Several factors (steric, electronic, orbital interactions etc.) can affect the inversion barrier of an amine. In the given pair which data is correctly placed ?

 

(a) The barrier to inversion for i-Pr₂ NMe is less than for Me₃N because going from sp³ (tetrahedral) to sp² (trigonal planar) spreads the bulky isopropyl groups further apart and relieves steric crowding. In other words the isopropyl groups destabilize the pyramidal amine more than the planar transition structure.

 

(b) The sp² transition state has ideal bond angle of ~120°. The three member ring T.S. is highly strained because it cannot achieve 120° due to the small ring locking the angles at ~60°. The 5-membered ring can easily adopt l20° angle.
 

(c) As the nitrogen goes from sp³ $\to$ sp² , it becomes effectively more electronegative. (Remember : more s-character means more electronegative) The electronegative chlorine atoms pull electron density away from N more than the methyl groups, creating an electron - deficient N, and destabilising the T.S.