H NMR Inconsistency


While attempting to compare the H-NMR from the simulation to known database or a simple analysis on my own, it is noticeable that many NMR’s for several products formed are not consistent. Either the multiplicities and/or the integrations are incorrect for the structure. Part of the lab write for my students is to take an IR, NMR, and mass spectrum and do a complete analysis as well as assigning the corresponding H’s to their peaks in the spectrum. Many students will probably come across this issue of inconsistency.

I’m not sure if the H-NMR database can be update somehow but one such example includes 2,3-dibromobutane which in the simulation shows two peaks at around 2.0ppm and 4.0ppm respectively as doublets. The 2.0 ppm is fine as it belongs to the CH3 and is split by the CH but the CH proton at 4.0 ppm appears as a doublet when in reality it should appear as a multiplet and not a doublet. The integrations are 2:1 when they should be 3:1.

I believe there may be many more molecules for which the H-NMR is not consistent with what is expected. I hope that the NMR database can be updated to reflect more accurate representations of known spectra for compounds being synthesized in the simulation.

Thank you.

Hi jeanpinto,

The NMR spectra were taken from real data collected on the molecules. The resolution of the data is a little low, so some of the multiplets are not fully resolved. You are correct about anti-2,3-dibromobutane, which falls under this group.

The program generates random values for the peak heights that are in the correct ratio (for determining the number of protons corresponding to each peak). I collected the proton spectrum of anti-2,3-dibromobutane four different times, and each time the integrations gave me a ratio of ~ 3/1.