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tb13 - Chaoter 13 Structure Determination Nuclear Maflnetic...

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1. Chaoter 13 - Structure Determination: Nuclear Maflnetic Resonance Soectroscoov For a nucleus to exhibit the nuclear magnetic resonance phenomenon, it must be magnetic. Magnetic nuclei include: Answer: 2. a. b. c. d. all nuclei with even numbers of neutrons and protons all nuclei with odd-numbers of protons all nuclei with odd numbers of neutrons both b and c d Nuclear magnetic resonance spectroscopy provides information about a molecule's: Answer: 3. a. b. c. d. conjugated pi electron system size and formula. carbon-hydrogen fTamework. functional groups. c Answer: Explain why all protons in a molecule do not absorb rf energy at the same fTequency. All nuclei in molecules are surrounded by electron clouds. When a uniform external magnetic field is applied to a molecule, the circulating electron clouds set up tiny local magnetic fields of their own. These local magnetic fields act in opposition to the applied field, so that the effective field actually felt by a nucleus is a bit smaller than the applied field. Beffective = Bapplied - Blocal This effect is termed shielding. Each nucleus is shielded to a slightly different extent, so each unique kind of proton in a molecule resonates at a slightly different fTequency and gives rise to a unique NMR signal. The following questions pertain to the charting of NMR spectra. MATCH a term to each description below. Place the letter of the term in the blank to the left of the description. a. b. c. TMS high-field or upfield side MHz delta (8) low-field or downfield side chemical shift specific absorption d. e. f. g. Answer: 4. - When looking at an NMR chart the right-hand part of the chart is the -. b Answer: 5. - The exact place on the chart at which a nucleus absorbs is called its - . f 110 Chapter 13: Structure Determination: Nuclear Magnetic Resonance Spectroscopy
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6. - The calibration standard for 1 H and 13C NMR is: Answer: a 7. - The NMR charts are calibrated using an arbitrary scale that is divided into - units. Answer: d For each of the compounds below tell how many signals you would expect the molecule to have in its normal, broadband decoupled 13C NMR spectra. 80cQ Answer: 1 iliree d) 3 3 CH3 9. I H3C- C- CH3 I CH3 Answer: two 2 CH3 2 11 2 H3C- C- CH3 I CH3 2 ()O ?" 10. I I ~ ~ plane of symmetry Answer: five 4 3 5 11. ad Test Itemsfor McMurry's Organic Chemistry, Seventh Edition 111
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Answer: .CH3 12. am, Answer: ~& 13. () Answer: H3C CH2CH3 \ / 14. C= C / \ H H Answer: CH3 15. < ~ H Answer: three 112 six Cl five I 4 0; 3 2/CH3 CH3 I 5 3 4 three & B~ Br 2 -:?' I I 3 ~ 2 3 five 145 H3C CH2CH3 \ 2 3/ C=C / \ H H I CH3 3~~ 3 H Chapter 13: Structure Determination: Nuclear Magnetic Resonance Spectroscopy
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IdentitY the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic. 16. ifo H H:: I I ~ ~ Answer: homotopic 17. ~H H~ V Cl Cl~ Answer: enantiotopic 18. ~H CH3 " )~< ~H H Answer: diastereotopic ~H H H o .CH3 1~ ' H CH3 Answer: unrelated For each compound below tell how many types of none qui valent protons there are.
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