The microscopic capture cross section sigma for the proton-transfer reaction of the ammonia cation has been studied by using the classical trajectory method. The orientation of the pulsing dipole moment of the ammonia cation affects sigma considerably. This orientational effect is inherent in the vibrating ion-polar molecule reaction. The reactivity is further depressed as the rotational energy of the polar molecule increases, where sigma is a monotone decreasing function of rotational energy.