J-RESolved Spectroscopy (JRES)

A 1D proton NMR spectrum can contain a great deal of information, including:

(a) chemical shifts (what types of protons you have);
(b) peak integrations (the number of each type of proton you have); and
(c) splitting patterns (how each type of proton is connected in the chain – i.e., what its neighbours are).

This is a wealth of information about molecular structure and, realistically, is why 1H NMR is such a tremendous characterization technique. However, if all three pieces of information are not taken into account simultaneously, OR, if overlap or spectral crowding somehow masks them, there can be errors in structural assignments. In order to help simplify and/or clarify spectral assignment, we can reach into our 2D NMR toolbox to help deconvolute the data!

I’ve already discussed the better-known homonuclear 2D experiment – the COSY, which correlates chemical shifts of nearby protons through their scalar couplings.

The less common JRES experiment (also introduced by Ernst in 1976)[1], on the other hand, separates the scalar coupling and the chemical shift from a particular resonance multiplet and projects these two orthogonal pieces of data onto two different axes. Multiplicity is displayed along f1 and chemical shift is displayed along f2. This helps to simplify and de-convolute insufficiently resolved data. Although this experiment can be used to resolve both heteronuclear (1H-X) and homonuclear (1H-1H) multiplicities, I will limit this discussion to homonuclear couplings.

In its simplest case, the JRES experiment is:

(a) an interscan relaxation delay
(b) a 90o pulse to induce transverse magnetization
(c) a time delay (t1/2), which is incremented on successive iterations of the sequence, that allows evolution of chemical shift and coupling constants
(d) a 180o pulse to flip spins
(e) a second time delay (t1/2), also incremented, during which evolution under chemical shifts (but not under J-couplings) is refocused
(f) FID detection

The 1H NMR of p-methoxy-trans-ethylcinnamate is shown below. The triplet and quartet of the ethyl group and the methoxy group’s singlet are pretty well resolved, but the aromatic/olefin region has some overlap. Given the symmetry in the ring, we would expect 2 unique aromatic signals that are coupled to each other in the indicative para-substituted benzene pattern, an AB quartet. Additionally, both of the protons attached to the olefin moiety should also give rise to doublets (via coupling to each other)

The JRES spectrum, shown below, helps us to fully assign all coupling constants and to identify the coupled spin systems unambiguously. In particular, each olefinic proton appears as a doublet with large J value (15.9 Hz) that is indicative of a trans orientation around the double bond. These doublets are found on either side of the aromatic AB quartet, fully overlapping it around 7.5 ppm. The aromatic peaks have a coupling of 8.4 Hz, and the ethyl group has a nice quartet and triplet for the -CH2- and –CH3, respectively, with a coupling constant of 7.1 Hz.

These type of experiments have application in biology[2] and metabolomics[3] and are possibly even more important for low-field NMR spectra, where the chemical shift differences are smaller relative to corresponding J couplings( Dd/J is about 4-6 for the resonances shown here) and we observe more second order strong-coupling effects that make the spectrum more difficult to fully assign by just visual inspection.

[1] Aue, W.P.; Karhan, H.; Ernst, R. R. J. Chem. Phys. 1976, 64, 4226
[2] Huang, Y.; Zhang, Z.; Chen, H.; Feng, J.; Cai, S.; Chen, Z. Scientific Reports, 2015, 5, 8390
[3] Ludwig, C.; Viant, M. R. Phytochem. Anal. 2010, 21, 22