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proton NMR

proton NMR - Organic Chemistry Forum

proton NMR - Organic Chemistry Forum

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Old 12-04-2006, 08:15 AM
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Default proton NMR

I am just learning about proton NMR, and i understand why protons
adjacent to a benzene ring are deshielded and thus have a higher shift
than expected. I am having troubole understanding why this shift is so
much higher, than say, the terminal proton in acetylene. Any thoughts?

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Old 07-08-2011, 04:58 PM
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Default Re: proton NMR

This is because of the location of the pi electrons.
You must realize that the effect of shielding happens due to the presence of electrons. The more dense the electron cloud around the nucleus you are monitoring (in this case protons) the more shielded they are.
In case of the benzene ring, the pi electron cloud lies above and below the plane of the benzene ring. Therefore, the pi electrons do not cover the nuclei in the plane of the ring (which is where the hydrogens lie).
In case of acylene, there are two pi clouds, one perpendicular to the plane of the bond and one in line with the plane of the bond. The pi electrons of the pi cloud which is in line with the plane of the bond causes shielding of the attached hydrogens.
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