Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy

Problem 30

Chapter 14, Problem 30

Draw all of the possible spin states to explain why a hydrogen with four neighbors appears as a quintet (quint, five peaks).

Verified step by step guidance

Identify the hydrogen atom in question and its neighboring hydrogen atoms. In this case, the hydrogen has four neighboring hydrogens.

Understand the concept of spin-spin coupling in NMR spectroscopy. Each neighboring hydrogen can either align with or against the external magnetic field, creating different spin states.

Calculate the number of possible spin states using the formula 2nI + 1, where n is the number of neighboring hydrogens and I is the spin quantum number of hydrogen (I = 1/2). For four neighbors, this results in 2(4)(1/2) + 1 = 5 spin states.

Draw the possible spin states: Each neighboring hydrogen can be in either the +1/2 or -1/2 spin state. The combinations of these states result in the quintet pattern.

Explain how these spin states lead to the quintet: The different combinations of neighboring hydrogen spins result in five distinct energy levels, which correspond to the five peaks observed in the NMR spectrum.

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Key Concepts

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

Spin States and NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a technique used to determine the structure of organic compounds by observing the magnetic properties of atomic nuclei. Spin states refer to the different orientations of nuclear spins in a magnetic field, which result in distinct energy levels. In NMR, these spin states cause splitting patterns in the spectrum, providing insights into the molecular environment of the nuclei.

Coupling and Multiplet Formation

Coupling in NMR occurs when the magnetic field of one nucleus affects the magnetic field of neighboring nuclei, leading to splitting of NMR signals into multiple peaks, known as multiplets. The number of peaks in a multiplet is determined by the number of neighboring hydrogen atoms plus one (n+1 rule). A quintet, therefore, indicates the presence of four neighboring hydrogens affecting the observed hydrogen.

Pascal's Triangle in NMR

Pascal's Triangle is a mathematical tool used to predict the relative intensities of peaks in an NMR multiplet. Each row corresponds to the number of neighboring hydrogens, with the coefficients representing the relative intensities of the peaks. For a quintet, the fifth row of Pascal's Triangle (1, 4, 6, 4, 1) shows the pattern of peak intensities, helping to visualize the distribution of spin states.

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Related Practice

Textbook Question

Draw a spectrum for each of the following molecules, being sure to indicate the multiplicity, integration, and chemical shift of each peak. Label each signal based on the set of equivalent hydrogens to which it corresponds.

(b)

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Textbook Question

Being able to recognize patterns of integration and multiplicity for common functional groups makes structure identification more efficient. Draw the pattern of integration and multiplicity you'd expect to see for each common alkyl group.

(d)

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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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(c)