A 13 C-NMR spectrum. RF Frequency The intensity of the peak doesn t does not necessarily correlate to the number of carbons.

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1 13 -NMR We can examine the nuclear magnetic properties of carbon atoms in a molecule to learn about a molecules structure. Most carbons are 12 ; 12 has an even number of protons and neutrons and cannot be observed by NMR techniques. nly 1% of carbons are 13, and these we can see in the NMR. This makes 13 -NMR much less senstive than carbon NMR. This affects the how we see splitting patterns.

2 A 13 -NMR spectrum. ere is what s cool about 13 -NMR: Different carbons appear at different frequencies! - Now you know how many different types of arbons you have! RF absorbance An internal standard called TMS is used to set the scale to zero RF Frequency The intensity of the peak doesn t does not necessarily correlate to the number of carbons.

3 Example: Two alcohols (- compound) with formula ID = 0 2. Possible structures: Same! 2 2 3

4 The -NMR spectrum of isopropanol only shows two different carbons! These two carbons are identical by symmetry:

5 ow Many 13 NMR resonances? 7 F 7 4 l F 4 F 7 F 2 3

6 = equivalent resonance structures 1 Br Br Br Br Br Br 2 l Br Br

7 An isolated 13 atoms have the same chemical shift. (resonate at the same frequency) Nucleus Electron cloud The electron cloud of the atom partially shields the nucleus from the surrounding magnetic field When the carbon nucleus is adjacent to an electronegative atom, the carbon nuclei has fewer Electrons around it. F Nuclei with less electron density around them will resonate at a different frequency: It requires shorter wavelength radio waves (or a weaker magnetic field) to cause a carbon nuclei to resonate if it has less electron density around it.

8 Typical Values of hemical shift depend on the 1. ybridization 2. Electronegativity of attached atom(s) aromatic Br, (l) carbonyl sp sp 40-5 sp Some specific values are included in tables Section IV your text book!

9 onversely atoms that donate electron density cause nuclei to resonate at lower frequencies (higher field strengths). 3 3 Si 3 The silicon in tetramethylsilane (TMS) shields the carbon nuclei and makes them appear up-field 3 The chemical shift spectrum is measured relative to TMS. Shift TMS

10 TE δ SALE IS USED T MEASURE EMIAL SIFT δ= is used to signify chemical shift. Because the frequency depends on the field strength, one uses a relative units scale of parts per million or ppm Shift TMS hemical shifts reported as ppm units give the same values for the same compound regardless of the instrument used! δ = (v (compound) - v (TMS) )/ v (TMS)

11 50.2 ppm ppm F l Br 71.6 ppm 25.6 ppm 9.6 ppm l l l l l l l l l l

12 4 6

13 4 6

14 3 6

15 13 peaks are in reality split by bonded protons. Generally shown like this: Actually looks like this: A doublet Why?? Adjacent nuclear spin of the proton locally increases magnetic field felt by carbon nuclei. arbons next to down proton spin arbons next to up proton spin NTE: There are only two possible peaks for s having one adjacent proton. or

16 Appears as quartet. There are eight possibilities: The general rule is: The number of peaks observed is equal to the number of attached protons, (N), plus one. Splitting = N+1 FR 13 it is the number of protons directly attached to the carbon that cause splitting.

17 PAY ATTENTIN!: 13-NMR Splitting pattern reflects how many protons are directly attached to the carbon atom A Triplett 1-NMR Splitting pattern reflects how many protons are connected to adjacent carbon atom(s) A doublet A Triplett The chances that my neighbor is also a -13 is 1/100 there is a 99% chance that the attached has a spin

18 We typically observe decoupled Spectra More sensitive! less confusing. 18 taken from Maitland Jones 3rd Ed.

19 Why don t the spins on adjacent arbons split each nly 1% of arbons have a spin! (Most are 12 and are NMR inactive). Therefore only 1 in ten thousand pairs of s have both have spin and are coupled. That s too small to detect!

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

21 Note all of these split peaks can quickly get confusing: Am I looking at two different carbon resonances or a single doublet? For this reason we show our spectra without splitting. But we often see the splitting indicated over the peak:

22 Account for ID! = = 6 10 ID =??? 2 SYMMETRY?No, same number of resonances as formula

23 3 9 N 44.6 (t) 27.4 (t) 11.5 (q) N 2

24 3 7 N 139 (d) 113 (t) 44.8 (t) N 2

25 3 9 N 43.0 (d) 26.5 (q) N 2

26 It is helpful to know what typical values of unfunctinalized s Approximate hemical Shifts ranges for unsubstituted Alkanes R R R R R R R R R R 5-22 ppm ppm 25-35ppm = 66-90ppm ppm

27 If your molecules containing xygen: ppm 50-75ppm (Similar to alcohol but two of them!) ppm R = alkyl X = l, Br, I R R R R R NR R X doublet but singlet! ~ N

28 If your molecule contains Nitrogen: Amines: N 2 N N ppm Nitro- N Imine N R ppm ~155 ppm N ~

29 (d) 67.7 (t) 18.7 (q)

30 8 8 ID Symmetry What is the doing? What other groups can you identify?

31 68 ID Symmetry What is the doing? What other groups can you identify? an you account for the ID?

32 4 7 l s t t q PPM l

33 5 8 s 2 t t t t PPM

34 ow do you take an NMR?? I. onstant Radio frequency but change magnetic field LD TRADITINAL WAY!! Magnetic Field Stronger Magnetic Field Energy No Field Max Tofield Rogers 1956 RF transmitter N DWN FIELD RF Receiver de-shielded S + - UP FIELD shielded Know these terms!!

35 Modern NMR spectra are obtained at constant field strength NSTANT MAGNETI FIELD!--hange Frequency No Absorbtion not enough energy Absorbtion Energies Match! No Absorbtion Energy too high

36 ENERGY FT-NMR RF-Pulse FYI different RF frequencies 13 Technically speaking: you don t selectively excite all the nuclei but rather you saturate the exchange of nuclei between the upper an lower energy states. At equilibrium (in the absence of the RF energy) there are slightly more nuclei in the lower energy state than in the upper energy state based on the normal Boltzman distributions. RF-Pulse

37 onsider the population of like-nuclei in a sample: FYI Irradiate Should be more in the ground state! wait hv

38 FYI hv Spin is conserved: When a nuclei changes spin, some other particle in the universe must also change spin. The coupling of spin is more efficient the closer the two particles are and if they are connected by bonds.

39 Different nuclei require different amounts of time to relax. WY??? FYI slowest fastest

40 DEPT (JVERT): APT (Attached Proton Test) experiment is not as sensitive as a DEPT experiment, the APT has FYI the advantage of seeing all carbons present: 0 at 145 ppm and 2 at 29 ppm are negative, while 3 at 16 ppm and in the range of ppm are positive. Thus c and b are negative, while a, d, e, and f are positive.spectrum of Ethyl Benzene: 3 2

41 FYI Presentation of Two Dimensional NMR

42 FYI

43 FYI

44 FYI

45 NMR is being used to determine the structure of small proteins. FYI