The thermal effects on the ClCH2CN+Cl(-)S(N)2 reaction at 300 K have been studied by ab initio molecular dynamics. The role of the cyano-substituent is explained by the formation of a hydrogen bond and is discussed by comparison with the reactions of ClCH3 and Cl2CH2. It is shown that the interactions occurring in the prereactive complex are sufficiently strong to be effective also at high temperature. It is also shown that the temperature effects on the activation barrier are significant.