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SPECTROSCOPIC METHODS
OF ORGANIC MOLECULES
Professor Ioannis P. GerothanassisDepartment of Chemistry, University of Ioannina
FEBRUARY 2018
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SPECTROSCOPIC METHODS
OF ORGANIC COMPOUNDS
Professor Ioannis P. GerothanassisDepartment of Chemistry, University of Ioannina
Concepts and Applications of NMR
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What is NMR ?
• A spectroscopic technique that gives information aboutthe number and types of atoms in a molecule.
• Nuclear magnetic resonance spectroscopy is a powerfulanalytical technique used to characterize molecules byidentifying e.g. hydrogen-carbon frameworks withinmolecules.
MagneticNuclear Resonance
In the Nucleus Involves
MagnetsAt the Nucleus
3
http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Nuclear Spin• A nucleus with an odd atomic number or an odd
mass number has a nuclear spin.
MagneticNuclear
• The spinning charged nucleus generates a magnetic field e.g. acts like a bar magnets.
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Precessional Motion:Consider a spinning top, in which the spinning axis of the top moves slowly around the vertical. This is precessional motion and the top is said to be precessing around the vertical axis of earth’s gravitational field.
Precessional Frequency:Likewise the proton (tiny magnet) precesses about the axis of the external magnetic field in the same manner as above.
http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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The spinning proton in an external magnetic field
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The nuclear magnetic resonance process; absorption occurs
when ν =ω
ΒΟ
http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Two Energy States in Quantum Chemical
Terms
The magnetic fields of the spinning nuclei will align either with the external field, or against the field.
An r.f. with the right amount of energy can be absorbed and cause the spinning proton to flip.
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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E, Magnet Strength and
The energy difference ΔΕ is proportional to the magnetic field strength B0 and .
E = h = h B0
2
• Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1gauss-1 for 1H).
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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E
Bo
500 MHz
11.74 T2.35 T
100 MHz
60 MHz
1.41 T
E = h/2(Bo)
Rf Field vs. Magnetic Field for a Proton
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Rf / values for Some Nuclei
/2 (MHz/T)Bo (T)Rf (MHz)Nuclei
500.00 11.74 42.58
125.74 11.74 10.71
76.78 11.74 6.54
470.54 11.74 40.08
202.51 11.74 17.25
10
1H
13C
2H
19F
31P
http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Where Nuclei Resonate at 11.74T
470.54
~220 ppm
19F (CFCl3)
12 ppm1H (TMS)
500.00
76.78
12 ppm2H
220 ppm
13C
125.74
500 MHz400300200100
RCO2H
CH4
-SO2F -CF2-CF2-
202.51
~380 ppm
31P (H3PO4)
CPH2PX3
��RCH3R2C=O
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http://textlab.io/doc/9993536/the-basics-of-nmr--powerpoint-
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Magnetic Shielding
• If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained.
• But protons are surrounded by electrons that shield them from the external field.
• Circulating electrons create an induced magnetic field that opposes the external magnetic field.
Beff=Bexternal-Bshielding
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• Such shifting in position of NMR absorption signals whicharise due to the shielding or deshielding of proton bysurrounding electrons is called the Chemical shift.
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Shielding or Deshielding Protons in a Molecule
• Depending on electronic environment protons inmolecules are shielded or deshielded by differentamounts.
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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• Shielded proton shows absorption of signals to right side anddeshielded protons at left side of spectrum.
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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NMR Signals
• The number of signals shows how many different kinds of protons are present.
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• The location of the signals shows how shielded or deshielded the proton is.
http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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The NMR Graph
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Integral: 1 3
http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_13/Wade13.ppt
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NMR Signals
• The number of signals shows how many different kinds of protons are present.
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• The location of the signals shows how shielded or deshielded the proton is.
• The intensity of the signal shows the number of protons of that type.
http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_13/Wade13.ppt
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IR spectrum of methanol
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• The value of δ for a substance relative to TMS can be obtainedby Chemical shift, ppm.
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δ =
• Any proton or set of protons which absorb at higher frequencythan TMS is given a positive value for (δ).
• The value of chemical shift, δ expressed in ppm and their valueis between 0 to 10 in the δ scale.
Shift high frequency from TMS (in Hz)
Spectrometer frequency (in MHz)
http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Delta Scale
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Hydrogen Chemical Shifts
ppm
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http://rd.nctu.edu.tw/web.case/nctu-rd-2/upload/ckeditor/20150410151405.pdf
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Classification of the Chemical Shift of the -OH Functional Groups2424
Siskos et al., Org. Biomol. Chem, 2013, 11, 7400-74011
Kontogianni et al., Org. Biomol. Chem, 2013, 11, 2013-2025
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O
O
2
3
4
5
H
O O H
5 4
A C
17.5-20 ppm
12-13 ppm
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14.75 ppm 14.17 ppm
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Effect of Strength of Hydrogen Bond on 1H Chemical Shift26
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http://www.authorstream.com/Presentation/knowledgexplorers-2245188-anwar-nmr/
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http://www.authorstream.com/Presentation/knowledgexplorers-2245188-anwar-nmr/
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most deshielded
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http://www.authorstream.com/Presentation/knowledgexplorers-2245188-anwar-nmr/
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Hydrogen Chemical Shifts
ppm
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Aromatic Protons, 7-8 ppm
The aromatic protonsare deshielded!
http://rd.nctu.edu.tw/web.case/nctu-rd-2/upload/ckeditor/20150410151405.pdf
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Aldehyde Proton, 9-10 ppm
http://rd.nctu.edu.tw/web.case/nctu-rd-2/upload/ckeditor/20150410151405.pdf
Electronegative
oxygen atom
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Vinyl Protons, 5-6 ppm
http://rd.nctu.edu.tw/web.case/nctu-rd-2/upload/ckeditor/20150410151405.pdf
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Acetylenic Protons, 2.5 ppm
http://rd.nctu.edu.tw/web.case/nctu-rd-2/upload/ckeditor/20150410151405.pdf
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NMR Signals
• The number of signals shows how many different kinds of protons are present.
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• The location of the signals shows how shielded or deshielded the proton is.
• The intensity of the signal shows the number of protons of that type.
• Signal splitting shows the number of protons on adjacent atoms.
http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_13/Wade13.ppt
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1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
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Spin-Spin Splitting
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Doublet: 1 Adjacent Proton
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Triplet: 2 Adjacent Protons
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Spin-Spin Coupling Constants
• Distance between the peaks of multiplet.
• Measured in Hz.
• Not dependent on strength of the external field.
• Multiplets with the same coupling constants may come from adjacent groups of protons that split each other.
• Protons separated by four or more bonds will not couple.
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?
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Values for
Coupling Constants
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Splitting Tree
C C
H
H
Ha
b
c
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https://translate.google.gr/translate?hl=el&sl=en&u=http://bionmr.unl.edu/courses/chem991a_-_intro_to_nmr/lectures/chapter-3-NMR-coupling.ppt&prev=search
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Hydrogen
Chemical Shifts
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and Carbon
http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Carbon-13
• 12C has no magnetic spin.
• 13C has a magnetic spin, but is only 1% of the carbon in a sample.
• The gyromagnetic ratio of 13C is one-fourth of that of 1H.
• Signals are weak, getting lost in noise.
• Hundreds of spectra are taken, averaged.
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http://wps.prenhall.com/wps/media/objects/340/348272/Instructor_Resources/Chapter_13/Wade13.ppt
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The NMR Spectrometer
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continuous change in frequency
http://rd.nctu.edu.tw/web.case/nctu-rd-2/upload/ckeditor/20150410151405.pdf
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Periodic variation of experimental nuclear
shielding ranges
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Fourier Transform NMR
• Nuclei in a magnetic field are given a radio-frequency pulse close to their resonance frequency
• The nuclei absorb energy and process (spin) like little tops
• A complex time domain signal is produced, then decays as the nuclei lose energy
• Free induction decay is converted to spectrum
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http://www.chemistry-solutions.com/wp-content/uploads/2015/07/CHE-242-Unit-5-Chapter-13.ppt
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excitation pulse (μs)
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http://rutchem.rutgers.edu/~nmurali/nmr_course/Chem_542_Spring2010_Lecture_3.pdf
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1H NMR spectra of caffeine
8 scans ~12 secs
13C NMR spectra of caffeine
8 scans ~12 secs
13C NMR spectra of caffeine
10,000 scans ~4.2 hours
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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Spin population NMR - Sensitivity
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Nuclear Spins in Bo
• In an applied field strength of 7.05T the difference
in energy between nuclear spin states for
– 1H is approximately 0.120 J (0.0286 cal)/mol, which
corresponds to a frequency of 300 MHz (300,000,000
Hz).
– The population difference of the two nuclear spin
states is only ∼10-5 and, therefore, has meaning only
at the Avogadro number!
http://www.authorstream.com/Presentation/kesarwaniarti-1755809-nmr-spectroscopy/
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- intrinsic property of nucleus can not be changed
C)3 for 13C is 64x N)3 for 15N is 1000x
1H is ~ 64x as sensitive as 13C and 1000x as sensitive as 15N !
NMR Sensitivity
• Relative sensitivity of 1H, 13C, 15N and other nuclei NMR spectra depend on
Gyromagnetic ratio ()
Natural abundance of the isotope
Consider that the natural abundance of 13C is 1.1% and 15N is 0.37%relative sensitivity increases to ~6,400x and ~2.7x105x !!
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http://www.authorstream.com/Presentation/Darshantelange-1542377-nmr-spectroscopy-ppt/
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http://www.authorstream.com/Presentation/solairajan87-1493928-nmr-spectroscopy/
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N.O.E. Transient NOE and Distances
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Prof. Claudio Santi Dipartment of Chemistry and Technology of Drugs University of Perugia
N.O.E. Example “steady-state”
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Ha Hb
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NMR and Chemical Exchange
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What information can NMR spectroscopy provide
Structural Theory NMR equivalent
Qualitative composition Choice of nucleus / isotope
Quantitative composition Signal intensities / integration
Functional groups Chemical shifts
Connectivity J (scalar) couplings
Spatial relationship of atoms NOE (distance)
J couplings (dihedral angle)
Dynamics Chemical exchange, lineshape
broadening
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Books in NMR - Web Sites
Solving Problems with NMR Spectroscopy by Atta-ur-Rahman and Muhammad Iqbal Choudhary
http://www.elsevier.com/books/modern-nmr-techniques-for-chemistry-research/derome/978-0-08-
032513-2
Modern NMR Techniques for Chemistry Research by A.E. Derome, University of Oxford, UK
http://www.elsevier.com/books/modern-nmr-techniques-for-chemistry-research/derome/978-0-08-
032513-2
Modern NMR spectroscopy, a guide for chemists, by J. K. M. Sanders and B. K. Hunter
http://onlinelibrary.wiley.com/doi/10.1002/mrc.1260251217/abstract
Nuclear Magnetic Resonance : Volume 3 by R K Harris
http://pubs.rsc.org/en/Content/eBook/978-0-85186-272-9#!divbookcontent
NMR Spectroscopy: Basic Principles, Concepts and Applications in Chemistry, by H. Gunther
http://www.wiley-vch.de/publish/en/books/ISBN978-3-527-33000-3/
Carbon 13 NMR Spectroscopy, by Hans-Otto Kalinowski, Stefan Berger and Siegmar Braun
http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0471913065.html
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Some NMR Web Sites
The Basics of NMR by J.P. Hornak Hypertext based NMR course
http://www.cis.rit.edu/htbooks/nmr/nmr-main.htm
Integrated Spectral Data Base System for Organic Compounds
http://www.aist.go.jp/RIODB/SDBS/menu-e.html
Educational NMR Software All kinds of NMR software
http://www.york.ac.uk/depts/chem/services/nmr/edusoft.html
NMR Knowledge Base A lot of useful NMR links
http://www.spectroscopynow.com/
NMR Information Server News, Links, Conferences, Jobs
http://www.spincore.com/nmrinfo/
Technical Tidbits Useful source for the art of shimming
http://www.acornnmr.com/nmr_topics.htm
BMRB (BioMagResBank) Database of NMR resonance assignments
http://www.bmrb.wisc.edu/
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STRUCTURE EUCIDATION USING NMR
SPECTROSCOPY – AN EXAMPLE
THE CASE OF THE FLAVONOID QUERCETIN
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2.2 Hz 2.02 Hz 2.02 Hz8.5Hz, 2.2 Hz70
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TWO DIMENSIONAL SPECTRATHE 1H-1H COSY EXAMPLE
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Jacobsen, NMR Spectroscopy Explained, Wiley, 2007
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Homonuclear 2 Dimensional Experiment: 1H – 1H COSY (COrrelated SpectroscopY)
1H
1H
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Homonuclear 2 Dimensional Experiment: 1H – 1H TOCSY (TOtal Correlated SpectroscopY)
1H
1H
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Heteronuclear 2 Dimensional Experiment: 1H – 13C HSQC (Heteronuclear Single Quantum Spectroscopy)
13C
1H
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Heteronuclear 2 Dimensional Experiment: 1H – 13C HMBC (Heteronuclear Multiple Bond Correlation)
13C
1H
C-OH
C=O
A
B
C
H6H8H5’
C-OH
6
8
2’
1
4
5’
6’
H6’H7’
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