The geometries and energetics of transition states (TS) for radical deactivation reactions, including competitive combination and disproportionation reactions. have been studied for the modeled 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)mediated free-radical polymerization of acrylonitrile with quantum mechanical calculations at the DFT/TJB3-LYP/6-311+G(3df,2p)//(U)AM1 level of theory (where DFT is density functional theory, AMI is Austin model 1, and UAM1 is unrestricted Austin model 1). A method providing reasonable starting geometries for an effective search for TS between the TEMPO radical and 1-cyanopropyl radical mimicking the growing polyacrylonitrile macroradical is shown. For the hydrogen atom abstraction reaction by the TEMPO radical from the 1-cyanopropyl radical, practically one TS has been found, whereas for the combination reaction of the radicals, Several TS have been found, mainly differing in out-of-plane angle alpha of the N-O bond in the TEMPO structure. alpha in the TS is correlated with the activation energy, Lambda E-0(1), determined from the single-point calculation at the DFT UB3-LYP/6-311+G(3df, 2p)//UAM1 level for the combination reaction of CH(3)AN. with the TEMPO radical. The theoretical activation energy for the coupling reaction from DFT UB3-LYP/6-311+G(3df, 2p)//UAM1 calculations has been estimated to be 11.6 kcal mol (1), that is, only about 4.5 times Smaller than Delta E-0(double dagger) for the disproportionation reaction obtained with the DFT UB3-LYP/6-311+G(3df, 2p)//(U)AM1 approach. (c) 2005 Wiley Periodicals, Inc.