13c-nmr - university of · pdf filehere is what’s cool about 13c-nmr: different carbons...
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13C-NMRWe can examine the nuclear magnetic properties of carbon atoms in a molecule to learn about a molecules structure.
Most carbons are 12C; 12C has an even number of protons and neutrons and cannot be observed by NMR techniques.
Only 1% of carbons are 13C, and these we can see in the NMR.
This makes 13C-NMR much less senstive than carbon NMR.
This affects the how we see splitting patterns.
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A 13C-NMR spectrum. RF
abs
orba
nce
RF Frequency
Here is what’s cool about 13C-NMR:
Different carbons appear at different frequencies! - Now you know how many different types of Carbons you have!
The intensity of the peak doesn’t does not necessarily correlate to the number of carbons.
An internal standard called TMS is used to set the scale to zero
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Example: Two alcohols (-OH compound) with formula C3H8O1. IHD = 02. Possible structures:
OH
OH
O
CH3CH2
H2C
O
H
Same!
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OH
These two carbons are identical by symmetry:
The C-NMR spectrum of isopropanol only shows two different carbons!
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F
OH
Cl
FF F
How Many 13C NMR resonances?
7 7 4
4 7
2 3
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equivalent resonance structures
=
1
BrBr
Br
Br
Br
Br
OH
4 3 42
Cl Br
Br3 10 5
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An isolated 13C atoms have the same chemical shift. (resonate at the same frequency)
CNucleus
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.
C 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.
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C O C C C C C
050100150200
40 -5
sp3
90 70sp150 105
sp2
150 120
aromatic
210 160
carbonyl
Typical Values of Chemical shift depend on the 1. Hybridization 2. Electronegativity of attached atom(s).
C O
C Br, (Cl)
75 50
Some specific values are included in tables Section IV your text book!
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Conversely atoms that donate electron density cause nuclei to resonate at lower frequencies (higher field strengths).
Si
CH3
CH3
CH3CH3
The silicon in tetramethylsilane (TMS) shields the carbon nuclei and makes them appear “up-field”
The chemical shift spectrum is measured relative to TMS.
TMSShift
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TMSShift
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
Chemical 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)
THE δ SCALE IS USED TO MEASURE CHEMICAL SHIFT
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HCHH
F
HCHH
Cl
HCHH
Br
HCHH
OH
HCHH
CH3
7.3 ppm
50.2 ppm
71.6 ppm
25.6 ppm
9.6 ppm
HCHH
ClClCHH
ClClCClH
ClClCClCl
Cl
25.6 54.0 77.2 96.1
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C4H6
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C4H6
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C3H6O
OH
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13C peaks are in reality split by bonded protons.
Generally shown like this:
Actually looks like this:
HC
Why??
C HAdjacent nuclear spin of the proton locally increases magnetic field felt by carbon nuclei.
Carbons next to up proton spin
Carbons next to down proton spin
NOTE: There are only two possible peaks for C’s having one adjacent proton. or
A “doublet”
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HCHH
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
FOR 13C it is the number of protons directly attached to the carbon that cause splitting.
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PAY ATTENTION!: 13C-NMR 1H-NMR
C CH
HH
CH
H
Splitting pattern reflects how many protons are directly attached to the carbon atom
A Triplett
Splitting pattern reflects how many protons are connected to adjacent carbon atom(s)
A doublet
A Triplett
C CH
HHThe chances that
my neighbor is also a C-13 is 1/100
there is a 99% chance that the
attached “H” has a spin
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We typically observe decoupled Spectra
18
taken from Maitland Jones 3rd Ed.
•More sensitive!•less confusing.
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Why don’t the spins on adjacent Carbons split each
C COnly 1% of Carbons have a spin! (Most are 12C and are NMR inactive). Therefore only 1 in ten thousand pairs of Cs have both have spin and are coupled. That’s too small to detect!
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Protons can couple (cause splitting) of C-13 peaks but we “don’t see” splitting of proton peaks by carbon!!C-H splitting is very small because only 1% of Carbon has a spin!.
C-13 satellites
You can see carbon-proton coupling if you look very closely!
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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:
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C6H10OIHD = ??? 2
C=O
C=C
Account for IHD!
SYMMETRY ?No, same number of resonances as formula
O
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C3H9N44.6 (t)
27.4 (t)
11.5 (q)
NH2
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C3H7N139 (d)
113 (t)
44.8 (t)
NH2
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C3H9N43.0 (d)
26.5 (q)
NH2
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It is helpful to know what typical values of unfunctinalized C’s
Approximate Chemical Shifts ranges for unsubstituted Alkanes
5-22 ppm
15-33 ppm
25-35ppm
C=C 110-150ppm
66-90ppm
R CH
HH
R CR
HH
R CR
RH R C
R
RR
C C
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If your molecules containing Oxygen:
OCR H
OCR R
OCR O
OCR NHR
R = alkyl
OC
R X
X = Cl, Br, I
195 - 220doublet
195 - 220but singlet!
165-180 165-180 165-180
OC
H3C O
OC
H3C NH
60.4 34.4
OC
O O
~155
C OH
C O C
50 - 75 ppm 50 - 75ppm (Similar to alcohol but two of them!)
CO O
90 - 100 ppm
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If your molecule contains Nitrogen:
C NH2
28-47 ppm
CN
C
H
CN
C
CAmines:
C NO
O61 - 85 ppm
Nitro-
CN RImine
~155 ppm
C NC
14-30
~120
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C3H8O2
72.7 (d)
67.7 (t)
18.7 (q)
OHHO
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C8H8O
IHDSymmetryWhat is the O doing?What other groups can you identify?
O
133128129
13719727
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C6H8O
IHDSymmetryWhat is the O doing?What other groups can you identify?Can you account for the IHD?
O198129145
4130
47
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O
Cl
020406080100120140160180200220PPM
s qt t
C4H7ClO
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O
O
020406080100120140160180PPM
C5H8O2s t t tt
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How do you take an NMR??
+ -
N S
RFtransmitter
RFReceiver
Ener
gy
No Field Magnetic Field StrongerMagnetic Field
I. Constant Radio frequency but change magnetic field
! OLD TRADITIONAL WAY!
DOWNFIELD
UPFIELD
Know these terms!!
Max Tofield Rogers 1956
de-shielded shielded
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Modern NMR spectra are obtained at constant field strength
CONSTANT MAGNETIC FIELD!--Change Frequency
No Absorbtion not enough energy
Absorbtion Energies Match!
No Absorbtion Energy too high
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RF-Pulse
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.
ENER
GY
RF-Pulse
FT-NMR
different RF frequencies13C
FYI
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Consider the population of like-nuclei in a sample:
Should be more in the ground state!
Irradiate
wait
hv
FYI
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hvSpin 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.
FYI
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fastestslowest
Different nuclei require different amounts of time to relax.
WHY???
FYI
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DEPT (JVERT): APT (Attached Proton Test) experiment is not as sensitive as a DEPT experiment, the APT has the advantage of seeing all carbons present: CH0 at 145 ppm and CH2 at 29 ppm are negative, while CH3 at 16 ppm and CH in the range of 126-129 ppm are positive. Thus Cc and Cb are negative, while Ca, Cd, Ce, and Cf are positive.Spectrum of Ethyl Benzene:
CCH2
CHCH3
FYI
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Presentation of Two Dimensional NMR FYI
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O
O
FYI
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HO
FYI
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FYI
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NMR is being used to determine the structure of small proteins.
FYI