Upload
glen
View
144
Download
0
Tags:
Embed Size (px)
DESCRIPTION
Nuclear Magnetic Resonance Spectroscopy. An Introduction. Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as NMR, has become the preeminent technique for determining the structure of organic compounds. Nuclear magnetic resonance (NMR) spectroscopy: - PowerPoint PPT Presentation
Citation preview
Nuclear Magnetic Resonance Spectroscopy
AnIntroduction
Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as NMR, has become the preeminent technique for determining the structure of organic compounds.
Nuclear magnetic resonance (NMR) spectroscopy: A spectroscopic technique that gives us information
about the number and types of nuclei in a molecule.For example, about the number and types of – Hydrogen nuclei using 1H-NMR spectroscopy.– Carbon nuclei using 13C-NMR spectroscopy.– Phosphorus nuclei using 31P-NMR spectroscopy.
Nuclear Magnetic Resonance spectroscopy involves the transition of a nucleus from one spin state to another with the resultant absorption of electromagnetic radiation by spin active nuclei when they are placed in a magnetic field.
The following features lead to the NMR phenomenon:
1. A spinning charge generates a magnetic field, as shown by the animation on the right.
The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.
The nuclei of many elemental isotopes have a characteristic spin (I). Some nuclei have integral spins (e.g. I = 1, 2, 3 ....), some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ....), and a few have no spin, I = 0 (e.g. 12C, 16O, 32S, ....).
Isotopes of particular interest and use to organic chemists are 1H, 13C, 19F and 31P, all of which have I = 1/2. Since the analysis of this spin state is fairly straightforward, our discussion of NMR will be limited to these and other I = 1/2 nuclei.
NUCLEAR SPIN STATES - HYDROGEN NUCLEUS
+ 1/2 - 1/2
The two statesare equivalentin energy in theabsence of amagnetic or anelectric field.
+ +
The spin of the positivelycharged nucleus generatesa magnetic moment vector, m.
m
m
TWO SPIN STATES
In the presence of an external magnetic field (B0), two spin states exist, +1/2 and -1/2.
The magnetic moment of the lower energy +1/2 state is alligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field. Note that the arrow representing the external field points North.
Nuclear Spins in Strong External Magnetic Fields
In a strong magnetic field (Bo) the two spin states differ in energy.
Bo
N
S
-1/2
+1/2
Bo
DE
+ 1/2
- 1/2
= kBo = hndegenerate at Bo = 0
increasing magnetic field strength
THE ENERGY SEPARATION DEPENDS ON Bo
The Larmor Equation!!!
g is a constant which is different for each atomic nucleus (H, C, N, etc)
DE = kBo = hn can be transformed into
gyromagnetic
ratio g
strength of themagnetic field
frequency ofthe incomingradiation thatwill cause atransition
gn = 2p
Bo
• More nucleons will be in the lower energy state aligned with the magnetic field.
• A nucleon can absorb a quantum of energy in the radio frequency range and align against the magnetic field.
• It emits a radio frequency when it drops back to its original position.
Absorption of Energy
Bo
+1/2
-1/2
+1/2
-1/2
DE = hnDE
quantized
Radiofrequency
AppliedField
Aligned
Opposed
WHEN A SPIN-ACTIVE HYDROGEN ATOM ISPLACED IN A STRONG MAGNETIC FIELD
….. IT BEGINS TO PRECESS
A SECOND EFFECT OF A STRONG MAGNETIC FIELD
OPERATION OF AN NMR SPECTROMETER DEPENDS ON THIS RESULT
If rf energy having a frequency matching the Larmor frequency is introduced at a right angle to the external field (e.g. along the x-axis), the precessing nucleus will absorb energy and the magnetic moment will flip to its I = -1/2 state. This excitation is shown in the following diagram.
Nuclear Magnetic Resonance
• Resonance: In NMR spectroscopy, resonance is due to the absorption of energy by a precessing nucleus and the results in “flip” of its nuclear spin from a lower energy state to a higher energy state.
• The precessing spins induce an oscillating magnetic field that is recorded as a signal by the instrument.– Signal: A recording in an NMR spectrum of a nuclear
magnetic resonance.
Strong magnetic fields are necessary for NMR spectroscopy.
The earth's magnetic field is not constant, but is approximately 10-4 T at ground level. Modern NMR spectrometers use powerful magnets having fields of 1 to 20 T.
Even with these high fields, the energy difference between the two spin states is less than 0.1 cal/mole.
1H 99.98% 1.00 42.6 267.531.41 60.02.35 100.07.05 300.0
2H 0.0156% 1.00 6.5 41.17.05 45.8
13C 1.108% 1.00 10.7 67.282.35 25.07.05 75.0
19F 100.0% 1.00 40.0 251.7
Resonance Frequencies of Selected Nuclei Isotope Abundance Bo (Tesla) Frequency(MHz) g(radians/Tesla)
4:1
For NMR purposes, this small energy difference (ΔE) is usually given as a frequency in units of MHz (106 Hz), ranging from 20 to 900 Mz, depending on the magnetic field strength and the specific nucleus being studied.
Irradiation of a sample with radio frequency (rf) energy corresponding exactly to the spin state separation of a specific set of nuclei will cause excitation of those nuclei in the +1/2 state to the higher -1/2 spin state.
The nucleus of a hydrogen atom (the proton) has a magnetic moment μ = 2.7927, and has been studied more than any other nucleus
NMR SPECTROMETER - INSTRUMENTATION
DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS
valence electronsshield the nucleusfrom the full effectof the applied field
B induced (opposes Bo)
Bo applied
magnetic fieldlines
The applied fieldinduces circulationof the valenceelectrons - thisgenerates amagnetic fieldthat opposes theapplied field.
Anisotropy
fields subtract at nucleus
All different types of protons in a moleculehave a different amounts of shielding.
This is why an NMR spectrum contains useful information(different types of protons appear in predictable places).
They all respond differently to the applied magnetic field and appear at different places in the spectrum.
PROTONS DIFFER IN THEIR SHIELDING
UPFIELDDOWNFIELD
Highly shielded protons appear here.
Less shielded protonsappear here.
SPECTRUM
It takes a higher fieldto cause resonance.
CHEMICAL SHIFT
• To standardise measurements on different NMR instruments, a standard reference sample is used in each experiment. This is tetramethylsilane (TMS).
This is a symmetrical and inert molecule. All H atoms have the same chemical environment and a single peak is produced from this molecule.
• The difference in energy needed to change the spin state in the sample is compared to TMS and is called the CHEMICAL SHIFT.
• The chemical shift of TMS is defined as zero• The symbol d represents chemical shift and is measured in
ppm. The chemical shift scale is measured from right to left on the spectrum.
chemical shift
= d = shift in Hz
spectrometer frequency in MHz= ppm
parts permillion
This division gives a number independent of the instrument used.
A particular proton in a given molecule will always come at the same chemical shift (constant value).
NMR Correlation Chart
12 11 10 9 8 7 6 5 4 3 2 1 0
-OH -NH
CH2FCH2ClCH2BrCH2ICH2OCH2NO2
CH2ArCH2NR2
CH2SC C-HC=C-CH2
CH2-C-O
C-CH-CC
C-CH2-CC-CH3
RCOOH RCHO C=CH
TMS
HCHCl3 ,
d (ppm)
DOWNFIELD UPFIELDDESHIELDED SHIELDED
Ranges can be defined for different general types of protons.
IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPESOF HYDROGENS COME IN SELECTED AREAS OFTHE NMR CHART
aliphatic C-H
CH on Cnext to pi bonds
C-H where C is attached to anelectronegative atom
alkene=C-H
benzene CH
aldehyde CHO
acidCOOH
2346791012 0
X-C-H X=C-C-H
MOST SPECTRA CAN BE INTERPRETED WITH A KNOWLEDGE OF WHAT IS SHOWN HERE
Factors influencing the Chemical Shift
• Inductive effect by Electronegative groups• Magnetic Anisotropy• Hydrogen Bonding
highly shieldedprotons appearat high field
“deshielded“protons appear at low field
deshielding moves protonresonance to lower field
C HClChlorine “deshields” the proton,that is, it takes valence electron density away from carbon, whichin turn takes more density fromhydrogen deshielding the proton. electronegative
element
DESHIELDING BY AN ELECTRONEGATIVE ELEMENT
NMR CHART
d- d+
d- d+
Electronegativity Dependence of Chemical Shift
Compound CH3X
Element X
Electronegativity of X
Chemical shift d
CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si
F O Cl Br I H Si
4.0 3.5 3.1 2.8 2.5 2.1 1.8
4.26 3.40 3.05 2.68 2.16 0.23 0
Dependence of the Chemical Shift of CH3X on the Element X
deshielding increases with theelectronegativity of atom X
TMSmostdeshielded
ANISOTROPIC FIELDSDUE TO THE PRESENCE OF PI BONDS
The presence of a nearby pi bond or pi system greatly affects the chemical shift.
Benzene rings have the greatest effect.
Secondary magnetic fieldgenerated by circulating pelectrons deshields aromaticprotons
Circulating p electrons
Ring Current in BenzeneRing Current in Benzene
Bo
DeshieldedH H fields add together
C=CHH
H H
Bo
ANISOTROPIC FIELD IN AN ALKENE
protons aredeshielded
shifteddownfield
secondarymagnetic(anisotropic)field lines
Deshielded
fields add
Bo
secondarymagnetic(anisotropic)field
H
HCC
ANISOTROPIC FIELD FOR AN ALKYNE
hydrogensare shielded
Shielded
fields subtract
HYDROGEN BONDING DESHIELDS PROTONS
O HR
O R
HHO
RThe chemical shift dependson how much hydrogen bondingis taking place.
Alcohols vary in chemical shiftfrom 0.5 ppm (free OH) to about5.0 ppm (lots of H bonding).
Hydrogen bonding lengthens theO-H bond and reduces the valence electron density around the proton- it is deshielded and shifted downfield in the NMR spectrum.
NMR Spectrum of Acetaldehyde
offset = 2.0 ppm
CCH3
O
H
SPIN-SPIN SPLITTING
Often a group of hydrogens will appear as a multipletrather than as a single peak.
Multiplets are named as follows:
Singlet QuintetDoublet SeptetTriplet OctetQuartet Nonet
This happens because of interaction with neighboring hydrogens and is calledSPIN-SPIN SPLITTING.
C CHCl
Cl H
HCl
integral = 2
integral = 1
triplet doublet
1,1,2-TrichloroethaneThe two kinds of hydrogens do not appear as single peaks,rather there is a “triplet” and a “doublet”.
The subpeaks are due tospin-spin splitting and are predicted by the n+1 rule.
C CH H
HC CH H
Htwo neighborsn+1 = 3triplet
one neighborn+1 = 2doublet
singletdoublettripletquartetquintetsextetseptet
MULTIPLETSthis hydrogen’s peakis split by its two neighbors
these hydrogens aresplit by their singleneighbor
SOME COMMON SPLITTING PATTERNS
CH2 CH2X Y
CH CHX Y
CH2 CH
CH3 CH
CH3 CH2
CH3CH
CH3
( x = y )
( x = y )
INTENSITIES OF MULTIPLET PEAKS
PASCAL’S TRIANGLE
1 2 1
PASCAL’S TRIANGLE
11 1
1 3 3 11 4 6 4 1
1 5 10 10 5 11 6 15 20 15 6 1
1 7 21 35 35 21 7 1
singlet
doublet
triplet
quartet
quintet
sextet
septet
octet
The interiorentries arethe sums ofthe two numbersimmediatelyabove.
Intensities ofmultiplet peaks
C C
H H
C C
H HA A
upfielddownfield
Bo
THE CHEMICAL SHIFT OF PROTON HA IS AFFECTED BY THE SPIN OF ITS NEIGHBORS
50 % ofmolecules
50 % ofmolecules
At any given time about half of the molecules in solution willhave spin +1/2 and the other half will have spin -1/2.
aligned with Bo opposed to Bo
neighbor aligned neighbor opposed
+1/2 -1/2
C C
H H
C C
H H
one neighbor n+1 = 2 doublet
one neighbor n+1 = 2 doublet
SPIN ARRANGEMENTS
yellow spins
blue spins
The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.
C CH H
HC CH H
H
two neighbors n+1 = 3 triplet
one neighbor n+1 = 2 doublet
SPIN ARRANGEMENTS
methylene spinsmethine spins
three neighbors n+1 = 4 quartet
two neighbors n+1 = 3 triplet
SPIN ARRANGEMENTS
C C
H H
H
H
H
C C
H H
H
H
H
methyl spinsmethylene spins
J J
J
J J
THE COUPLING CONSTANT
The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet.
J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet.
CH
HC HH
H
J
APPLICATIONS
• GEOPHYSICAL: Used to determine the water content in the geophysical samples.
• Engineering Applications: It is used to study the Process engineering aspects like the kinetic and equilibrium studies of Formaldehyde – water – methanol systems.
• Non- destructive testing of DNA, proteins etc.• This is very much useful in Data acquisition in
Petroleum Industry. Here NMR probes are developed and Used.
MRI• Magnetic resonance imaging, noninvasive• “Nuclear” is omitted because of public’s fear
that it would be radioactive.• Only protons in one plane can be in resonance
at one time.• Computer puts together “slices” to get 3D.• Tumors readily detected.