主题：NMR Basics FAQ 5

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 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.
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.

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 Varian 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 program you use depends on the type of experiment you ran. If you ran a series of normal spectra at different temperatures, then you need to simulate the lineshape. This is done using the DNMR5 program. There is another program, dnmr5input, to help you create the input file for DNMR5. Instructions are in the folders by the Sun computers.
If you ran a series of
selective inversion pulse -- delay -- hard pulse -- acquire
experiments to use magnetization transfer information to determine rate constants, we have a program provided by Prof. Brian Mann of Sheffield University that you can use to analyse your data.


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