The carbon-bromine distance in
the transition state in the tert-butyl system is longer than that
in the methyl system, (2.9Å compared to 2.5Å). This leads
to an increase in charge separation.
For each of the transition states,
select Surfaces from the Display menu and double click
on the line "density potential..." in the dialog which results.
Electrostatic potential maps
show greater charge separation (a greater range of colors) in transition
state for addition to tert-butyl bromide than in the transition
state for addition to methyl bromide. Electrostatics and not steric crowding
is the cause of the decrease in reaction rate.