Proton (1H) NMR Spectroscopy (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define spin-spin splitting.
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Cards in this collection (18)
Define spin-spin splitting.
A spin-spin splitting is the splitting of an NMR peak into sub-peaks caused by a proton's spin interacting with the spin states of protons on adjacent, non-equivalent carbons.
State the n+1 rule for 1H NMR and explain what n represents.
The n+1 rule states that a proton with n protons on adjacent carbons splits into n+1 sub-peaks. n is the number of non-equivalent protons on the directly neighbouring carbon atom(s).
True or False?
In 1H NMR, equivalent protons cause spin-spin splitting of each other's signals.
False. Equivalent protons do not cause spin-spin splitting of each other's signals. Only non-equivalent protons on adjacent carbons cause splitting.
In the 1H NMR spectrum of an ethyl group (CH3CH2–), the CH3 signal appears as a .......... and the CH2 signal appears as a ..........
In the 1H NMR spectrum of an ethyl group (CH3CH2–), the CH3 signal appears as a triplet and the CH2 signal appears as a quartet.
What information does the area under a peak in a 1H NMR spectrum provide?
The area under a peak is proportional to the number of protons in that particular molecular environment, allowing the ratio of different types of hydrogen to be determined.
Define molecular environment (1H NMR).
A molecular environment in 1H NMR is the unique chemical surrounding of a proton; chemically equivalent protons share the same environment and produce a single peak at the same chemical shift.
In which chemical shift range would you expect the signal for a proton on a carbon adjacent to a C=O group?
Protons on a carbon adjacent to a carbonyl group (alkyl next to C=O) appear in the range 2.2–3.0 ppm.
True or False?
All four protons in 1,2-dichloroethane are in the same 1H NMR environment and produce a single peak.
True. In 1,2-dichloroethane (Cl–CH2–CH2–Cl), all four protons are chemically equivalent and produce one singlet on the 1H NMR spectrum.
Ethanol (CH3CH2OH) has .......... molecular environments in 1H NMR, giving peaks at approximately 1.2 ppm (–CH3), 3.7 ppm (–CH2–) and .......... ppm (–OH).
Ethanol (CH3CH2OH) has 3 molecular environments in 1H NMR, giving peaks at approximately 1.2 ppm (–CH3), 3.7 ppm (–CH2–) and 5.4 ppm (–OH).
Define tetramethylsilane (TMS).
Tetramethylsilane (TMS) is Si(CH3)4, used as the reference compound in NMR spectroscopy. Its single sharp peak is assigned a chemical shift of 0 ppm.
Why does TMS produce only one sharp peak in an NMR spectrum, despite containing 12 hydrogen atoms?
All 12 hydrogen atoms in TMS are in the same molecular environment (four identical CH3 groups bonded to Si), so they all absorb at the same frequency and produce one high-intensity peak.
True or False?
TMS is used as a reference in NMR because it is reactive and has a high boiling point.
False. TMS is used because it is inert and volatile — properties that prevent it from reacting with samples and allow easy removal after analysis.
In NMR, deuterium oxide (D2O) is added to identify –OH or –NH signals. The proton in these groups exchanges with .......... atoms, causing the signal to .......... from the spectrum.
In NMR, deuterium oxide (D2O) is added to identify –OH or –NH signals. The proton in these groups exchanges with deuterium atoms, causing the signal to disappear from the spectrum.
Why is CDCl3 preferred over an ordinary solvent when dissolving a sample for 1H NMR?
CDCl3 contains deuterium (2H) instead of hydrogen (1H). Deuterium absorbs radio waves in a different region, so the solvent peak does not interfere with the proton signals from the sample.
Define deuterated solvent.
A deuterated solvent is a solvent in which hydrogen atoms (1H) are replaced by deuterium (2H), preventing solvent peaks from interfering with the proton NMR signals of the dissolved sample.
True or False?
After adding D2O to a sample, the –OH signal disappears from the 1H NMR spectrum.
True. Adding D2O causes the proton in –OH groups to exchange with deuterium, replacing 1H with 2H. The signal disappears because 2H does not absorb in the 1H NMR region.
Why can –OH and –NH peaks in 1H NMR be difficult to identify without using D2O exchange?
–OH and –NH peaks are often broad, their chemical shift ranges can overlap with other proton types, and proton exchange with traces of water means only one average peak appears — making confident assignment difficult.
CCl4 is a suitable NMR solvent because it contains no .......... atoms and therefore produces no .......... NMR signals.
CCl4 is a suitable NMR solvent because it contains no hydrogen atoms and therefore produces no 1H NMR signals.
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