NMR
Kinetics
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Department
of Chemistry
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(A PDF version of this page is available from the Handouts site) |
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Resources
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An
arrayed experiment allows the user to run multiple experiments
in a single session with no need for changing parameters each time
a value needs modification. Why would you want to run an arrayed
experiment? There are many circumstances where an arrayed experiment
is useful. Did you know that the DEPT macro
is an arrayed experiment? Or perhaps you wish to determine a 90° pulse
for a HMQC experiment or you would like to acquire spectra of a
compound at multiple temperatures or maybe you want to take a spectrum
of a reaction every 10 minutes and you don’t want to sit
around for 3 hours. I will use the last example to set up an arrayed
experiment for a kinetics experiment.
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Handouts | |||
CREATING AN ARRAYED EXPERIMENT FOR A KINETICS RUN
IMPORTANT TIPS FOR A KINETICS EXPERIMENT:
Internal Standard:
Instructions for Kinetics Experiments Performed in NMR Probe:
There are two possibilities for running a simple array: using a macro or
using command line execution.
USING THE ARRAY MACRO: This will allow you to array parameters
with set intervals between scans.
USING
COMMAND
LINE
EXECUTION
FOR
AN
ARRAY: This allows you to have differing values
between each data point or spectrum.
Important
Commands
for
Arrayed
Spectra:
ds(#) -
displays
spectrum
#,
where
#
is
the
spectrum
number
in
the
array.
The
first
spectrum
is
1,
the
second
is
2,
etc.
(e.g.
ds(1) to
display
first
spectrum).
dssa -
displays
all
spectra
vertically
dssh -
displays
all
spectra
horizontally
dssl -
displays
the
spectrum
number(#)
to
be
used
with
ds(#)
pl(‘all’) - print
all
stacked
spectra
Kinetics Data Analysis:
Instructions for Determination of T1:
The constant for relaxation, T1, is known as longitudinal or spin-lattice relaxation. This is the time for magnetization along the z-axis to relax. Many factors influence T1's including temperature, paramagnetic impurities, solvent viscosity and dielectric, molecular size and structure. Within a given molecule, the individual resonances will have different T1's depending on their molecular environment. As a rough rule of thumb, more protons for a resonance means shorter T1's. Thus, alkynes and aromatic protons will have longer T1's than alkyl groups. Carbon T1's are generally longer than a proton's.
If insufficient time is allowed for all protons to completely relax (5*T1) between pulses in an FT NMR experiment, one will obtain inaccurate integration data. Let's say, for example, that a molecule has two resonances possessing T1's of 1 and 10 seconds, respectively. We decide to pulse every 5 seconds (i.e. at=5 and d1=0). We will get all the signal from the first resonance, but clearly not from the second (it still has 5 more seconds of relaxation and signal to give us). The result is incorrect integration!
Data Processing:
The output you receive will look something like the following for 2-phenylbutanoic acid. The picture on the left shows a portion of the output after typing t1. The first table shows the determined T1 and its error. For example, peak 1, which is an aromatic peak, has a T1 of 2.754 seconds with an error of 0.01343 seconds. The methyl T1 is peaks 25-27 and has a T1 at 1.1(0.007) seconds.
Instructions for Variable Temperature (VT) Operation:
Variable temperature NMR is an excellent way to investigate dynamic behavior of molecules. Both qualitative and quantitative information about inversion, ring-flip, and other barriers can be extracted from a series of spectra at different temperatures.
VT operation is available on many of our spectrometers. We have set limits of 0 to 50 °C for spectrometers. These temperatures were selected to protect the probes from damage due to either a solvent freezing and cracking the NMR tube (D20 at 0 °C) or a solvent boiling and spilling in the probe (CDCl3 at 62 °C). Both could cause significant probe damage. This has occurred, not because of the user who did the VT experiment, but because of the next user who was unaware of the probe temperature.
IMPORTANT: BE SURE THAT YOU DO NOT GO ABOVE THE BOILING POINT OR BELOW THE FREEZING POINT OF YOUR SOLVENT. IF YOU DO NOT KNOW THESE VALUES, CLICK HERE TO FIND OUT. IF YOU REQUIRE TEMPERATURES OUTSIDE THESE RANGES, PLEASE CONTACT US (3537).
YOU MUST ALLOW AMPLE TIME FOR THE PROBE TO RETURN TO ROOM TEMPERATURE (UP TO 15 MINUTES) PRIOR TO THE NEXT USER'S TIME. FAILURE TO DO SO MAY LEAD TO PROBE DAMAGE.
Temperature Calibration:
The temperature that is displayed on the instrument console and on the VNMR screen display is not necessarily the actual temperature. Therefore, it is important to calibrate the temperature. This calibration can be done a week in advance on the experiment or a week after, but it is best done within a few days. If you will be using a series of temperature, you should obtain a calibration curve.
Step by Step Instructions for Mercuryplus: