prelab 2 - Review Assessment NMR Submitted Name Status...

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Review Assessment: NMR Submitted 1/18/08 1:12 PM Name NMR Status Completed Score 150 out of 150 points Instructions Question 1 10 out of 10 points What does the (n +1) rule state? Selected Answer: a proton with n equivalent neighbors will have its signal split into n+1 peaks Answer Feedback: Correct! Feedback: The simple "n+1 rule" only applies to signals whose neighboring protons are all of one type (equivalent) and, therefore, will all have the same coupling constant (J value). Complex splitting patterns arise when a proton has different coupling constants with different proton neighbors. Question 2 10 out of 10 points How many possible orientations do spin 1/2 nuclei have when they are located in an applied magnetic field? Selected Answer: 2 Answer Feedback: Correct. Feedback: Each nucleus can be aligned either with or against the field, giving TWO possible orientations. When an NMR sample is pulsed with radiofrequency energy, it causes the nuclei aligned with the field to "flip" to the other orientation (the higher energy alignment against the field). Relaxation back to the lower energy state is what gives rise to a signal in the NMR. Question 3 10 out of 10 points What does the integral trace on an NMR spectrum indicate? Selected Answer: the relative number of protons in each magnetic environment Answer Feedback: Correct. Feedback: The integral trace indicates the area under each peak, which is proportional to the number of protons contributing to that signal. Only the RATIO of protons is given by the integration, not the absolute number of protons. Question 4 10 out of 10 points In an NMR, the term ppm refers to: Selected Answer: parts per million Answer Feedback: Correct. Feedback: The chemical shift is given in ppm (parts per million), which tells us about the electronic environment in which a proton resides.
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