# How to explain two NMR spectra of the (allegedly) same compound with slightly different shifts?

For my synthetic lab work I synthesised a Horner-Wadsworth-Emmons phosphonate; for the sake of the question let’s assume it to be the one presented in the image below.

When first synthesising it, I of course recorded NMR data. I performed a Horner-Wadsworth-Emmons reaction using a tiny bit of the phosphonate and kept the rest in the fridge.

One month later I retried the HWE and got very bad yields, so I decided to record another NMR spectra. The two spectra themselves look identical and the signals (those that aren’t multiplets) also show the same coupling constants. However, when referencing the spectrum to the chloroform signal ($7.26~\mathrm{ppm}$), all signals are shifted by the constant value of $\approx 0.07~\mathrm{ppm}$.

I decided to check the old literature spectrum from 1986, but it was unhelpful: It only reported multiplets such that both of my spectra comfortably fall well within the multiplets’ limits.

Considering that the two spectra look identical and that the difference between two signals corresponding to the same hydrogens is identical as are the coupling constants, I want to assume that both spectra show an identical compound. However, the $0.07~\mathrm{ppm}$ shift for everything save chloroform still irritates me.

Both spectra were recorded at $500~\mathrm{MHz}$. The parameters of the earlier one state a temperature of $25~\mathrm{^\circ C}$, the later one was apparantly measured at $25.1~\mathrm{^\circ C}$. However, I don’t think that that could explain the shift.

What could explain this shift difference?

(My first thoughts were some changes to the compound that only affect the phosphorus atom, but aside from deoxygenation being chemically unlikely shouldn’t it also cause random chemical shift differences including slightly different coupling constants? In fact, shouldn’t any chemical modification cause the shifts to change in a seemingly random fashion depending on the proximity of hydrogen in question and entity modified?)

After checking because I was asked in the comments: The later spectrum (the one that has higher shift values) has broadened peaks. That includes the chloroform peak.

• Do the shifts change for carbon and phosphorous? – Lighthart Apr 18 '16 at 17:04
• For reference, the later spectrum included a dichloromethane peak that stemmed from the bulb of the pipette I used for pipetting chloroform. The former didn’t, so I wasn’t able to check if dichloromethane was shifted, too. A very minor peak at $\approx -0.02$ or $\approx -0.09$ was shifted. – Jan Apr 18 '16 at 17:04
• If the peak widths are different, that means the machine's field has drifted a little. And as a result, somewhat wider peaks (or worse shim) may be modestly adjusting the digital calculation of peak maximum (and hence chemical shift). Wider peaks would provide evidence that this is a measurement problem, not a chemical purity issue. That being said, you indicate you are not telling us exactly the compound synthesized, so we can't really judge if 0.1C will impact for reasons of conformational/rotomer changes. – Lighthart Apr 18 '16 at 17:31
• Another possible reason is that the shift is caused because of aggregation. Not unreasonable to happen for a phosphonate but on the other hand the shift you observe is too wide to be explained by aggregation alone? Have a look at this [paper ](pubs.acs.org/doi/pdf/10.1021/jm400535b) for example. I only know about this from its use as an assay in med chem so I couldn't help with more specific info. – K_P Apr 18 '16 at 22:36
• That just seems so strange. How could the magnetic properties of one nucleus change without any others changing? Even if the probe had drifted, I would expect everything to move, not just the solvent peak. – SendersReagent Apr 19 '16 at 12:18