9701 · 37.4
Proton (¹H) NMR spectroscopy flashcards
Revision flashcards for Cambridge 9701 Proton (¹H) NMR spectroscopy (syllabus 37.4). Flip, recall, then mark a real past-paper question.
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What is chemical shift (δ)?
The position of a signal in an NMR spectrum, relative to a reference standard (TMS). It is measured in parts per million (ppm) and depends on the proton's local electronic environment.
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Why is Tetramethylsilane (TMS) used as a reference standard?
TMS (Si(CH₃)₄) is used because its 12 protons are chemically equivalent and highly shielded, producing a single, sharp signal defined as 0 ppm. It is also chemically inert and volatile, making it easy to remove from the sample.
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What is shielding in ¹H NMR?
Shielding is the effect where electron density around a proton creates a small magnetic field that opposes the external applied field. This causes the proton to resonate at a lower frequency (upfield), resulting in a smaller chemical shift (δ) value.
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What is deshielding in ¹H NMR?
Deshielding is the effect where electron-withdrawing groups reduce the electron density around a proton. This reduces the opposition to the external field, causing the proton to resonate at a higher frequency (downfield), resulting in a larger chemical shift (δ) value.
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What does the integration trace on a ¹H NMR spectrum represent?
The integration trace indicates the relative area under each peak, which is proportional to the number of protons responsible for that signal. It gives the ratio of protons in each distinct chemical environment.
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What are chemically equivalent protons?
Protons that are in identical chemical environments within a molecule. They can be interchanged by a symmetry operation (like rotation or reflection) and have the same chemical shift, contributing to the same signal.
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What is spin-spin splitting?
The phenomenon where the signal for a set of protons is split into multiple peaks (a multiplet) due to the magnetic influence of non-equivalent protons on adjacent atoms.
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What is the n+1 rule?
A rule used to predict the splitting pattern of a signal. A signal for a proton with 'n' non-equivalent protons on adjacent atoms will be split into 'n+1' peaks. For example, a CH₂ group next to a CH₃ group will appear as a quartet (3+1=4).
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What is D₂O exchange used for?
D₂O exchange is a technique used to identify labile protons, such as those in -OH and -NH groups. Adding D₂O to the sample causes these protons to be replaced by deuterium, making their corresponding peaks disappear from the ¹H NMR spectrum.
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What is a labile proton?
A proton, typically in an O-H or N-H bond, that can be easily removed or exchanged, for example, with deuterium from D₂O. These protons often appear as broad singlets in an NMR spectrum.
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What is a quartet in ¹H NMR?
A signal that is split into four peaks with a characteristic intensity ratio of 1:3:3:1. According to the n+1 rule, it indicates that the proton(s) are adjacent to a group containing three non-equivalent protons (e.g., a CH₃ group).
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Why do -OH protons often appear as broad singlets?
Due to rapid chemical exchange with other labile protons (e.g., from trace water), the coupling to adjacent protons is averaged out to zero. This results in a single, often broad, peak that does not follow the n+1 rule.