NMR Basics / FAQ.
All the really basic NMR questions you were too embarrassed to ask!
(Although the questions below are of a general nature, the instruments
mentioned are the Agilent spectrometers currently used in the NMR Centre
of the Australian National University).
Please email your ideas for other questions to be included in this list
to Chris Blake.
- Rolling baseline? Strange lineshapes? Go to Michigan State University's
NMR Artifacts pages.
- There is a comprehensive
Basics of NMR document
available at the Center for Imaging Science, Rochester Institute of
- Michigan State University has a good introduction to NMR.
- University of Saskatchewan has some good pages on the
theory and practice
- Lecture notes on NMR
theory (PDF format) by James Keeler.
- The NMR Wiki
has lots of information.
- If you haven't used any NMR spectrometers at the NMR Centre before.
Come down to room G48 or G42 in the Chemistry Faculties building, and
ask Chris or Peta to arrange a training session for you. Even if you
have used Agilent spectrometers before, our setup here is likely to be
slightly different, so in the long run you will save yourself time by
having a short lesson. More information is available in the
introduction to the NMR Centre.
It is NMR Centre policy that you will not be given a license to operate
a spectrometer until we assess your competence with Agilent spectrometers.
- How much solvent volume should I use?
To get good resolution you need about 0.7 ml of solvent in a 5 mm NMR
tube. The minimum solvent volume on newer spectrometers is 0.55ml or 40mm length.
If you have a limited amount of sample you can increase its
effective concentration by reducing the solvent volume to 0.4 - 0.5 ml
however you will need to spend more time shimming. If you have a sample
that will give a proton spectrum in 15 minutes when disolved in 0.7 ml,
there is little point reducing sample volume if it means you need to
spend an extra 10 minutes shimming! On the other hand if you then want
to run a carbon spectrum of that sample it would certainly be worth
reducing the sample volume. For 13C HMQC or HMBC runs with
very small amounts of material, a 4mm tube and rotor is available. 3mm spinners
are available for the Bruker instruments.
- What facilities are available for variable temperature operation?
Note: people browsing this site from outside the RSC should not use
the temperature ranges given below as a guide to the abilities of Agilent
spectrometers, since they reflect our desire not to stress probes that are
in some cases over 10 years old. Check the
Agilent web site or contact Agilent
for the temperature specifications of current probes.
Temperature control is available on the Inova and Avance spectrometers,
but not on the Mercury.
Users must not attempt variable temperature operation unless they have
received training in this from an NMR staff member. Probe damage could result!
You should not raise the temperature of your sample higher than 10°C
below the boiling point of your solvent. You should also not freeze samples
disolved in H2O or D2O. Ask NMR staff about thick-walled NMR tubes if this
is a problem.
The available / safe temperature range depends on which probe is used on
the instrument in question.
- Avance 600 and 800
These spectrometers use cryoprobes with a temperature range of 0°C to
Users should not attempt to take the probe all the way to +80°C however,
since a small transient rise in temperature above +80°C will cause the
cryoplatform to warm the probe up.
- Inova 500
- New pulsed field gradient probe: from -100°C to +130°C
- Old pulsed field gradient probe: from -10°C to +50°C
- Old 1H/19F probe: from -150°C to +200°C
- Inova 300
- 1H / 15N - 31P probe:
from -100°C to +120°C
(Brief runs at higher temperatures may be allowed under the supervision of
an NMR staff member).
- New pulsed field gradient probe: from -100°C to +130°C
- What does the "ADC overflow" error message mean?
The signal recieved from the NMR sample is first amplified by the
reciever and then digitised by the analog to digital converter (ADC).
If the signal is too strong for them to handle, either the receiver or
ADC will "overflow", causing a RECEIVER OVERFLOW or
ADC OVERFLOW message to be displayed. The acquired FID is
likely to be clipped, resulting in a distorted spectrum. The solution
is to use autogain (type gain='n')
or to type in a lower value for the receiver gain. If overflow still
occurs when the gain is set to zero, reduce the observe pulsewidth
(PW) to half its present value. If overflow still occurs
dilute your sample, or if the solvent signal is causing the ADC overflow
use a solvent suppression technique.
- How do I shim / tune the spectrometer?
First of all, let's get our terminology straight. Shimming is
adjusting the resolution of the signal by optimizing the homogeneity of
the magnetic field. Tuning is adjusting the
impedance of the probe. A poorly tuned probe reflects a lot of the
power of the pulses, so that what should be a 90 degree pulse is in
reality only (say) a 50 degree pulse. Probe tuning does not affect the
resolution, however the signal to noise of a standard spectrum will be
worse. Also, experiments such as DEPT or COSY that rely on accurate 90
degree pulses may produce artefacts or not work at all. (Note however
that it is possible to adjust
the pulse width to give a 90 degree pulse on a poorly tuned probe).
Poor shimming on the other hand, results in broad NMR resonances.
People often talk about "tuning the resolution" which is where some
confusion between shimming and probe tuning arises. Shimming is
adjusting the homogeneity of the magnetic field, so that every part of
the sample in the NMR tube experiences exactly the same field strength.
- OK, so how do I tune the probe?
If you're using the Mercury or MR400 spectrometers, you
never need to tune the probe. Tuning the probes on these spectrometers
is a specialised operation which should only be performed by NMR Centre staff.
For best results, you should tune the probes of all other spectrometers
before acquiring a spectrum. Frequency, solvent and sample height all
affect probe tuning. If you were running a set of similar samples in
the same solvent, you might only bother to tune the probe before
running the first spectrum. If however, half your samples were
disolved in chloroform and half in D2O, you might run all
of the chloroform samples and then quickly adjust the tuning after
inserting the first D2O sample. Tuning involves setting up
for the nucleus of interest and minimizing the reflected power shown
on the meter on the magnet leg. Some recabling is required. Do not
attempt to do this unless an NMR staff member has given you a lesson.
This doesn't mean that the Mercury and MR400 have "automatic tuning"
technology. It just means they are left in a state of tune that is
good enough for the run-of-the-mill experiments they were designed
for. On other spectrometers, tuning is necessary because
- You can get the best possible tuning for your sample,
- You may not know what nucleus the previous user left the
probe tuned to, or whether he/she completely messed up the tuning,
- More sophisticated experiments such as HMQC, HMBC etc. work best
when the probe is tuned and short 90 degree pulses are required.
- How do I tune for carbon or phosphorus?
As mentioned above, if you're using the Mercury or MR400 spectrometers,
you don't need to tune the probe. First, check whether the probe you are
using requires a tuning stick to be inserted. Tuning sticks are kept separate
from the probe, and have a small capacitor on the end to change the
tuning range of the probe. If a tuning stick is required, select the
stick for the observe frequency and screw it gently all the way into the
probe. You can find the observe frequency by setting up for the nucleus
of interest and reading the value of sfrq from the dg
display. Then make the cable connections for tuning, and adjust both
the tuning and matching rods. These two tuning rods affect each other,
so it is usually necessary to go back and forth between them to get a
good minimum. There is a bit of a knack to it, so persevere!
(Hint: make the tuning worse with one rod, then better with the
other. Each dual operation should result in better tuning than before).
Also note that if you are decoupling protons while observing carbon
or phosphorus, it is a good idea to check the proton tuning. If the
probe is poorly tuned to protons, some decoupler power may be reflected,
resulting in an improperly decoupled spectrum. On the Inova spectrometers
you can tune the observe and decoupler channels at the same time. On older
spectrometers you need to set up for and tune protons, then set up
for and tune carbon or phosphorus.
(Hint: always tune the highest frequency first and the lowest
- Which spectrometer should I use for carbon?
When measuring carbon spectra, the main concern is usually signal to
noise. You would expect higher field spectrometers to have a decisive
advantage - for example a 500 Mhz spectrometer when compared to a 300
MHz spectrometer should have an advantage of (5/3) squared, or 2.8 times
the signal to noise. However there are other considerations, including
for example the type of probe. An indirect detection probe has the
proton observe coil on the inside (that is, closer to the sample than the
coil used for carbon). This improves the proton signal to noise, however
if you use an indirect detection probe for directly observing carbon,
the signal to noise will of course be worse than a standard probe which
has the carbon coil on the inside. Regardless of the probe design,
carbon and protons use different coils, and since the electronic
circuit for the two nuclei is different it makes no sense to compare
proton signal to noise on two instruments and extrapolate the results
Also, signal to noise tests are usually performed by
collecting a single scan on a concentrated sample, however this does not
give the best indication of the results obtainable on "real" samples
where the sample is scanned for several hours. When a sample is
repeatedly pulsed, the relaxation times of the various carbons must be
taken into consideration. Nuclei take longer to relax at higher fields,
so the gain in signal to noise is less than expected. Also note that
carbons that do not have directly bonded protons (i.e. carbonyls and
quaternaries) have much longer relaxation times than protonated carbons.
A rough comparison was obtained using a standard menthol sample, and
measuring the signal to noise of the CDCl3 solvent and one of the menthol
| CDCl3 || CH3 |
Also remember that
- if there is not much sample available, you should reduce
the amount of solvent.
(See How much solvent volume should I use?)
- if you are interested in quarternary carbons, a longer D1
delay of 3 seconds or more is advisable.
- If the signal to noise of your carbon spectrum is too low,
try running a short and/or long range proton-carbon 2D correlation
experiment. It has been known for a long time that this can give
dramatic improvements in S/N. See J. Am. Chem. Soc. 101, 4481 - 4484 (1979).
- Why are some of the peaks in my APT missing?
The APT experiment relies as much on the size of the
1JCH coupling as the number of attached protons
to generate the spectral pattern. This is because the delays in the
experiment are matched to the inverse of the size of
1JCH. If 1JCH is much larger
than the default 1JCH of the experiment (usually
set to 140 Hz which is the average of 1JCH for
sp3 and sp2 carbons) then peaks will either
disappear or appear with incorrect phase. Carbons that may show this
behaviour are terminal ethynyl groups (1JCH =
250 Hz approx.), epoxide carbons (1JCH = 175 Hz),
furan, pyrone and isoflavone carbons (1JCH = 200 Hz),
2-unsubstituted pyridine and pyrolle carbons
(1JCH = 180 Hz) and 2-unsubstituted imidazole
and pyrimidine carbons (1JCH > 200 Hz).
- I can't lock on.
- You are using a deuterated solvent aren't you?
- Can you see a lock signal? If not, click the "OFF" button in
the lock display, turn the lock power and lock gain to their maximum
values, and look for a sine wave by adjusting Z0. If you find
a sine wave, adjust Z0 until its frequency becomes zero. Then
reduce the lock power (to avoid saturating the lock) and
click the "ON" button.
- If it loses lock as soon as you try to lock on, turn the lock
off and adjust the lock phase as shown in the manual.
- Is your tube spinning? It might not be spinning because you
inserted the tube too quickly, causing it to break. Take the
tube out and check that it is in one piece. While you have it
out, use a depth gauge to check that the sample is centred in
- It won't shim.
- Often when a sample won't shim, it is because the lock is
saturated. Go to the lock power/gain/phase display in the shim
window, and reduce the lock power by 4. The lock level should
drop by 10 units or more. If it does not drop, or it increases,
you are saturating the lock. Keep reducing the lock power until
the lock level drops.
- Check the linewidth of the narrowest line in your
spectrum. If there are some broad lines and some narrow lines,
the broad lines are probably broad because they are undergoing
chemical exchange, not because the resolution is poor. Broad lines
may also be caused by quadrupolar broadening if your compound has
a transition metal.
- If you have not already done so, load the standard shims. You
don't know what sort of state the previous user left the shims
in! All Agilent spectrometers in the NMR Centre have a macro
rtss which loads the standard shims. This macro is equivalent
to typing rts('stdshm') su. Also check that
the probe parameter is set to the probe you are using, since
the rtss macro uses this value to determine which shims to load.
- If it still won't shim, take the tube out and inspect your
sample. Is the tube scratched? Is there anything floating in
the sample? Is the sample centred in the coil? If you are using
a small amount of solvent to improve the concentration, you may
need to add some more solvent to make it easier to shim.
- Do you have paramagnetic ions in your sample?
- Have other people been getting poor resolution? If so, report
it to a member of the NMR staff. If not, change NMR tubes,
filter your sample, and try again. If changing tubes solves the
problem, throw the old tube away.
- Have you placed your NMR tube in an oven to dry? If so, throw
the tube away as it has distorted. (Remember, glass is a liquid.
It flows at high temperature). The correct way to dry a tube is
via a stream of dry nitrogen through a glass wool filter.
- My tube broke when I inserted it into the magnet.
If this happens, it is absolutely essential to leave a note to warn
others not to use the spectrometer. If someone lowers their tube on top
of your broken tube, not only will it break their sample, but it will
force your samle further into the probe, and the probe will then have
to be sent away for repair. A single broken tube is unlikely to damage the
probe, but two broken tubes will!
During the day, phone an NMR staff member. Tell them the solvent and
any hazards posed by your compound. After normal working hours
tell the watchmen who will call in someone.
- I can't phase correct my spectrum.
The aph (automatic phase correction) command usually does a good
job of correcting the phase, and should be the first thing you try.
Sometimes (for example in noisy spectra) the aph command is unable to
correct the phase, and in these situations it often leaves lp at
a high value (say one or two thousand). In these situations you will have
to correct the phase manually. First a couple of obvious things:
if you ran a DEPT or APT experiment or something similar, there will be
some positive and some negative peaks, so don't try and phase them all
positive! Similarly in a 1:1 binomial solvent suppression sequence,
half the spectrum will be positive and half negative.
Having established that you are not running an exotic pulse sequence
that produces strange phases, the next thing to consider is foldback.
Are you sure that you used a large enough spectral width when acquiring
the spectrum? If one or more resonances occurred outside the observe
region, the method used to digitise the signal results in these resonances
appearing within the observed spectral width, but with a phase error. If
in doubt, double or triple the spectral width, run the spectrum again, and
see if the resonance that could not be phase corrected remains at the
same chemical shift as before.
To perform manual phase correction on Agilent spectrometers,
proceed as follows:
Type lp=0 rp=0. This sets the left
phase and right phase to zero. On Agilent spectrometers, "right phase"
and "left phase" equate very roughly to the zero-order and first-order
phase adjustments respectively. The zero-order phase affects the
entire spectrum equally, while the first-order phase is frequency
dependent. The zero-order phase should always be in the range
-360° to +360° and the first order phase should also usually
be in this range. If you have a first order phase correction of more
than a thousand degrees, not only is it probably incorrect, but you
will also probably be generating baseline roll. Click the "phase"
icon with the mouse. Perform a zero-order phase correction on the
largest peak as described in the manual for the spectrometer you are
using. Now choose another peak some distance from the largest peak,
and adjust the first-order phase. On Sun based systems you only get
one shot at adjusting the zero-order phase - all subsequent
corrections are made to the first-order phase, so there is no point
clicking on the largest peak again. If you want to readjust the
zero-order phase, get out of the phase-correction routine (by for
example typing ds) then click on the phase button again. If
for some reason a large first-order phase correction is required, it
may be easier to choose a peak for the first-order adjustment that is
close to the peak you used for the zero-order adjustment. On Sun based
systems, set the phasing parameter to 100. This causes the
effect of the phase values to be shown for the entire spectrum as you
are making the adjustments, thus making it easier to see what you are
- I need to run my spectrum at a higher field to get better resolution.
No you don't! The resolution of a high field spectrometer may even be
worse than a low field spectrometer. What a high field instrument has
more of is dispersion. This means that resonances with different
chemical shifts are further apart. Multiplets due to coupling will
not show any improvement unless the higher field instrument
separates overlapping multiplets with different chemical shifts,
or the multiplet showed strong coupling effects at lower field.
Some nuclei such as 31P may have worse resolution because
of a property called chemical shift anisotropy which increases with field
- There are no parameters for the solvent I want to use.
If you're running a proton spectrum, set up for 1H /
CDCl3, double the spectral width, run a quick spectrum,
and put the two cursors around the spectrum. Then do a movesw
and acquire the final spectrum. If you're running a carbon spectrum,
set up for 13C / CDCl3, increase
the spectral width by 20 percent, and run as normal. If your solvent
has carbon nuclei which show up quickly, reference the solvent and
check that the observed spectral range is correct.
If you are running a phosphorus spectrum, set up for
31P / CDCl3, increase the spectral
width by 20 percent, and run as normal. Then supply an NMR tube
containing the solvent to the NMR staff so that they can set up
H3PO4 referencing parameters for you.
- How can I suppress a strong solvent resonance in a proton
If the solvent signal is less than two or three times the size of the
largest signal from your compound, it may not be worth bothering. One
method is to presaturate the solvent signal (instructions for doing
this are in the folders near the spectrometer). Although it is simple,
this method has the disadvantage that NH or OH protons that are
exchanging with water also have their signals reduced or
eliminated. Another method is the 1:1 binomial pulse sequence. The
signals on one side of the solvent resonance are of opposite phase to
the other side when this method is used. On the Inova spectrometers,
the method of choice if a pulsed field gradient probe is in use, is
watergate solvent suppression. If chemical exchange is very rapid,
watergate may not be suitable, in which case a binomial pulse sequence
is the best choice.
- What is nuclear spin?
All nuclei carry a charge. In some nuclei this charge "spins", causing
the nucleus to behave like a tiny bar magnet. This is why it aligns
with or against the magnetic field of an NMR spectrometer. However
unlike a bar magnet, the low energy state is aligned with the field and
the high energy state is aligned against the field. Up to now we have
been talking about nuclei with a uniform spherical charge distribution.
These nuclei are said to have a spin of ½. Protons,
13C and 31P are all spin half nuclei. Note that
the most common isotope of carbon, 12C, has no spin and can
therefore not be observed using NMR. Nuclei with a non-spherical charge
distribution have a spin number I of 1, 3/2 or higher (in steps
of ½ ), and are referred to as quadrupolar nuclei.
Spin ½ nuclei have two orientations (with or against the
field). Spin 1 nuclei have three orientations, spin 3/2 nuclei have 4
orientations, etc. Deuterium is an example of a spin 1 nucleus.
Although deuterium is chemically the same as hydrogen, for the purposes
of NMR it is completely different. For example a carbon spectrum of
CDCl3 is a 1:1:1 triplet regardless of whether you turn on
the proton decoupler. This is because the deuterium attached to the
carbon can have three orientations, and occurs at a different frequency
- What is a double quantum coherence?
When you put your sample in the magnet, all the spin half nuclei align
either with or against the magnetic field. The population difference
between these two orientations (known as the Boltzman distribution) is
field dependent, and is determined by their energy difference.
An NMR signal is observed
when nuclei flip from one orientation to the other. This is a single
quantum coherence. When two nuclei are coupled, they can flip together
as though they were a single unit. If they flip in opposite directions,
the flips "cancel each other out" (sort of) resulting in a zero quantum
coherence. If they both flip the same way, you get a double quantum
coherence. The frequency of a zero quantum coherence is between zero
and a few kilohertz, so it is not directly observed. Similarly the
frequency of a double quantum coherence is roughly twice the normal
observe frequency, so that is not observed directly either. You can
also have triple quantum coherences from groups of three coupled nuclei.
The effect of double and triple quantum coherences can only be observed
by inserting pulses or delays into a pulse sequence to convert them to
single quantum coherences before acquisition of the NMR signal. Do not
confuse double quantum coherences with coupling in a normal spectrum. A
doublet for example, arises when there are two coupled spins, but only
one of these spins flips.
- What are pulsed field gradients?
Imagine if you could really mess up the Z1 resolution for a few
milliseconds then restore it to its proper value during the course of
the pulse sequence. This is an oversimplification, since pulsed field
gradients do not use the normal shim circuits. A special PFG probe, and
a PFG amplifier are necessary. By applying a gradient to the magnetic
field, the top of the sample experiences a slightly different magnetic
field to the bottom of the sample. Since magnetisation precesses at
different rates in different fields, it is possible after a 90 degree
pulse and a PFG of a few milliseconds to have the magnetisation vectors
along the length of the tube pointing in all directions instead of
nicely aligned along one axis of the rotating frame. Obviously if the
magnetisation vectors are pointing in all directions, there is no net
signal. The vectors are said to be dephased. If you now apply a
PFG of opposite sign for the same time, you will rephase the
magnetisation, and get your signal back. You could achieve the same
thing by giving the dephased vectors a 180 degree pulse, then applying a
PFG of the same sign. The other thing to be aware of is that
double quantum coherences dephase at twice the rate of normal single
quantum coherences, so by adjusting the strength or duration of pulsed
field gradients, you can select single, double or triple quantum
coherences. The "old fashioned" way of selecting certain types of
coherences is to use elaborate phase cycles which cause the unwanted
signals to cancel out on successive scans. The PFG method acquires only
the desired signal on each scan, resulting in fewer artifacts and allowing
fewer scans. The old method can be thought of as "cancellation of unwanted
signals over time" whereas the PFG method can be thought of as
"cancellation of unwanted signals over space" where "time" refers to
successive scans, and "space" refers to the physical length of the
sample in an NMR tube.
- What is the Nuclear Overhauser Effect?
Glad you asked. Have a look at our NOE guide.
- How do I run a quantitative spectrum?
A quantitative spectrum is simply a spectrum where you can trust the
integral ratios. In other words, if the integral of resonance A
is twice the height of the integral of resonance B, you can say
with certainty that resonance A is due to twice the number of
nuclei as resonance B. Why do we use integrals? Because it is
the area of the resonances that is proportional to the number
nuclei. The height of a broad line may be less than that of a sharp
line, but its area may be greater. How do we get accurate integrals? By
ensuring that all resonances are equally excited, well digitised, and
It is harder to obtain quantitative carbon spectra, because carbon
relaxes more slowly than protons, is less intense, and steps have
to be taken to eliminate the Nuclear Overhauser Effect which
builds up when protons are decoupled.
- Equally excited : if the pulse power is not high enough,
some resonances far from the observe frequency may experience a reduced
flip angle, resulting in a smaller observed signal.
- Well digitised : if the number of data points in the
spectrum is too low, there will not be enough points to accurately
define each resonance, resulting in inaccurate integrals (and peak
- Properly relaxed : resonances that are not fully
relaxed give a weaker signal than fully relaxed resonances. The
nuclei in your compound will not all relax at the same rate, so if
you pulse too rapidly the quickly relaxing resonances will appear
stronger than the slowly relaxing ones. To be sure of obtaining
accurate integrals, you need to measure the relaxation times of
your compound, and set a delay equal to 5 times the longest
relaxation time. Fortunately it is easy to run an inversion -
recovery experiment to measure relaxation times.
- What is digital resolution?
Digital resolution is simply the separation in hertz between each data
point in your spectrum. It has nothing to do with shimming!
Say, for example, you set the number of points np to 32,768
and acquire a normal 1 dimensional FID. The number of points in
the spectrum you see will be 16384, since half the data points are
imaginary. Now if the spectral width (sw) is 6000, the digital
resolution will be 6000/16384, or 0.366 hz per point. (Before you
grab your calculator to measure your own digital resolution, note
that the number of points in the spectrum is not always simply
np/2. See the section below on the Fourier number). The Vnmr command
to display the digital resolution is dres. If you place the
cursor on a peak and type dres, two values will be displayed:
The dres command may give a different linewidth value for every peak
you put the cursor on, but the digital resolution value will always be
the same, unless you change the Fourier number fn and do
another Fourier transform. If the natural linewidth of a resonance is
comparable to the digital resolution, the resonance may only be defined
by one or two data points. If you expand a line like this, it will look
more like a spike than a proper Lorentzian line. Consequently the height
of the line may appear less then it really is, the integral will be
inaccurate, and even the chemical shift value will be less accurate than
it should be. Also, if the separation between two resonances is
comparable to the digital resolution, they may appear as a single resonance
in the spectrum, because no data point falls in the space between the
tops of the two peaks.
the linewidth which is the width of the peak at half-height, and
depends on shimming, weighting functions and the natural width of the
line. Also the
- digital resolution, which is what this section is all about.
- What is the fourier number?
Mathematicians can do a Fourier transform of any number of points. NMR
spectrometers speed things up by using the Cooley-Tukey fast fourier
transform algorithm. As implemented on NMR spectrometers, this requires
the number of points to be a power of two. So what happens if the
number of points np is not a power of two? On Agilent
spectrometers this can be controlled by the Fourier number (fn)
parameter. If it is used, fn can only be set to powers of 2, and the
value of fn is the number of points that are actually used in the
Fourier transform. If fn is less than np, some points on the end of the
FID are not used in the Fourier transform. If fn is greater than np,
the end of the FID is padded with zeros to increase the number of
points. This is referred to as zero filling. Zero filling does not
write extra zeros on to the end of the FID file on the disk where the
FID is stored, it merely adds the zeros in memory just before doing the
transform. It is also possible to set the Fourier number to n
(not used). In this case, the spectrometer uses the first power of 2
which is higher than np when doing the Fourier transform. So for
example if np was 16385 (that is, 214 + 1) it would use
32768 (i.e. 215) points for the Fourier transform.
- What is the relaxation time?
It would be an oversimplification to say that the relaxation time
is the time taken for a nucleus to relax to equilibrium.
After a pulse, a nucleus relaxes toward its equilibrium value at an
exponential rate. The value quoted as the relaxation time is actually the
time constant of this exponential curve. It takes five time
constants for the magnetisation to relax to 95% of its equilibrium
value. There are two basic types of relaxation, T1 and
T2. In the T1 process, the magnetization remaining
along the z-axis relaxes back to its equilibrium value. This is also
known as spin-lattice relaxation because relaxation occurs by the
loss of energy from the excited nuclear spins to the surrounding
molecular lattice. In the T2 process, the magnetization
in the x-y plane fans out out until the net magnetization is zero.
This is also known as spin-spin relaxation because it is due to
the excited spins exchanging energy with each other.
- What NMR Simulation Programs are Available?
- To simulate a normal (non-exchanging) spin system,
you can perform the simulation using the same Vnmr program that you
use for data processing. There are instructions in the folders.
The first step is to decide what sort of spin system you have -
AB, A2X, ABCXY etc. The letters are not important to Vnmr, so it
doesn't matter whether you tell it that you have an ABC or an AMX
system. Vnmr only needs to know the values of the chemical shifts
and coupling constants.
- To simulate a dynamic (exchanging) spin system, the usual
procedure is to run a series of normal spectra at different temperatures,
and then simulate the lineshapes. This is done using the DNMR5 program,
an implementation of which is available in Bruker's Topspin program, version
2 or later.
Australian National University NMR Centre