NMR Lecture 5. Factors affecting 13 C NMR spectra Proton decoupler always on, no pulse delay How do...
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Transcript of NMR Lecture 5. Factors affecting 13 C NMR spectra Proton decoupler always on, no pulse delay How do...
NMR Lecture 5
Factors affecting 13C NMR spectra
Proton decoupler always on, no pulse delay
How do you know 3 microsec = 22°
Pulse delay 400 s; gated decoupler-on only when acquiring signal
Effects of the pulse delay between pulses, 400 s
Proton decoupler always on (NOE)
Information obtained by examining nmr spectra
1. Chemical shift (identifies nature of nucleus2. Area (identifies relative number of nuclei)3. Multiplicity ( (NOE)
2D NMR basically provides information about connectivity and proximity
Applications of 1H and 13C NMR
Molecular formula C9H10O
IR: 1720 cm-1
area: 5: 2: 3
CH2 CH3
O
Molecular formula C7H14O2
IR 1720 cm-1
1H NMR
CCH3 CH2
CCH3
O OCH3
CH33:2:3:6
F NMR of F3
CF24
CF25H
=CF3-
F
FF
F
NH2
H
Returning to Dynamic Nuclear Magnetic Resonance
For a simple exchange process coalescence /21/2
Most chemical shift differences are of the order of a few hundred Hz or less. Rate constants are of the order of few hundred sec-1. Large in comparison to k 10-4 – 10-5 sec-1, measured by conventional kinetics but small relative to many dynamic processes occurring in molecular systems such as rotations about bonds.
Assign the spectrum and explain the coupling observed
Let’s do a thought experiment
What would the NMR spectrum of the methyl resonances of 4-dimethyl-3-hydroxycyclobutenone look like if all the methine hydrogens of this molecule had the same spin?
HO
OCH3
H
CH3
HO
OCH3
H
CH3
J13, J15 are the same sign
J13 J15 are of opposite sign
HO
OCH3
H
CH3
J13, J15 are the same sign
J13, J15 are of opposite sign
What are some mechanisms by which the two methyl groups can become identical?
HO
OCH3
H
CH3
O
H
CH3
CH3
H
O
O
CH3
H
CH3
OH
H2O
HO
OCH3
H
CH3
O
CH3
H
CH3
OH
O
O
H
CH3
H
CH3
HO
OCH3
H
CH3
HO CH3
OHCH3
O
CH3
H
CH3
OH
H2O
HO
OCH3
H
CH3
O
CH3
H
CH3
OH
HO CH3
OHCH3
HO
OCH3
H
CH3
OH
O CH3
H
H3C
1,3-hydrogen shift
HO
OCH3
H
CH3
OH
O CH3
H
H3C
1,3-hydrogen shift
HO
OCH3
H
CH3
J is the same sign
J is of opposite sign
OH
O CH3
H
H3C
HO
OCH3
H
CH3
HO CH3
OHCH3
O
CH3
H
CH3
OH
HO
OCH3
H
CH3
O
CH3
H
CH3
OHO
O
H
CH3
H
CH3
1,3-hydrogen shift; JAX
and JAM of the same sign O
CH3
H
CH3
OH
HO
OCH3
H
CH3
HO
OCH3
H
CH3
1,3-hydrogen shift; JAX and JAM of the opposite signs
O
CH3
H
CH3
OH
Problem Set DNMR
By line shape analysis, determine the activation energy for rotation about a carbon nitrogen bond in an amide.
Go to my gateway and down load the Excel file.
A plot of ln k vs 1/T should result in a straight line and the slope of the line should be - Ea/R. Your problem will be to fit the lineshapes as best you can and evaluate k for a series of temperatures. Remember that the most accurate k values will be when the line shapes change the most.
k = rate constant
