Thermochemical properties, Δ_fH°(298), S°(298), and C_P°(T), for n-butyl and n-pentyl hydroperoxides, the alkyl and peroxy radicals, transition states and kinetics for intramolecular hydrogen shift reactions to the peroxy radicals

摘要

Alkyl radicals undergo a rapid association with molecular oxygen (~3O_2) to form an alkyl peroxy radical (ROO·). One very important reaction of the peroxy radicals is the intramolecular H-shift (Abstraction) to form a hydroperoxide alkyl radical (·R'COOH), where the new alkyl radical undergoes a further reaction with O_2 (chemical activation) which may result in chain branching at moderate to low temperatures. The thermochemistry and trends in kinetic parameters for the hydrogen shift reactions from each carbon (four to seven-membered-ring TS) in n-butyl and n-pentyl peroxy radicals (CCCCOO· and CCCCCOO·) are analyzed using density functional and ab initio calculation methods. Thermochemical properties, Δ_fH°(298 K), S°(298 K), and C_P°(T) of saturated linear C4 and C5 aliphatic peroxides (ROOH) as well as the corresponding hydroperoxide-alkyl radical (·R'COOH) are determined. The Δ_fH°(298 K) are obtained from isodesmic reactions and the total energies at the B3LYP/6-311++G(2d,p)//B3LYP/6-31G(d,p) level, with the butyl system also analyzed at the CBS-QB3 level to calibrate the B3LYP accuracy. Contributions to the entropy and the heat capacity from translation, vibration, and external rotations are calculated using the rigid-rotor-harmonic-oscillator approximation based on the B3LYP/6-31G(d,p) structures. Hindered internal rotational contributions to entropies and heat capacities are calculated. The pre-exponential A-factors (T) decrease with the increase of the ring size (4- to 8-member rings in TS, H is included). The pre-exponential factors in the pentyl-peroxy radical are slightly lower than those in n-butyl-peroxy system. The Ea for 4,5, 6, and 7-membered-rings in n-butyl (and n-pentyl) peroxy are 39.8 (41.7), 30.2 (31.0), 19.2 (19.6), 21.1-p (19.6) kcal/mol, the Ea for the 8-membered-ring in n-pentyl peroxy is 22.2-p kcal/mol, respectively. All abstractions are from secondary (-CH_2-) groups except those marked (-p) are primary. It is seen that a significant fraction of the stronger C-H bond on the primary carbons is carried into the transition state Ea. BHandHLYP/6-311G(d,p) and CBS-QB3 methods are compared with the B3LYP method for results n-butyl-peroxy system, and the higher level CBS-QB3 calculation method shows good agreement with the B3LYP results; but not the BHandH for these peroxy radical abstractions.