Question

In the H NMR spectrum of most aromatic compounds, the aromatic protons exhibit a chemical shift of 7-8 ppm. However, in ferrocene, the chemical shift of the aromatic protons is 4.15 ppm. Explain what factors cause the upfield shift.

Answer 1

The peak at 4.15 ppm belongs to the protons in ferrocene. Due to the five-fold symmetry of the cyclopentadienyl rings, the protons are all chemically equivalent, hence there is only one signal. The rapid rotation of the rings around the Fe centre results in the rapid equilibration of the protons in ferrocene. Furthermore, there are no neighbouring protons to couple with thus the signal is a singlet. The chemical shift value of 4.15 ppm is lower than that for aromatic protons at 7 ppm because there is an increased electron density in the two rings that increases the shielding effect, causing a decrease in the chemical shift value.