Textbook Question
For the hydrogen(s) screened in blue, draw the signal you would expect to see in a ¹H NMR spectrum. At which chemical shift would the signal appear?
(c)
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15. Analytical Techniques:IR, NMR, Mass Spect
1H NMR:Spin-Splitting Simple Tree Diagrams
Problem 40b
Textbook Question
Draw the expected signal for a hydrogen with the following coupling constants.
(b) Hₐ : δ 3.34 (Jₐ꜀ = 9 , Jₐ₆ = 4 )
Verified step by step guidance1
Identify the type of hydrogen environment for Hₐ. The chemical shift (δ) at 3.34 ppm suggests it is likely in a deshielded environment, possibly near an electronegative atom or a π-system.
Determine the splitting pattern using the given coupling constants. The coupling constants Jₐ꜀ = 9 Hz and Jₐ₆ = 4 Hz indicate that Hₐ is coupled to two different sets of hydrogens.
Apply the n+1 rule for each coupling constant to predict the splitting pattern. For Jₐ꜀ = 9 Hz, if Hₐ is coupled to n hydrogens, the splitting will be (n+1). Similarly, apply this for Jₐ₆ = 4 Hz.
Combine the splitting patterns from each coupling to determine the overall multiplicity. The signal will be a doublet of doublets (dd) due to the two different coupling constants.
Sketch the expected NMR signal for Hₐ, showing a doublet of doublets with the larger coupling constant (9 Hz) causing the larger separation and the smaller coupling constant (4 Hz) causing the smaller separation within each doublet.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Chemical Shift (δ)
Chemical shift (δ) is a measure of the resonance frequency of a nucleus relative to a standard in a magnetic field. It is expressed in parts per million (ppm) and provides information about the electronic environment surrounding the nucleus. In this question, δ 3.34 indicates the position of the hydrogen signal on the NMR spectrum, suggesting the hydrogen is in a relatively deshielded environment.
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Coupling Constants (J)
Coupling constants (J) are a measure of the interaction between nuclear spins and are expressed in hertz (Hz). They provide information about the number of neighboring hydrogens and their spatial relationship. In this question, Jₐ꜀ = 9 Hz and Jₐ₆ = 4 Hz indicate the hydrogen is coupled to two different sets of neighboring hydrogens, leading to a splitting pattern in the NMR signal.
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NMR Splitting Patterns
NMR splitting patterns arise from spin-spin coupling between non-equivalent hydrogens. The number of peaks in a splitting pattern is determined by the n+1 rule, where n is the number of neighboring hydrogens. In this case, the hydrogen with coupling constants Jₐ꜀ = 9 Hz and Jₐ₆ = 4 Hz will exhibit a complex splitting pattern, reflecting its interactions with two different sets of neighboring hydrogens.
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