Fig. 3. Decomposition path of CF3I at the
B3LYP/LanL2DZ level of theory.
During CF3I dissociation, two possible products can be
formed when different bonds break. The most favorable is that
CF3I can break the
C─I bond to form CF3· + I, and the reaction barrier is
57.526 kcal·mol−1. In this process, the length of C─I
bond gradually increases from 2.172Å of CF3I to 2.559Å
until the fracture. In addition, two kinds of isomerization of
CF3I can occur to produce the same substance,
CF2· + IF. One of the lower energy barrier is that atom
I on atom C is transferred to an atom F of atom C, and the product
CF2· + IF (P2) is formed through the transition state
TSa2, and the reaction energy barrier is 80.904
kcal·mol−1. In this process, atom F on molecule
CF3I is close to atom I. The I atom and F atom gradually
combine and separate from the CF3I molecule to form a
triplet of difluorocarbine CF2· and iodine fluoride. The
other is that CF3I generates product P1 through the
transition state TSa1 with an energy barrier of 89.763
kcal·mol−1. The potential energy curve of the
decomposition path of CF3I is shown in Fig.4 .
It can be seen that the energy barriers of C─I bond fracture are all
lower than that of C─F bond fracture, that is, the generation of
CF3· from C─I bond fracture is more likely to occur
kinetically. The reaction rate constants of each decomposition path of
CF3I at 1 atm and at different temperatures (280-2000K)
are shown in Fig.5 . It can be seen that the rate constants of
C─I bond homolysis reaction are all higher than that of C─F bond at
different temperatures, which further indicates that C─I bond fracture
is easier than C─F bond from the perspective of reaction rate.