Theoretical study on the reaction mechanism of 2,4-D butyl ester with OH radical

The degradation mechanism of 2,4-D butyl ester initiated by OH radical is investigated using a dual-level direct dynamics method. Seventeen reaction channels were established according to the different reaction mechanisms. The equilibrium geometries and the corresponding harmonic vibrational frequencies of all stationary points were obtained at the level of M06-2X/6-31+G(d,p). At the same level, the minimum energy paths were calculated, the energy values were further refined by the higher level of M06-2X/6-311++G(3df,2p). The rate constants of all reaction channels were calculated by the improved canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200–1000 K. The total rate constants are in good agreement with the available experimental data, and the three-parameter expression k ( T ) = 3.66 × 10 −22 T 3.84 exp (877.84/ T ) cm 3 molecule −1 s −1 was given. The calculated results indicated that the H-abstraction reaction channel R5a on the side chain is the major degradation channel, while the addition to the benzene ring of 2,4-D butyl ester is the most competitive channels.