Resolution Booster

Resolution is a key concept in high resolution NMR and considerable effort (e.g. shimming) is usually devoted to ensure optimum resolution. High spectral resolution is important for the measurement of NMR parameters, especially for signal intensities, chemical shifts, and coupling constants. However, in many areas of high-resolution NMR the observed resonance lines are broadened in some undesirable way that may complicate, if not preclude, the accurate analysis of e.g. scalar couplings. Moreover, you can directly measure accurate values of J only when the splitting is much larger than the linewidth. The classical solution to the line broadening problem, other than using higher magnetic fields and assuming proper shimming, is multiplication of the FID by a resolution-enhancement function. More sophisticated procedures involve nonlinear data-processing methods such as the much-discussed maximum entropy method. The Filter Diagonalization Method (FDM) and Reference Deconvolution are other well known processing procedures that can be used for resolution enhancement.

The most frequently used methods for resolution enhancement are those which involve appropriate mathematical treatment of the FID (weighting). For example, one popular, routinely used weighting function is the Lorentzian/Gaussian transformation. Other well-known weighting functions include the TRAF, convolution difference, and sine-bell functions. Most of these methods have in common that they alter the FID such that its beginning is deemphasized, relative to the latter parts. Although these methods can improve resolution significantly, caution must be taken because information can be destroyed or artefacts created by extreme enhancement, such as that which results in large negative lobes on both sides of a signal.

The Resolution Booster is a new frequency domain post processing method which yields a considerable resolution enhancement with a fast calculation, while avoiding the negative artefacts of the other resolution enhancement functions.

In order to illustrate the power of our Resolution Booster method, we will use as an example the spectrum of dimethyl pyridine-2,5-dicarboxylate acquired at 250 MHz:

This spectrum has been processed without applying any weighting function and the resolution is 0.13 Hz/pt. If we look at the signals corresponding to proton 2 in the structure, we can appreciate a small splitting due to 4 bond coupling with proton 6. Proton 6 shows a large splitting due to 3 bond coupling with proton 5 and a small splitting due to the 4 bond coupling with proton 2. Proton 5 appears as a double doublet because of the 3 bond coupling with proton 6 and a small 5 bond coupling with proton 2. The latter is barely appreciated in the figure because of the lack of resolution. In fact, the same splitting should show in proton 2 but this can not be seen at this resolution level. It is important to remember that you can measure couplings constants accurately when the splitting is much larger than the line width.

Following the menu 'Processing/More Processing/Resolution Booster' will display the 'Resolution Booster' dialog box, where the user will be able to select a noise cutoff option:

In the picture below you will be able to see the result of applying the Resolution Booster method:

In this case, resolution has been increased by ~230% making possible the calculation of the weak, long range coupling constants.

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Note:

It is important to notice that this procedure may change relative intensities. On isolated lines, in principle, it is approximately proportional to the second derivative which, when all lines have the same line width (as they often do) is proportional to the line height. However, broad lines can get suppressed and unresolved humps and shoulders get resolved (+) but their intensities and, to some extent, positions can not be trusted (-).

Please bear in mind that we can not integrate the spectrum after having applied Resolution Booster as this algorithm is sensitive to the signals linewidths. If all the lines in the spectrum would have exactly the same line width, Resolution Booster could also be considered a valid tool for qNMR analysys. But in real life, this is never true.

Thus, RB should be considered as a Resolution Enhancement tool which you can use for any of the following purposes:

• to reveal signals which could otherwise be hidden and badly resolved because of spectral overlap

The Resolution Booster can also be used with 2D-NMR spectra (even with phase sensitive experiments):