NMR of Naphthalene: why are the alpha-protons more downfield than the beta- protons?

[flan318]

Hi, can you please help me explain why the alpha-protons of naphthalene are further downfield? I know that the protons at the alpha position must be more deshielded, but I don't know how to explain why they have less electron density compared to the beta protons. Does this have to do with the number of double bonds that can be drawn in different resonance structures? Thanks for your help!Thanks so much!

[Cubbi5362]

In aromatics, the protons are deshielded due to ring currents. The alpha protons in naphthalene are close enough to the other ring to feel BOTH ring currents. The beta protons only feel one ring's current. Look at anthracene, where the protons on the central ring are even further deshielded (into 8.4 ppm), under the combined effect of three rings.Here's a recent publication on this topic: http://www.biomedsearch.com/nih/Understanding-ring-current-effects-magnetic/18711723.html

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