McMinn
Study Guide -- Lab 6
After reading Chapter 10 in Vollhardt and Schore and pages 871-888 in Pavia, you should be able to answer the following: 1. NMR spectroscopy applies to atoms (within molecules) which have nuclear magnetic moments. Which two atoms are most important to organic chemists? 2. What symbols are used to designate the available nuclear spin states of 1H? The spin states of the nucleus are of the same energy (said to be DEGENERATE) in the absence of a magnetic field but when a magnetic field is applied, they are no longer degenerate. How is this diagrammed? (see the left side of figure NMR.1, p. 871 in Pavia). 3. How is the lower energy spin state (of a 1H nucleus) aligned with respect to the magnetic field? 4. What is meant by the "resonance condition" in NMR? 5. How is spin conversion accomplished (i.e. what is the source of energy)? 6. True/False The applied radio frequency signal required for a spin conversion depends on the strength of the applied magnetic field. 7. What is a H-NMR spectrum? An NMR spectrum is ...... 8. Why is TMS used as a standard in NMR spectroscopy? 9. What is meant by the term "chemical shift"? 10. Where does the signal due to TMS appear in a typical NMR spectrum? Is this the high energy end or low energy end of the spectrum? 11. What is meant by the phrase "a proton absorbs 3 ppm downfield from TMS?" What would be the chemical shift for such an absorption? 12. What is the equation relating chemical shift (*) to the observed shift and the magnitude of the radio frequency radiation?
13. Chemical shifts can be tabulated for various kinds of protons (i.e. protons in various environments). Are these numbers dependent on the strength of the applied magnetic field?
14. What is meant by the term "chemical equivalence"? What is the effect of chemical equivalence on the proton NMR spectrum in terms of a) # of peaks and (b) size (area) of the peaks?
15. What is meant by the phrase "integrate the spectrum"? What information does an integration provide?
16. Describe what is meant by the term "deshielded". What role do electronegative elements play in determining which protons are more deshielded than others?
17. True/False: The greater the electronegativity of a substituent (E), the greater the chemical shift of a proton under consideration. (i.e. as in R-CH2-E). 18. What is meant by the "ring current" in aromatic systems? What is the effect of this ring current on the chemical shift of protons attached to an aromatic ring?
19. Explain the N+1 rule that applies to spin-spin splitting? How is the coupling constant related to this? What symbol is given to the coupling constant?
20. True/False: Magnetically equivalent protons do not split each other.
21. See Section NMR.11. True/False: a) Protons attached to O do not give an absorption in the proton NMR spectrum b) Protons attached to N give absorptions that are so broad they sometimes are hard to detect
22. The instrument we have is a 300 MHz instrument. The spectra in Pavia were recorded (for the most part) on a 60 MHz instrument. Look at figure NMR.3 (p. 873) and the equation for chemical shift at the top of p. 874. Do the following: a) Locate the peak due to the internal standard (TMS) placed at 0 Hz. b) Determine the chemical shift of the "3 proton singlet" (this is the peak closest to the TMS peak). c) Using the equation on p. 874, calculate the observed shift (in ppm) given that the spectrum was determined on a 60 MHz instrument. d) State the chemical shift (in *) if it were run on a 300 MHz instrument. e) Calculate the observed shift of the "3 proton singlet" (in ppm) if it were run on a 300 MHz instrument? f) Figure NMR.3 has several scales across the top portion of the spectrum. It looks like the numbers 500,400,300,200,100,0 cps are darker than the others. (Figure NMR.9 is more apparent). What is the significance of these numbers? After reading pages 816-821 in Pavia, you should be able to answer the following: 1. Why do organic chemists usually use deuterated chloroform as a solvent for NMR?
2. How do you prepare a routine sample for NMR?
3. How do you clean the NMR tube?
4. What are the health hazards associated with NMR solvents?
13. Chemical shifts can be tabulated for various kinds of protons (i.e. protons in various environments). Are these numbers dependent on the strength of the applied magnetic field?
14. What is meant by the term "chemical equivalence"? What is the effect of chemical equivalence on the proton NMR spectrum in terms of a) # of peaks and (b) size (area) of the peaks?
15. What is meant by the phrase "integrate the spectrum"? What information does an integration provide?
16. Describe what is meant by the term "deshielded". What role do electronegative elements play in determining which protons are more deshielded than others?
17. True/False: The greater the electronegativity of a substituent (E), the greater the chemical shift of a proton under consideration. (i.e. as in R-CH2-E). 18. What is meant by the "ring current" in aromatic systems? What is the effect of this ring current on the chemical shift of protons attached to an aromatic ring?
19. Explain the N+1 rule that applies to spin-spin splitting? How is the coupling constant related to this? What symbol is given to the coupling constant?
20. True/False: Magnetically equivalent protons do not split each other.
21. See Section NMR.11. True/False: a) Protons attached to O do not give an absorption in the proton NMR spectrum b) Protons attached to N give absorptions that are so broad they sometimes are hard to detect
22. The instrument we have is a 300 MHz instrument. The spectra in Pavia were recorded (for the most part) on a 60 MHz instrument. Look at figure NMR.3 (p. 873) and the equation for chemical shift at the top of p. 874. Do the following: a) Locate the peak due to the internal standard (TMS) placed at 0 Hz. b) Determine the chemical shift of the "3 proton singlet" (this is the peak closest to the TMS peak). c) Using the equation on p. 874, calculate the observed shift (in ppm) given that the spectrum was determined on a 60 MHz instrument. d) State the chemical shift (in *) if it were run on a 300 MHz instrument. e) Calculate the observed shift of the "3 proton singlet" (in ppm) if it were run on a 300 MHz instrument? f) Figure NMR.3 has several scales across the top portion of the spectrum. It looks like the numbers 500,400,300,200,100,0 cps are darker than the others. (Figure NMR.9 is more apparent). What is the significance of these numbers? After reading pages 816-821 in Pavia, you should be able to answer the following: 1. Why do organic chemists usually use deuterated chloroform as a solvent for NMR?
2. How do you prepare a routine sample for NMR?
3. How do you clean the NMR tube?
4. What are the health hazards associated with NMR solvents?