nmr-I-1H-05 Organic Chemistry
-
Upload
sarra-mood-iman -
Category
Documents
-
view
223 -
download
0
Transcript of nmr-I-1H-05 Organic Chemistry
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
1/43
Spectroscopy:Spectroscopy:
Nuclear Magnetic ResonanceNuclear Magnetic Resonance
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
2/43
electronicelectronic
vibrationalvibrational
rotationalrotational
nuclear spinnuclear spin
UV-VisUV-Vis
InfraredInfrared
microwavemicrowave
Radiofrequency (NMR)Radiofrequency (NMR)
Spectroscopic Energy StatesSpectroscopic Energy States
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
3/43
An NMR Spectrometer
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
4/43
11H andH and 1313CC
both have spin = 1/2both have spin = 1/2
11H is 99% at natural abundanceH is 99% at natural abundance
1313
C is 1.1% at natural abundanceC is 1.1% at natural abundance
The nuclei that are most useful toThe nuclei that are most useful to
organic chemists are:organic chemists are:
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
5/43
Nuclear SpinNuclear Spin
A spinning charge, such as the nucleus ofA spinning charge, such as the nucleus of11H orH or1313C,C,
generates agenerates a magnetic fieldmagnetic field. The. The magnetic fieldmagnetic fieldgenerated by a nucleus of spin +1/2 is opposite ingenerated by a nucleus of spin +1/2 is opposite in
direction from that generated by a nucleus of spin direction from that generated by a nucleus of spin
1/2.1/2.
+ +
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
6/43
++
+
+
+
The distribution ofThe distribution of
nuclear spins isnuclear spins is
random in therandom in theabsence of anabsence of an
external magneticexternal magnetic
field.field.
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
7/43
++
+
+
+
An external magneticAn external magnetic
field causes nuclearfield causes nuclear
magnetic moments tomagnetic moments to
align parallel andalign parallel and
antiparallel to appliedantiparallel to applied
field.field.
HH00
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
8/43
++
+
+
+
There is a slightThere is a slight
excess of nuclearexcess of nuclear
magnetic momentsmagnetic momentsaligned parallel toaligned parallel to
the applied field.the applied field.
HH00
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
9/43
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
10/43
no difference in absence of magnetic fieldno difference in absence of magnetic field
proportional to strength of external magnetic fieldproportional to strength of external magnetic field
Energy Differences Between Nuclear Spin StatesEnergy Differences Between Nuclear Spin States
+
+
EE EE''
increasing field strengthincreasing field strength
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
11/43
The energy difference between the two spin states
depends on the strength of the magnetic field
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
12/43
-spin states -spin states
absorb E
release E
Signals detected by NMR
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
13/43
Some important relationships in NMRSome important relationships in NMR
The frequency of absorbedThe frequency of absorbed
electromagnetic radiationelectromagnetic radiation
is proportional tois proportional to
the energy difference betweenthe energy difference between
two nuclear spin statestwo nuclear spin states
which is proportional towhich is proportional to
the applied magnetic fieldthe applied magnetic field
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
14/43
Some important relationships in NMRSome important relationships in NMR
The frequency of absorbedThe frequency of absorbed
electromagnetic radiationelectromagnetic radiation
is proportional tois proportional to
the energy difference betweenthe energy difference between
two nuclear spin statestwo nuclear spin states
which is proportional towhich is proportional to
the applied magnetic fieldthe applied magnetic field
UnitsUnits
HzHz
kJ/molkJ/mol
(kcal/mol)(kcal/mol)
tesla (T)tesla (T)
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
15/43
Some important relationships in NMRSome important relationships in NMR
The frequency of absorbed electromagneticThe frequency of absorbed electromagnetic
radiation is different for different elements,radiation is different for different elements,
and for different isotopes of the same element.and for different isotopes of the same element.
For a field strength of 4.7 T:For a field strength of 4.7 T:11H absorbs radiation having a frequencyH absorbs radiation having a frequency
of 200 MHz (200 x 10of 200 MHz (200 x 1066 ss-1-1))1313C absorbs radiation having a frequencyC absorbs radiation having a frequency
of 50.4 MHz (50.4 x 10of 50.4 MHz (50.4 x 1066 ss-1-1))
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
16/43
Some important relationships in NMRSome important relationships in NMR
The frequency of absorbed electromagneticThe frequency of absorbed electromagnetic
radiation for a particular nucleus (such asradiation for a particular nucleus (such as 11H)H)
depends on its molecular environment.depends on its molecular environment.
This is why NMR is such a useful toolThis is why NMR is such a useful tool
for structure determination and medical diagnoses.for structure determination and medical diagnoses.
QuickTime and aGIF decompressor
are needed to see this picture.
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
17/43
Nuclear Shieldingand
1H Chemical Shifts
What do we mean by "shielding?"What do we mean by "shielding?"
What do we mean by "chemical shift?"What do we mean by "chemical shift?"
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
18/43
The electrons surrounding a nucleus affect the effective
magnetic field sensed by the nucleus
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
19/43
ShieldingShielding
An external magnetic fieldAn external magnetic field
affects the motion of theaffects the motion of the
electrons in a molecule,electrons in a molecule,
inducing a magnetic fieldinducing a magnetic fieldwithin the molecule.within the molecule.
CC HH
HH00
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
20/43
ShieldingShielding
An external magnetic fieldAn external magnetic field
affects the motion of theaffects the motion of the
electrons in a molecule,electrons in a molecule,
inducing a magnetic field withininducing a magnetic field within
the molecule.the molecule.
The direction of the inducedThe direction of the induced
magnetic field is opposite tomagnetic field is opposite to
that of the applied field.that of the applied field.
CC HH
HH00
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
21/43
ShieldingShielding
The induced field shields theThe induced field shields the
nuclei (in this case, C and H)nuclei (in this case, C and H)
from the applied field.from the applied field.
A stronger external field isA stronger external field isneeded in order for energyneeded in order for energy
difference between spin statesdifference between spin states
to match energy of rf radiation.to match energy of rf radiation.
CC HH
HH00
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
22/43
Chemical ShiftChemical Shift
Chemical shift is a measure ofChemical shift is a measure of
the degree to which a nucleusthe degree to which a nucleus
in a molecule is shielded.in a molecule is shielded.
Protons in differentProtons in differentenvironments are shielded toenvironments are shielded to
greater or lesser degrees;greater or lesser degrees;
they have different chemicalthey have different chemical
shifts.shifts.
CC HH
HH00
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
23/43
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift ( , ppm), ppm)
measured relative to TMSmeasured relative to TMS
UpfieldUpfield
Increased shieldingIncreased shielding
DownfieldDownfield
Decreased shieldingDecreased shielding
(CH(CH33))44Si (TMS)Si (TMS)
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
24/43
Chemical Shift
The reference point of an NMR spectrum is defined by
the position of TMS (zero ppm)
The chemical shift is a measure of how far the signal is
from the reference signal
he common scale for chemical shifts = (ppm)
=distance downfield from TMS (Hz)
operating frequency of the spectrometer (MHz)
Si CH3H3C
H3C
H3C
CH3
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
25/43
01.02.03.04.05.06.07.08.09.010.0
Chemical shift (Chemical shift ( , ppm), ppm)
7.28 ppm7.28 ppmHH CC
ClCl
ClCl
ClCl
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
26/43
Effects of Molecular Structure
on1
H Chemical Shifts
protons in different environments experience different
degrees of shielding and have different chemical
shifts
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
27/43
The chemical shift is independent of the operating
frequency of the spectrometer
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
28/43
Electron withdrawal produces NMR signals downfield
athigher frequency (at larger values)
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
29/43
Electronegative substituents decreaseElectronegative substituents decrease
the shielding of methyl groupsthe shielding of methyl groups
CHCH33FF 4.3 ppm4.3 ppm
CHCH33OOCHCH33 3.2 ppm3.2 ppmCHCH33NN(CH(CH33))22 2.2 ppm2.2 ppm
CHCH33CHCH33 0.9 ppm0.9 ppm
CHCH33SiSi(CH(CH33))33 0.0 ppm0.0 ppm
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
30/43
Electronegative substituents decreaseElectronegative substituents decrease
the shielding of methyl groupsthe shielding of methyl groups
CHCH33FF 4.3 ppm4.3 ppm least shielded Hleast shielded H
CHCH33OOCHCH33 3.2 ppm3.2 ppmCHCH33NN(CH(CH33))22 2.2 ppm2.2 ppm
CHCH33CHCH33 0.9 ppm0.9 ppm
CHCH33SiSi(CH(CH33))33 0.0 ppm0.0 ppm most shielded Hmost shielded H
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
31/43
Effect is cumulativeEffect is cumulative
CHCHClCl33
7.3 ppm7.3 ppm
CHCH22ClCl22 5.3 ppm5.3 ppm
CHCH33ClCl 3.1 ppm3.1 ppm
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
32/43
Protons attached to spProtons attached to sp22 hybridized carbonhybridized carbon
are less shielded than those attachedare less shielded than those attached
to spto sp33 hybridized carbonhybridized carbon
HH HH
HHHH
HH
HH
CC CC
HHHH
HH HH
CHCH33CHCH33
7.3 ppm7.3 ppm 5.3 ppm5.3 ppm 0.9 ppm0.9 ppm
Di ti A i t
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
33/43
Diamagnetic Anisotropy
The electrons are less tightly held by the nuclei than
are electrons; they are more free to move in responseto a magnetic field
Causes unusual chemical shifts for hydrogen bonded to
carbons that form bonds
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
34/43
Chemical Shift TableChemical Shift Table
Type of protonType of proton Chemical shift (Chemical shift ( ),),
ppmppm
Type of protonType of proton Chemical shift (Chemical shift ( ),),
ppmppm
CCHH RR 0.9-1.80.9-1.8
1.6-2.61.6-2.6CCHH CCCC
2.1-2.52.1-2.5CCHH CC
OO
2.52.5CCHH CCCC
CCHH ArAr 2.3-2.82.3-2.8
CC CC
HH
4.5-6.54.5-6.5
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
35/43
Chemical Shift TableChemical Shift Table
Type of protonType of proton Chemical shift (Chemical shift ( ),),
ppmppm
Type of protonType of proton Chemical shift (Chemical shift ( ),),
ppmppm
6.5-8.56.5-8.5
9-109-10
2.2-2.92.2-2.9
3.1-4.13.1-4.1
CCHH BrBr 2.7-4.12.7-4.1
3.3-3.73.3-3.7
HH ArAr
CC
OO
HH
CCHH NRNR
CCHH ClCl
CCHH OO
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
36/43
Chemical Shift TableChemical Shift Table
Type of protonType of proton Chemical shift (Chemical shift ( ),),
ppmppm
1-31-3HH NRNR
0.5-50.5-5HH OROR
6-86-8HH OArOAr
10-1310-13CC
OO
HHOO
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
37/43
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
38/43
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
39/43
Characteristic Values of
Chemical Shifts
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
40/43
1H NMR spectrum / Integration
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
41/43
1H NMR spectrum / Integration1-bromo-2,2-dimethylpropane
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
42/43
The area under each signal is proportional to the numberof protons that give rise to that signal.
The height of each integration step is proportional to thearea under a specific signal.
The integration tells us the relative number of protons
that give rise to each signal, not absolute number.
Integration
1H NMR spectrum / Integration
-
8/3/2019 nmr-I-1H-05 Organic Chemistry
43/43
1H NMR spectrum / Integration1-bromo-2,2-dimethylpropane
11
4.54.5
(2)(2)
(9)(9)