1. NMR Lecture 5

2. Factors affecting 13C NMR spectra
How do you know 3 microsec = 22°
Proton decoupler always on, no pulse delay

3. Pulse delay 400 s; gated decoupler-on only when acquiring signal

4. Effects of the pulse delay between pulses, 400 s
Proton decoupler always on (NOE)

5. Information obtained by examining nmr spectra
1. Chemical shift (identifies nature of nucleus
2. Area (identifies relative number of nuclei)
3. Multiplicity ( (NOE)
2D NMR basically provides information about connectivity and proximity

6. Applications of 1H and 13C NMR

7. Molecular formula C9H10O
area: 5: 2: 3
IR: 1720 cm-1

8. 3:2:3:6
1H NMR
Molecular formula C7H14O2
IR cm-1

10. F NMR of F3

11. CF24
CF25H
=CF3-

13. Returning to Dynamic Nuclear Magnetic Resonance
For a simple exchange process coalescence  /21/2
Most chemical shift differences are of the order of a few hundred Hz or less. Rate constants are of the order of few hundred sec-1. Large in comparison to k  10-4 – 10-5 sec-1, measured by conventional kinetics but small relative to many dynamic processes occurring in molecular systems such as rotations about bonds.

19. Assign the spectrum and explain the coupling observed
Let’s do a thought experiment

20. What would the NMR spectrum of the methyl resonances of 4-dimethyl-3-hydroxycyclobutenone look like if all the methine hydrogens of this molecule had the same spin?

21. J13 J15 are of opposite sign
J13, J15 are the same sign

22. J13, J15 are of opposite sign
J13, J15 are the same sign

23. What are some mechanisms by which the two methyl groups can become identical?

24. H2O

26. H2O

28. 1,3-hydrogen shift

29. 1,3-hydrogen shift

30. J is of opposite sign
J is the same sign

35. 1,3-hydrogen shift; JAX
and JAM of the same sign

36. 1,3-hydrogen shift; JAX
and JAM of the opposite signs

38. Problem Set DNMR
By line shape analysis, determine the activation energy for rotation about a carbon nitrogen bond in an amide.
Go to my gateway and down load the Excel file.
A plot of ln k vs 1/T should result in a straight line and the slope of the line should be - Ea/R. Your problem will be to fit the lineshapes as best you can and evaluate k for a series of temperatures. Remember that the most accurate k values will be when the line shapes change the most.
k = rate constant