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 hydrogen atom in question, Hₐ, and note its chemical shift, δ 3.34, which indicates where the signal will appear on the NMR spectrum.
Understand the coupling constants provided: Jₐ꜀ = 9 Hz and Jₐ₆ = 4 Hz. These values indicate the splitting pattern of the signal due to neighboring hydrogens.
Determine the number of neighboring hydrogens that cause the splitting. The coupling constant Jₐ꜀ = 9 Hz suggests a strong coupling with one set of hydrogens, while Jₐ₆ = 4 Hz suggests a weaker coupling with another set.
Predict the splitting pattern based on the coupling constants. A larger coupling constant typically indicates a doublet, while a smaller one might suggest further splitting. Consider the possibility of a doublet of doublets.
Draw the expected NMR signal for Hₐ, showing the splitting pattern as a doublet of doublets, with the larger coupling constant (9 Hz) causing the initial split and the smaller constant (4 Hz) causing further splitting of each peak.
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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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