In contrast to oxygen, fluorine is able to easily migrate to the silicon atom and to destabilize the C4Si2H6N12F12 cage, see Fig. 2b. So, the C4Si2H6N12F12 system is much less stable. To evaluate its lifetime t before the decomposition, we adopt the Arrhenius formula
\(t=\left(\frac{1}{w}\right)\exp\left(-\frac{E_a}{kT}\right)\)
where w is the frequency factor, Ea is the activation energy, k is the Boltzmann’s constant, T is the temperature. In our evaluation, we assume the activation energy is equal to the energy barrier (0.18 eV, see Fig. 2b). Frequency factor w can be defined from the Vineyard formula \cite{Vineyard_1957}
\(w=\frac{\Pi w_i}{\Pi w'_i}\)     .
Here ω and ω’ are the real eigenfrequencies (normal modes) for the ground and transition states, respectively. Note that the numerator in this formula contains an additional factor because the transition state has one imaginary frequency, which is not taken into account. So, we obtain Ea = 0.18 eV and w = 1.93·1015 1/s. In accordance with the Arrhenius formula, t ~ 0.5 ps at T = 300 K. So, CSi5H6N12O12 is unstable at room temperature and is barely suitable for practical applications.
We also stress the fact that the kinetic stability of any cage compound is determined by the energy barrier preventing its decomposition rather than the strain energy enclosed in its framework. For example, methylcubanes demonstrate an inverse relationship between their strain energies and kinetic stabilities.\cite{Katin_2016} Moreover, initial processes leading to the CL-20 decomposition do not necessarily involve any framework transformation. For these reasons, cage strain energy is not a suitable measure of the stability of the CL-20 derivatives, as it is discussed in Ref. \cite{Tan_2014}. Although the CL-20 derivatives containing silicon possess higher strain energies,\cite{Tan_2014} they are not necessarily kinetically unstable.