Chapter 13: NMR 1 Dr. Sivappa Rasapalli Chemistry and
Biochemistry University of Massachusetts Dartmouth CARBON-13 ( 13
C) NMR SPECTROSCOPY
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both give us information about the number of chemically
nonequivalent nuclei (nonequivalent hydrogens or nonequivalent
carbons) both give us information about the environment of the
nuclei (hybridization state, attached atoms, etc.) Carbon-13 NMR
Spectroscopy H 1 and C 13 NMR Spectroscopy
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SPIN PROPERTIES OF ATOMIC NUCLEI What is spin? The Simple
explanation Spin is a fundamental property of nature like
electrical charge or mass. Spin is a measure of angular momentum
(rotation about an axis) hence the term Spin comes in multiples of
1/2 (0, 1/2, 1, 3/2, 2, 5/2) and can be + or -. Protons, electrons,
and neutrons possess spin. Individual unpaired electrons, protons,
and neutrons each possesses a spin of 1/2 Atomic nuclei composed of
neutrons and protons may also possess spin. The spin of an atomic
nucleus is determined by the number of protons and neutrons in the
nucleus. Atoms with an odd number of protons will have spin Atoms
with an odd number of neutrons will have spin Atoms with an odd
number of both protons and neutrons will have spin Atoms with an
even number of both protons and neutrons will not have spin The
value of nuclear spin is represented by the symbol I, the nuclear
spin quantum number. (I = 0, 1/2, 1, 3/2, 2, 5/2.) A nucleus with
spin of I can exist in (2I+1) spin states. The shell model for the
nucleus tells us that nucleons (protons and neutrons), just like
electrons, fill orbitals. When the number of protons or neutrons
equals 2, 8, 20, 28, 50, 82, and 126, orbitals are filled. Because
nucleons have spin, just like electrons do, their spin can pair up
when the orbitals are being filled and cancel out. Odd numbers mean
unfilled orbitals, that do not cancel out.
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A moving perpendicular external magnetic field will induce an
electric current in a closed loop An electric current in a closed
loop will create a perpendicular magnetic field A Basic Concept in
Electromagnetic Theory- Direct Application to NMR
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Carbon-13 NMR Spectroscopy 5 Wheres Waldo?
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13C NMR Spectroscopy 6 E= B o h 2 B o = external magnetic field
strength = magnetogyric ratio 1 H= 26,752 13 C= 6.7 One carbon in 3
molecules of squalene is 13 C
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Nucleus (10 6 rad/Tesla sec) Field strength B 0 (Tesla)
Frequency (MHz) 1H1H267.531.0042.6 4.70200. 7.05300.
2H2H41.11.006.5 13 C67.281.0010.7 4.7050.0 7.0575.0 19
F251.71.0040.0 13C Transition Energy
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BoBo = h / 4 In the absence of external field, each nuclei is
energetically degenerate Add a strong external field (B o ) and the
nuclear magnetic moment: aligns with (low energy) against
(high-energy) Magnetic alignment RANDOM ORIENTATION
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Nuclear Spin (cont.) to convert lower energy spin state into
higher energy spin state, require external energy
source.........irradiate with radiofrequency (rf) radiation! 1. At
zero external magnetic field, spins are degenerate! 2. Apply an
external magnetic field spins states will differ in energy
depending upon relative orientation with respect to external field.
-nuclei with I = will adopt two specific orientations with respect
to an externally- applied magnetic field... external magnetic field
BoBo -spin state +1/2 (lower energy) -spin state -1/2 (higher
energy) radiofrequency energy source
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Spins Orientation in a Magnetic Field (Energy Levels) to
convert lower energy spin state into higher energy spin state,
require external energy source.........irradiate with
radiofrequency (rf) radiation! 1. At zero external magnetic field,
spins are degenerate! 2. Apply an external magnetic field spins
states will differ in energy depending upon relative orientation
with respect to external field. -nuclei with I = will adopt two
specific orientations with respect to an externally- applied
magnetic field... radiofrequency required depends on E E depends on
strength of B o h E apply magnetic field increasing B o -spin: +1/2
lower energy -spin: -1/2 higher energy no external magnetic
field
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Spins Orientation in a Magnetic Field (Energy Levels)
radiofrequency required depends on E E depends on strength of B o h
E apply magnetic field increasing H 0 -spin: +1/2 lower energy
-spin: -1/2 higher energy no external magnetic field Difference in
energy between the two states is given by: E = h B o / 2 E = h
where: B o = external magnetic field h = Plancks constant;
gyromagnetic ratio B o =0 B o >0
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= Bo/2 The value, , is the magnetogyric ratio
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Frequency of absorption: = B o / 2 Transition from the low
energy to high energy spin state occurs through an absorption of a
photon of radio-frequency (RF) energy RF Spins Orientation in a
Magnetic Field (Energy Levels)
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Nuclear Spin (cont.) Difference in energy between the two
nuclear spin states: depends on strength of external magnetic field
(MHz) For nucleus of H atom (proton), spin energy differences: 0
4.736.358.4611.75 -1/2 +1/2 100 200300360500 2.34 H 0 (Tesla) Thus,
at H 0 = 4.7 T (Tesla), use rf radiation of 200 MHz for 1 H Nuclei
ENERGY OF A PHOTON E = h SPIN STATE ENRGY DIFFERENCE E = hB 0 /2
WHEN E = E, SPIN FLIP OCCURS h hB 0 /2 THE NECESSARY FREQUENCY IS:
B 0 /2
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Nuclear Spin (cont.) Difference in energy between the two
nuclear spin states: depends on strength of external magnetic field
(MHz) For nucleus of H atom (proton), spin energy differences: 0
4.736.358.4611.75 -1/2 +1/2 25 507590125 2.34 H 0 (Tesla) Thus, at
B 0 = 4.7 T (Tesla), use rf radiation of 50 MHz etc for 13 C
nuclei. ENERGY OF A PHOTON E = h SPIN STATE ENRGY DIFFERENCE E = hB
0 /2 WHEN E = E, SPIN FLIP OCCURS h hB 0 /2 THE NECESSARY FREQUENCY
IS: B 0 /2
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Fourier Transform NMR 13 C-spectra of CH 3 CH 2 CH 2 CH 2 CH 2
OH average of 200 scans after one scan
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Features of 13 C NMR Spectra Each unique C in a structure gives
a single peak in the spectrum; there is rarely any overlap. The C
NMR spectrum spans over 200 ppm; chemical shifts only 0.001 ppm
apart can be distinguished; this allows for over 2x10 5 possible
chemical shifts for carbon. The intensity (size) of each peak is
NOT directly related to the number of that type of carbon. Other
factors contribute to the size of a peak: Peaks from carbon atoms
that have attached hydrogen atoms are bigger than those that dont
have hydrogens attached. Carbon chemical shifts are usually
reported as downfield from the carbon signal of tetramethylsilane
(TMS).
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H 1 and C 13 NMR Spectroscopy Number of peaks Chemical shifts
Integration Spin-Spin Splitting Number of peaks Chemical shifts
Integration Spin-Spin Splitting
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13 C NMR 19
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13 C NMR 20
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Predicting 13 C Spectra
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Predicting 13 C NMR
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Each unique carbon in a molecule gives rise to a 13 C NMR
signal. Therefore, if there are fewer signals in the spectrum than
carbon atoms in the compound, the molecule must possess symmetry.
Examples: Symmetry in C-13 NMR
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How many signals would you expect?
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* * * * * Enantiotopic vs Diastereotopic carbonss
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Determine the number of signals in the proton-decoupled C-13
NMR spectrum of each of the following compounds: Predicting 13 C
NMR
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1H and 13C NMR compared: 13 C signals are spread over a much
wider range than 1 H signals making it easier to identify and count
individual nuclei The following slides show the 1 H NMR and the 13
C spectrum of 1-chloropentane. It is much easier to identify the
compound as 1-chloropentane by its 13 C spectrum than by its 1 H
spectrum.
Chemical shift ( , ppm) Carbon Spectrum ClCH 2 CH 2 CH 2 CH 2
CH 3 020406080100120140160180200 13 C CDCl 3 a separate, distinct
peak appears for each of the 5 carbons
13C-NMR: Integration 1 H-NMR: Integration reveals relative
number of hydrogens per signal 13 C-NMR: Integration reveals
relative number of carbons per signal Rarely useful due to slow
relaxation time for 13 C time for nucleus to relax from excited
spin state to ground state
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13 C Chemical shifts are most affected by: electronegativity of
groups attached to carbon hybridization state of carbon
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13 C NMR Chemical Shifts Several functionalities appear
directly on 13 C NMR which are not visible in 1 H NMR: - Quaternary
carbons - ipso carbons - Carbonyl carbons downfield (ppm) upfield
deshielded shielded higher E lower E
220200180160140120100806040200.0 carbonyl carbons aromatic carbons
alkene carbons alkyne carbons sp 3- EWG sp 3 carbon
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13 C NMR
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Chemical Shift - 13 C-NMR Trends RCH 3 < R 2 CH 2 < R 3
CH Electronegative atoms cause downfield shift Pi bonds cause
downfield shift C=O 160-210 ppm
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36 Chemical Shift Range of 13 C Note the carbonyl range
Chemical Shift - 13 C-NMR
Spin-Spin Coupling in 13 C NMR Homonuclear coupling of 13 C- 13
C is possible in theory. However, due to the low natural abundance
of 13 C, it is rare to find two 13 Cs in the same molecule, let
alone adjacent to one another. No need to consider 13 C- 13 C
coupling except for enrichment studies! Heteronuclear coupling
between 13 C and the 1 H atoms attached to them is observed ( 1 H
abundance ~99%). Because the 1 H atoms are directly attached, the
coupling constants ( 1 J)are large, typically 100-250 Hz. When such
spectra are observed, they are referred to as proton coupled
spectra (or non-decoupled spectra).
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Carbon-13 NMR Spectroscopy 50 Wheres Waldo?
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Spin-Spin Coupling in 13 C NMR Homonuclear coupling of 13 C- 13
C is possible in theory. However, due to the low natural abundance
of 13 C, it is rare to find two 13 Cs in the same molecule, let
alone adjacent to one another. No need to consider 13 C- 13 C
coupling except for enrichment studies! Heteronuclear coupling
between 13 C and the 1 H atoms attached to them is observed ( 1 H
abundance ~99%). Because the 1 H atoms are directly attached, the
coupling constants ( 1 J)are large, typically 100-250 Hz. When such
spectra are observed, they are referred to as proton coupled
spectra (or non-decoupled spectra).
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1 H NMR (Proton with Carbon-13 coupling)
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13 C NMR Spectrum Carbon 13 and Proton-Coupled
Proton-Coupled
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1 H 13 C Splitting The splitting follows the simple N+1 rule:
The multiplet analysis gives useful information, but there are two
major limitations: 1) If the 13 C signal is weak (common) the outer
peaks of the multiplet may be lost in the noise of the spectrum. 2)
Due to the large J-constants, the multiplets quickly begin to
overlap and become congested. quaternary singlet methine doublet
methylene triplet quaternary quartet
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Effect of Coupling Coupling can cause 13 C NMR spectra to
become very complicated (convoluted) quite easily. 1 H Coupled
Three equal intensity lines at 77 ppm CDCl 3 solvent 13 C- 2 D
coupling
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1 H Decoupling To simplify the 13 C spectrum, and to increase
the intensity of the observed signals, a decoupler is used to
remove the spin effects of the 1 H nucleus. A second RF generator
irradiates at the 1 H resonance frequency causing the saturation
effectively averaging all their spin states to zero 1 H channel- 13
C channel 13 C pulse 13 C FID
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Proton-decoupled mode, a sample is irradiated with two
different radiofrequencies. One to excite all 13 C nuclei, a second
to cause all protons in the molecule to undergo rapid transitions
between their nuclear spin states. On the time scale of a 13 C-NMR
spectrum, each proton is in an average or effectively constant
nuclear spin state, with the result that 1 H- 13 C spin-spin
interactions are not observed and they are decoupled.
Decoupling
H1 Decoupling Techniques J values for C-H are typically 110-300
Hz (C-C-H and C-C-C-H are 0-60Hz). Thus a CH3 group would appear as
a quartet, CH2-triplet CH-doublet etc. The H1 nuclei are irradiated
with a broadband Rf to remove coupling to Carbon.
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CH 3 OH Attached Protons Affect T1 and Signal Intensity 7
carbons give 7 signals, but intensities are not equal Chemical
shift ( , ppm) 020406080100120140160180200
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Example: Ethanol 1 H- 13 C spin-spin coupling: spin-spin
coupling tells how many protons are attached to the 13 C nuclei.
(i.e., primary, secondary tertiary, or quaternary carbon) 13 C
spectra are usually collected with the 1 H- 13 C coupling turned
off (broad band decoupled). In this mode all 13 C resonances appear
as singlets.
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Other isomers of C 5 H 12 pentane CH 3 CH 2 CH 2 CH 2 CH 3 3
peaks 2,3-dimethylpropane(CH 3 ) 4 C2 peaks There are four
chemically different carbon atoms in the molecule so there are four
peaks in the C-13 nmr spectrum. NO SPLITTING WITH C-13 ONLY ONE
PEAK FOR EACH CARBON NO SPLITTING WITH C-13 ONLY ONE PEAK FOR EACH
CARBON chemically equivalent carbon atoms H C C C C H H H CH3CH3 H
H H H H 2-methylbutane (CH 3 ) 2 CHCH 2 CH 3
Attached Proton Test APT Acquiring a 13 C after 1/J seconds:
methine and methyl Cs produce negative peaks (odd number of
attached Hs) methylene and quaternary Cs produce positive peaks
(even number of attached Hs) Remember 1 J CH is essentially the
same for all tetrahedral carbons Thus acquiring the C signal after
a pre-determined time can give positive peaks, negative peaks, or
even no peaks at all, depending on how many Hs are attached. APT
has been superseded by DEPT Distortionless Enhancement by
Polarization Transfer Complex pulse sequence allowing selective
reception of signals from different C types: -C, -CH, -CH 2, -CH 3
DEPT spectra (Distortionless Enhancement by Polarization Transfer)
a modern 13 C NMR spectra that allows you to determine the number
of attached hydrogens.
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Measuring a 13C NMR spectrum involves In DEPT, a second
transmitter irradiates 1 H during the sequence, which affects the
appearance of the 13 C spectrum. some 13 C signals stay the same
some 13 C signals disappear some 13 C signals are inverted
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13 C NMR - DEPT Distortionless enhancement by polarization
transfer (DEPT) spectra permit identification of CH 3, CH 2, and CH
carbon atoms. DEPT 45 shows 1 o, 2 o,and 3 o carbons. DEPT 90 shows
only 3 o carbons. DEPT 135 shows 1 o and 3 o carbons as positive
peaks and 2 o carbons as negative peaks.
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Using DEPT to Count Hydrogens Attached to 13C
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Chemical shift ( , ppm) 020406080100120140160180200 Proton
Decoupled SpectrumOC C CH CHCH CH 2 CH 3 CCH 2 CH 2 CH 2 CH 3
O
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Chemical shift ( , ppm) 020406080100120140160180200 DEPT 135
SpectrumCH CHCH CH 2 CH 3 CCH 2 CH 2 CH 2 CH 3 O CH and CH 3
unaffected C and C=O nulled CH 2 inverted
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94 CH 3 CH 2 Broad-band decoupled DEPT 6 5 24 1 7 8 3 CH CH 2 s
give negative resonances CHs and CH 3 s give positive resonances
Quaternary carbon (no attached Hs) are not observed DEPT 135
Spectrum
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DEPT DEPT Distortionless Enhancement by Polarization Transfer
Allows us to observe the number of hydrogens attached to a
particular carbon. DEPT Pulse Sequence
methylmethylenemethinequaternary DEPT-45Positive peak Not observed
DEPT-90No obs. peak Positive peakNot observed DEPT-135Positive
peakNegative Peak Positive PeakNot observed
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13C Citronellol
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DEPT-135 CH 3 positive CH 2 negative CH positive C not observed
DEPT-135 Ipsenol
DEPT Spectra of 1-phenyl-1-butanone DEPT 135 DEPT 90 DEPT 45 CH
3, CH 2, CH (+), C (none) CH 3, CH 2 (none), CH (+), C (none) CH 3
(+), CH 2 (-), CH (+), C (none)
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DEPT Spectra of CH 3, CH 2 (none), CH (+), C (none) CH 3 (+),
CH 2 (-), CH (+), C (none)
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Summary of Edited 13 C NMR
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Summary Number of signals indicates the number of types of
carbon in the sample. (Is symmetry present?) Chemical shifts show
what types of carbons are in the sample. Quaternary/ipso carbons
will be smaller than carbons with protons attached. DEPT
differentiates between primary, secondary, and tertiary
carbons.
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How many peaks would you expect there to be in the carbon-13
spectrum of butaneCH 3 CH 2 CH 2 CH 3 2-methylpropaneCH 3 CH(CH 3
)CH 3 butanalCH 3 CH 2 CH 2 CHO butanoneCH 3 COCH 2 CH 3
pentan-2-oneCH 3 COCH 2 CH 2 CH 3 pentan-3-oneCH 3 CH 2 COCH 2 CH 3
cyclohexaneC 6 H 12
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How many peaks would you expect there to be in the carbon-13
spectrum of butaneCH 3 CH 2 CH 2 CH 3 2 2-methylpropaneCH 3 CH(CH 3
)CH 3 2 butanalCH 3 CH 2 CH 2 CHO4 butanoneCH 3 COCH 2 CH 3 4
pentan-2-oneCH 3 COCH 2 CH 2 CH 3 5 pentan-3-oneCH 3 CH 2 COCH 2 CH
3 3 cyclohexaneC 6 H 12 1 19
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Identify the isomers of C 4 H 8 O
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A butanal B butanone C 2-methylpropanal
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The 13 C nucleus is present in only 1.08% natural abundance.
Therefore, acquisition of a spectrum usually takes much longer than
in 1 H NMR. The magnetogyric ratio of the 13 C nucleus is about 1/4
that of the 1 H nucleus. Therefore, the resonance frequency in 13 C
NMR is much lower than in 1 H NMR. (75 MHz for 13 C as opposed to
300 MHz for 1 H in a 7.04 Tesla field). At these lower frequencies,
the excess population of nuclei in the lower spin state is reduced,
which, in turn, reduces the sensitivity of NMR detection. Unlike 1
H NMR, the area of a peak is not proportional to the number of
carbons giving rise to the signal. Therefore, integrations are
usually not done. Each unique carbon in a molecule gives rise to a
13 C NMR signal. Therefore, if there are fewer signals in the
spectrum than carbon atoms in the compound, the molecule must
possess symmetry. When running a spectrum, the protons are usually
decoupled from their respective carbons to give a singlet for each
carbon atom. This is called a proton-decoupled spectrum. C 13
NMR-Important points
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2D NMR: COSY AND HETCOR
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2D NMR Terminology 1D NMR = 1 frequency axis 2D NMR = 2
frequency axes COSY = Correlated Spectroscopy 1 H- 1 H COSY
provides connectivity information by allowing one to identify
spin-coupled protons. x,y-coordinates of cross peaks are
spin-coupled protons
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1H-1H COSY CH 3 CCH 2 CH 2 CH 2 CH 3 O 1H1H 1H1H
Slide 114
HETCOR 1 H and 13 C spectra plotted separately on two frequency
axes Coordinates of cross peak connect signal of carbon to protons
that are bonded to it.
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1H-13C HETCOR CH 3 CCH 2 CH 2 CH 2 CH 3 O 13 C 1H1H
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Solving Combined Spectra Problems: 116 Mass Spectra: Molecular
Formula Nitrogen Rule # of nitrogen atoms in the molecule M+1 peak
# of carbons Degrees of Unsaturation: # of rings and/or -bonds
Infrared Spectra: Functional Groups C=OO-H C=CN-H C CCO-OH C N 1 H
NMR: Chemical Shift ( ) chemical environment of the H's Integration
# of H's giving rise to the resonance Spin-Spin Coupling
(multiplicity) # of non-equivalent H's on the adjacent carbons
(vicinal coupling). 13 C NMR: # of resonances symmetry of carbon
framework Type of Carbonyl Each piece of evidence gives a fragment
(puzzle piece) of the structure. Piece the puzzle together to give
a proposed structure. The proposed structure should be consistent
with all the evidence.
121 Infrared (IR): Characteristic OH stretching absorption at
3300 to 3600 cm 1 Sharp absorption near 3600 cm -1 except if
H-bonded: then broad absorption 3300 to 3400 cm 1 range Strong CO
stretching absorption near 1050 cm 1 O-H C-O cm -1 % T
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122 = 1.5, q, 2H = 0.9, d, 3H = 3.65, t, 2H = 1.7, m, 1H =
2.25, br s, 1H CDCl 3 41.7 61.2 24.7 22.6 1 H NMR: protons attached
to the carbon bearing the hydroxyl group are deshielded by the
electron-withdrawing nature of the oxygen, 3.3 to 4.7 O-HC-O
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123 Usually no spin-spin coupling between the OH proton and
neighboring protons on carbon due to exchange reaction The chemical
shift of the -OH proton occurs over a large range (2.0 - 5.5 ppm).
It chemical shift is dependent upon the sample concentration and
temperature. This proton is often observed as a broad singlet (br
s). Exchangable protons are often not to be observed at all.
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124 13 C NMR: The oxygen of an alcohol will deshield the carbon
it is attached to. The chemical shift range is 50-80 ppm DMSO-d 6
(solvent) CH 3 CH 2 CH 2 CH 2 OH 62 35 19 14
Magnetic Resonance Imaging (MRI) 126 MRI uses the principles of
nuclear magnetic resonance to image tissue MRI normally uses the
magnetic resonance of protons on water and very sophisticated
computer methods to obtain images. Other nuclei within the tissue
can also be used ( 31 P) or a imaging (contrast) agent can be
administered
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127 Normal 25 years old Normal 86 years old Alzheimers Disease
78 years old fMRI: MRI images