Ab initio molecular dynamics simulations were carried out for the borderline reaction pathways in the reaction of CH2O.- with CH3Cl. The simulations reveal distinctive features of three types of mechanisms passing through the S(N)2-like transition state (TS): (i) a direct formation of S(N)2 products, (ii) a direct formation of ET products, and (iii) a two-step formation of ET products via the S(N)2 valley. The direct formation of the ET product through the S(N)2-like TS appears to be more favorable at higher temperatures. The two-step process depends on the amount of energy that goes into the C-C stretching mode. (C) 2002 Elsevier Science B.V. All rights reserved.