Upload
ann
View
120
Download
5
Embed Size (px)
DESCRIPTION
Proton NMR Spectroscopy. The NMR Phenomenon. Most nuclei possess an intrinsic angular momentum , P . Any spinning charged particle generates a magnetic field. P = [I(I+1)] 1/2 h/2 p where I = spin quantum # I = 0, 1/2, 1, 3/2, 2, …. Which nuclei have a “spin”?. - PowerPoint PPT Presentation
Citation preview
Proton NMR Spectroscopy
The NMR Phenomenon
• Most nuclei possess an intrinsic angular momentum, P.
• Any spinning charged particle generates a magnetic field.
P = [I(I+1)]1/2 h/2where spin quantum #
I = 0, 1/2, 1, 3/2, 2, …
Which nuclei have a “spin”?• If mass # and atomic # are both even, I = 0 and the
nucleus has no spin. e.g. Carbon-12, Oxygen-16
• For each nucleus with a spin, the # of allowed spin states can be quantized:
• For a nucleus with I, there are 2I + 1 allowed spin states.
1H, 13C, 19F, 31P all have I = 1/2E = h/2)Bo
Spin states split in the presence of B0
no field applied field
E
+1/2 parallel
-1/2 antiparallel
Bo
When a nucleus aligned with a magnetic field, B0, absorbs radiation frequency (Rf), it can change spin orientation to a higher energy spin state. By relaxing back to the parallel (+1/2) spin state, the nucleus is said to be in resonance. Hence,
NMR
Presence of Magnetic Field
NMR instruments typically have a constant Rf and a variable B0.
A proton should absorb Rf of 60 MHz in a field of 14,093 Gauss (1.4093 T).
Each unique probe nucleus (1H perhaps) will come into resonance at a slightly different - and a very small percentage of - the Rf.
All protons come into resonance between 0 and 12/1,000,000 (0 – 12 ppm) of the B0.
• Nuclei aligned with the magnetic field are lower in energy than those aligned against the field
• The nuclei aligned with the magnetic field can be flipped to align against it if the right amount of energy is added (DE)
• The amount of energy required depends on the strength of the external magnetic field
NMR Spectrometer• Schematic diagram of a nuclear magnetic
resonance spectrometer.
Energy Difference (E) Between Two Different
Spin States of a Nucleus With I=1/2
+1/2
-1/2
E 400 MHz300 MHz200 MHz100 MHz
23,500 47,000 70,500 104,000
parallel
antiparallel
inc. magnetic field strength, Gauss
B0
What Does an NMR Spectrum Tell You?
• # of chemically unique H’s in the molecule # of signals
• The types of H’s that are present e.g. aromatic, vinyl, aldehyde …
chemical shift• The number of each chemically unique H
integration• The H’s proximity to eachother
spin-spin splitting
Chemical EquivalenceHow many signals in 1H NMR spectrum?
O OO
O
CH3
Number of Equivalent Protons
O
OOO
6 4
CH3
1 3 5 2 4
Homotopic H’s– Homotopic Hydrogens
• Hydrogens are chemically equivalent or homotopic if replacing each one in turn by the same group would lead to an identical compound
Enantiotopic H’s• If replacement of each of two
hydrogens by some group leads to enantiomers, those hydrogens are enantiotopic
Diastereotopic H’s• If replacement of each of two hydrogens
by some group leads to diastereomers, the hydrogens are diastereotopic
– Diastereotopic hydrogens have different chemical shifts and will give different signals
Vinyl Protons
Typical 1H NMR Scale is 0-10 ppm
The Scale
Tetramethylsilane (TMS)
CH3SiCH3
CH3
CH3TMS
Arbitrarily assigned a chemical shift of 0.00
Chemical Shift Ranges, ppm
Diamagnetic AnisotropyShielding and Deshielding
Deshielding in Alkenes
Shielding in Alkynes
Integration
CH3
CH3
CH3
Br
H H
Methyl t-butyl ether (MTBE)
Toluene at Higher Field
Splitting patterns in aromatic groups can be confusing
A monosubstituted aromatic ring can appear as an apparent singlet or a complex pattern of
peaks
Integral Trace
Signal Splitting; the (n + 1) Rule
• Peak:Peak: The units into which an NMR signal is split; doublet, triplet, quartet, multiplet, etc.
• Signal splitting:Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens.
• ((nn + 1) rule: + 1) rule: If a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks.
Spin-Spin Splitting
The Doublet in 1H NMR
C C
HH
B0
a b
Ha splits into a 1:1 doublet peak
Hb is parallel or anti-parallel to B0
Ha is coupled to Hb
Hb in 1,1,2-Tribromoethane
The Triplet in 1H NMR
C C
H
H
H
B0
a b
Ha splits into a 1:2:1 triplet peak
Hb can both be parallel, anti-parallel Ha is coupled to Hb and Hb
b
or one parallel and one anti-parallel
Ha in 1,1,2-Tribromoethane
1,1,2-Tribromoethane
The Quartet in 1HMR
B0
C
H
CH
HH
proton splits into n+1
n = # adjacent H'squartet 1:3:3:1
shieldeddeshielded
Chemical Shift
Summary of Signal Splitting• The origins of signal splitting patterns. Each arrow
represents an Hb nuclear spin orientation.
1,1-Dichloroethane
Ethyl benzene
CH3CH2OCH3
Equivalent Protons do not Couple
Pascal’s Triangle
Signal Splitting (n + 1)
ProblemProblem: Predict the number of 1H-NMR signals and the splitting pattern of each.
CH3CCH2CH3O
CH3CH2CCH2CH3O
CH3CCH(CH3)2O
(a)
(b)
(c)
Differentiate using 1H NMR
Methyl Isopropyl Ketone
1-Nitropropane
Para Nitrotoluene
Coupling Constants (J values)
Bromoethane
Coupling Constants– An important factor in vicinal coupling is the angle
between the C-H sigma bonds and whether or not it is fixed.
– Coupling is a maximum when is 0° and 180°; it is a minimum when is 90°.
Physical Basis for (n + 1) Rule• Coupling of nuclear spins is mediated through
intervening bonds.– H atoms with more than three bonds between them
generally do not exhibit coupling.– For H atoms three bonds apart, the coupling is called
vicinal coupling.
Physical Basis for (n + 1) Rule
• Coupling that arises when Hb is split by two different nonequivalent H atoms, Ha and Hc.
More Complex Splitting Patterns
– Complex coupling that arises when Hb is split by Ha and two equivalent atoms Hc.
More Complex Splitting Patterns– Since the angle between C-H bond determines the extent of
coupling, bond rotation is a key parameter.– In molecules with free rotation about C-C sigma bonds, H
atoms bonded to the same carbon in CH3 and CH2 groups are equivalent.
– If there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded to the same carbon may not be equivalent.
– Nonequivalent H on the same carbon will couple and cause signal splitting.
– This type of coupling is called geminal couplinggeminal coupling.
More Complex Splitting Patterns
– In ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent.
More Complex Splitting Patterns– Tree diagram for the complex coupling seen for the
three alkenyl H atoms in ethyl propenoate.
More Complex Splitting Patterns– Cyclic structures often have restricted rotation about
their C-C bonds and have constrained conformations.– As a result, two H atoms on a CH2 group can be
nonequivalent, leading to complex splitting.
More Complex Splitting Patterns– A tree diagram for the complex coupling seen for the
vinyl group and the oxirane ring H atoms of 2-methyl-2-vinyloxirane.
More Complex Splitting Patterns
• Complex coupling in flexible molecules.– Coupling in molecules with unrestricted bond rotation often
gives only m + n + I peaks.– That is, the number of peaks for a signal is the number of
adjacent hydrogens + 1, no matter how many different sets of equivalent H atoms that represents.
– The explanation is that bond rotation averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar; for example in the 6- to 8-Hz range for H atoms on freely rotating sp3 hybridized C atoms.
More Complex Splitting Patterns– Simplification of signal splitting occurs when
coupling constants are the same.
More Complex Splitting Patterns
– Peak overlap occurs in the spectrum of 1-chloro-3-iodopropane.
– Hc should show 9 peaks, but because Jab and Jbc are so similar, only 4 + 1 = 5 peaks are distinguishable.
Styrene
Ha splitting in Styrene“Tree” Diagram
C CH
HHa
b
c
In the system below, Hb is split by two different sets of hydrogens : Ha and Hc
– Theortically Hb could be split into a triplet of quartets (12 peaks) but this complexity is rarely seen in aliphatic systems
0.750.50
60 MHz
100 MHz
300 MHz
300 240 180 120 60 0 Hz
300 240 180 120 60 0 Hz
300 240 180 120 60 0 Hz
Why go to a higher field strength?
0.75 (t, 2H, J=10)0.50 (t, 2H, J=10)
60 MHz
100 MHz
300 MHz
300 240 180 120 60 0 Hz
300 240 180 120 60 0 Hz
300 240 180 120 60 0 Hz
Homonuclear Decoupling
Allyl Bromide
CH2=CHCH2Br
CH2=CHCH2Br
irradiate
1-Phenyl-1,2-dihydroxyethane
C
H
OH
C
H
H
OH
a b
c
1-Phenyl-1,2-dihydroxyethane
Irradiate at 4.8