Clickers. a. I watched all 5 videos b. The dog ate my iphone

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1 Clickers a. I watched all 5 videos b. The dog ate my iphone

2 40% 33% 33% 40% 59%

3 67% of you: Watch youtube! PRBLEMS: Complete end of chapter 13 problems 1 10 from Lab Manual Answers

4 1 NMR Protons (nucleus of a hydrogen atom) also have a net spin and can be observed in the NMR. ydrogen atoms are more than 99% 1. ther isotopes have special names 2 = deuterium 3 = Tritium (radioactive) Remember 13 C is only 1% of carbon; 1 NMR is much more sensitive. Practically speaking for a typical lab molecule, you can obtain a 1 -NMR wit 1-2 mg of compound in a matter of minutes but the same sample may require a few hours to obtain a 13 C spectrum.

5 No Field Magnetic Field Stronger Magnetic Field Energy Energy hv } ΔΕ = hv In a strong magnetic field, the energy level difference corresponds to the energy of radio waves

6 FYI: Q&A ow strong is a magnetic field in a typical NMR? 300 Mz NMR = tesla 600 Mz NMR = tesla What other nuclei are commonly observed by NMR? 15N, 19 F, 31 P What type of NMR is used in MRI? MRI methods typically look at 1 -NMR of water in the different environments of the body.

7 Protons resonate at a different frequency than 13 C s. Typical compounds lie within a smaller range of frequencies than 13 C. PRTNS N TYPICAL RGNIC CMPUNDS 1-12 ppm Typical locations of 1 -NMR resonances. Down Field C Up Field C C C (X =, al) C X C TMS

8 See Table 13.1 in your text (lab manual) or Table 1 (page in spec book).

9 C C C

10 3 C a. 1.2 b. 1.6 c. 2.0 d. 2.6 e. 3.0 Cl a. 1.2 b. 2.1 c. 3.0 d. 3.4 e. 4.2 a. 1.2 b. 2.1 c. 3.0 d. 3.4 e. 3.8 b d e

11 Example: C C 3 C C 2 C 3 C 3 Solvent CDCl 3 TMS

12 WAT IS S SPEC IAL ABUT PRTN NMR? You can accurately integrate your spectra to know how many of each proton type you have. Through bond coupling tells us about what is adjacent to the protons of a particular resonance.

13 1-NMR peaks can be integrated!! { 5 : Tells you the ratio of the number of protons that compose each resonance. This is particularly helpful when you have symmetry.

14 Protons can couple (cause splitting) of C-13 peaks but we don t see splitting of proton peaks by carbon!! C- splitting is very small because only 1% of Carbon has a spin! You can see carbon-proton coupling if you look very closely! C-13 satellites

15 Protons can split other protons by through bond J-coupling. Coupling can be observed between two protons. The coupling originates because in a magnetic field, the nuclear spin will perturb the local distribution of electrons. 1 C 2 C 3 Protons on adjacent carbons are said to be Vicinal protons Are coupled over three bonds. 1 2 C Splitting over two bonds is usually not observed. Geminal coupling is only observed when the two protons are in different chemical environments, Note: there are exceptions when the protons are diastereotopic (more on this later). Protons on the same carbons are said to be geminal protons

16 Proton NMR signals are also split following the N+1 rule for proton NMR, N typically represents the number of adjacent protons. 3 C singlet (no adjacent s) quartet (3 adjacent s) 2 C C 3 triplet (2 adjacent s) N+1 RULE STILL APPLIES (note: it can get more complicated)

17 N+1 rule extends beyond quartets Br F Br a pentet (quintet) t pent q 2, 2, 3

18 PAY ATTENTIN!: 13C-NMR Splitting pattern reflects how many protons are directly attached to the carbon atom C A Triplet 1-NMR Splitting pattern reflects how many protons are connected to adjacent carbon atom(s) C C A doublet A Triplet

19 clickers Br Br Br Br x a. singlet b. doublet c. triplet d. quartet a. singlet b. doublet c. triplet d. quartet

20 Rules for J-coupling Nuclei must be chemical shift NN-EQUIVALENT to show (obvious) coupling to each other: Cl 1,2-dichloroethane is a singlet 2 C C 2 Cl The coupling is mutual: A splits B the same amount as B splits A J AB = J BA a a b a : 3.90 ppm, d (J = 7.1 z) Cl b : 5.77 ppm, t (J = 7.1 z) Cl Cl Coupling constants are reported in z and will have the same value on different instruments.

21 C 5 10 CLICKERS Which is it? A B C note: 6:4 ratio = 3:2 ratio

22 C a b c d 3 3 2

23 C a b c d 3 s (3) q t (2, 3) s t pent q q pent t s s 1 2, 2, 3 3, 2, 2 1 q (6) hept (1 ) 3 2

24 Br D

25 C a b 3 C C 3 c d

26 C a b 3 C C 3 c d

27 60 Mz NMR 1 ppm = 60 z 8 hz Same compound run on a 60 Mz and a 300 Mz spectrum 300 Mz NMR 1 ppm = 300 z 8 z Coupling constant is measured by taking the difference between peaks (in ppm)within a multiplet and multiplying by the field strength in z.

28 Common Splitting Patterns Ethyl triplet quartet Cl C 2 C PPM 1 0 Isopropyl doublet Br heptet 3 2 PPM 1 0

29 Cl PPM aromatic protons have (almost) the same chemical shift

30 a b/c b a Br c b a PPM 3 2 1

31 C 4 10 ID?

32 C 4 10 ID?

33 C 10 14

34 C 8 15 N N C s 46.9 t 46.5 t 44.2 t 32.6 q 27.4 s 25.9 q (2) s t s t s 6

35 Problem: C (sept) from: structural database for organic compounds

36 Protons in different chemical structures have different amounts of splitting or coupling constants. Bad News: Life gets more complicated Good News: Splitting tells us more about the chemical structure.

37 Coupling Constants Depends on Structure and Geometry Approximate Coupling Constants. Special Couplings over more than 3 bonds C C 7 z (free rotation) 2 z C C C C C C 10 z 15 z 2 z C C C C C C C 6 z C C 0-3 z depends on geometry 8 z C 1 z

38 Karplus Equation: J = 2-3 z J = 8-10 z

39 3 C N

40 C E D A B (i.e. Yuck!)

41 400 Mz J = 2-3 z J = 8-10 z N 2 Ac Ac Ac Ac S

42 400 Mz Ac Ac 6 2 Ac Ac 1 S R 1 4

43 Why is A a singlet? ow do you explain the splitting pattern of B? ow do you Explain the splitting pattern of C? A B C

44 Magnet'analogy' observe coupled protons C observe lower energy state high energy state two "degenerate" (same energy) states

45 Spin splitting diagrams a a b b Split by b Split by 2nd b

46 Spin splitting diagrams a a b b Split by b Split by 2nd b

47 Spin splitting diagrams a a b b Split by b J = 8.0 z Split by 2nd b J = 8.0 z J = 8.0 z J = 8.0 z J = 8.0 z

48 What if we have a strong and weak magnet? observe + coupled protons big magnet attractive little magnet repulsive little magnet attractive big magnet repulsive different energy!

49 What if we have a strong and weak magnet? observe + coupled protons big magnet attractive little magnet repulsive little magnet attractive big magnet repulsive different energy!

50 What if the s are different, and have very different coupling constants??! a a J AB 10 z J AC 2 z b c Split by b J = 10.0 z J = 8.0 z J = 8.0 z J = 8.0 z J = 8.0 z

51 What if the s are different, and have very different coupling constants??! a a J AB 10 z J AC 2 z b c Split by b J = 10.0 z Split by c 2.0 z 2.0 z A doublet of doublets

52 Aromatic (o,m,p) coupling constants 3 doublets of doublets a 7.58 ppm b 7.29 ppm c 6.92 ppm b a Br Br J( ab ) 1.9 z J( bc ) 8.0 z J( ac ) 0.3 z c b b 1.9 z 8.0 z a c 0.3 z 1.9 z 0.3 z 8.0 z 7.58 ppm 7.29 ppm 6.92 ppm

53 Aromatic (o,m,p) coupling constants 3 doublets of doublets a 7.58 ppm b 7.29 ppm c 6.92 ppm b a Br Br J( ab ) 1.9 z J( bc ) 8.0 z J( ac ) 0.3 z c b b 1.9 z 8.0 z a c 0.3 z 1.9 z 0.3 z 8.0 z 7.58 ppm 7.29 ppm 6.92 ppm

54 Doublets of triplets and doublets of quartets where J ab J ac c a b b c a b b b

55 What if Jab ~ Jac 2 N c 8.1 z a a Cl b 8.0 z a Split by b J = 8.1 z Split by c Still looks like a triplet, even on a high field spectrometer: Referred to as an apparent triplet J = 8.0 z J = 8.0 z J = 8.0 z J = 8.0 z

56 When the going gets really tough we call things a multiplet (m) situation 1: coupling pattern is very complex coupled to 7 different protons, with a different coupling constant Me described as: (m, 1)

57 When the going gets really tough we call things a multiplet (m) situation 2: your peak overlaps with other resonances C overlapping s! (m, 14)

58 skip to slide 70 58

59 igher order effects in NMR When chemical shifts are different: 2 doublets A X When chemical shifts are the same, coupling is not observed A A What if chemical shifts are similar but not identical? A B We observe collapsed doublets

60 300 Mz 240 Mz WEN PRTNS DN T SPLIT!! A B 180 Mz X C C Y V 1 V Mz J J 60 Mz

61 Dynamic NMR What happens when we take an NMR spectrum of a molecule in conformational equilibrium? Try taking a picture of a fan spinning ~40 times a second (i.e. 40 z). Your picture will depend on your shutter speed Shutter speed 1/10th Second - total blur You photograph the average 1/40th Second - similar to the frequency of the fan coalescence (in NMR speak) is the point where the fan speed and the shutter speed are ~equal 1/200th Second - no movement In NMR, the movement of the fan is analogous to movement of the molecule, and the frequency of the magnet is equivalent to shutter speed

62 Dynamic NMR axial equatorial The rate of interconversion (fan speed) is slower at low temperature eq ax

63 Dynamic NMR axial equatorial The rate of interconversion (fan speed) is slower at low temperature eq ax

64 Dynamic NMR axial equatorial The rate of interconversion (fan speed) is slower at low temperature eq ax

65 Dynamic NMR axial equatorial The rate of interconversion (fan speed) is slower at low temperature 72 C 80 C eq ax

66 Dynamic NMR

67 Diastereotopic Protons a a Same chemical shift No coupling a b Different! diastereotopic protons Diastereotopic protons: C 2 nearby a chiral center Chemical shifts differ (but may overlap in some cases) Coupling can often be observed

68 Diastereotopic Protons: not just in cyclic systems a b x a b 4.8 z 6.8 z 8.0 z Still diastereotopic a : 3.6 ppm,dd, J = 8.0 z, 4.8 z b : 3.9 ppm, dd, J = 6.8, 4.8 z x : 4.5 ppm, dd, J= 6.8, 8.0 z

69 3 diastereotopic methylenes Ph Ph Notice the coupling! Ph Ph

70 bserved nly Sometimes Depends on Concentration Solvent used Coupling to etero Atoms Rarely bserved C C N WY: hydrogen atoms exchange due to hydrogen bonding R R R R R R Easy Test C D 2 C D The hydrogen isotope deuterium ( 2 ) has no spin and cannot couple

71 dilute solution of ethanol t dq concentrated solution of ethanol q broad s from M. Denk

72 Big C C small R BIG

73 a ow can coupling constants help you assign the spectrum of vinyl acetate? c b

74 Assign the aromatic protons: int: 8 z 2 z a b c

75 Assembling Structures Using 1 and 13 C NMR. 1. Determine and Assign ID (Mol formula/c13) 2. Identify symmetry (C13) 3. Identify Functional Groups (C13 or other techniques). 4. Explore around functional groups

76 C 9 18 C t 68.7 d 36.9 d 26.6 t (2) 23.4 t (2) 23.0 q (2) d sept m m m d 6 Why Multiplets? Should be 1.83 t,t,t = multiplet Should be 1.47 d,d,d,d = multiplet

77 Why Multiplets? Should be 1.83 t,t,t = multiplet Should be 1.47 d,d,d,d = multiplet

78 13.3 C C- NMR: 173.6, s 51.3, q 42.0, t 34.9, d 33.1, t (2) 26.2, t (2) 26.1, t 1- NMR: 3.67, s, , d, J = 6.4 z, , m, , m, 5

79 13.4 C 8 11 N 127.1, s 126.3, d 117.7, s 28.0, t 5.79, t, J = 6.2 z, , s, , m, , m, , t 25.1, t 22.5, t 21.8, t C N

80 1 NMR 214.9, s 173.5, s 73.6, d 61.7, t 55.1, d 44.1, t 29.9, t 29.2, t 28.1, t 24.0, t 14.1, q C C-NMR 4.20, q, J = 6.0 z, , d, J = 3.5, 1 2.9, d, bs, 1 (exchanges) , m, , m, , t, J = 3

81 s C d t t t t t q PPM

82 4 3 PPM PPM PPM Difficult to determine structure by 1 and 13 C NMR only.

83 Big C C small R BIG

84 C 8 9 Br Br

85 C 7 12 d d d t t t q PPM Where is the double bond? Is the double bond cis or trans?

86 C 7 12 d d d t t t q PPM NMR 9.72 t J = 8.1 z, dq, J = 15.9, 6.2 z, , dt, J = 15.9, 5.9, dt, J = 8.1, 7.4 z, , td, J = 7.9, 5.9z, , quint, J = 8.0 z, , d, J = 6.2 z, 3

87 Carbon NMR essentials C methane ethane 3 C C propane butane pentane hexane heptane octane nonane decane isobutane isopentane isohexane neopentane 2,2-dimethylbutane methylpentane 2,3-dimethylbutane 2,2,3-trimethylbutane

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