WS4Q_NMRintro

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Old Dominion University *

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213

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Chemistry

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Apr 3, 2024

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pdf

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Uploaded by SargentRose15797

WS4 Questions Key, CHEM 213, Dr. Poutsma 1 Worksheet 4 Answer Key Nuclear Magnetic Resonance Spectroscopy Identifying the structures of molecules. Model 1: Nuclear Spin Questions 1. If a larger magnetic field is applied to your sample, will the energy difference (  E) between the two spins (aligned with and aligned against) increase or decrease? Increase 2. Which spin is lower in energy, the one aligned with the magnetic field or the one aligned against? The one aligned with the magnetic field. 3. If A and B are different nuclei and  E A is larger than  E B , will a higher or lower frequency radio wave be required to “flip” the spin of A compared to B? higher frequency 4. If the applied magnetic field is increased, will a higher frequency or lower frequency radio wave be required to “flip” the spins? higher frequency 5. In NMR spectra, the peaks correspond to the frequencies that converted the low energy spins to high energy spins. Do the peaks correspond to frequencies that were absorbed or transmitted? absorbed Model 2: Shielding Questions 6. a) Does electron “shielding” result in a higher or lower  E between the two nuclei spin states than predicted from just B applied ? lower  E b) Do you think more electrons around a nucleus will increase or decrease shielding? Increase 7. Electron density can be thought of as the cloud of electrons around a molecule. A partial negative charge on an atom means the electron density has increased (i.e. the electron cloud is thicker), while a partial positive charge means the density has decreased. a) Would increased electron density around a nucleus increase or decrease electron shielding of the nucleus? increase electron shielding b) Would increased electron density result in a higher or lower B effective ? Higher or lower  E? lower B effective and lower  E c) Would a higher or lower frequency radio wave be absorbed to “flip” the spin of a nucleus surrounded by increased electron density? lower frequency 8. a) Would a hydrogen bond have more electron density around it if it was bonded to a carbon or an oxygen? carbon atom (less partial positive charge on hydrogen)

WS4 Questions Key, CHEM 213, Dr. Poutsma 2 b) Which hydrogen nucleus (bonded to carbon or oxygen) would absorb a higher frequency radio wave? hydrogen bonded to an oxygen Model 3: Chemical Shifts Questions 9. a) Carbon is more electronegative than silicon. What affect do you think this will have on the electron density around the hydrogens in TMS? The hydrogens will have higher electron density around them and more shielding than normal hydrogens bonded to carbon. b) TMS is used as a reference compound because the H nuclei absorb at a different frequency from the H nuclei of most compounds. Do you think the absorption frequency of TMS is higher or lower than most compounds? lower frequency Model 4: Delta,  , or parts per million, ppm Questions 10. a) Based on the equation for  , what  value will TMS have? 0 b) Will most compounds have  values that are positive or negative? Positive 11. a) On the spectrum below  increases from right to left . Assuming a constant magnetic field, does frequency increase from right to left or left to right? increases right to left b) Based on your answers to questions 7 and 8, does electron shielding increase from right to left or left to right? increases from left to right Model 6: NMR Spectrometer Questions 12. Do the peaks on the NMR spectrum above occur upfield or downfield of the TMS peak? Downfield 13. a) Will the three hydrogens on the carbon have similar electron densities or different ones? (Remember that the carbon is tetrahedral.) similar b) Does your answer to 13a explain why only one peak is observed for those three hydrogens? Explain. Yes, they will all have the same electron densities and thus absorb at the same frequency. 14. Why does the peak for the H attached to the oxygen come downfield from the peak for the hydrogens attached to the carbon? The H on the oxygen is more deshielded by the electronegative oxygen.

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