Chemical Shift In NMR Spectroscopy | Summary and Q&A

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December 11, 2018
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Chemical Shift In NMR Spectroscopy

TL;DR

Chemical shift in NMR spectroscopy measures the displacement of signals on the spectrum due to electron withdrawal, electronegativity of atoms, and proximity to functional groups.

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Questions & Answers

Q: How does the presence of a halogen affect the chemical shift in NMR spectroscopy?

The presence of a halogen increases the chemical shift, with methylfluoride having the highest shift followed by methyl chloride, methyl bromide, and methyl iodide. This is due to the inductive effect, where more electronegative atoms result in higher chemical shifts.

Q: What factors influence the chemical shift in NMR spectroscopy?

The chemical shift is affected by the presence of electronegative atoms or electron-withdrawing groups, as well as the proximity of the protons to functional groups. Carbon atoms with more electron-withdrawing groups result in higher chemical shifts.

Q: How does the relative position of protons to an electron-withdrawing group impact the chemical shift?

Protons located closer to electron-withdrawing groups have higher chemical shifts, appearing more downfield on the NMR spectrum. Protons further away from the electron-withdrawing group have lower chemical shifts and appear more upfield.

Q: What are the chemical shifts of common functional groups in NMR spectroscopy?

Some common chemical shifts include 10-12 ppm for carboxylic acids, 9-10 ppm for aldehydes, 6.5-8.5 ppm for benzene protons, and 4.5-6.5 ppm for protons attached to alkenes. Methyl groups attached to benzene rings have chemical shifts around 2.1-2.3 ppm.

Summary & Key Takeaways

  • Chemical shift in NMR spectroscopy is represented by the symbol delta, which is the ratio of observed chemical shift to the spectrophotometer's operating frequency.

  • The reference signal, Tetramethylsilane (TMS), has the lowest chemical shift value, while other compounds with electronegative atoms or electron-withdrawing groups have higher chemical shift values.

  • Presence of halogens increases the chemical shift, with methylfluoride having the highest shift and methyl iodide having the lowest shift.

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