GEMSTONE - Ultra selective excitation

Interpretation of NMR spectra is often hampered by the overlap of signals. Particularly for compounds with numerous complex multiplets, assigning individual signals can be tricky. However, a recent improvement in selective excitation techniques, GEMSTONE, offers a method to tease apart the overlapping signals.

Assigning a 19F spectrum

Recently I was asked to help assign a ¹⁹F spectrum. The Skaggs NMRs are not capable of recording ¹⁹F spectra so I have not had much experience interpreting them. The extent of my ¹⁹F knowledge was an awareness of a large chemical shift range and large scalar couplings. Nevertheless, what follows is our rationalisation of the spectrum. Please let me know if you have a better explanation!

Comparing processing in Mnova and TopSpin

Occasionally users have told me that spectra they have collected on the Skaggs NMRs do not look as good when they take them back to their own computers. This could be because they did not process their data with optimised parameters. Or, since most users use Mnova, it may be due to real processing differences between Mnova and TopSpin. Here I examine a few spectra processed with both packages to see if there is a real difference between processing with Mnova and TopSpin.

Extracting 1H-1H coupling constants

When publishing NMR data ¹H-¹H couplings are often reported. These are most easily obtained from a 1D ¹H spectrum. Measuring the couplings can be made easier, and more accurate, if the spectrum is processed to enhance resolution.

Care and cleaning of NMR tubes

Every NMR user ends up with a collection of used NMR tubes. These tubes are often cleaned and used over and over, however, some care needs to be taken when cleaning tubes to prevent them from being damaged. Damaged tubes give poorer quality spectra, and may even break the spectrometer probe. This post provides some guidelines on best practices for cleaning NMR tubes.

NMR tube specifications

NMR sample tubes come in a variety of grades. Good quality tubes will not only give you better quality spectra, but are less likely to damage the instrument. In this post the specifications used to distinguish a good NMR tube from a bad one are explained and how these parameters affect your spectra are discussed.

NUS for ASAP-HSQC experiments

Non Uniform Sampling (NUS) speeds the collection of multidimensional NMR spectra by measuring only a fraction of the data and predicting what was omitted. The quality of the reconstructed data depends on many factors and what works well for one experiment many not work well for others. The last two posts examined how NUS affects COSY and HMBC experiments. This post examines if NUS impacts the ASAP-HSQC experiment more than the traditional HSQC.

NUS for COSY experiments

Non Uniform Sampling (NUS) reduces the time taken to acquire multi-dimensional NMR spectra by predicting a fraction of the normal data instead of measuring it. The most commonly used algorithm for reconstructing the missing data requires the collected data to be properly phased in the indirect dimension. For this reason I have not recommended using NUS with HMBC and gCOSY experiments. However, last month's post showed that unphaseable HMBC experiments cope with NUS just as well as the phaseable LR-HSQMBC. In this post I compare the unphaseable gCOSY experiment with the phaseable CLIP-COSY to see how they are impacted by NUS

NUS for HMBC and LR-HSQMBC experiments

Non Uniform Sampling (NUS) of multidimensional NMR data can greatly reduce the time taken to record a spectrum by recording only a subset of the normal data. A variety of algorithms are available to reconstruct the omitted data based on the data that was recorded. The most commonly used algorithm is Iterative Soft Thresholding (IST). Most implementations of the IST algorithm rely on the peaks in the detected dimension being phased correctly and positive. For most modern experiments this is not a problem, but in the HMBC experiment it is not possible to phase the peaks. For this reason I have not recommended using NUS with HMBCs. The LR-HSQMBC experiment, however, can be phased and I recommend using NUS with it. In this post, spectra recorded with different levels of NUS were recorded to determine how NUS affects HMBC and LR-HSQMBC experiments.

Ξ values for indirect referencing

In the previous post I mentioned that indirect referencing was required to compare spectra collected in methanol-d₄ and methanol-d₃. Indirect referencing relies on precisely determined ratios of the ¹H gyromagnetic ratio and the gyromagnetic ratio of the nucleus being referenced. Unfortunately, many different values of this ratio, known as Xi (Ξ), have been reported. Here I test several of these Ξ values to see how closely they match referencing to an internal standard.

Locating hydroxyls by deuterium exchange

The position of hydroxyl groups can often be inferred from chemical shifts, but for complex, unknown molecules chemical shift arguments may not be conclusive. Chemical reaction of the alcohol is one method often used to identify the location of the group, but it is also possible to use the change in chemical shift induced by exchanging protons for deuterons. This can be as simple as changing the solvent as shown in the example below.

Water suppression

In NMR spectra solvent peaks often obscure the peaks of interest. Using deuterated solvents is one way to avoid this problem, but solubility or the need to observe exchangeable protons such as hydroxyls and amides sometimes requires using a protonated solvent. This is often the case when trying to use physiological conditions, as in biomolecular NMR or metabolomics. To detect the solute in aqueous solutions the water peak must be suppressed and there are several different ways this can be achieved.