Post on 30-Dec-2015
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Other Magnetic Nuclei than 1H2H (Deuterium): I = 1; simplifies proton spectrum as H-D coupling is smallX-CH2-CH2-CH2-COY X-CH2-CH2-CD2-COYtriplet, quintet, triplet triplet, slightly broad triplet
31P: I = 1/2 (100% natural abundance)large coupling constants P-H = 200-700 Hz
19F: I = 1/2 (100% natural abundance)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)
13C: I = 1/2 (1.1% Natural abundance)C-H coupling (about 100-200 Hz) is not seen unless enriched with 13C
Fluoroacetone, CH3COCH2F
19F,H coupling (I = ½)
2J 4J
13C-NMR Spectroscopy
12C not magnetically active but13C has I = ½; its natural abundance is 1.1%;
The sensitivity of 13C is only 1/5700 of 1H; this sensitivity problem is overcome with Fourier Transform (FT) NMR instrumentation (1970’s);
Usually, the peak splitting due to couplings with protons are removed by broadband decoupling in a double resonance experiment;broadband decoupling can also enhance the 13C signal intensity caused by the Nuclear Overhauser Effect (NOE)
13C chemical shifts are reported relative to TMS;
300 MHz for 1H-NMR = 75.5 MHz for 13C-NMR;
10 mg in 0.5 mL of solvent in 5 mm tube
Double Resonance: Spin-Spin Decoupling
CH2
CH2
OHH3C
triplet - sextet - triplet
irradiate
CH2
CH2
OHH3C
triplet - quartet
CH2
CH2
OHH3C
irradiateCH2
CH2
OHH3C
singlet - singlet
CH2
CH2
OHH3C
irradiateCH2
CH2
OHH3C
triplet - triplet
Protons can be readily decoupled if they are about 100 Hz apart
13C-NMR of diethylphthalate
proton coupled
13C{1H} NMR of diethylphthalate
proton decoupled
why low intensity?
13C{1H}-NMR of diethylphthalate
Proton decoupled10-s delay
Peak Intensities in 13C-NMR
• the relaxation times in 13C-NMR vary over a wide range so peak areas do not integrate for the correct number of nuclei;
• long delays would work but the time required is prohibitive;
• NOE response is not uniform for all C atom environments;
• C atoms without protons attached give low intensity;
• substitution of D for H results in decreased intensity;
• deuterium has I = 1 so 13C is split into 3 lines ratio 1:1:1 when coupled to one deuterium (possible spin states for D are -1, 0, +1)
• thus CDCl3 exhibits a 1:1:1 triplet in 13C-NMR
Chemical Shifts in 13C-NMR
Cl
Carbon chemical shifts parallel (generally) proton shifts but with a much broader range
1 2
4 4
3 6
4 6
Number of different aromatic 13C resonances in substituted benzene molecules
Cl Cl
Br Cl
Cl
BrCl Cl
Br Cl
Cl
Cl
Diamagnetic shielding (electrons in s- and p-orbitals) and paramagnetic shielding (electrons in p-orbitals with angular momentum) contributes to the shift of C-atoms.
13C shifts of functional groups
Well defined for acyclic, saturated hydrocarbons
= -2.5 + ∑Aini
Methane = -2.1replacement of H by C (CH3, CH2, CH, C) causes a +9.1 shift in the -position, +9.4 in the -position, and -2.5 in the -position.
Replacement of hydrogen causes a relative constant shift that depends primarily on the electronegativity of X.
Calculation of 13C shifts
Calculation of 13C shifts
Well defined for acyclic, saturated hydrocarbons
= -2.5 + ∑Aini
t-butyl alcohol
2,2,4-trimethyl-1,3-pentanediol