In substitution reactions the tertiary substrates cannot undergo substitution via neighboring group participation (NGP) due to the steric impediment, this means that the volume occupied by the substituents is very large and makes it impossible to attack the nucleophile to the substrate carbon.
The forces of attraction between particles in their gaseous state seems to be nonexistential. therefore scientist would care less. however another state after gas which is plasma has a lesser force of attraction than the gaseous state.