Proton NMR

Proton NMR, generally referred to as ¹H NMR, uses hydrogen nuclei to generate signals between 0 and 12ppm.

⊕An ¹H spectra of propanoyl chloride

Similar to how carbons in ¹³C NMR have different environments, the same hold true in ¹H NMR. Different hydrogens can exist within the same environment. The peak area (listed in the diagram under each corresponding peak) is proportional to how many hydrogens exist within that environment. Within actual NMR spectra, the peak area is found by taking the integral l of the range the peak exists in.

The area under the peak, not the height of the peak, shows the density of the environment.

Hydrogen Equivalence

⊕Despite both the third and fourth carbon having two hydrogen bonds, their hydrogens are not chemically equivilent.

Hydrogen are almost always equivalent to all other hydrogen attached to an atom (ie all four hydrogen in a methane molecule are chemically identical). However, not all similar chemical groups are equivalent

Not all hydrogens with the same chemical structure are chemically equivalent. On the above structure, the far left and far right CH₃ groups are not chemically equivalent and therefor have separate peaks on a ¹H NMR spectrum.

If H_a and H_a^` are chemically equivalent, they have to be bonded to chemically equivalent atoms. The hydrogens on the opposing CH₃ are chemically equivalent however the hydrogens on the CH₂ are not.

Exercise

For each of these structures, identify chemically equivalent hydrogens

¹H Splitting

Splitting in ¹H NMR is determined differently than in ¹³C NMR. The amount of splits is determined by the by the amount of non-equivalent hydrogens within three bonds from the Hydrogen.

Fill in the table describing the relationship between peak multiplicity and the number of neighboring hydrogens within three bonds

⊕chemical structure and ¹H NMR spectrum of propane

The peak cluster at 0.9ppm for propane corresponds to H_a hydrogen atoms. The peak is split twice because within the 3 bond distance, are both of the H_b hydrogen atoms.

The peak cluster at 1.3ppm corresponds to the H_b hydrogen atoms. The peak is split three times despite having 6 hydrogens within the 3 bond distance. this is because both CH₃ groups within its range are chemically equivalent and therefor, only one is counted for splitting the peaks.

Similarly to how in ¹³C NMR, the number of peaks is equal to N+1 where N is the number of bonded hydrogens, in ¹H NMR, the number of peaks is equal to N+1 where N is the number of hydrogens within the three bond radius

PPM Ranges for ¹H NMR

below are the most common molecular structures and their corresponding chemical shift (ppm range)

Alcohols do not always follow the rules for predicting multiplicities.

Based on integration and chemical shift alone, assign each of the three peaks in the proton NMR spectrum of ethanol? Which of the peaks above does not follow the peak multiplicity rules we have learned so far?

Special Considerations

• Under normal conditions, the H of an alcohol or amine does not participate in splitting
• When D₂O (Deuterium oxide, a common solvent in NMR solutions) is added to an NMR sample, a peak due to a hydrogen attached to oxygen or nitrogen will disappear
• In typical proton NMR samples, the solvent is present in much higher concentrations than the molecule being analyzed
• Why are solvents with hydrogen (Such as CHCl₃, C₆H₆, CH₂O, etc) not useful as proton NMR solvents?
• CDCl₃ (deuterated chloroform, or chloroform-d) is the most common solvent used as its effective at dissolving many organic molecules and has no hydrogen atoms.
• most CDCl₃ is contaminated with a small amount of CHCl₃, the peak of which appears as a singlet at 7.24 ppm

Cis and Trans Splitting

Splitting between hydrogens of trans of one another is larger than splitting between hydrogens that are cis of each other.

This can be used to determine whether the molecule is the trans (Z) or cis (E) version of its self

Conclusion

• Proton NMR works in much the same way as carbon NMR, but again interpretation of the spectra is a much more important skill for a chemist than understanding the complex physics behind the instrument.

• Equivalent H’s do NOT split each other - The rule itself is not hard to follow, but people sometimes forget to check for symmetry and so do not realize that two neighboring H’s are equivalent

• Peak area, not peak height, tells you the relative number of H’s associated with a peak cluster - This leaves open the possibility that a tall skinny peak could be smaller (in terms of area) than a short fat peak

• Splitting in carbon NMR tells you the number of H’s attached to a given carbon. For this reason, students incorrectly assume that splitting in proton NMR tells you the number of H’s associated with a peak cluster - In fact, it is a bit more complicated

• ppm or chemical shift (given by location along the x axis) is a function of the amount of electron density around an H

• The closer the H is to an electronegative element, the more “deshielded” it is and therefore the higher the ppm number of its peak cluster (farther left on the spectrum)

• Multiple bonds also cause the signal of nearby H’s to be shifted to the left

• Each chemically distinct H is expected to have a unique chemical shift, though in practice different peak clusters sometimes overlap just by coincidence - This is a bigger problem in proton NMR than in carbon NMR (especially in the so-called “aromatic region” from 7-9 ppm), since most proton NMR peaks are squeezed into just an 8 ppm range (1-9 ppm)

• Integration or peak cluster area (given by a number above or below a peak cluster, or by a line stepping up from left to right called the integration line) tells you the relative area of each peak and therefore the relative number of equivalent H’s represented by each peak

• Note that integration (especially the integration line) gives you only a ratio of peak areas - This means the same integration may be reported, for example, on the spectrum of a molecule with 1H to 3H ratio and a 2H to 6H ratio

• Multiplicity is the number of peaks in a peak cluster (also called splitting or proton-proton coupling)

• It tells you the number of nonequivalent neighbor H’s within three bonds

• For example, a doublet (two peaks) tells you there is exactly one non- equivalent H within three bonds of the H responsible for this signal