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Chapter 9: Problem 53

List the factors that can influence the chemical shift of a hydrogen. Which of these factors do you suppose accounts for the fact that cyclopropane hydrogens appear at unusually high field, near \(\delta 0.2\) ppm?

Short Answer

Expert verified
The ring current effects in cyclopropane cause its hydrogens to appear at an unusually high field, around \(\delta 0.2\) ppm.

Step by step solution

01

- Define Chemical Shift

Explain what chemical shift is in nuclear magnetic resonance (NMR) spectroscopy. It is the resonance frequency of a nucleus relative to a standard in a magnetic field.
02

- List Factors Influencing Chemical Shift

List the factors that influence chemical shift, such as electronegativity of neighboring atoms, hydrogen bonding, hybridization of the carbon atom, and ring current effects.
03

- Explain Ring Current Effects

Discuss how ring current effects, which refer to induced magnetic currents in ring systems, can lead to shifts in NMR signals.
04

- Relate Ring Current Effects to Cyclopropane

Explain that cyclopropane hydrogens are influenced by ring current effects, causing the hydrogens to appear at an unusually high field.
05

- Conclusion

Summarize that the ring current effects in cyclopropane are responsible for the observed chemical shift near \(\delta 0.2\) ppm.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Factors Influencing Chemical Shift
In NMR spectroscopy, the chemical shift of a hydrogen nucleus can be influenced by several factors. Understanding these factors can help interpret NMR spectra more accurately.
  • **Electronegativity:** Atoms with high electronegativity, such as oxygen and nitrogen, deshield nearby hydrogen atoms, causing a downfield shift in their chemical shift.
  • **Hydrogen Bonding:** Hydrogen bonding can also impact the chemical shift by altering the electron density around the hydrogen nucleus.
  • **Hybridization:** The hybridization state of the carbon atom attached to the hydrogen can affect the electron density and, thus, the chemical shift. For instance, sp2 hybridized carbons typically cause hydrogens to appear downfield in comparison to sp3 hybridized carbons.
  • **Magnetic Anisotropy:** This factor includes contributions from ring currents in aromatic compounds which create local magnetic fields affecting the chemical shift.
Understanding these factors gives insight into why certain hydrogens display unique chemical shifts in NMR spectra.
Ring Current Effects
Ring current effects occur when circulating π-electrons in conjugated ring systems, like benzene, create a local magnetic field. This induced magnetic field can either shield or deshield hydrogen nuclei depending on their position relative to the ring plane.
If a hydrogen atom lies above or below the aromatic ring, it experiences shielding and appears upfield (lower δ value). Conversely, if it lies within the ring plane, it experiences deshielding and appears downfield (higher δ value).
Ring current effects are significant in explaining anomalies in chemical shifts of hydrogens in aromatic and non-aromatic ring systems.
Cyclopropane in NMR
Cyclopropane provides an interesting case in NMR spectroscopy. Typically, cyclopropane hydrogens have an unusually high field shift, around \(\delta 0.2\) ppm.
This can mainly be attributed to the unique ring strain and geometric configuration of cyclopropane. The significant angular strain in cyclopropane causes unique electron distribution, affecting the local magnetic environment of the hydrogen atoms. Consequently, the induced magnetic fields shift the hydrogen resonance to higher fields than expected for typical alkyl hydrogens.
Electron Shielding in NMR
Electron shielding in NMR spectroscopy refers to the phenomenon where electron clouds around a nucleus protect it from the external magnetic field. This shielding effect results from circulating electrons generating a local opposing magnetic field.
When electron shielding is significant, the proton experiences less net magnetic field and resonates at a higher field (lower δ value). Various factors, such as the electronegativity of neighboring atoms and molecular geometry, influence electron shielding. High electronegativity atoms like fluorine or oxygen reduce shielding, causing the hydrogen nucleus to resonate at a lower field (higher δ value).
Hybridization in NMR
Hybridization impacts the electron density around a hydrogen nucleus, influencing its chemical shift in NMR spectroscopy. The hybridization state of the carbon atom attached to the hydrogen plays a key role.
  • **sp3 Hybridization:** Hydrogens attached to sp3 hybridized carbons (like in alkanes) typically resonate upfield, often between \(\delta 0.5-2\) ppm due to higher electron density.
  • **sp2 Hybridization:** Hydrogens attached to sp2 hybridized carbons (like in alkenes and aromatic rings) resonate further downfield, usually between \(\delta 5-10\) ppm.
  • **sp Hybridization:** Hydrogens attached to sp hybridized carbons (like in alkynes) appear near \(\delta 2-3\) ppm, owing to distinct electron distributions.
Understanding these hybridization-induced shifts aids in the correct interpretation of NMR spectra.

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Most popular questions from this chapter

The room-temperature \({ }^{1} \mathrm{H}\) NMR spectrum of cyclooctane is a single peak at \(\delta=1.53\). What does this observation tell you about the structure of cyclooctane?

Describe the signals for the methylene hydrogens (underlined) of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHPh}(\mathbf{1})\) in its \({ }^{1} \mathrm{H} \mathrm{NMR}\) spectrum taken at low temperature.

The wavelength of light used in UV/vis spectroscopy ranges from 200 to \(800 \mathrm{~nm}\). Calculate the energy (in \(\mathrm{kcal} / \mathrm{mol}\) ) corresponding to these two wavelengths.

The carbon-bound hydrogens of pyrrole (p. 236) appear at \(\delta 6.05\) and \(6.62 \mathrm{ppm}\) in the \({ }^{1} \mathrm{H} \mathrm{NMR}\) spectrum, upfield of the typical aromatic signal region. Does this observation mean that pyrrole is not aromatic?

(a) A hydrogen attached directly to the carbon of a carbonyl group (aldehyde hydrogen) appears at \(\delta\) 9-10 ppm and must be strongly deshielded. Explain why. (b) Vinylic hydrogens that are on the \(\beta\) position of a conjugated carbonyl compound are also substantially downfield. Use resonance structures to explain why.
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