Proton NMR spectroscopy is a powerful method for figuring out molecular structure by studying hydrogen atoms in a magnetic field. It matters because many organic molecules contain many different hydrogen environments, and NMR can distinguish them without destroying the sample. A 1H NMR spectrum gives clues about where hydrogens are located, how many there are, and what atoms are nearby.
Chemists use these clues to identify unknown compounds and confirm reaction products.
In 1H NMR, protons absorb radiofrequency energy at slightly different frequencies depending on their electronic surroundings. These differences appear as chemical shifts on the x axis, usually measured in parts per million. The area under a signal, called integration, shows the relative number of hydrogens producing that signal, while splitting patterns reveal neighboring hydrogens through the n + 1 rule.
By combining chemical shift, integration, and splitting, a chemist can build a consistent picture of the molecule.
Key Facts
- Chemical shift is reported in parts per million: delta = (frequency difference from reference / spectrometer frequency) x 10^6.
- More deshielded protons appear farther downfield, at larger ppm values.
- Integration gives the relative number of equivalent protons responsible for each signal.
- For simple first-order splitting, number of peaks = n + 1, where n is the number of equivalent neighboring protons.
- Common 1H NMR regions include alkyl H at 0.8 to 2 ppm, H next to electronegative atoms at 3 to 4.5 ppm, alkene H at 4.5 to 6.5 ppm, and aromatic H at 6.5 to 8.5 ppm.
- Tetramethylsilane, TMS, is often used as a reference at delta = 0 ppm.
Vocabulary
- Proton NMR
- A spectroscopy technique that detects hydrogen nuclei in a molecule and uses their signals to infer molecular structure.
- Chemical shift
- The position of an NMR signal on the ppm scale, caused by how strongly a proton is shielded by surrounding electrons.
- Integration
- The measured area under an NMR signal that shows the relative number of protons producing that signal.
- Spin-spin splitting
- The division of an NMR signal into multiple peaks caused by magnetic interactions with neighboring nonequivalent protons.
- Equivalent protons
- Hydrogen atoms in the same chemical environment that produce the same NMR signal.
Common Mistakes to Avoid
- Treating peak height as the number of hydrogens, which is wrong because integration area, not height, represents relative proton count.
- Applying the n + 1 rule to all nearby hydrogens without checking equivalence, which is wrong because simple splitting usually counts only equivalent neighboring protons on adjacent atoms.
- Reading the ppm axis backward, which is wrong because larger chemical shift values are downfield and usually appear on the left side of an NMR spectrum.
- Assuming one signal always means one hydrogen, which is wrong because one signal can represent several equivalent hydrogens.
Practice Questions
- 1 A 1H NMR signal integrates to 3 units while another integrates to 2 units. If the molecule has 10 total hydrogens and only these two types of protons, how many hydrogens does each signal represent?
- 2 A proton has 2 equivalent neighboring protons. Using the n + 1 rule, how many peaks should its signal be split into?
- 3 A compound has a 1H NMR signal near 7.2 ppm, another near 3.8 ppm, and an integration ratio of 5:2. Explain what structural features these signals might suggest and why the chemical shifts matter.