We present an ab initio molecular dynamics simulation study of a CH3CN-(H2O)(40) cluster with an excess electron (EE) injected vertically in this work. Instead of surface bound or internally solvated electron, a hydrated CH3CN- is first formed as the CN transient after geometrical relaxation. The driving forces for the formation of CH3CN- are bending vibration of angle CCN angle, which initiates transfer of an extra charge to the CH3CN LUMO, and hydration effect of the immediate water molecules, which plays a stabilizing role. Solvent thermal fluctuation can lead to different resonances (the quasi-C2-resonance versus quasi-N-resonance) from the CN transient and further cause the hydrated CH3CN- system to evolve via two distinctly different pathways featuring spontaneous proton transfer to the central C and N sites, producing two different protonation products, respectively. The solvent thermal fluctuation induced formation of hydrogen bonding with the corresponding sites (C-2 versus N) is responsible for the quasi-resonances and interconversion between three resonant structures and further proton transfers featuring spontaneous transfer of a proton to C-2 or to N from its interacting water molecule. The duration of CH3CN- for either of the two proton transfer processes is less than 200 fs. On the basis of experimental ESR results in which only the CH3CHN radical was found and present theoretical calculations, it is suggested that the trans-CH3CNH radical can be further converted to the CH3CHN radical via a water-mediated hydrogen atom transfer path.