Which of the following compound will not undergo amine inversion?

• A

  
• B

  $\mathrm{C}{\mathrm{H}}_3-\mathrm{N}\mathrm{H}-\mathrm{C}{\mathrm{H}}_3$
• C

  $\mathrm{N}{\mathrm{H}}_3$
• Correct answer
  D

  

Solution

In  flipping is restricted due to rigidity of the ring system.

Hence, the correct answer is option: 'D'.