16 ch203 fall 2014 lecture 16 october 10.pdf
TRANSCRIPT
The previous slide may be found at h5p://www.jkwchui.com/2011/12/interpre@ng-‐proton-‐nmr-‐overview/
CH203 Lecture 16 October 10, 2014 Complex coupling 13C NMR
1960: Bruker KIS-‐1 NMR (25 MHz)
How much informa@on have we go5en out of the NMR so far?
1. The loca@on of the hydrogen resonance in ppm (chemical shiY). Tells us the local electronic environment of the hydrogen.
2. The number of different hydrogen resonances. Tells us how many sets of nonequivalent hydrogens are present.
3. The number of hydrogens in each nonequivalent set by integra@on.
4. The number of neighboring hydrogens. 5. The orienta@on of the neighboring hydrogens
(coupling constant).
Spin-‐spin coupling in a vicinal two hydrogen system
Vocabulary
C C
HH
C C
H H
C C
H
H
C C
H
H
H
H
C C
H
C H
H
H
H
H
H
H
3J, vicinal 6-‐8 Hz
3J, vinylic cis 5-‐10 Hz
3J, vinylic trans 11-‐18 Hz
2J, vinylic geminal 0-‐5 Hz
2J, geminal 0-‐5 Hz
4J, allylic 0-‐1 Hz
2J, ortho 7-‐9 Hz
3J, meta 2-‐3 Hz
4J, para 0-‐1 Hz
Repor@ng NMR data
10
The ethyl group
11
The isopropyl group
Complex coupling
The Hb signal is split by Ha (Jab) and then by Hc (Jbc) to give a doublet of doublets. (This only applies if Jab and Jbc are not equal or close to equal.)
Complex coupling
The Hb signal is split by Ha (Jab) and then by two Hc (Jbc) to give a doublet of triplets. (This only applies if Jab and Jbc are not equal or close to equal.)
What happens when the Js are equal or close to equal?
When Jab and Jbc are close to the same magnitude, the spliang tree does not generate the expected nine signals of a triplet of triplets. Only five peaks are seen due to overlap.
What happens when the Js are equal or close to equal?
In this case, Jab and Jbc are close to the same magnitude because they are in a flexible chain. Rota@on averages the coupling constants to about the same value. In general, where you would expect to see (n +1) x (m +1) peaks, a flexible system like this will give (n + m +1) peaks. (n +1) x (m +1) = 9 peaks expected (n + m +1) = 5 peaks observed
What happens when the Js are equal or close to equal?
In 1-‐chloro-‐3-‐bromopropane, Jab and Jbc are almost equal. We do not see the expected nine peaks [(n +1) x (m + 1)] of a triplet of triplets for Hc. Only five peaks (n + m +1) are observed.
What happens when the Js are equal or close to equal?
In 1-‐chloropropane, Jab and Jbc are almost equal. We do not see the expected twelve peaks [(n +1) x (m + 1)] of a triplet of quartets for Hb. Only six peaks (n + m +1) are observed.
Geminal coupling in an alkene
NMR of ethyl propenoate. The region around 6 ppm needs to be expanded to see the spliang pa5erns and measure the coupling constants.
Geminal coupling in an alkene
NMR of ethyl propenoate. The signals for the three vinylic hydrogens are each a doublet of doublets. The pa5ern changes as the coupling constant changes.
Geminal coupling in a ring
2-‐methyl-‐2-‐vinyloxirane. The two H atoms on the oxirane ring are nonequivalent, so they exhibit geminal coupling.
Topicity
Hydrogens are homotopic if replacement does not generate a new chiral center. D (2H) is some@mes used as the test replacement. In dibromomethane, the hydrogens are homotopic and so are always iden@cal.
BrC
Hb
BrHa
BrC
D
BrHa
BrC
Hb
BrD
Topicity
Hydrogens are enan@otopic if replacement does generate a new chiral center. In bromofluoromethane, the hydrogens are enan@otopic and so are iden@cal in achiral environments.
BrC
Hb
FHa
BrC
D
FHa
BrC
Hb
FD
Topicity
Groups are diastereotopic if replacement does generate a new chiral center in a molecule with at least one exis@ng chiral center. In 3-‐methyl-‐2-‐butanol, the two methyl groups bonded to C3 are diastereotopic and so are not equivalent. Diastereotopic groups will exhibit different chemical shiYs.
C
OH
CH3H3C
H3C
C C
OH
CH3H3C
D3C
C C
OH
CH3D3C
H3C
Topicity
O
CH3H3C
H3C
The two methyl groups at C3 in 3-‐methyl-‐2-‐butanone are not diastereotopic and appear as a clean doublet at about 1 ppm.
Topicity
The two methyl groups at C3 in 3-‐methyl-‐2-‐butanol are diastereotopic and appear as two doublets at about 1 ppm.
OH
CH3H3C
H3C
Topicity
Homotopic: replacement does not generate a new chiral center always iden@cal same chemical shiYs
Enan@otopic: replacement does generate a new chiral center
iden@cal in achiral environments same chemical shiYs in achiral NMR experiment
Diastereotopic: replacement does generate a new chiral center in a molecule
with at least one exis@ng chiral center different chemical shiYs
What is an achiral NMR experiment?
Ways to make an achiral NMR environment Chiral solvent: not many available, very expensive Chiral addi@ve: “shiY reagents” are chiral molecules which complex with the target molecule and shiY the proton signals in one enan@omer Chiral deriva@ves: chemically add a chiral group to the mixture of enan@omers to generate a mixture of diastereomers which will have different NMRs
What is an achiral NMR experiment?
Sodium [(R)-‐1,2-‐Diaminopropane-‐N,N,N',N'-‐tetraacetato]samarate(III), hydrate
13C NMR and the Fourier Transform
There are two kinds of NMR spectrometers: con@nuous wave spectrometers and pulse spectrometers.
In a CW spectrometer, the sample is placed in a sta@c magne@c field and swept with radiofrequency (Rf) energy. The CW NMR spectrum shows peaks at frequencies where there is absorp@on of Rf energy.
In a pulsed spectrometer, the sample is placed in a sta@c magne@c field and then hit with a pulse of RF waves powerful and wide enough to simultaneously excite all nuclei in the sample. AYer the pulse the nuclei return to their ground states by radia@ng the absorbed energy. The pulsed spectrometer detects this reradiated energy as a Free Induc@on Decay (FID) signal. The computer uses the Fourier Transform (FT) to convert the FID @me data into frequency data which is presented like the original CW spectra.
13C NMR
Problems with obtaining 13C NMR spectra 13C makes up only 1% of the carbon in a sample. The gyromagne@c ra@o of 13C is one-‐fourth of that of 1H. This means that the 1H signal is 1000 @mes stronger than the 13C signal. To get a useable spectrum, the FT-‐NMR pulses the sample mul@ple @mes and the computer sums the FIDs to enhance peaks and minimize noise.
13C NMR
The 13C resonance signal is also split by the protons a5ached to it. In the top spectrum of norbornane, carbons 2,3,4, and 6, which are all equivalent and all have two hydrogens bound, appear as a triplet. A pulse spectrometer is able to irradiate the sample with a second radiofrequency energy which is absorbed by all the hydrogens and decouples their spin states from the carbon spin states. The spin decoupled spectrum is on the bo5om. Each set of equivalent carbons is seen as one peak.
13C chemical shiYs
13C chemical shiYs compared to 1H
=>
Pulse sequences
DEPT
Goto
66 Bruker AVANCE User’s Guide
Figure 18: DEPT Pulse Sequence
Acquisition and Processing 6.3
Insert the sample in the magnet. Lock the spectrometer. Readjust the Z and Z2 shimsuntil the lock level is optimized. Tune and match the probehead for 13C observation,1H decoupling.
Reference spectraSince DEPT is a 13C-observe, 1H-decouple experiment, the first step would be toobtain a reference 1H spectrum of the sample to determine the correct o2 for 1Hdecoupling. The second step would then be to obtain a 1H-decoupled 13C spectrumto determine the correct o1 and sw for the DEPT experiments. However, both ofthese steps were already carried out in Section 4.3 starting on page 37. So, a 1H-decoupled 13C reference spectrum of this sample can be found in carbon/3/1. (Theone thing to be aware of is that broadband decoupling was used in carbon/3/1, buthere the cpd sequence WALTZ-16 will be used).
Create a new file directory for the data setEnter re carbon 3 1 to call up the reference spectrum. Enter edc and change thefollowing parameters:
NAME deptEXPNO 1PROCNO 1 .
Click SAVE to create the data set dept/1/1.
Set up the acquisition parametersEnter eda and set the acquisition parameters as shown in Table 22. Use the valuesdetermined in Chapter 5 ‘Pulse Calibration’ for the parameters pl1 and p1 (13Cobserve high power level and 90° pulse time), pl2 and p3 (1H decouple high powerlevel and 90° pulse time), and pl12 and pcpd2 (1H decouple low power level and
cpd
acq
!
! "
!2
!2
13C
1H12JXH
12JXH
12JXH
trd
p1 p2
p3 p4 p0
d2d1 d2d2
Simple pulse sequence
Pulse sequence for a DEPT (Distor@onless Enhancement by Polariza@on Transfer) experiment.
Pulse sequences -‐ DEPT
Pulse sequences -‐ APT
What is it?
C6H8O4
1H 13C
(13C signals are not propor@onal to the number of carbons at that ppm.)
2H
6H
What is it?
C6H8O4
1H 13C
(13C signals are not propor@onal to the number of carbons at that ppm.)
2H
6H
H
H
O
O
OCH3
OCH3Dimethyl maleate
What is it?
C4H4S (13C signals are not propor@onal to the number of carbons at that ppm.)
1H
2H 2H 13C
What is it?
C4H4S (13C signals are not propor@onal to the number of carbons at that ppm.)
1H
2H 2H 13C
S
Thiophene