Three isoelectronic reactions, proton transfer (PT), hydrogen abstraction (HA), and electron transfer (ET), of NH; with NH,, H,O, and HF have been studied using ab initio molecular orbital calculations. For the reaction of N H l + H,O, the energy of the transition state (TS) is higher than that of the reactants. This is consistent with the experimental observation that the rate constant is less than the average dipole orientation (ADO) rate constant. It seems reasonable that the reaction rate for the reaction N H i + H,O would hardly depend on the u, mode of N H l at least for low-lying excited states (E,,, I 0.714 eV) of the v, mode, because the u2 mode contributes mainly to the normal mode orthogonal to the reaction coordinate at the TS. This is consistent with experimental observation. A similar prediction can be made for the NH;+ HF reaction. The electrontransfer processes for the HA reactions have been examined in terms of the intrinsic reaction coordinate (IRC). The order of reactivity with NH; is NH, > H,O > HF. It is found that the degree of the electron transfer and the reactivity are correlated with the absolute hardness ( r ] ) of NH,, H,O, and HF. This is in accord with the softness as the chemical reactivity index in the density functional theory. 0 1996 John Wiley & Sons, Inc. vibrational energies of the v2 umbrella-bending mode of NH:; the proton transfer (PT) reaction NH; + NH, + NH, + NH: is suppressed as the vibrational energy NH : increases, whereas the hydrogen abstraction (HA) reaction NH; + H,O + NH: + OH hardly depends on the vibrational energy of NH;. Chesnavich and Bowers [2] studied these two reactions using phase-space theory; their calculations have overestimated the rate constant with the isoelectronic reaction NH; + NH, [l-101. For the reactions of NH: + NH, and NH: + H20, Chupka and Russell [ 13 studied the dependence on