Proton NMR differs from 13C NMR in a number of ways.

• ¹H is the major isotope of hydrogen (99.985% natural abundance), while ¹³C is only a minor isotope (1.1%).

• ¹H NMR is quantitative: the area under the peak tells us the number of hydrogen nuclei, while ¹³C NMR may give strong or weak peaks from the same number of ¹³C nuclei.

• Protons interact magnetically (‘couple’) to reveal the connectivity of the structure, while ¹³C is too rare for coupling between ¹³C nuclei to be seen.

• 1H NMR shifts give a more reliable indication of the local chemistry than that given by ¹³C spectra.

We shall examine each of these points in detail and build up a full understanding of proton NMR spectra. Proton NMR spectra are recorded in the same way as ¹³C NMR spectra: radio waves are used to study the energy level differences of nuclei in a magnetic field, but this time they are ¹H and not ¹³C nuclei. Hydrogen nuclei in a magnetic fi eld have two energy levels: they can be aligned either with or against the applied magnetic field.

¹H and ¹³C spectra have many similarities: the scale runs from right to left and the zero point is given by the same reference compound, although it is the proton resonance of Me4Si rather than the carbon resonance that defines the zero point. You will notice at once that the scale is much smaller, ranging over only about 10 ppm instead of the 200-ppm needed for carbon. This is because the variation in the chemical shift is a measure of the shielding of the nucleus by the electrons around it. There is inevitably less change possible in the distribution of two electrons around a hydrogen nucleus than in that of the eight valence electrons around a carbon nucleus. Here is the ¹H NMR spectrum of acetic acid, which you fi rst saw in Chapter 3.

مواضيع ذات صلة

Protons on saturated carbon atoms Chemical shifts are related to the electronegativity of substituents

Regions of the proton NMR spectrum

Integration tells us the number of hydrogen atoms in each peak

Kinetic versus thermodynamic products

Catalysis in carbonyl substitution reactions

What does third-order kinetics mean

Rate equations

Catalysis

Reaction intermediates

Why some reactions are done at low temperature

The route from reactants to products: the transition state

How fast do reactions go? Activation energies