# Why does the proton NMR of ferrocenecarboxaldehyde appear to be four singlets?

One singlet stems from the aldehyde H, and another from the Cp ring with 5 hydrogens attached.

However, the ring with the aldehyde group attached has hydrogens in two chemical environments which are just three bonds apart from each other. Why do they not couple? (400 MHz, CDCl3)

• I'm thinking: A is the aldehyde H, D is the 5 H's on the Cp ring without the aldehyde, B the 2 H's closer to the aldehyde on the ring and C the remaining two, right? Feb 1 '19 at 1:11
• Related: 1H NMR of Acetylferrocene I generally agree with your assignments. As long writes in the linked question: due to magnetic inequivalence, this is an AA'XX' system where the spectrum will exhibit second order effects. Why this fine structure isn't manifested, though, I don't know. Feb 1 '19 at 1:19
• Judging from your residual chloroform peak, your sample is highly concentrate. Also, for good mesure, it is at least $\pu{0.1 ppm}$ broad ($\pu{40 Hz}$ since you used $\pu{400 MHz}$ machine). That means your peaks looks broad singlets, instead of having splitted. Feb 1 '19 at 2:16
• @MathewMahindaratne that explanation is very reasonable, but if you look at a c13 spectrum (eg Aldrich) you only see one peak for the substituted ring. What's that all about? Feb 1 '19 at 10:28
• @Try Hard: I posted some date to answer your question a few days ago. I apologize for the delay. Feb 5 '19 at 2:54

I like to address some interesting NMR features of ferrocene derivatives. Try Hard's question about $$\mathrm{^{13}C-NMR}$$ of ferrocenecarboxaldehyde showing one signal for the substituted cyclopentyl ring prompt me to find some solid evidence. However, it seems like most of ferrocene derivatives give similar results regardless of how strong magnetic field you used. Therefore, Aldrich NMR spectra may not be the best spectra with high definition (e.g., they use $$\lt \pu{300 MHz}$$ machine for $$\mathrm{^{1}H-NMR}$$).
Following $$\mathrm{^{13}C-NMR}$$ values from very recent paper, which published NMR data of few ferrocene derivatives using a $$\pu{400 MHz}$$ (but $$\pu{100 MHz}$$ for $$\mathrm{^{13}C}$$) machine (Ref.1):
The values in left hand side is for $$\mathrm{C_p}$$ ring (substituted) while single value at the bottom is for all 5 $$\ce{C}$$s of unsubstituted $$\mathrm{C_{p'}}$$ ring. If you look closely, you'd see monosubstituted rings have so close values that they may overlap in spectra taken in low resolution machines.