2 ch2 ch 2 - university of windsormutuslab.cs.uwindsor.ca/eichhorn/59-330 lecture notes...
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Magnetic Nuclei other than 1H2H (Deuterium): I = 1H,D-Exchange might be used to simplify 1H-NMR spectra since H-D couplings are generally small;
-CH2 - CH2 - CH2 - CO - -CH2 - CH2 - CD2 - CO -triplet of triplets slightly broadened triplet
31P: I = 1/2 (100% natural abundance)Chemical shift range between 270 to -480 ppm;Large P-H (1J) coupling constants between 200-700 Hz;
19F: I = 1/2 (100% natural abundance)Chemical shift range between 276 to -280 ppm;coupling constants F-CH = 50-100 Hz;
29Si: I = 1/2 (100% Natural abundance)Si-CH coupling constant is about 6 Hz; only low intensity (satellites)(Si-H is about 215 Hz)13C: I = 1/2 (1.1% Natural abundance)C-H coupling (about 100-200 Hz) is not resolved unless the molecule is enriched with 13C
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Fluoroacetone, CH3COCH2F19F,H coupling (I = ½)
2J 4J
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13C-NMR Spectroscopy
Some Facts:• 12C is not magnetically active but 13C is (I = ½); its natural
abundance is 1.1%;• The low abundance of 13C causes a sensitivity problem (only
1/5700 of 1H), which has been overcome with the development of Fourier Transform (FT) NMR instrumentation in the 1970’s; However, higher concentrations are usually used for solution NMR (10 mg in 0.5 mL of solvent for a 5 mm tube);
• 13C chemical shifts are reported relative to TMS;• 300 MHz for 1H-NMR equals 75.5 MHz for 13C-NMR;• Peak splittings due to couplings with protons are usually
removed by broadband decoupling in a double resonanceexperiment;
• Broadband decoupling can also enhance the 13C signal intensity caused by the Nuclear Overhauser Effect (NOE);
• The range of chemical shift values is much wider than for 1H (typically between 0-200 ppm); Therefore, chemical shift is of high analytical value in 13C-NMR;
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Peak Intensities in 13C-NMR
• Relaxation times in 13C-NMR vary over a wide range so that peak areas do not integrate for the correct number of nuclei; Long delays between each acquisition would resolve this problem but the required measurement time is prohibitive;
• NOE response is not uniform for all C atom environments; Carbon atoms without protons attached to them have low intensities because of the missing NOE and for other reasons;
• Substitution of H by D results in decreased intensity of the 13C signal;
• Deuterium has I = 1 so that a 13C signal is split into 3 lines ratio 1:1:1 when coupled to one deuterium (possible spin states of D are -1, 0, +1; CDCl3 exhibits a 1:1:1 triplet in 13C-NMR!)
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Double Resonance: Spin-Spin Decoupling
CH2
CH2
OHH3C
triplet - sextet - triplet
irradiate
CH2
CH2
OHH3C
triplet - quartet
CH2
CH2
OHH3Cirradiate
CH2
CH2
OHH3C
singlet - singlet
CH2
CH2
OHH3Cirradiate
CH2
CH2
OHH3C
triplet - triplet
Protons can be readily decoupled individually if their deferens in resonance frequency (chemical shift) is ≥100 Hz;
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13C-NMR of diethylphthalate
proton coupled
why triplet?
q of t(q not resolved)
t of q(t not resolved)
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13C{1H} NMR of diethylphthalate
proton decoupled
why low intensity?
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13C{1H}-NMR of diethylphthalate
proton decoupledwith 10 seconds delay
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Chemical Shifts in 13C-NMR
Cl
Carbon chemical shifts parallel (generally) proton shifts but with a much broader range
64
63
44
21
Number of different aromatic 13C resonances in substituted benzene molecules
Cl Cl
Br Cl
Cl
BrCl Cl
Br Cl
Cl
Cl
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Diamagnetic shielding (electrons in s- and p-orbitals) and paramagnetic shielding (electrons in p-orbitals with angular momentum) contribute to the shift of C-atoms.
Group Specific Chemical Shifts in 13C-NMR
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Well defined for acyclic, saturated hydrocarbons
δ = -2.5 + ∑Aini
δMethane = -2.5 ppmreplacement of H by C (CH3, CH2, CH, C) causes a shift of +9.1 ppm in the α-position, +9.4 ppm in the β-position, and -2.5 ppm in the γ-position;
Replacement of hydrogen causes a relative constant shift that depends primarily on the electronegativity of X.
Calculation of 13C shifts
corrections for branching
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XAll tables from “Silverstein & Webster”
X
αβ
γ
αβ
γ
Functional group X attached to internal carbon
Functional group X attached to terminal carbon
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t-butyl alcohol
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2,2,4-trimethyl-1,3-pentanediol
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CH3
CH3H3C
The equation for alkanes is based on a weighted average for open chain conformers. A complete new set of parameters is necessary for cyclic alkanes as the average conformation is different.
Cyclopropane = -2.6, cyclobutane = 23.3, and cyclohexane = 27.7;all other rings have 27.7 ± 2 ppm
β-ax = 5.2
β-eq = 8.9
β-gem = -1.2; extra correction factor for branching at β-C (2 methyl groups);
27.7 + 5.2 + (2 x 8.9) + -1.2 = 49.5 (obs. 49.9)
Cycloalkanes
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AlkenesChemical shift ranges of alkenecarbon atoms strongly depend on their degree of substitutionC=CH2 = 104-115 ppm; C=CHR = 120-140 ppm;C=CR2 = 140-165 ppm;
α,β,γ represent substituents on the same end of the double bond while α , β’, γ’ are on the far side.
Example:
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Starting with benzene = 128.7
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Comprehensive Table of aromatic 13C-NMR chemical shift increments
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OH
Cl Cl
δ = 16 + 31 – 4 = 43 (obs. 42)
δ = 27 + 8 = 35 (obs. 34)
O
128.4149.8
Starting with ethene = 123.3
NO2
Starting with benzene = 128.7 or 128.5 (depending on table)
148.3123.4
129.5
134.7
Example Calculations (Predictions)
δ1 = -2.5+9.1+9.4+(-2.5x2)+0.3 = 11.3[1α,1β,2γ,1δ]δ2 = -2.5+(9.1x2)+(9.4x2)+(-2.5)+(-2.5) = 29.5[2α,2β,1γ,2º(3º)]δ3 = -2.5+(9.1x3)+(9.4x2)+(-3.7x2) = 36.2[3α,2β,3º(2º)]
1 2 3
4
-M effect
O153.2
84.2+M effect
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CH, CH3
CH2
CH
Distortionless Enhancement by Polarization Transfer (DEPT)
α-TerpineneConventional 13C-NMR