How can I draw NMR splitting diagrams?

Question

Ryan Andrews · Accepted Answer

The general rule for #""^1"H"# NMR spectra is: n neighbouring protons with the same coupling constant J will split a signal into n+1 lines.

We draw a "splitting tree" for each proton or group of protons.

No Adjacent Protons

No splitting. Draw a single line.

1 Adjacent Proton

Draw 2 lines (a doublet) separated by #J_(ax)#.

2 Adjacent Protons

From each line of the doublet, draw two more lines separated by #J_(ax)#. Stack the overlapping lines. This gives a triplet with area ratios 1:2:1.

3 Adjacent Protons

From each line of the triplet, draw two more lines separated by #J_(ax)#. Stack the overlapping lines. This gives a quartet with area ratios 1:3:3:1.

Repeat the process as many times as needed.

The relative intensity ratios are given by Pascal's triangle.

Here are some splitting patterns that you should be able to draw quickly.

AX doublet-doublet: -CH-CH-

AX₂ doublet-triplet: -CH-CH₂-

AX₃ doublet-quartet: CH₃CH-

AX₆ doublet-septet: (CH₃)₂CH- (an isopropyl group!)

Note: A septet often looks like a quintet with the wrong intensities, because the two outermost lines are so weak.

A₂X₃ triplet quartet: CH₃CH₂- (an ethyl group!)

Here's a video that gives an introduction to NMR splitting patterns.

Addison Armstrong · Answer

undefined To draw NMR splitting diagrams, follow these steps:

1. Identify the hydrogen atoms in the molecule that you want to analyze.
2. Determine the number of neighboring hydrogen atoms (N+1 rule) for each hydrogen atom. This number represents the number of peaks that will appear in the splitting pattern.
3. Apply the N+1 rule to determine the multiplicity of each peak: singlet (1 peak), doublet (2 peaks), triplet (3 peaks), quartet (4 peaks), etc.
4. Assign the relative intensities of the peaks according to Pascal's triangle: 1, 1, 1, 1, 2, 1, 1, 1, 3, 3, 1, ...
5. Draw the splitting pattern by placing the appropriate number of peaks at the chemical shift corresponding to each hydrogen atom.
6. Label the peaks with the appropriate multiplicity (e.g., s, d, t, q) and integrate the areas under the peaks to determine the relative abundance of each type of hydrogen atom.