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AFB QO I 2007/08 1
Química Orgânica I
Ciências Farmacêuticas
Bioquímica
Química
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Nuclear Magnetic Resonance Spectroscopy (II)
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� Adaptado de Organic Chemistry, 6th Edition; L.G. Wade, Jr.
� Leitura complementar recomendada:
� Espectroscopia de RessonânciaMagnética Nuclear; A.M.d’A.R. Gonsalves, T.M.V.D. Pinho e Melo, 2007
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(rev)
� Desvio químico (ambiente electrónico)
� Nº de sinais (grupos de protões equivalentes)
� Intensidade (nº de protões equivalentes)
� Multiplicidade de sinal (protões adjacentes)
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Chemical shift
2.70 ppm2.70 ppmδδδδ value:
270 Hz162 HzShift From TMS:
100 MHz60 MHzOperating Frequency:
B0 = 2.35 TeslaB0 = 1.41 TeslaStrength of Field:
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Me-H; Me-F
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4.263.052.682.160.23Chemical Shift δδδδ
4.03.12.82.52.1Electronegativity of X
FClBrIHElement X
CH3FCH3ClCH3BrCH3ICH3HCompound CH3X
Electronegativity effects on proton chemical shifts
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δ
=>
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Number of Signals
� Equivalent hydrogens have the same chemical shift.
=>
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Intensity of Signals
� The area under each peak is proportional to the number of protons.
� Shown by integral trace.
=>
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1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
=>
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Doublet: 1 Adjacent Proton
=>
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Triplet: 2 Adjacent Protons
=>
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Explain observed signals
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distance between split peaks = 1.26 ppm - 1.14 ppm = 0.12 ppm J = 0.12 ppm * (60 Hz / 1 ppm) = 7.2 Hz
J
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Values for Coupling Constants
=>
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The N + 1 Rule
If a signal is split by N equivalent protons,
it is split into N + 1 peaks.
=>
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Range of Magnetic Coupling
� Equivalent protons do not split each other.
� Protons bonded to the same carbon will split each other only if they are not equivalent.
� Protons on adjacent carbons normally will couple.
� Protons separated by four or more bonds will not couple.
=>
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Splitting for Ethyl Groups
=>
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Splitting for Isopropyl Groups
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Coupling Constants (rev.)
� 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.
=>
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Complex Splitting
� Signals may be split by adjacent protons, different from each other, with different coupling constants.
� Example: Ha of styrene which is split by an adjacent H trans to it (J = 17 Hz) and an adjacent H cis to it (J = 11 Hz).
=>
C C
H
H
Ha
b
c
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Splitting Tree C C
H
H
Ha
b
c
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Spectrum for Styrene
=>
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Stereochemical Nonequivalence
� Usually, two protons on the same C are equivalent and do not split each other.
� If the replacement of each of the protons of a -CH2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other.
=>
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Some Nonequivalent Protons
C C
H
H
Ha
b
c
OH
H
H
H
a
b
c
d
CH3
H Cl
H H
Cl
a b
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(S)-1,2-dichloropropane
H Cl
H H
Cl
3.96
1.59
3.74 3.49
=>
01234PPM
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2 conformers
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Time Dependence
� Molecules are tumbling relative to the magnetic field, so NMR is an averaged spectrum of all the orientations.
� Axial and equatorial protons on cyclohexaneinterconvert so rapidly that they give a single signal.
� Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule.
=>
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Hydroxyl Proton
� Ultrapure samples of ethanol show splitting.
� Ethanol with a small amount of acidic or basic impurities will not show splitting.
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N-H Proton
� Moderate rate of exchange.
� Peak may be broad.
=>
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Identifying the O-H or N-H Peak
� Chemical shift will depend on concentration and solvent.
� To verify that a particular peak is due to O-H or N-H, shake the sample with D2O.
� Deuterium will exchange with the O-H or N-H protons.
� On a second NMR spectrum the peak will be absent, or much less intense.
=>
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Modern NMR
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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 FT-NMR spectra are taken, averaged.=>
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Fourier Transform NMR
� Radio-frequency pulse given.
� Nuclei absorb energy and precess (spin) like little tops.
� A complex signal is produced, then decays as the nuclei lose energy.
� Free induction decay is converted to spectrum. =>
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Hydrogen and Carbon Chemical Shifts
=>
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Combined 13C and 1H Spectra
=>
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Differences in 13C Technique
� Resonance frequency is ~ one-fourth, 15.1 MHz instead of 60 MHz.
� Peak areas are not proportional to number of carbons.
� Carbon atoms with more hydrogens absorb more strongly.
=>
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Spin-Spin Splitting
� It is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible.
� 13C will magnetically couple with attached protons and adjacent protons.
� These complex splitting patterns are difficult to interpret.
=>
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O
O
7.14
7.07
7.14
7.06
7.06 3.51
4.12
1.30
O
O
129.2
127.6
129.2
129.8
134.8
129.8 46.0
171.361.3
14.1
calculated
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Proton Spin Decoupling
� To simplify the spectrum, protons are continuously irradiated with “noise,”so they are rapidly flipping.
� The carbon nuclei see an average of all the possible proton spin states.
� Thus, each different kind of carbon gives a single, unsplit peak.
=>
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Two 13C NMR Spectra
=>
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Proton-decoupled 13C spectra for ethyl phenylacetate
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Off-Resonance Decoupling
� 13C nuclei are split only by the protons attached directly to them.
� The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks.
=>
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Off-resonance proton-coupled 13C spectra for ethyl phenylacetate
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DEPT
� Distortionless Enhancement by Polarization Transfer
� 1H magnetization is generated first, then transferred to 13C.
� This "polarization transfer" enhances sensitivity.
� Also, the experiment repetition rate is dependent on relaxation of 1H, rather than 13C, so a shorter delay is needed.
http://www.acornnmr.com/codeine/dept.htm
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DEPT - codeine
DEPT-135 CH and CH3 peaks up, CH2 peaks invertedDEPT-90 CH peaks onlyDEPT-45 all protonated carbonsnormal 13C spectrum
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solvents
4.82 (singlet)water - d2
5.32 (singlet)methylene chloride - d2
3.31 (singlet), 4.78 (singlet)methanol - d4
1.11, 3.56, 5.19ethanol - d6
2.49 (singlet)dimethylsulfoxide - d6
7.25 (singlet)chloroform - d
nonecarbon tetrachloride
7.15 (singlet)benzene - d6
1.93 (singlet)acetonitrile - d3
2.09 (singlet)acetone - d6
2.0 (singlet), 11.7(singlet)acetic acid - d4
δδδδ (ppm)Solvent
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Solvents for 13C-NMR
49.0methanol
66.5dioxane
39.5dimethylsulfoxide
77.0chloroform
128.0benzene
206.0, 24.8acetone
δδδδ (ppm)Solvent
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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.
=>
AFB QO I 2007/08 56
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� http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr1.htm#nmr1
� http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html
� http://orgchem.colorado.edu/hndbksupport/nmrtheory/main.html
� http://www.chemie.uni-erlangen.de/oc/research/NMR/music.html
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