All the H atoms in the last example were attached to sp3 carbons, so you will expect them to fall between 0 and 5 ppm. However, you can clearly see that H atoms that are nearer to oxygen are shifted downfi eld within the 0–5 ppm region, to larger δ values (here as far as 3.3 and 3.9 ppm). We can use this fact to build some more detail into our picture of the regions of the ¹H NMR spectrum.

These regions hold for protons attached to C: protons attached to O or N can come almost anywhere on the spectrum. Even for C–H signals the regions are approximate and overlap quite a lot. You should use the chart as a basic guide, and you should aim to learn these regions. But you will also need to build up a more detailed understanding of the factors affect ing proton chemical shift. To help you achieve this understanding, we now need to examine the classes of proton in more detail and examine the reasons for their particular shifts. It is important that you grasp these reasons. In this chapter you will see a lot of numbers—chemical shifts and differences in chemical shifts. We need these to show that the ideas behind ¹H NMR are securely based in fact. You do not need to learn these numbers. Comprehensive tables can be found at the end of Chapter 18, which we hope you will find useful for reference while you are solving problems.

مواضيع ذات صلة

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

Integration tells us the number of hydrogen atoms in each peak

The differences between carbon and proton NMR

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