The relaxation delay

The relaxation delay is one of the basic NMR parameters. Optimising the relaxation delay can help improve the appearance of your spectra and increase the accuracy of your integrals. In this post its impact on 1D ¹H spectra is shown.

Fill volume for 1.7mm NMR tubes

The SSPPS NMR Facility routinely uses a 1.7mm micro-cryoprobe that takes capillary NMR tubes with a diameter of 1.7mm instead of the standard 5mm NMR tubes. These capillary tubes require much less sample volume than a standard tube, but exactly how much should be used? And what happens if there is too little? Or too much?

Fluorine and phosphorus heteronuclear coupling

The most commonly encountered coupling in NMR spectra is due to ¹H, but other nuclei can induce splitting of signals as well. The previous post discussed how coupling to ¹³C at natural abundance produces the small ¹³C satellite peaks. The other heteronuclear couplings that are most likely to be observed are due to ¹⁹F and ³¹P. Examples of these in 1D ¹H and  ¹³C spectra are shown below.

13C satellites

Recently I was asked about the small peaks appearing symmetrically around a large peak in a 1D ¹H spectrum. These peaks are called ¹³C satellites and arise from heteronuclear coupling to the small amount of ¹³C naturally present.

Receiver gain

The receiver gain is one of the parameters that should be optimised before recording an NMR spectrum. Here I'll explain what the receiver gain is and why it needs to be optimised.

Methanol and DMSO solvent peaks

Two common deuterated NMR solvents, methanol and dimethyl sulfoxide, produce very similar, characteristic peaks with an unusual splitting pattern. What is the cause of these peaks?

Determining stoichiometry from NMR titrations

NMR titrations are often used to demonstrate binding, locate active sites, and measure affinities. By measuring changes in chemical shifts or linewidths as the concentration of one of the binding partners is changed, quantitative data, such as dissociation constants and dissociation rates, may be obtained. In most cases the systems studied involve 1:1 binding. Systems with multiple binding sites are more difficult to analyse but, with some simplifying assumptions, quantitative information, such as stoichiometry, may still be obtained.

i-HMBC : Identifying two bond correlations

The HMBC experiment provides ¹H-¹³C correlations over multiple bonds and is essential for assigning small molecules. The main drawback to the HMBC is that it does not identify the length of the correlation and in some cases may be ambiguous. A recent publication describes a method, called i-HMBC, for distinguishing two bond HMBC correlations from longer ones.

Cleaning a cryoprobe

One of the Skaggs NMRs is normally fitted with a 1.7mm cryoprobe. This piece of equipment is essential for our natural products and metabolomics users who have limited amounts of their samples. The capillary sample tubes for the 1.7mm cryoprobe use special shuttles that do not grip the tube. If the probe gets dirty then the tube can fail to enter the top of the probe and instead gets pushed up out of the shuttle. This becomes apparent when the sample fails to produce a lock signal. Repeatedly ejecting and inserting the sample tube can sometimes get the tube to drop into place by luck, but this is not really a viable solution. If the 1.7mm tubes are not entering the probe reliably then the probe needs to be cleaned.

NMR assignment exercises

Interpreting NMR spectra is a skill that every chemist needs, but it requires practice and experience. A recent paper describes a new interactive website that offers many NMR assignment exercises based on experimental one- and two-dimensional spectra. Working through this website allows a user to practice and improve their NMR assignment skills. In addition, there are several other websites available that provide NMR spectra, along with MS and IR data, for users to try their hand at. This post lists some of the currently available resources.

Assigning stereochemistry with NOEs

Relative stereochemistry can often be determined by analysis of nuclear Overhauser effects or NOEs. I have seen many papers where the stereochemistry was defined based on the observation of one particular peak, but this may not be sufficient and some care must be taken when interpreting NOEs. Here I offer a few pointers on how best to interpret the crosspeaks in NOESY and ROESY spectra.

GEMSTONE-NOESY

GEMSTONE is a new highly selective NMR technique. Combining it with traditional homonuclear 2D experiments creates exceptionally selective 1D versions of these experiments, which can be useful when trying to characterize highly overlapped spectra. In this post I compare the GEMSTONE-NOESY experiment with the standard 1D selective NOESY.