To Do s. Answer Keys are available in CHB204H

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1 To Do s Read Chapters 2, 3 & 4. Complete the end-of-chapter problems, 2-1, 2-2, 2-3 and 2-4 Complete the end-of-chapter problems, 3-1, 3-3, 3-4, 3-6 and 3-7 Complete the end-of-chapter problems, 4-1, 4-2, , 4-8, 4-10, 4-11, 4-13, 4-16, 4-17, 4-18, 4-20 and 4-21 Answer Keys are available in CB204

2 NMR (Nuclear Magnetic Resonance) - NMR looks at magnetically active nuclei that is nuclei with non-zero nuclear spin. - We will learn 1 - and 13 C-NMR

3 NMR Gives Detailed Structural Information Molecular structure Peak assignment Structure elucidation NMR spectra Recording NMR is equivalent of seeing the molecule (almost).

4 Magnetically Active Nuclei (a) Organic molecules usually contain many hydrogens (b) More sensitive than the others (c) istorically, it s done first. ~ 6000 times less sensitive than 1 -NMR. Friebolin: Ein und zweidimensionale NMR-Spektroskopie VC (1988) - The relative sensitivity is given at constant magnetic field and equal number of nuclei. - The absolute sensitivity is the product of relative sensitivity multiplied by natural abundance. - Another common NMR nuclei is 19 F. 100% natural abundance and its γ is nearly as large as 1.

5 What Do We Look for in 1 -NMR? # of unique protons Peak positions (chemical shift) Shielding by bonding electrons Magnetic anisotropy Peak splitting (spin-spin coupling) Integrals Peak area are proportional to # of protons

6 Ethyl p-ydroxybenzoate, 1 -NMR 3 1, singlet E (triplet) 2 2 A C B 2 D(quartet) Chemical shift scale

7 What do we look for in 13 C-NMR? # of unique carbons Peak positions 13 C-NMR chemical shifts are spread in a much wider range than 1 -NMR, due to the presence of p-electrons.

8 Ethyl p-ydroxybenzoate, 13 C-NMR symmetry A wider chemical shift range

9 Nuclear Spin and Magnetic Moment

10 Nuclear Spin and Spin States 1 and 13 C have a spin quantum number of 1/2. They can have two different spin states - ± 1/2 or α/β or The different spin states have the same energy unless they are put in an external magnetic field

11 Small Magnet in an External Magnetic Field or ΔE = B 0 γh 2π h : Plank s constant γ : gyromagnetic ratio B 0 : external magnetic field

12 Energy Differences ΔE = B 0 γh 2π

13 Population Difference and spin state β ΔE 100,000 CW-NMR The energy gap between the two spin states creates a small population difference. When the system is irradiated with electromagnetic radiation, energy is absorbed at the frequency that matches to the energy gap. E α 100,001 in 200 Mz instrument E = hν hν = B γh 0 2π ν = γb 0 2π ν frequency

14 FT-NMR Sample Magnetization Response Data Spectrum Magnet Perturbation (Rf pulse) Detection Fourier Transformation Storage

15 Precession of Magnetic Moment and Net Magnetization ω 0 = γb 0 (rad/sec) ω 0 : Larmor frequency (angular velocity) B 0 ω 0 animation M : net magnetization

16 Pulsed Experiments B 1 sum of + and - vectors + : clockwise rotation - : counter clockwise rotation ν = γb 0 2π ω 0 = 2πν Energy is absorbed, and M will be tilted around the x-axis θ = γb 1 t p t p : pulse duration (usually ~ msec)

17 Relaxation and Free Induction Decay (FID) z M time x y Animation Magnetization on x-y plane

18 Rf Pulse and FID Animation Tilt and recovery Animation x-y plane magnetization

19 FID Signals O 3 C C 3 O 3 C O a C 3 b O O a b c d O

20 Fourier Transform of FID Single FT-NMR experiment takes ~ 5 sec or less ( 1 -NMR) Many FID signals can be accumulated and averaged Better S/N ratio

21 Chemical Shift Scale Different NMR instruments have different magnets, and operate at different frequencies. The same proton would resonate at different frequencies. Chemical shift (δ) = ν obs - ν ref ν instrument x 10 6 ppm

22 1 -NMR of Menthol

23 NMR Reference Compounds 3 C C 3 Si C 3 C 3 Chemically stable Can be removed easily (b.p. = 27 C) Most protons appear to the left of TMS Tetramethylsilane (TMS) Positive chemical shifts C 3 3 C Si SO 3 For D 2 O C 3 4,4-dimethyl-4-silapentane-1-sulfonic acid(dss)

24 Common NMR Solvents Do not use them as references

25 Chemical Shift Range for 1 - NMR TMS (δ=0)

26 Why Different Chemical Shifts Are Observed? Shielding by bonding electrons Magnetic anisotropy This will be discussed later in details. First, we will learn how to identify different protons in a given structure.

27 Chemical Shifts and Chemical Equivalence Chemically equivalent protons have the same chemical shift. ow do we find chemically equivalent(or non-equivalent) protons? Z substitution Z Z Z Z Z Z Same compound Chemically equivalent (homotopic)

28 Toluene and p-xylene b c a d b c b b a a b b

29 13 C-NMR of Toluene and p- Xylene

30 Methylacetate

31 alogenated Methanes a a a a C a a a C Cl a a C Cl Cl Enantiotopic protons a a C Cl Br Z C Cl Br Z C Cl Br A pair of enatiomers Enantiotopic protons have the same chemical shift although they are not chemically equivalent.

32 alogenated Ethanes a a C a a C a a Cl a C a b C b b Cl a C a a C a Cl Enantiotopic protons omotopic protons Diastereotopic protons Cl b C c a C Br Cl Cl Z C C Br Cl Cl C Z C Br Cl A pair of diastereomers Diastereotopic protons have different chemical shift.

33 Trityl-serine lactone

34 Typical 1 Chemical Shift Ranges

35 13 C-NMR Chemical Shift Range Both s- and p-electrons are responsible for shielding

36 Spin-spin Coupling A C C X Spin-spin coupling is commonly observed between protons that are separated by 3 bonds or less.

37 1,1,2-Trichloroethane Cl Cl C C Cl

38 1,1,2-Trichloroethane a b a Cl Cl C C Cl a b A doublet A triplet

39 1,1,2-Trichloroethane a Cl 2 a Cl C C Cl a b Spin-spin coupling 1 b TMS a : doublet (d) b : triplet (t)

40 Diethyl Ether a b O C C b a b a 3 C C 2 O C 2 C 3 b Jab a 3 3 Less shielded More shielded 1 1 A quartet

41 N+1 Rule and Pascal s Triangle singlet doublet triplet quartet quintet sextet septet etc A proton with N adjacent neighbors will split into N+1 lines. omotopic or enatiotropic protons do NOT split each other (there are exceptions) The intensity of the lines may be determined using Pascal s triangle. Spin-spin couplings are commonly observed between protons that are separated by three bonds or less.

42 Commonly Observed Splitting Patterns

43 When two J values are different J ac a Cl c Cl Cl C C Cl C C < J ac a b a b b b J ac J ac J ac J ac Triplets are a special case of dd. A doublet of doublets (dd)

44 Vinyl Acetate dd singlet dd dd Important to recognize splitting patterns

45 Typical Coupling Constants

46 Cis and Trans Alkenes

47 Ethyl-trans-Crotonate 3 C O O C 2 C 3

48 Alkenic Protons a 3 C c b b O O C 2 C 3 c = 7 z J ac = 1 z J bc = 16z a doublet of quartets

49 Allylic Protons a 3 C c b O O C 2 C 3 = 7 z J ac = 1 z J bc = 16z a doublet of doublets (dd)

50 Trityl-serine lactone a : doublet b : ddd c : doublet of doublets (dd) d : dd

51 Trityl-serine lactone

52 ydrogen Bonded Protons Usually appear as a broad peak Their chemical shifts depend on concentration, temperature, solvent etc. Generally do not show spin-spin coupling. They are exchangeable, and can be identified by D 2 O shake. * R-O + D 2 O R-OD + DO + D 2 O (excess) DO

53 Ethanol

54 N-Methyl Aniline N C 3 N

55 Problem 2.4(a)

56 Problem 2.4(a) Br ID = 0 quintet sextet Br triplet triplet 3 Br Br Don t be afraid of writing wrong structures!

57 Problem 2.4 (d)

58 ID = 5 Problem 2.4 (d) X Y Z C 2 C 3 3 N 2

59 Problem 2.4 (d) X Y N 2 Z C 2 C 3 C 9 12 NO 2 CO 2 missing 2 N O C 2 C 3 O 2 N O O C 2 C 3

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