7.S: Summary

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Feb 11, 2023
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- 7.7: 2-D NMR Problems
- 8: Structure Elucidation Problems

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7.1: Chapter Objectives and Preview of Correlation NMR Spectroscopy

Complex molecules can be hard to elucidate solely with 1-D NMR because it doesn't quite solve the entire picture. This is where correlation NMR spectroscopy can be used.

7.2 Theory

The splitting of resonances indicates that groups were "correlated" to each other due to the spins within each group.
A simple 2-D experiment pulse sequence consists of a relaxation delay, a pulse, a variable time interval (t₁), a second pulse, and acquisition (t₂).
In 2-D experiments, the signal detected during acquisition is a function of acquisition time (t₂), which has been modulated as a function of the time interval (t₁). This means that magnetization evolves around one frequency during t₁ and a different frequency during t₂.
The output once Fourier transformed is a 2-D spectrum with two axes.
One axis (v₂) represents the nucleus detected during acquisition (t₂), while the other axis (v1) can represent the same nucleus or a different nucleus.
With two axes, it leads to cross peaks along a diagonal connecting coupled nuclei.

7.3 Two Dimensional Homonuclear NMR Spectroscopy

Homonuclear 2-D NMR spectroscopy is looking at the correlation of the same nuclei in a molecule.
COSY looks at ¹H coupling to ¹H through bonds typically 3 bonds away and relies on the J-coupling to provide spin-spin correlation to indicate which protons are close to each other.
TOCSY obtains correlations between all protons within a given spin system and begin to chain together fragments of a molecule.
A molecule can have just one spin system or hundreds in more complex systems.
The goal in TOCSY is to transfer the magnetization beyond directly coupled spins.
NOESY determines which signals arise from protons athar are close to each other in space, even if they are not bonded.

7.4 Two Dimensional Heteronuclear NMR Spectroscopy

Heteronculear 2-D NMR is the correlation between different nuclei, such as a ¹H to ¹³C.
HSQC is used to determine the proton to carbon or heteroatom (often nitrogen) single bond correlations.
The purpose of a HSQC is to determine which protons are coupled to what other specific carbon or heteroatom in the molecule through bonds.
HMBC is used to determine long range ¹H to ¹³C connectivity.
HMBC gives the correlation between ¹H and ¹³C when separated by two, three, and even four (if through a conjugated system) bonds away.

7.5 Uses of 2-DNMR Spectroscopy

Complex structure elucidation often requires 2-D NMR spectroscopy.
HSQC can be used to determine the profile of metabolites in low concentrations (microMolar) accurately.
TOCSY has been utilized to show changes in tumor cells and identify biomarkers associated with these cells.
Molecular dynamics can be studied using 2-D NMR spectroscopy to map the molecule's internal mobility patterns.

Skills to Master

Skill 7.1 Distinguish between 1-D and 2-D techniques.
Skill 7.2 Learn to read COSY, TOCSY, NOESY, HSQC, HMBC spectra.
Skill 7.3 Know what type of experiment to use to gain the information needed.
Skill 7.4 Understand applications of 2-D NMR spectroscopy.
Skill 7.5 Solve unknown structure determination problems with 2-D spectroscopy.

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