1fe1ccaa7b2","cardType":"basic","front":"What does the number of signals in an NMR spectrum indicate?","back":"The number of different chemical environments for that nucleus.","points":1,"mcqOptions":[{"text":"The number of different chemical environments for that nucleus.","correct":true,"explanation":"Nuclei in distinct chemical environments experience different amounts of shielding, so they resonate at different chemical shifts and produce separate signals in the NMR spectrum."},{"text":"The total number of nuclei of that element in the molecule.","correct":false,"explanation":"Counting every nucleus ignores equivalence, as several nuclei can share one signal. The number of signals instead shows the number of different chemical environments for that nucleus."},{"text":"The number of neighbouring nuclei causing spin-spin splitting.","correct":false,"explanation":"This confuses the number of signals with peak splitting. Neighbouring nuclei affect multiplicity, while the number of signals indicates the number of different chemical environments."},{"text":"The relative abundance of nuclei contributing to each resonance.","correct":false,"explanation":"This confuses signal count with integration, which can show relative numbers of nuclei in proton NMR. Signal count instead identifies the number of different chemical environments."}]},{"id":"c491b7cf-1357-4c83-a250-4c4afe8a552f","cardType":"basic","front":"Why does propanone ($CH_3COCH_3$) produce only two peaks in its $^{13}C$ NMR spectrum?","back":"The two methyl groups are equivalent by symmetry.","points":2,"mcqOptions":[{"text":"The two methyl groups are equivalent by symmetry.","correct":true,"explanation":"The molecule is symmetrical, so both methyl carbons have identical chemical environments and produce one signal, while the carbonyl carbon produces the second signal."},{"text":"The two methyl groups are equivalent by rapid bond rotation.","correct":false,"explanation":"Rotation around individual carbon-carbon bonds does not exchange the methyl groups across the carbonyl group. Their equivalence arises from molecular symmetry, giving one shared methyl-carbon signal."},{"text":"The three carbon atoms are equivalent by symmetry.","correct":false,"explanation":"The carbonyl carbon has a different environment from the methyl carbons, so all three carbons are not equivalent. Only the two methyl groups are symmetry-equivalent."},{"text":"The carbonyl carbon is absent because of rapid relaxation.","correct":false,"explanation":"Carbonyl carbons are detected in carbon-13 NMR and produce a characteristic signal. Propanone has two peaks because symmetry makes its two methyl carbons chemically equivalent."}]}]}],"examCountdown":{"examDate":"2027-05-28","paperLabel":"Paper 1"},"heading":"Nuclear magnetic resonance spectroscopy (A-level only)","title":"Nuclear magnetic resonance spectroscopy (A-level only) Flashcards","topicSidebar":"$L41"}]}],"topicId":"431a4594-7523-5900-8260-4cf75f86308a","walled":"$L42"}],"$L43"]