dynamic effects in nmr - university of missouri-st. louischickosj/c365/lecturenmr4.pdf4. off...
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The timescale in nmr
is fairly long; processes occurring at frequencies of the order of chemical shift differences will tend
to average out.
For a simple exchange process coalescence /21/2
This suggests that if proton spins can be made to change of the order of 20 to 40 Hz, coupling could be averaged out and its effects eliminated (recall the decoupling observed in the alcohol OH)
Effects of a Resonance frequency on a Nuclear Spin State
2
Irradiate 2
but observe 1
11
Processes occurring during double resonance1. Spins change2. Ratio of populations of ground and excited states 13. System reacts by redistributing populations of other
spin states
decoupling
Nuclear Overhauser
Effect
We will return to other aspects of dynamic NMR later but first lets apply double resonance to 13C spectra.
13
C NMR Spectra
Unlike 1H nuclei, 13C are rare nuclei. The probability of finding a 13
C nucleu is approximately 1/100. The probability of finding 2 13
C next to each other is 2*.01*.01 = 2*10-4
In a molecule like n-butyl vinyl ether, the probability of finding a 13
C nucleus at any of the carbon positions is equal. The problem is that 1H will couple with 13C rendering a weak signal even weaker.
Summary: Irradiation of the all the protons using a second broadband series of frequencies simultaneously while acquiring 13C spectrum as well causes?
Double resonance:
1.
Multiplicity is lost and some structural information is lost (JCH
)
2.
When the protons are irradiated, the Boltzman
distribution of spin states is perturbed, resulting in more H in the excited state than usual; if we apply Le Chatelier’s
principle, the system
responds to minimize the perturbation; if a 13C is next to one of the protons being irradiated, this perturbation results in more 13C nuclei returning to their ground state. This is a T1
process, meaning it will take a few seconds or longer (5 T1
)to achieve this new equilibrium state. Once equilibrium is achieved, this leads to an enhancement of the 13C signal and is called the Nuclear Overhauser
effect
Gated Decoupling: using the decoupler
to effect characteristic changes
in the spectrum by turning the decoupler
frequency on and off at specific intervals
1.
Broadband decoupling at protons; observe 13C
Effect: decoupling, NOE effect;
Gated Decoupling: using the decoupler
to effect characteristic changesin the spectrum2.
Gated decoupling to collape
coupling without any NOE
NOE builds up with a time constant associated with 13C T1 values. If the rf
frequency that irradiates the protons is left on, NOE is observed
in a minute or so.
Why would you want gated decoupling without NOE?Interested in area under the curves (quantitative analysis)
4. Off resonance decoupling: some coupling is retained so that the multiplicity is retained providing information regarding neighbors; the NOE effect is partially retained; information regarding the magnitude of the JCH
coupling is lost. The closer a nucleus is to the irradiating field, the more the coupling constant is reduced.
The use of ACD to predict 13C NMR spectra
1.
Estimation of : CH3
CH2
CH2
CH2
OCH=CH2
2. Estimation of : CHO
OC2H5
Coupling constants in 13C NMR
3. The relationship between hybridization and coupling constant
4. 1JCH
CHCl3
: 209 Hz; CH2
CH2
: 178; CH3
Cl 150; CH2
=CH2
156 Hz
cyclopropane
Measurement of T1
’s
In a pulse experiment, if the rf
field is left on long enough, the magnetization can be tipped 90°. What happens if the strong rf
field
is left on longer?
Suppose we
•
Measure the signal immediately after the 180 °
pulse by using a second weak pulse to tip the nuclei and generate a signal in the xy
plane. Wait 5 T1
•
Repeat the experiment, now waiting
seconds after the 180 ° pulse.
•
Vary
= 0 after 180 °
pulse
and weak second pulse
= 5T1
repeat but wait
sec
before second pulse
wait 5 T1
repeat varying
population of ground and excited states are equal
The decrease in intensity and then buildup again is a first order rate process. The change in ln(magnetization) plotted against time results in a straight line. The slope of the line is the rate constant and 1/slope = T1
Any other uses ?
Solvent suppression: T1
’s for small molecules such as solvents are usually longer than for other nuclei for both 13C and 1H
rf
generator
signal coil
signal coil, rf
generator
NS
2. apply 2nd
180°
pulse
red: faster rotating
blue: slower rotating
rf
generator
signal coil
signal coil, rf
generator
NS
1. apply 90 Hrf
pulse
2. apply 2nd
180°
pulse
blue: slower rotating
red: faster rotating
rf
generator
signal coil
signal coil, rf
generator
NS
1. apply 90 Hrf
pulse
2. apply 2nd
180°
pulse
blue: faster rotating
red: slower rotating
Measurement of T2 Spin Echo Technique
Suppose we give a 90 rf
pulse
to a set of identical uncoupled
nuclei. Magnetization is developed
in the xy
plane. After a period τ
a 180 °
pulse is given. An echo is
observed at 2 τ
Suppose that we repeat this experiment varying the length of of time
between the original pulse and the second 180 °
pulse.
The intensity of the spin echo will decrease as a result of magnetic inhomogeneity
and this decrease will follow first
order kinetics. The reciprocal of the rate constant is equal to T2
Now consider a 13
CH fragment. The 13
C will signal will be a doublet due to the fact that half of the H’s will be and the others will be . Suppose our rotating frame of reference is at the chemical shift of the 13
C. Some of the magnetization of the 13
C signal will be moving J/2 faster than our rotating frame and half will be moving J/2 slower.
Chemical shift of 13
C
= 0
= Ta
=6Ta
180°
pulse
= 0
= 2Ta
A spin echo 180 °out of phase will be observes at Ta
later
Following an initial 90 °
pulse
180 °
pulse
The phase of the spin echo of a 13
CH can be both positive and negative.
The spin echo of a 13
C is always has the same phase (quaternary carbon)
Lets now consider a 13
CH2 and use for our rotating frame the chemical shift of the
13
C
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