Chromatography and Spectroscopy (OCR A-Level Chemistry A): Revision Notes

Proton NMR Spectroscopy

Introduction to proton NMR spectroscopy

Proton nuclear magnetic resonance (¹H NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic molecules. Similar to carbon-13 NMR, proton NMR examines how hydrogen nuclei in molecules absorb radiofrequency radiation in a strong magnetic field. The resulting spectrum provides detailed structural information that helps chemists identify unknown compounds and confirm molecular structures.

Chemical shifts and the TMS reference

In proton NMR, the chemical shift (symbol: $\delta$) measures how far a proton's absorption is from a reference compound. The reference standard used is tetramethylsilane (TMS), which is defined as having a chemical shift of $\delta = 0$ ppm. All other chemical shifts are measured relative to this reference point.

The chemical shift range in proton NMR extends from approximately 0 to 12 ppm, which is much narrower than the carbon-13 range (which spans about 220 ppm). This narrower range means that proton NMR peaks are more closely spaced, but the technique provides additional information through splitting patterns that compensate for this limitation.

Different proton environments absorb at characteristic chemical shift values. The main categories of proton environments, in order of increasing chemical shift, are:

• $\delta$ = 0-1 ppm: Protons bonded to alkyl chains (HC-R groups)
• $\delta$ = 2-4 ppm: Protons bonded to electronegative atoms such as oxygen, nitrogen, chlorine, or bromine (e.g., HC-O, HC-N, HC-Cl), and protons in benzene rings or adjacent to C=O groups
• $\delta$ = 5-6 ppm: Protons bonded to carbon-carbon double bonds (HC=C) or aromatic rings
• $\delta$ = 7-8 ppm: Aromatic protons (protons attached directly to benzene rings)
• $\delta$ = 10-12 ppm: Carboxylic acid protons (COOH groups)

Additional proton environments include O-H protons (alcohols, phenols, carboxylic acids) and N-H protons (amines, amides, amino acids), which can appear at variable chemical shifts across a wide range.

Equivalent and non-equivalent protons

Understanding which protons in a molecule are equivalent is crucial for predicting and interpreting proton NMR spectra. This concept directly affects the number of peaks you will observe in a spectrum.

Defining equivalent protons

Protons are considered equivalent when they are in the same chemical environment. Equivalent protons absorb radiofrequency radiation at the same chemical shift value, which means they produce a single peak in the NMR spectrum. The size of this peak increases with the number of equivalent protons contributing to it.

Conversely, protons of different types exist in different chemical environments and are non-equivalent. These protons absorb at different chemical shift values and therefore produce separate peaks in the spectrum.

Using symmetry to identify equivalent protons

Let's examine two examples that illustrate this principle clearly.

Integration and relative numbers of protons

While carbon-13 NMR peak areas are not directly proportional to the number of carbon atoms, proton NMR is different. In proton NMR, the area under each peak directly relates to the number of protons responsible for that peak.

Integration traces

The NMR spectrometer measures the area under each peak using a mathematical process called integration. The integration trace is displayed either as an additional line on the spectrum or as printed numbers showing the relative peak areas. This integration provides invaluable information for identifying unknown compounds because it reveals the ratio of different types of protons in the molecule.

The integration trace appears as a stepped line that rises at each peak. The height of each step is proportional to the area under that peak, which in turn is proportional to the number of protons causing the peak.

Spin-spin coupling

So far, we have seen that proton NMR spectra provide information about the structure of molecules through peak positions and integration values. However, proton NMR offers an additional level of structural detail through a phenomenon called spin-spin coupling.

What is spin-spin coupling?

Spin-spin coupling occurs when a proton's magnetic field interacts with the magnetic fields of nearby protons in different chemical environments. This interaction causes the main NMR peak to split into multiple sub-peaks, creating a characteristic splitting pattern. These patterns provide information about the number of protons bonded to adjacent carbon atoms, revealing how different parts of the molecule are connected.

The n+1 rule

The number of sub-peaks in a splitting pattern follows a simple rule called the n+1 rule:

Common splitting patterns

The table below shows the splitting patterns you will encounter most frequently in proton NMR spectra:

n	n+1	Splitting pattern	Relative peak areas	Structural feature
0	1	singlet	1	no H on adjacent atoms
1	2	doublet	1:1	adjacent CH
2	3	triplet	1:2:1	adjacent CH₂
3	4	quartet	1:3:3:1	adjacent CH₃

These patterns are very common in NMR spectra. The relative peak areas within each splitting pattern follow a mathematical sequence called Pascal's triangle. As the number of adjacent protons increases, the degree of splitting continues beyond what is shown here. For example, a CH proton adjacent to six protons (such as in CH(CH₃)₂) produces a heptet (seven-peak pattern) with relative areas of 1:6:15:20:15:6:1.

Splitting patterns occur in pairs

An important principle to understand is that splitting patterns always occur in pairs in an NMR spectrum. If you observe one splitting pattern, there must be another pattern elsewhere in the spectrum. This occurs because each proton splits the signal of neighbouring protons, and those neighbouring protons split the original proton's signal in return.

This pairing makes structural analysis easier. Several very common splitting combinations appear repeatedly in spectra, and learning to recognise these will speed up your analysis considerably. While you can work out any splitting pattern using the n+1 rule, you will quickly learn to recognise the triplet/quartet combination that indicates a CH₃CH₂ unit because it appears so frequently in organic molecules.

Hydroxyl and amino protons

Organic compounds frequently contain protons that are not bonded to carbon atoms. The most common examples are hydroxyl (OH) protons and amino (NH) protons.

Functional groups containing OH and NH protons

These protons appear in several important functional groups:

• Alcohols (ROH)
• Phenols (ArOH)
• Carboxylic acids (RCOOH)
• Amines (RNH₂)
• Amides (RCONH₂)
• Amino acids (RCH(NH₂)COOH)

Characteristics of OH and NH peaks

In solution, OH and NH protons frequently participate in hydrogen bonding with other molecules. This hydrogen bonding causes the NMR peaks for these protons to be broad rather than sharp, and their chemical shifts can vary considerably depending on concentration, solvent, and temperature.

The chemical shift ranges shown in data sheets indicate that OH and NH peaks can occur at almost any chemical shift value, though carboxylic acid COOH protons are more predictable and typically absorb at $\delta = 10-12$ ppm.

Proton exchange with deuterium oxide

Chemists have developed a clever technique to identify OH and NH protons unambiguously. This method, called proton exchange, involves the following steps:

1. A proton NMR spectrum is recorded as normal
2. A small volume of deuterium oxide (D₂O, also called heavy water) is added to the sample, the mixture is shaken, and a second spectrum is recorded

Deuterium is an isotope of hydrogen with one proton and one neutron in its nucleus. When D₂O is added to the sample, the OH and NH protons exchange with deuterium atoms according to equilibria such as:

$\text{CH}_3\text{OH} + \text{D}_2\text{O} \rightleftharpoons \text{CH}_3\text{OD} + \text{HOD}$

Since deuterium does not absorb in the chemical shift range used for proton NMR, the peaks from OH and NH protons disappear in the second spectrum. By comparing the two spectra, you can definitively identify which peaks correspond to exchangeable OH and NH protons.