Two solvents, two different spectra - Aromatic Solvent Induced Shifts

In my opinion, one of the most helpful papers ^[1] in the field of NMR spectroscopy in Organic Chemistry consists of ‘just’ two tables. In these, the chemical shifts (¹H and ¹³C) of as many as forty-two common impurities in twelve different deuterated solvents are listed. This is gold! Why? We know, that the signals of one and the same compound can show a rather high discrepancy in its chemical shifts in different solvents. But did you also know, that there is a concept called Aromatic Solvent Induced Shifts (ASIS), which benefits from this fact?

Although the background is not completely understood yet, empirical observations support the hypothesis that polar compounds show significant different chemical shifts in aromatic solvents like C₆D₆ compared to less magnetically interactive solvents like CDCl₃.^[2] This property can be used for the deconvolution of overlapping signals, which is a recurring topic especially in benchtop NMR spectroscopy.

One example for this is tropolone (Figure 1). You can see that the overlapping of the signals is way more prominent in CDCl₃ (top) than it is in C₆D₆(bottom). Please check the 500 MHz spectrum of this compound in the same solvents in Dr. Glenn Facey’s Blog _[3] here. As the proton of the hydroxy group undergoes a dynamically exchange between the two oxygen atoms, it is observed as a broad peak (yellow).

FIGURE 1. 60 MHZ ¹H NMR SPECTRA OF TROPOLONE IN (CDCL₃(TOP) AND C₆D₆(BOTTOM)).

In Figure 2 you can find the ¹H, ¹H-COSY NMR spectra of eugenol in C₆D₆ and CDCl₃, respectively along with the assignment.

FIGURE 2. 60 MHZ ¹H,¹H-COSY NMR SPECTRA OF EUGENOL IN A) CDCL₃ AND B) C₆D₆.

Two things can be observed in these spectra: First, in the aromatic section in spectrum a) is more convoluted, while in spectrum b) we can clearly distinguish at least the two aromatic H atoms in ³J distance (red and magenta), which show a cross signal between the baseline separated signals. So, for the aromatic protons, in regards of minimizing signal overlapping, the aromatic solvent appears to be favorable. Second, if we look at the aliphatic region of both spectra we can see, that the aliphatic CH₂ group ("yellow) and the methoxy group (lime) do overlap in benzene, while in chloroform, these signals are separated.

You see, sometimes it’s not only a question of solubility but if you encounter a little complicated spectrum, you could try these solvents to see if you can shed some light on the signal separation just as easy like that.

References

^[1]G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb, A. Nudelman, B. M. Stoltz, J. E. Bercaw, K. I. Goldberg, Organometallics 2010, 29, 2176–2179.
^[2]H. J. Reich, University of Wisconsin, Chem. 605, HMR-5.02.11 – 5.02.13, improve-your-chemical-shift-resolution.html">(accessed on 2019-08-30)
^[3]G. A. Facey, University of Ottaw, U of O NMR Blog, http://u-of-o-nmr-facility.blogspot.com/2007/10/improve-your-chemical-shift-resolution.html(accessed on 2019-08-30)