1. Dynamic Effects in NMR

2. The timescale in nmr is fairly long; processes occurring at frequencies of the order of chemical shift differences will tend to average out. For a simple exchange process coalescence   /2 1/2 This suggests that if proton spins can be made to change of the order of 20 to 40 Hz, coupling could be averaged out and its effects eliminated (recall the decoupling observed in the alcohol OH)

3. Effects of a Resonance frequency on a Nuclear Spin State 2 Irradiate 2 but observe 1 1 1 Processes occurring during double resonance 1. Spins change 2. Ratio of populations of ground and excited states  1 3. System reacts by redistributing other populations of spin states decoupling Nuclear Overhauser Effect

5. We will return to other aspects of dynamic NMR later but first lets apply double resonance to 13 C spectra.

6. 13 C NMR Spectra Unlike 1 H nuclei, 13 C are rare nuclei. The probability of finding a 13 C nucleu is approximately 1/100. The probability of finding 2 13 C next to each other is 2*.01*.01 = 2*10 -4 In a molecule like n-butyl vinyl ether, the probability of finding a 13 C nucleus at any of the carbon positions is equal. The problem is that 1 H will couple with 13 C rendering a weak signal even weaker.

7. Advantage: signal to noise goes up Disadvantage: spin coupling lost

8. Summary: Irradiation of the all the protons using a second broadband series of frequencies simultaneously while acquiring 13 C spectrum as well causes? Double resonance: 1.Multiplicity is lost and some structural information is lost (J CH ) 2.When the protons are irradiated, the Boltzman distribution of spin states is perturbed, resulting in more H in the excited state than usual; if we apply Le Chatelier’s principle, the system responds to minimize the perturbation; if a 13 C is next to one of the protons being irradiated, this perturbation results in more 13 C nuclei returning to their ground state. This is a T 1 process, meaning it will take a few seconds or longer (5 T 1 )to achieve this new equilibrium state. Once equilibrium is achieved, this leads to an enhancement of the 13 C signal and is called the Nuclear Overhauser effect

9. NOE observed when the decoupler is left on

10. Gated Decoupling: using the decoupler to effect characteristic changes in the spectrum by turning the decoupler frequency on and off at specific intervals 1.Broadband decoupling at protons; observe 13 C Effect: decoupling, NOE effect;

12. Gated Decoupling: using the decoupler to effect characteristic changes in the spectrum 2.Gated decoupling to collape coupling without any NOE NOE builds up with a time constant associated with 13 C T 1 values. If the rf frequency that irradiates the protons is left on, NOE is observed in a minute or so.

13. Why would you want gated decoupling without NOE? Interested in area under the curves (quantitative analysis)

14. 3. Gated decoupling with NOE without loss of coupling; retains NOE enhancement and coupling

15. 4. Off resonance decoupling: some coupling is retained so that the multiplicity is retained providing information regarding neighbors; the NOE effect is partially retained; information regarding the magnitude of the J CH coupling is lost. The closer a nucleus is to the irradiating field, the more the coupling constant is reduced.

17. Carbon chemical shifts

18. The use of ACD to predict 13 C NMR spectra 1.Estimation of : CH 3 CH 2 CH 2 CH 2 OCH=CH 2 2. Estimation of :

21. Coupling constants in 13 C NMR Typical coupling constants

22. Coupling constants in 13 C NMR 2. Long range CH coupling

23. Coupling constants in 13 C NMR 3. The relationship between hybridization and coupling constant

24. Coupling constants in 13 C NMR 3. The relationship between hybridization and coupling constant 4. 1 J CH CHCl 3 : 209 Hz; CH 2 CH 2 : 178; CH 3 Cl 150; CH 2 =CH 2 156 Hz cyclopropane

25. Measurement of T 1 ’s In a pulse experiment, if the rf field is left on long enough, the magnetization can be tipped 90°. What happens if the strong rf field is left on longer?

26. N S Before the rf pulse

27. rf generator signal coil N S Just after a 90 ° rf pulse pulse width = τ

28. Just after a 180 ° pulse; no signal generated in detector coil pulse width 2 τ N S

29. The result of applying a second short rf pulse shortly after the 180° pulse S N

30. Weak rf pulse turned off

31. 1.Measure the signal immediately after the 180 ° pulse by using a second weak pulse to tip the nuclei and generate a signal in the xy plane. Wait 5 T 1 2.Repeat the experiment, now waiting  seconds after the 180 ° pulse. 3.Vary 

32.  = 0 after 180 ° pulse and weak second pulse  = 5T 1 repeat but wait  sec before second pulse wait 5 T 1 repeat varying  population of ground and excited states are equal

33. Inversion recovery method is a way of measuring T 1

34. The decrease in intensity and then buildup again is a first order rate process. The change in ln(magnetization) plotted against time results in a straight line. The slope of the line is the rate constant and 1/slope = T 1 Any other uses ? Solvent suppression: T 1 ’s for small molecules such as solvents are usually longer than for other nuclei for both 13 C and 1 H

35. 3-fluoroalanine

36. Measurement of T 2 Spin Echo Technique Suppose we give a 90 rf pulse to a set of identical uncoupled nuclei. Magnetization is developed in the xy plane. After a period τ a 180 ° pulse is given. An echo is observed at 2 τ

37. rf generator signal coil signal coil, rf generator N S 1. apply 90 H rf pulse

38. rf generator signal coil signal coil, rf generator N S 2. apply 2 nd 180° pulse red: faster rotating blue: slower rotating

39. rf generator signal coil signal coil, rf generator N S 1. apply 90 H rf pulse 2. apply 2 nd 180° pulse blue: faster rotating red: slower rotating

40. rf generator signal coil signal coil, rf generator N S 1. apply 90 H rf pulse 2. apply 2 nd 180° pulse blue: faster rotating red: slower rotating

41. Suppose that we repeat this experiment varying the length of of time  between the original pulse and the second 180 ° pulse. The intensity of the spin echo will decrease as a result of magnetic inhomogeneity and this decrease will follow first order kinetics. The reciprocal of the rate constant is equal to T 2

42. Now consider a 13 CH fragment. The 13 C will signal will be a doublet due to the fact that half of the H’s will be  and the others will be . Suppose our rotating frame of reference is at the chemical shift of the 13 C. Some of the magnetization of the 13 C signal will be moving J/2 faster than our rotating frame and half will be moving J/2 slower. Chemical shift of 13 C

43.  = 0  = Ta  = 3Ta Observing a CH

44.  = 0  = Ta  =6Ta  180° pulse  = 0  = 2Ta A spin echo 180 °out of phase will be observes at T a later Following an initial 90 ° pulse 180 ° pulse

45. The phase of the spin echo of a 13 CH can be both positive and negative. The spin echo of a 13 C is always has the same phase (quaternary carbon) Lets now consider a 13 CH 2 and use for our rotating frame the chemical shift of the 13 C

46.  = 0  = Ta  = 3Ta

47.  = 0  180 ° pulse  = 0 Net magnetization never out of phase

48. This forms the basis of the DEPT experiment also called APT and other similar experiments. It recovers the information lost when using broadband decoupling (ie. The number of attached protons) Summary Quaternary carbons and CH 2 behave differently from CH and CH 3 groups.