Application Note 4: Aromatic Systems
and JRES Experiments
One of the features of working on a lower field instrument is the narrower spectral
dispersion which is measured. This means that the spectra of most compounds are
susceptible to second order effects of varying degrees. Second order effects arise when
the difference in chemical shift becomes similar to the coupling constant. J-resolved(JRES) experiments allow the chemical shift to be separated from the J-couplingby displaying them in different dimensions. As a result, overlapping peaks can bedeconvoluted. Some aromatic compounds are examples where the peaks overlap due tothe similarity between the chemical shift and coupling frequencies.

356OH

4-10
OH-5I

62

5

30

45
10
8.5

8.0

7.5

7.0

6.5
f2 (ppm)

6.0

5.5

5.0

4.5

f1 (Hz)1

4-Iodophenol has two aromatic proton resonancesin the 1D proton NMR spectrum. The spectrum
obtained on the Spinsolve (42.5 MHz) contains allof the same information (coupling constant andchemical shift) as the spectrum obtained at300 MHz, but the multiplets occupy more of theppm scale, meaning the multiplets appear more
spread out (Figure 1).

300 MHzOH
126354I

42.5 MHzOH2,63,5CDCl3

8.0

7.5

7.0

6.5

6.0
f1 (ppm)

5.5

5.0

4.5

Figure 1: Spinsolve (42.5 MHz) and high field (300 MHz) data for 4-iodophenol.
1

H NMR (42.5 MHz, Chloroform-d) δ 7.51 (d, J = 9.0 Hz, 2H, H-3, H-5), 6.62 (d, J = 9.0 Hz, 2H, H-2, H-6).

1

H NMR (300 MHz, Chloroform-d) δ 7.51 (d, J = 8.9 Hz, 2H, H-3, H-5), 6.61 (d, J = 8.9 Hz, 2H, H-2, H-6).

Second order effects can make the spectra ofcompounds that have multiple protons withsimilar chemical shift and strong coupling quite
complicated. An example of this is 2-iodophenol,
which has four proton resonances in a similar

chemical shift range, with strong J-coupling. As
a result, the peaks overlap and it is difficult todetermine the chemical shift and coupling patternfrom the 1D proton NMR spectrum.

For 2-iodophenol: Consider the green and purple regions (Figure 2).
For 42.5 MHz:Δδ = 7.01-6.69 ppm = 13.6 Hz
3JH-H = 8 Hz
Δδ = 1.7 J
The chemical shift difference and coupling
are comparable, which explains why thespectrum is so complicated (Figure 2,bottom).

564

3

8.0

For 300 MHz:Δδ = 7.01-6.69 ppm = 96 Hz
3
JH-H = 8 Hz
Δδ = 12 J
The difference in chemical shift is muchlarger than the coupling frequency, so themultiplets are well separated in the spectrum(Figure 2, top).

7.5

7.0

6.5
f1 (ppm)

6.0

5.5

Figure 2: 300 MHz (top) and Spinsolve (bottom) data for 2-iodophenol. The high field data has been overlaid
as coloured lines below the Spinsolve data to show the origin of the splitting pattern observed.

5.0

observed in the spectrum from the high field
instrument can be resolved (Figure 3).

The Spinsolve system is capable of performing
JRES experiments which separate chemical shift
from J-coupling. This means all of the information

56

JRES-1H1H

43

-30
-200

f1 (Hz)-10102030
8.5

8.0

7.5

7.0

6.5

6.0 5.5
f2 (ppm)

5.0

4.5

4.0

3.5

3.0

Figure 3: JRES data for 2-iodophenol obtained on a Spinsolve system with the 1D proton NMR above the f2 trace.

The JRES experiment allows the proton NMR spectral data of 2-iodophenol to be reported, which would
be extremely difficult given only the 1D proton NMR spectrum.
H NMR (42.5 MHz, Chloroform-d) δ 7.66 (dd, J = 7.6, 1.6 Hz, 1H, H-3), 7.23 (td, J = 7.3, 1.6 Hz, 1H, H-5),
6.93 (dd, J = 8.2, 2.0 Hz, 1H, H-6), 6.60 (td, J = 7.5, 2.3 Hz, 1H, H-4), 5.23 (s, 1H, OH).
1

Conclusion
Working on a 42.5 MHz system means that most spectra will display some second order effects
which become more complicated as the difference in chemical shift approaches the magnitude of theJ-coupling. For compounds with simple spectra such as 4-iodophenol, the 1D proton spectrum contains
the same information as that obtained on a high field instrument, but each multiplet occupies more of
the ppm scale. For more complicated systems such as 2-iodophenol, the second order effects make thespectrum difficult to interpret. The JRES experiment was able to separate J-coupling from chemical shift
which allowed the individual resonances to be identified and reported.

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