We theoretically investigated collision cross sections due to the attractive forces between NH4+(NH3)(n-1) (n=2-8) and ND3. We found that the dependence of the collision cross sections on collision energy and cluster size are comparable to those of measured fusion cross sections. The kinetic magic number, n=5, is related to the structure of the pentamer. Namely, the center ion in the pentamer is surrounded by first-shell ammonia molecules.