Which of the following cannot be represented by resonance structures?(a) Dimethyl ether(b) Nitrate anion(c) Carboxylate anion(d) Toluene

The molecule that cannot be represented by resonance structures is (a) Dimethyl ether.

Resonance structures are used to represent the delocalization of electrons in certain molecules where a single Lewis structure is not sufficient to describe the electron configuration.

(a) Dimethyl ether (CH3-O-CH3) does not have any delocalized electrons. It is a simple ether where all the atoms have complete octets and there are no charges, so no resonance is possible.

On the other hand, (b) Nitrate anion (NO3-), (c) Carboxylate anion (COO-) and (d) Toluene (C6H5CH3) all have delocalized electrons and can be represented by resonance structures.