Question

Why do these protons appear at 6.9 and 6.93 ppm, at lower ppm value? I assumed it was due to the anisotropic effect, specifically I thought the circulating (pi) electrons induced a magnetic field which causes them to experience a stronger net field, and those that were lower were not being affected by the induced magnetic field. This is instead apparently due to resonance. How come it is due to resonance? Is the circulation of electrons (ring current) not affecting them all or instead is resonance the primary reason why some protons come lower than others?

Answer 1

Protons appear at different ppm values in NMR spectra primarily due to their chemical environment which influences their resonance frequency in nuclear magnetic resonance (NMR), rather than just the ring current effects.

Step-by-step explanation:

The appearance of protons at 6.9 and 6.93 ppm in NMR spectra can be attributed to nuclear magnetic resonance (NMR), which is a resonance phenomenon sensitive to the chemical environment of a nucleus.

While the circulation of pi electrons and resulting ring current can indeed affect the local magnetic field experienced by protons in aromatic systems, leading to anisotropic effects, the primary reason for the variation in chemical shift among protons is due to their different resonance environments.

In resonance, the specific frequency of the radio waves that are absorbed and re-emitted by protons depends on the type of nucleus and its chemical environment, which involves the electron density around the nucleus and can be affected by electronic effects such as resonance and inductive effects.