Why Are the Top Methyl Groups Not Chemically Equivalent?

[silversurf]

Homework Statement

I attached a picture of the molecule. My question is why are the methyl group at the top not chemically equivalent. The answer says that this molecule makes three singlets in HNMR but I was thinking it should make only 2 because the top methyl groups are chemically equivalent aren't they?

Homework Equations

The Attempt at a Solution

 

[lurflurf]

They are not equivalent because each of the methyl groups are in different positions (ie different distance from the oxygen). Also they cannot rotate.

 

[chemisttree]

The distance of the two bridge methyls are certainly different distances (through space) from the oxygen but the largest effect is due to the position of one of the methyls in the deshielding zone of the pi system.

 

[silversurf]

Oh I see. Thank you so much!

 

[DeeNos]

Chemically equivalent hydrogens refer to hydrogen atoms in a molecule that have the same chemical environment and therefore exhibit the same chemical shifts in nuclear magnetic resonance (NMR) spectroscopy. In the molecule provided, the two methyl groups at the top may appear to be in the same position and have the same chemical environment, but they are actually not chemically equivalent. This is because they are connected to different carbon atoms, which affects their chemical environment and therefore their NMR signals. Additionally, the two methyl groups may have different orientations or conformations, which can also affect their NMR signals. Therefore, even though they may appear to be in the same position, they are not chemically equivalent and will produce different NMR signals, resulting in three singlets in the HNMR spectrum.