NMR Practice: Videos & Practice Problems

Draw the approximate positions that the following compound might show in its 1H NMR absorptions?

In proton nuclear magnetic resonance (NMR) spectroscopy, the typical range for chemical shifts is from 0 to 10 parts per million (ppm), with some compounds, like carboxylic acids, extending up to 13 ppm. To analyze a compound's proton NMR spectrum, one must first identify the number of unique proton signals, determine their multiplicities, and then ascertain their positions on the spectrum.

When examining a compound, it is essential to recognize the different types of hydrogen environments present. For instance, in a symmetrical molecule with multiple CH₂ groups, each unique environment will produce a distinct signal. The multiplicity of each signal can be predicted using the n + 1 rule, where 'n' represents the number of neighboring protons. For example, a proton adjacent to two other protons will appear as a triplet (2 + 1 = 3).

In terms of chemical shifts, the position of each signal is influenced by nearby electronegative atoms. For instance, protons connected to an oxygen atom (as in alcohols) are deshielded and typically resonate at higher ppm values. A CH₂ group adjacent to an -OH group will shift upfield, often around 3.7 ppm, while a CH₂ group that is further away may resonate closer to 1.7 ppm. The most deshielded protons, such as those directly attached to an oxygen, will appear around 5 ppm.

Integration is another critical aspect of interpreting NMR spectra, as it reflects the number of protons contributing to each signal. The height of each peak corresponds to the number of equivalent protons, allowing for a ratio to be established among the signals. For example, if there are two alcohol protons, four equivalent CH₂ protons, and another set of four equivalent CH₂ protons, the integration ratio would be 2:4:4. This ratio can be simplified to 1:2:2.

In summary, constructing a proton NMR spectrum involves identifying the number of unique signals, determining their multiplicities, calculating their chemical shifts based on neighboring groups, and considering integration for peak heights. This comprehensive approach enables a clearer understanding of the molecular structure and the environment of the protons within the compound.

Draw the approximate positions that the following compound might show in its 1H NMR absorptions?

In analyzing a compound's proton NMR (Nuclear Magnetic Resonance) spectrum, the first step is to determine the number of distinct proton signals. Each unique set of protons in a molecule contributes to a different signal. For instance, in a compound with a carbonyl group, the protons directly attached to the carbonyl will produce distinct signals based on their environment. In this case, we identify four signals: one from a methyl group (CH₃) directly connected to the carbonyl, one from a methylene group (CH₂) also connected to the carbonyl, another CH₂ between a CH₂ and a CH₃, and finally, another CH₃ group.

Next, we assess the multiplicity of each signal, which indicates how many neighboring protons influence the signal. The first CH₃ group, with no neighboring protons, appears as a singlet. The second CH₂ group, influenced by two neighboring protons, appears as a triplet. The third signal, a CH₂ group adjacent to a CH₃ and another CH₂, results in a sextet due to the cumulative effect of neighboring protons. Lastly, the final CH₃ group, with two neighboring protons, also appears as a triplet.

After determining the number of signals and their multiplicities, we can approximate the chemical shifts for each signal. Chemical shifts are measured in parts per million (ppm) and are influenced by the electronic environment of the protons. For example, CH₃ groups typically resonate around 0.9 ppm, but when adjacent to a carbonyl, this shifts to approximately 1.9 ppm. Similarly, CH₂ groups resonate around 1.2 ppm, but those that are alpha to a carbonyl shift to about 2.2 ppm. For CH₂ groups that are beta to the carbonyl, the shift is less pronounced, resulting in a value around 1.4 ppm. The final CH₃ group, being gamma to the carbonyl, remains at approximately 0.9 ppm.

When plotting these signals on an NMR spectrum, which typically ranges from 0 to 10 ppm, it is essential to consider both the chemical shifts and the integration of each signal. The integration reflects the number of protons contributing to each signal, influencing the height of the peaks. For instance, the singlet from the CH₃ group should reach a height corresponding to three protons, while the triplet and sextet signals must be adjusted accordingly based on their respective integrations. This careful consideration of signals, multiplicities, chemical shifts, and integrations ensures an accurate representation of the compound's NMR spectrum.