Why can quaternary ammonium ions have stable mirror-image forms?

Quaternary ammonium ions can have stable mirror-image forms because they possess a rigid, tetrahedral geometry that prevents the rapid inversion seen in standard amines.1

In typical amines, the nitrogen atom has a nonbonded electron pair that allows the molecule to undergo a rapid inversion process—similar to an umbrella turning inside out—which interconverts mirror-image configurations. Because of this constant flipping, individual optical isomers (or enantiomers) cannot be isolated.1

In contrast, a quaternary ammonium ion replaces that nonbonded electron pair with a fourth substituent, resulting in a nearly regular tetrahedral arrangement. Because there is no lone pair to facilitate inversion, the nitrogen atom is locked into its configuration. When all four substituents attached to the nitrogen are different, the nitrogen becomes chiral, and its left-handed and right-handed mirror-image forms remain indefinitely stable and isolable.12