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Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 15 - Structural Identification II: Nuclear Magnetic Resonance SpectroscopyProblem 21
Chapter 14, Problem 21

(a) Calculate the resonance frequency of an aldehydic proton ( δ 9.3 ppm) if it is detected on a 60-MHz NMR spectrometer.
(b) What if it were detected on a 300-MHz instrument?

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1
Understand that the resonance frequency of a proton in NMR is determined by the chemical shift (δ) and the operating frequency of the NMR spectrometer.
The chemical shift (δ) is given in parts per million (ppm) and represents the difference in resonance frequency of the proton relative to a reference compound, usually TMS (tetramethylsilane).
To calculate the resonance frequency, use the formula: \( \text{Resonance Frequency} = \delta \times \text{Operating Frequency} \).
For part (a), substitute the given values into the formula: \( 9.3 \text{ ppm} \times 60 \text{ MHz} \).
For part (b), substitute the given values into the formula: \( 9.3 \text{ ppm} \times 300 \text{ MHz} \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) Spectroscopy is a technique used to observe the local magnetic fields around atomic nuclei. It provides detailed information about the structure, dynamics, and environment of molecules. The resonance frequency of a nucleus in an NMR spectrometer depends on the strength of the external magnetic field and the chemical environment of the nucleus.
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Chemical Shift

Chemical shift is a key concept in NMR spectroscopy, representing the resonance frequency of a nucleus relative to a standard reference. It is measured in parts per million (ppm) and provides insight into the electronic environment surrounding the nucleus. The chemical shift is influenced by factors such as electronegativity and hybridization of nearby atoms.
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Resonance Frequency Calculation

Resonance frequency in NMR is calculated using the formula: frequency = chemical shift × spectrometer frequency. For an aldehydic proton with a chemical shift of δ 9.3 ppm, the resonance frequency can be determined by multiplying the chemical shift by the spectrometer's operating frequency, such as 60 MHz or 300 MHz, to find the specific frequency at which the proton resonates.
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Related Practice
Textbook Question

The methyl hydrogens in propane appear at a chemical shift of 0.9 ppm, whereas the methyl hydrogens of propene appear around 2.5 ppm. Explain.

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Textbook Question

For the molecules shown, give the number of signals expected and the relative ratio of the signal integrations.

(b)

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Textbook Question

An unknown compound (C₇H₆O) gives the IR spectrum shown here. At what chemical shifts would you expect to see signals in the ¹H NMR spectrum?

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Textbook Question

In Figure 15.34, Ha was assumed to be 'up.' How does the analysis change if we assume instead that Ha is down?

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Textbook Question

Without worrying about the relative location of the signals (i.e., the chemical shift) or the splitting patterns, draw a spectrum of the following molecule, being sure to indicate the integration of each peak. Label each signal based on the set of equivalent hydrogens to which it corresponds. [We expand on this question in future assessments.]

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Textbook Question

Without worrying about the relative location of the signals (i.e., the chemical shift) or the splitting patterns, draw a spectrum of the following molecule. Be sure to label each signal based on the set of equivalent hydrogens to which it corresponds.

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