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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 67b
Chapter 14, Problem 67b

Draw the 13C NMR spectrum you would expect to see for each of the molecules shown.
(b) Chemical structure of a bicyclo[3.1.0]hexane molecule, featuring a six-membered ring with a three-carbon bridge.

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1
Identify the number of unique carbon environments in the molecule. Each unique environment corresponds to a distinct signal in the ¹³C NMR spectrum.
Consider the symmetry of the molecule. Symmetrical molecules may have fewer unique carbon environments due to equivalent positions.
Analyze the chemical shifts expected for each carbon environment. Factors such as electronegativity of nearby atoms, hybridization, and resonance effects influence the chemical shift.
Determine the splitting pattern for each signal. In ¹³C NMR, splitting is less common, but coupling with nearby hydrogen atoms can occur, leading to multiplets.
Sketch the spectrum, placing each signal at the appropriate chemical shift and indicating the relative intensity based on the number of equivalent carbons contributing to each signal.

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Key Concepts

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

¹³C NMR Spectroscopy

¹³C NMR spectroscopy is a technique used to study the carbon atoms in organic molecules. It provides information about the number of unique carbon environments in a molecule by measuring the resonance frequencies of carbon-13 nuclei in a magnetic field. Each distinct carbon environment typically appears as a separate peak in the spectrum, allowing chemists to deduce structural information about the molecule.
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Chemical Shift

Chemical shift in NMR spectroscopy refers to the position of an NMR signal relative to a standard reference point, usually tetramethylsilane (TMS). It is measured in parts per million (ppm) and provides insight into the electronic environment surrounding a nucleus. In ¹³C NMR, chemical shifts can indicate the presence of functional groups, hybridization states, and electronegative atoms near the carbon atom.
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Symmetry and Equivalent Carbons

Symmetry in a molecule can lead to equivalent carbon atoms, which appear as a single peak in a ¹³C NMR spectrum. Equivalent carbons are those that experience the same electronic environment due to the molecule's symmetry. Identifying symmetry elements, such as planes or axes, helps predict the number of unique carbon signals, simplifying the interpretation of the NMR spectrum.
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Related Practice
Textbook Question

Draw the ¹³C NMR spectrum you would expect to see for each of the molecules shown.

(a)

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Textbook Question

A graduate student ran a reaction that produced a mixture of the following two compounds. After painstaking purification, she is able to separate the two compounds. Using ¹H NMR, how can she determine which diastereomer is in which separated sample?

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Textbook Question

For the molecules in Assessment 15.58, give an approximate chemical shift for each indicated carbon. [The range of correct answers is large here.].

(a)

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Textbook Question

Assign the peaks in the ¹H NMR spectrum for the molecule shown.

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Textbook Question

For the molecules in Assessment 15.58, give an approximate chemical shift for each indicated carbon. [The range of correct answers is large here.].

(c)

Textbook Question

Draw the 13C NMR spectrum you would expect to see for each of the molecules shown.

(c)

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