Why does dimethyl ether have a bond angle bigger than 109.5 degrees?

Question

Why does dimethyl ether have a bond angle bigger than 109.5 degrees, in contradiction with VSEPR theory?

Answer 1

In case of $\ce{Me2O}$, the positive deviation of the bond angle can be explained on the basis of steric crowding of two methyl groups.

There are other contradiction too like this:

In case of $\ce{(SiH3)2O}$, the huge bond angle increase is mainly due to back bonding and partly due to steric repulsion.

Answer 2

Let us compare the $\ce{C-O-C}$ bond angle in dimethyl ether (110.3°) with H-O-H bond angle in $\ce{H2O}$ (104.5°).

The bond angle in dimethyl ether is greater due to the following reason. The $\ce{CH3}$ functional group is many times heavier than the H atom (both act as side atoms). So the effect of lone pair - lone pair repulsion in $\ce{H2O}$ is pronounced in $\ce{H2O}$ whereas due to heavy molecular weight the repulsive forces aren't able to fully force $\ce{CH3}$ functional groups to compress them.